# Reference Array and Design Consideration for the next-generation Event
Horizon Telescope

Sheperd S. Doeleman Center for Astrophysics $|$ Harvard & Smithsonian, 60
Garden Street, Cambridge, MA 02138, USA  Black Hole Initiative at Harvard
University, 20 Garden Street, Cambridge, MA 02138, USA  John Barrett
Massachusetts Institute of Technology Haystack Observatory, 99 Millstone Road,
Westford, MA 01886, USA  Lindy Blackburn Center for Astrophysics $|$ Harvard &
Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA  Black Hole Initiative
at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA  Katherine
Bouman  California Institute of Technology, 1200 East California Boulevard,
Pasadena, CA 91125 USA  Avery E. Broderick Perimeter Institute for Theoretical
Physics, 31 Caroline Street North, Waterloo, ON, N2L 2Y5, Canada  Department
of Physics and Astronomy, University of Waterloo, 200 University Avenue West,
Waterloo, ON, N2L 3G1, Canada  Waterloo Centre for Astrophysics, University of
Waterloo, Waterloo, ON, N2L 3G1, Canada  Ryan Chaves  Center for Astrophysics
$|$ Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA  Vincent
L. Fish Massachusetts Institute of Technology Haystack Observatory, 99
Millstone Road, Westford, MA 01886, USA  Garret Fitzpatrick  Center for
Astrophysics $|$ Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138,
USA  Antonio Fuentes  Instituto de Astrofísica de Andalucía-CSIC, Glorieta de
la Astronomía s/n, E-18008 Granada, Spain  Mark Freeman  Center for
Astrophysics $|$ Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138,
USA  José L. Gómez Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la
Astronomía s/n, E-18008 Granada, Spain  Kari Haworth  Center for Astrophysics
$|$ Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA  Janice
Houston  Center for Astrophysics $|$ Harvard & Smithsonian, 60 Garden Street,
Cambridge, MA 02138, USA  Sara Issaoun  Center for Astrophysics $|$ Harvard &
Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA  Michael D. Johnson
Center for Astrophysics $|$ Harvard & Smithsonian, 60 Garden Street,
Cambridge, MA 02138, USA  Black Hole Initiative at Harvard University, 20
Garden Street, Cambridge, MA 02138, USA  Mark Kettenis Joint Institute for
VLBI ERIC (JIVE), Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
Laurent Loinard Instituto de Radioastronomía y Astrofísica, Universidad
Nacional Autónoma de México, Morelia 58089, México  Instituto de Astronomía,
Universidad Nacional Autónoma de México (UNAM), Apdo Postal 70-264, Ciudad de
México, México  Neil Nagar Astronomy Department, Universidad de Concepción,
Casilla 160-C, Concepción, Chile  Gopal Narayanan Department of Astronomy,
University of Massachusetts, 01003, Amherst, MA, USA  Aaron Oppenheimer
Center for Astrophysics $|$ Harvard & Smithsonian, 60 Garden Street,
Cambridge, MA 02138, USA  Daniel C. M. Palumbo Center for Astrophysics $|$
Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA  Black Hole
Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
Nimesh Patel Center for Astrophysics $|$ Harvard & Smithsonian, 60 Garden
Street, Cambridge, MA 02138, USA  Dominic W. Pesce Center for Astrophysics $|$
Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA  Black Hole
Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
Alexander W. Raymond Black Hole Initiative at Harvard University, 20 Garden
Street, Cambridge, MA 02138, USA  Center for Astrophysics $|$ Harvard &
Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA  Freek Roelofs Center
for Astrophysics $|$ Harvard & Smithsonian, 60 Garden Street, Cambridge, MA
02138, USA  Black Hole Initiative at Harvard University, 20 Garden Street,
Cambridge, MA 02138, USA  Ranjani Srinivasan  Center for Astrophysics $|$
Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA  Paul Tiede
Center for Astrophysics $|$ Harvard & Smithsonian, 60 Garden Street,
Cambridge, MA 02138, USA  Black Hole Initiative at Harvard University, 20
Garden Street, Cambridge, MA 02138, USA  Jonathan Weintroub Center for
Astrophysics $|$ Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138,
USA  Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge,
MA 02138, USA  Maciek Wielgus Max-Planck-Institut für Radioastronomie, Auf dem
Hügel 69, D-53121 Bonn, Germany

###### Abstract

We describe the process to design, architect, and implement a transformative
enhancement of the Event Horizon Telescope (EHT). This program - the next-
generation Event Horizon Telescope (ngEHT) - will form a networked global
array of radio dishes capable of making high-fidelity real-time movies of
supermassive black holes (SMBH) and their emanating jets. This builds upon the
EHT principally by deploying additional modest-diameter dishes to optimized
geographic locations to enhance the current global mm/submm wavelength Very
Long Baseline Interferometric (VLBI) array, which has, to date, utilized
mostly pre-existing radio telescopes. The ngEHT program further focuses on
observing at three frequencies simultaneously for increased sensitivity and
Fourier spatial frequency coverage. Here, the concept, science goals, design
considerations, station siting and instrument prototyping is discussed, and a
preliminary reference array to be implemented in phases is described.

##  1 Introduction

On April 10, 2019, the Event Horizon Telescope project (EHT) released images
of the supermassive black hole at the heart of galaxy M87 (Event Horizon
Telescope Collaboration et al., 2019a, b, c, d, e, f). The observed ring of
emission, formed by radio waves lensed in the gravitational field of a 6.5
billion solar mass black hole, has dimensions that match the predictions of
General Relativity. Images of Sgr A∗, the 4 million solar mass black hole at
the center of the Milky Way, also exhibit a ring morphology with diameters
anticipated by theory (Event Horizon Telescope Collaboration et al., 2022a, b,
c, d, e, f). These results confirm that the EHT has observed the strong
gravitational lensing signature of supermassive black holes (Bardeen, 1973;
Luminet, 1979; Falcke et al., 2000; Takahashi, 2004; Broderick & Loeb, 2006),
and these images have opened a new field of precision black hole studies on
horizon scales.

This work built upon decades of technical development and precursor
observations. Pioneering first Very Long Baseline Interferometry (VLBI)
experiments at wavelengths of 1.3mm (Padin et al., 1990; Krichbaum et al.,
1998) demonstrated that observations with the required resolution were
possible at frequencies where AGN are likely to be optically thin. Discovery
of horizon-scale structure in both Sgr A* and M87 with purpose-built ultra-
high bandwidth systems on early EHT arrays (Doeleman et al., 2008, 2012)
confirmed that imaging these sources was feasible. Subsequent observations
revealed time-variability and ordered magnetic fields on Schwarzschild radius
dimensions (Fish et al., 2011; Johnson et al., 2015). Emergence of the EHT to
a full imaging array grew from building community support through a decadal
review processes (Doeleman et al., 2009), efforts to modify large scale
international facilities, such as ALMA, through global cooperation (Doeleman,
2010; Matthews et al., 2018), and work to enable VLBI capability at the most
remote observatories on the planet (Inoue et al., 2014; Kim et al., 2018).
Over the course of two decades, all the technical, logistical, organizational
and analytical aspects of the full EHT were implemented by an expert team that
grew from a few 10’s to over 200 collaborators worldwide.

Building upon this legacy, the next-generation EHT (ngEHT) provides a roadmap
to greatly accelerate the development of the EHT, envisaging a transformative
new instrument capable of delivering real-time black hole movies. Where the
EHT used existing mm/sub-mm facilities to form a first imaging array, the
ngEHT will take the next step by designing and locating new dishes to optimize
performance and scientific return. This vision offers excellent opportunities
to engage the curious public on many levels. It is estimated that over a
billion people have now seen the M87 image (Christensen et al., 2019). We
anticipate that the long term public and STEM education engagement as the
ngEHT builds to its goal of black hole ‘cinema’ will be similar in scope.

For the purposes of this paper, the term ngEHT is used to describe a program
to explore and define a long-term plan to enhance the EHT to realize a new set
of transformative science goals. This paper describes that vision by outlining
improvements in bandwidth, frequency range, new antenna deployment and new
operating modes that enable increases in angular resolution, Fourier spatial
frequency coverage, sensitivity and temporal resolution. For brevity, ngEHT
will also be used as shorthand for the future arrays that will emerge through
these plans, as well as for the constellation of improvements that constitute
the ngEHT concept.

Technical advances in several areas make design and implementation of the
ngEHT within this decade a realistic goal. Over most of the past two decades,
the bandwidth of VLBI systems has kept pace with Moore’s Law - a doubling of
capacity and speed approximately every 18 month (see, e.g., Event Horizon
Telescope Collaboration et al. (2019b) and Figure 10 below). This is primarily
due to the migration of VLBI instrumentation development to designs that adopt
industry-standard components, including CPUs, Analog to Digital Converters,
Field Programmable Gate Arrays (FPGA) and commercial data transmission
protocols (e.g., Vertatschitsch et al. (2015)). The increased bandwidth of
these components and systems match the analog bandwidth improvements planned
for international and national submm facilities, including ALMA (Carpenter et
al., 2020) and the Submillimeter Array (Grimes et al., 2020). Meanwhile, the
transport of larger data volumes captured by next-generation VLBI systems can
be accommodated either by high-speed internet connections (Selina et al.,
2018, 2022), or increased capacity of hard disk and solid-state disk, which
can be shipped by commercial carriers. Once gathered at a central computing
facility, the many 10’s of Petabytes anticipated for next-generation EHT array
observations can be correlated by purpose-built clusters, allocated time on
national super-computing centers, or through virtual machine creation using
cloud architectures (e.g., Gill et al. (2019)). Once correlated, data analysis
options include a growing number of video reconstruction algorithms that can
render the dynamics of supermassive black hole activity on horizon scales
(Johnson et al., 2017; Bouman et al., 2018; Arras et al., 2022). These
developments, combined with a positive mention of the ngEHT project (Doeleman
et al., 2019) in the Radio/Millimeter/sub-Milimeter panel of the most recent
US Astronomy Decadal Review (National Academies of Sciences, Engineering, and
Medicine, 2021), imply that implementing the ngEHT is both feasible and
timely.

##  2 ngEHT Concept

The first images of M87 and Sgr A* revealed a clear ring morphology, but they
achieved a dynamic range of only ${\sim}10$ (Event Horizon Telescope
Collaboration et al., 2019d, 2022c). Image fidelity from the 2017 data sets
was primarily limited by sparse interferometric baseline coverage. The
shortest baselines, between telescopes located at the same geographic location
(ALMA-APEX in Chile and JCMT-SMA in Hawai’i), probe arc second scale
structures. There is a large gap between these “intra-site” baselines and the
first “inter-site” baseline that links LMT-SMT, which creates a baseline with
angular resolution corresponding to $\sim 150\,\mu$as. Furthermore, the 2017
observations included inter-site baselines between only five geographic
locations for M87, and six locations for Sgr A*, fundamentally limiting the
fidelity of image reconstruction on angular scales that resolve the black hole
shadow.

The ngEHT concept focuses on overcoming these limits through several key
developments. Foremost among these is the deployment of relatively modest
diameter radio dishes at optimized locations to increase baseline coverage.
Figure 1 shows that even a 6m diameter dish in marginal weather conditions can
detect long baseline correlated fluxes from Sgr A* and M87 when paired with a
large “anchor” aperture. This reflects the fact that the 2017 observations,
though using fringe detection algorithms limited to 2GHz bandwidth, achieved
signal-to-noise ratios that were typically in excess of $\sim 10$ and often
reached $\sim 100$. In other words, the current EHT is limited by baseline
coverage and not sensitivity considerations. Through this increased baseline
coverage, the ngEHT will reach image dynamic ranges that exceed 1000:1 for
full Earth rotation aperture synthesis observations of M87 and other AGN. Time
lapse movies that capture the dynamics of M87’s accretion flow and jet launch
by combining bi-weekly observations will achieve similar dynamic ranges (pre-
cursor multi-epoch observations are possible with the existing EHT at lower
imaging dynamic range). For Sgr A*, which has an Innermost Stable Circular
Orbit (ISCO) period of $\sim\frac{1}{2}\,$hour, the ngEHT snapshot baseline
coverage in 5 minute integrations will be sufficient for near real-time video
reconstruction. Figure 2 shows the current EHT array and the location of
potential new ngEHT sites.

Figure 1: Interferometric baselines between key anchor stations and modest-
diameter dishes have sufficient sensitivity to detect target flux densities on
time-scales of several seconds. A star marks the correlated flux expected for
SgrA* and M87 over long ngEHT baselines. Performance for 2017 is taken over 2
GHz of bandwidth and the observed median sensitivity of ALMA and LMT during
EHT April 2017 observations. ALMA-II assumes phase referencing using the
entire 8 GHz (64 Gbps) of EHT bandwidth, while LMT-II assumes 16 GHz of
bandwidth and aperture efficiency of $\eta_{A}=0.37$. NOEMA is projected for a
12-element array under nominal weather conditions, and the small ngEHT remote
site is evaluated at $\eta_{A}=0.5$ and line-of-sight opacity $\tau=0.6$.
Atmospheric phase tracked on rapid timescales at 86 GHz or 230 GHz can be
transferred to 345 GHz, allowing for longer coherent integration times and
robust measurement at the highest ngEHT observing frequencies.

Significant improvements in sensitivity will still be realized through
deployment of wider band receivers and backends, which can now typically
digitize 8GHz per sideband. For a given frequency band, the ngEHT targets
dual-sideband and dual-polarization, for a potential Stokes I fringe detection
that combines 32GHz aggregate bandwidth. For any given baseline, this advance
results in a net detection threshold that is four times lower than currently
achievable.

In addition to this increase in overall received bandwidth, the ngEHT
frequency coverage will include the 86 and 345 GHz bands. Routine multi-band
operation has several important consequences for ngEHT capability. Each
station pair probes distinct spatial frequencies when observing in different
bands, and multi-frequency imaging algorithms can make use of the aggregate
interferometric coverage to improve image fidelity (e.g., Chael et al., 2022).
Observing in the 345GHz band also improves angular resolution of the global
array by up to 50%. The EHT already offers 345GHz observing capability on a
subset of antennas (Crew et al., 2023, see also Table 2), but not yet
simultaneously with 230GHz. Additional frequency bands also enable analyses
and modeling that differentiate between gravitationally lensed achromatic
features (e.g., the photon ring) and structures whose appearance have a
spectral dependence (e.g., accretion flows and relativistic jets). And through
use of the frequency phase transfer technique (FPT; Rioja et al., 2023), VLBI
phase solutions determined at lower frequencies can be transferred to higher
frequency observations, effectively removing atmospheric phase effects to
extend coherent integration times for higher sensitivity. The full case for
adding 86 GHz capability that leverages FPT through simultaneous multi-band
systems is described in Issaoun et al. (2023).

Combined, these enhancements lead to profound increases in array capability.
The implementation roadmap for the ngEHT will proceed in two phases with a
goal of ultimately adding $\sim 10$ new dishes to the EHT. In Phase 1, a total
of six new sites will be developed: four radio dishes will be deployed to new
geographic locations (Section 4.4.1); and two existing facilities (the 37m
telescope at MIT Haystack Observatory and a 10m telescope at the Owens Valley
Radio Observatory) will be modified to participate in future observations
(see, e.g., Kauffmann et al., 2023). A Phase 2 will add four more telescopes,
either by deploying additional new purpose-built telescopes, or by
instrumenting planned single dish facilities due to come on-line by $\sim
2030$ (Section 4.4.2). These Phases, when complete, will double the number of
dishes in the array recently fielded in the 2022 and 2023 annual EHT observing
campaigns (see Section 4.4).

Figure 2: Current EHT sites (in black), other existing or near-future sites
that may join global observations (in red), and potential new ngEHT sites (in
magenta).

##  3 Next Generation Science Goals

The ngEHT design has been guided by a series of Key Science Goals (KSGs),
developed through a community-driven process of exploration, evaluation, and
prioritization. These goals and their associated instrument requirements are
presented via a Science Traceability Matrix (STM) in a companion paper
(Johnson et al., 2023); and a series of papers in a special issue of
Galaxies111https://www.mdpi.com/journal/galaxies/special_issues/ngEHT_blackholes
presents science topics in greater detail. Here, we briefly describe several
of the ngEHT KSGs, which define the target baseline array architecture, and
are summarized in Table 1.

###  3.1 Existence and properties of black hole horizons

By characterizing the central brightness depression region in black hole
images, the ngEHT can directly address the question of the existence of a
black hole’s horizon. For Magnetically Arrested Disk (MAD) accretion modes,
which are favored for M87 (Event Horizon Telescope Collaboration et al.,
2019e), emission in the innermost part of the flow originates primarily in the
equatorial plane, and the central depression (the “inner shadow”) is defined
by light paths that cross the event horizon without visiting the emitting
region of the accretion system (Dokuchaev & Nazarova, 2019; Chael et al.,
2021). Measuring the shape of this “inner shadow” to be smaller than the
photon orbit would correspond to observing the lensed event horizon, allowing
estimates of the black hole’s mass and spin (Chael et al., 2021). For both M87
and Sgr A*, this measurement requires an imaging dynamic range of
${\sim}100$:1. For Sgr A*, intrinsic variability presents an additional
challenge that will require future algorithm development. Furthermore,
enhancing the dynamic range of the images with the ngEHT will allow to obtain
improved constraints on the brightness ratio between the black hole shadow
interior and the observed emission ring. These constraints can be translated
to an argument supporting the existence of the event horizon, by putting most
stringent limits on the albedo of the surface of an exotic compact object
alternative to a black hole (Event Horizon Telescope Collaboration et al.,
2022f), ultimately limited only by the emission and absorption in the
foreground by the gas located out of the equatorial plane (e.g., Vincent et
al., 2022).

###  3.2 Measurements of the Spin of a SMBH

General relativity predicts that astrophysical black holes are described
solely by two properties: their mass and angular momentum (or “spin”). The
ngEHT has the opportunity to produce the direct secure measurements of a black
hole spin through distinctive image features that reflect the imprint of the
strongly curved spacetime near the horizon. In particular, images of GRMHD
simulations show several robust indicators of spin (for a review, see Ricarte
et al., 2023b). The most promising of these is the spiraling polarization
pattern around the emission ring (Palumbo et al., 2020). By producing time-
averaged polarimetric images of M87 and Sgr A* at both 230 and 345 GHz, the
ngEHT will be able to securely measure this pattern and decouple the effects
of the spacetime from those of the surrounding plasma (Faraday rotation and
conversion), which are steeply chromatic.

###  3.3 Evolution of Supermassive Black Holes.

Though the EHT has to date observed only two SMBHs (M87∗ and Sgr A∗) with
horizon-scale angular resolution, numerical simulations of black hole
accretion flows (e.g., Event Horizon Telescope Collaboration et al., 2019e,
2022e) predict that the “shadow” structure seen towards these sources should
be a generic image feature in sufficiently optically thin systems.
Measurements of the size of the SMBH shadow can be used to constrain the black
hole mass (e.g., Event Horizon Telescope Collaboration et al., 2019f, 2022d),
and measurements of the near-horizon linear polarization structure may also be
able to provide indirect constraints on the black hole spin (Palumbo et al.,
2020; Ricarte et al., 2023b, a). Access to a population of shadow-resolved
SMBHs would thus provide an opportunity to make uniquely self-consistent
measurements of these spacetime properties, permitting corresponding studies
of SMBH formation, growth, and co-evolution with their host galaxies.

The ngEHT is expected to be able to be able to detect up to several dozen
SMBHs with sufficient angular resolution and sensitivity to access their
masses and spins through measurements of their horizon-scale structure (Pesce
et al., 2021, 2022). A database of the most promising individual targets is
being compiled within the ETHER sample (Ramakrishnan et al., 2023).

###  3.4 Mechanisms of Black Hole Accretion

Despite decades of study, the mechanisms that drive accretion onto SMBHs are
still poorly understood (for a review, see Yuan & Narayan, 2014). The ngEHT
will make the first resolved movies of a black hole accretion flow, allowing a
direct study of the dynamics of the turbulent plasma and the role of magnetic
fields in providing an effective viscosity that drives infall (Balbus &
Hawley, 1991, 1998).

###  3.5 Heating and Acceleration of Relativistic Electrons

In low density, low accretion rate systems such as in M87⋆ and Sgr A⋆ the
Coulomb collision time for both electrons and protons is much larger than the
dynamical (accretion) time scale. As a consequence, protons and electrons
cannot redistribute their energy and a two-temperature plasma is occurs.
Assuming that the emission is mainly generated by electrons, their temperature
is determined by the interplay between cooling and heating processes
(Mahadevan, 1997). Given, that in low accretion rate systems the cooling
processes can be neglected (cooling time scale larger than the dynamical time
scale), the impact of possible electron heating processes on the observed
emission from M87⋆ and Sgr A⋆ can be probed.

Two of the main processes for the heating of electrons are turbulent heating
(see, e.g., Kawazura et al., 2019; Howes, 2010)) and magnetic reconnection
heating (see, e.g., Rowan et al., 2017). The results of two-temperature
general relativistic magneto-hydrodynamic (GRMHD) simulations showed that
magnetic reconnection heating leads to a disk dominated emission structure
while turbulent heating tends to a disk-jet structure (Ryan et al., 2018;
Chael et al., 2018a, 2019a; Mizuno et al., 2021). The ngEHT with its improved
u-v coverage and increased sensitivity will allow us to image and track at the
same time the disk and faint jet structures on scales of 100 M. Together with
the multi-frequency capabilities of the ngEHT movies of the total intensity
and the spectral evolution can be produced. These movies, in close combination
with detailed numerical simulations, can allow us to locate the heating sites
and distinguish between the different electron heating process in M87⋆ and Sgr
A⋆.

###  3.6 Energy Extraction from Black Holes

Energy from a spinning black hole can be extracted via the Blandford-Znajek
(BZ) process (Blandford & Znajek, 1977), an electromagnetic analog of the
classic Penrose process (Penrose, 1969). With the ngEHT we will probe this
energy extraction mechanism via the generated jet power or more precisely via
the so-called BZ jet power. The BZ-jet power is proportional to the square of
the black hole spin and to square of the magnetic flux crossing the horizon
(Tchekhovskoy et al., 2011). In addition, the jet power can be measured from
the observed spectral energy distribution or from the x-ray luminosity (see
Prieto et al., 2016, for a detailed disccusion on jet power estimates).

To compute a theoretical estimate for the BZ-jet power precise measurements of
the black hole spin and the magnetic flux are necessary. As mentioned in
Section 3.2 of this paper and in Broderick et al. (2022) combined ngEHT
observations will provide the black hole spin and black hole mass with
sufficient precision. The second quantity in the BZ-jet power, namely the
magnetic flux across the horizon can be obtained either via polarimetric ngEHT
observations (Event Horizon Telescope Collaboration et al., 2021) or via the
frequency dependent position of the core i.e., the core-shift using multi-
frequency observations (Zamaninasab et al., 2014). In both cases the superior
detection and imaging capabilities of the ngEHT will allow us to provide
answers to this long-standing question of energy extraction from black holes.
To perform this measurement for M87, Phase 2 of the ngEHT is required.

###  3.7 Jet Formation

Based on numerical GRMHD simulations we know that rotating black holes can
launch jets via the BZ process (see, e.g., Gammie et al., 2003; Tchekhovskoy
et al., 2011). However, once launched the jets need to be accelerated and
confined, whereas the associated physical processes behind this acceleration
and collimation as well as the jet composition are still on debate (see
Blandford et al., 2019, for a review). The jet composition electron-positron
or electron-proton plasma can be probed via circular polarisation and a
detailed review can be found in this special issue by Emami et al. (2023).

Details on the fluid structure and the formation process of the jet in M87 can
be derived from the velocity field and the jet-to-counter-jet ratio (Mertens
et al., 2016; Walker et al., 2018). The structure of the velocity field will
allow us to probe the stratification of the jet into a fast inner spine and
slow outer sheath (see, e.g., Komissarov, 1999). The ngEHT will enable such
studies in objects other than M87 through resolving the transversal jet
structure, e.g., in Centaurus A (Janssen et al., 2021). In addition to the
poloidal velocity field (spine-sheath structure), the toroidal velocity field
plays a crucial role in determining the formation process of the jet: Is the
jet anchored in the accretion disk (Blandford & Payne, 1982) or is the jet
launched from the ergosphere of a rotating black hole (Blandford & Znajek,
1977). Extracting the velocity field of the jet requires multi-frequency
observations and a high cadence of observations. To avoid the “contamination”
of the velocity field by secondary effects, i.e. by Kelvin-Helmholtz
instabilities or re-collimation shocks (triggered by changes in the ambient
medium) scales up to 100 M are sufficient. In order to determine the velocity
field in M87 the ngEHT in Phase 1 is required.

###  3.8 Constraining Properties of the Black Hole Photon Ring

One of the clearest predictions motivated by the first black hole images is
that the observed ring of emission should exhibit a fine sub-structure: nested
concentric rings, each formed by light rays that make successively more orbits
around the photon shell region, located very close to the black hole’s event
horizon (Johnson et al., 2020). Each sub-ring is a lensed image of the
surrounding accretion and jet emission with inner sub-rings becoming
exponentially fainter and narrower. The structure of the primary ($n=0$) ring,
observed by the EHT, depends on a combination of the local spacetime and the
detailed emission structure on Schwarzschild radius scales, while subsequent
sub-rings ($n\geq 1$) asymptotically approach the true photon orbit, which is
dependent exclusively on the spacetime metric (Bardeen, 1973). Detection of
the $n=1$ ring, formed by photons that make a half-orbit around the black
hole, would be important confirmation of this untested prediction of General
Relativity and lead to new tests of GR in highly curved space-time (Broderick
et al., 2022; Wielgus, 2021). Robust extraction of this feature with the ngEHT
will require the longest Earth baselines at 345 GHz and geometric model
fitting that uses multiple frequencies (Tiede et al., 2022). This science goal
would be a target of the fully realized (Phase 2) ngEHT.

Key Science Goal | Source | ngEHT Phase | Reference
---|---|---|---
Establish the existence |  M87∗ | Phase 1 |  Chael et al. (2021);
of black hole horizons |  Sgr A∗ | Phase 2 | Dokuchaev & Nazarova (2019)
Measure a SMBH’s spin |  M87∗ | Phase 2 | Palumbo et al. (2020)
|  Sgr A∗ | Phase 2 | Ricarte et al. (2023b)
Understanding Black Hole-Galaxy | AGN Survey | Phase 1 |  Pesce et al. (2021, 2022);
Formation, Growth and Coevolution |  |  | Ramakrishnan et al. (2023)
Reveal how black holes |  M87∗ | Phase 1 |  Balbus & Hawley (1998);
accrete material |  Sgr A∗ | Phase 2 | Yuan & Narayan (2014)
Observe localized electron |  M87∗ | Phase 1 |  Rowan et al. (2017);
heating and acceleration |  Sgr A∗ | Phase 2 | Ball et al. (2018)
Determine if BH jets are |  M87∗ | Phase 2 |  Blandford & Znajek (1977);
powered by spin energy |  Sgr A∗ | Phase 2 | Tchekhovskoy et al. (2011)
Determine jet formation |  M87∗ | Phase 1 | Blandford et al. (2019)
& launching mechanisms |  Sgr A∗ | Phase 2 |
Constraining Properties |  M87∗ | Phase 2 |  Johnson et al. (2020);
of the BH Photon Ring |  Sgr A∗ | Phase 2 | Tiede et al. (2022)
Table 1: Select ngEHT Key Science Goals. For the full Science Traceability
Matrix and additional details, see Johnson et al. (2023).

##  4 Optimizing the ngEHT Reference Array

The scientific performance of an array generically benefits from addition of
new stations, regardless of where those stations are located. However, when
constrained by a fixed budget or a fixed number of new dishes to be added,
determining the optimal placement of the new dishes is a challenge that
requires finding a balance between many – often conflicting – objectives. For
instance, science goals that require high angular resolution favor array
configurations with many long baselines, while goals that involve high-
fidelity imaging on large fields of view instead favor configurations
containing dense short-baseline coverage. Similarly, while atmospheric opacity
considerations favor the highest and driest locations, such sites are often
remote and lack critical infrastructure, significantly driving up construction
and operating costs. Any array configuration that one ultimately arrives at
necessarily hinges on a non-unique choice about what exactly constitutes
“optimality,” and the result can depend sensitively on how one weights the
many relevant considerations when doing so.

In this section, we detail some of the considerations that are entering into
the design process for the ngEHT array configuration. Section 4.1 describes
how we have selected an initial pool of candidate sites to consider, Section
4.2 describes our procedure for simulating realistic ngEHT observations, and
Section 4.3 details several metrics that we use to evaluate array quality.
Section 4.4 describes our evaluation of the many different candidate arrays
and discusses a strategy for translating array performance into site
selection. Various details relevant for the site selection procedure are
provided in Appendix A.

###  4.1 Candidate sites

From most locations on the surface of the Earth, atmospheric opacity prevents
observations at the primary ngEHT frequencies of 230 GHz and 345 GHz. We thus
take our starting pool of candidate sites from Raymond et al. (2021), who
identified sites with favorable atmospheric transmission properties for 230
GHz and 345 GHz observations during the March/April typical EHT observing
season; the candidate sites are shown in Figure 2 and listed in Table A1.

Given the selection on atmospheric opacity performed in Raymond et al. (2021),
the candidate sites are naturally situated in the highest and driest
locations. Figure 3 shows the highest-elevation locations around the globe,
and Figure 4 shows where the mean level of precipitable water vapor (PWV) is
lowest throughout the year. We have computed the PWV using atmospheric data
from the MERRA-2 database (Gelaro et al., 2017). The PWV at a particular
location is determined by integrating the water vapor through the column of
atmosphere above that location (see, e.g., Salby, 1996),

$\text{PWV}=\frac{1}{\rho g}\int_{0}^{P_{\text{surf}}}\frac{q(P)}{1-q(P)}dP.$
(1)

Here, $q(P)$ is the specific humidity, $P$ is the atmospheric pressure,
$P_{\text{surf}}$ is the atmospheric pressure at the surface, $\rho\approx 1$
g cm-3 is the mass density of water, and $g\approx 9.81$ m s-2 is the
acceleration of gravity at the surface of the Earth. MERRA-2 provides both $P$
and $q$ in 42 different atmospheric layers as a function of geographic
location and time.

Figure 3: Global elevation map. Locations with elevations above 1000 meters
are shaded red, with darker colors indicating higher elevations. Figure 4:
Locations around the globe with mean PWV less than 5 mm, in January (top),
April (second from top), July (second from bottom), and October (bottom). A
darker red coloring indicates a lower value of mean PWV, and only locations
with elevations above 50 meters are colored. We have determined the PWV via
Equation 1 using atmospheric data from MERRA-2 (Gelaro et al., 2017), and the
average is taken over all available data between 2012–2022.

###  4.2 Synthetic data generation

We evaluate candidate array performance using synthetic observations of the
key science targets M87∗ and Sgr A∗. For source models we use the results of
general relativistic magnetohydrodynamic (GRMHD) simulations that have been
post-processed using ray-tracing and radiative transfer codes to produce
images at the 230 GHz and 345 GHz observing frequencies appropriate for the
ngEHT. Our M87∗ source model comes from the simulations carried out in Chael
et al. (2019b), and our Sgr A∗ source model comes from the simulation library
produced in Event Horizon Telescope Collaboration et al. (2022e).

We generate synthetic datasets using the
ngehtsim222https://github.com/Smithsonian/ngehtsim library. Given a candidate
ngEHT array configuration and a source model, ngehtsim uses eht-imaging (Chael
et al., 2016, 2018b) to sample the Fourier transform of the source at the
$(u,v)$-coverage corresponding to the array. Thermal noise $\sigma_{ij}$ on a
baseline between stations $i$ and $j$ is determined by the radiometer
equation,

$\sigma_{ij}=\frac{1}{\eta_{q}}\sqrt{\frac{\text{SEFD}_{i}\text{SEFD}_{j}}{2\Delta\nu\Delta
t}},$ (2)

where $\Delta\nu$ is the observing bandwidth, $\Delta t$ is the integration
time, SEFD is the station system equivalent flux density, and $\eta_{q}=0.88$
is an efficiency factor associated with 2-bit quantization during data
collection (Thompson et al., 2017). We determine SEFDs for each station as a
function of time using

$\text{SEFD}=\frac{2kT_{\text{sys}}}{A_{\text{eff}}}e^{\tau},$ (3)

where $k$ is the Boltzmann constant, $\tau$ is the (time-dependent) line-of-
sight atmospheric opacity, $A_{\text{eff}}$ is the effective collecting area
of the telescope,

$T_{\text{sys}}=T_{\text{rx}}+T_{\text{atm}}\left(1-e^{-\tau}\right)$ (4)

is the system temperature, $T_{\text{rx}}$ is the receiver temperature, and
$T_{\text{atm}}$ is the temperature of the atmosphere. We determine
$T_{\text{atm}}$ using historical atmospheric data from the MERRA-2 database
(Gelaro et al., 2017), and $\tau$ is obtained by passing the atmospheric state
information from MERRA-2 through the am radiative transfer code (Paine, 2019).

For the synthetic datasets in this section, we use a bandwidth of
$\Delta\nu=16$ GHz (for ngEHT) or $\Delta\nu=2$ GHz (for EHT) at each of two
frequency bands, one centered at 230 GHz and the other centered at 345 GHz. We
use an integration time of $\Delta t=10$ minutes, which is assumed to be
enabled by suitable phase calibration, with a 50% duty cycle (i.e., 10 minutes
on-source followed by 10 minutes off-source); the total duration of each
observation is 24 hours. We assume receiver temperatures $T_{\text{rx}}$ of 50
K at 230 GHz and 75 K at 345 GHz.

To emulate fringe-finding, we apply two signal-to-noise ratio (SNR)
thresholding schemes to the generated visibilities. The first scheme emulates
the “fringe groups” strategy from Blackburn et al. (2019): if a visibility
does not achieve an equivalent SNR of 5 on an integration time of 10 seconds
(at 230 GHz) or 5 seconds (at 345 GHz), and if the stations comprising the
baseline associated with that visibility do not participate in other baselines
that achieve the requisite SNR, then that visibility is assumed to be a non-
detection and is flagged from the dataset. Note that for the stations with
dual-frequency capabilities, both of the frequency bands are checked
simultaneously; if either one of the two frequency bands has an SNR that
satisfies the threshold condition, then we assume that both bands can be
detected. This second scheme emulates frequency phase transfer across the
bands (see, e.g., Rioja & Dodson, 2020). Both of these fringe-finding schemes
are applied when simulating ngEHT data, while only the first (fringe groups)
scheme is applied when simulating EHT data.

###  4.3 Array performance metrics

The analysis methods utilized by the EHT for performing measurements of
physical interest using VLBI data are in general highly computationally
expensive to evaluate. Further, the added value of a particular set of new
sites is non-linear in the number of sites; the number of new baselines is
quadratic in the number of existing sites, and the value of an individual site
is sensitive to its position with respect to existing dishes.

To evaluate the performance of candidate ngEHT array configurations without
running computationally expensive analysis pipelines (such as imaging or
model-fitting), we utilize metrics of array performance that are based on pre-
analysis quantities. We primarily employ two metrics: one metric that
quantifies the $(u,v)$-coverage and another metric that quantifies the
aggregate baseline sensitivity. We compute the array performance metrics using
synthetic observations at frequencies of 230 GHz and 345 GHz, which drive the
key science goals of the ngEHT. While 86 GHz is an important addition that
enables improved detection prospects at the higher frequencies (see Section
7.1), it serves primarily a calibration-related role and thus is not included
in our $(u,v)$-coverage or baseline sensitivity metric computations.

Figure 5: Illustration of the $(u,v)$-filling fraction metric described in
Section 4.3 (see also Palumbo et al. (2019)). Given some $(u,v)$-coverage –
shown here by the blue points for a mock observation of M87 using the full
ngEHT Phase 1 array in April – the FF metric is a measure of how much area
within a circular region of radius $1/\theta_{\text{res}}$ is sampled, after
convolving the coverage with a disk of radius $1/\theta_{\text{FOV}}$. In this
case, $\theta_{\text{res}}=14$ $\mu$as and $\theta_{\text{FOV}}=200$ $\mu$as.
The convolved coverage is shaded in light blue, and takes up a fraction
$\mu_{\text{ff}}=0.5$ of the area of the outer circle.

We use as our quantification of $(u,v)$-coverage quality the $(u,v)$-filling
fraction metric (FF metric), defined in Palumbo et al. (2019) as the fraction
$\mu_{\rm ff}$ of the area enclosed by a bounding circle in $(u,v)$ of radius
$1/\theta_{\text{res}}$ that is covered by the two-dimensional convolution of
the coverage with a circular disk of radius $1/\theta_{\text{FOV}}$. Here,
$\theta_{\text{res}}$ and $\theta_{\rm FOV}$ are array performance
specifications based on imaging expectations, and they are not predicted
directly by the coverage; Figure 5 provides an illustration of how the FF
metric is calculated. Palumbo et al. (2019) found that as the filling fraction
increases, imaging performance in compact imaging examples improves steadily
until it flattens to a constant factor of the diffraction-limited image
fidelity near $\mu_{\rm ff}$ $\gtrsim 0.5$. The FF metric naturally demands
greater coverage for equivalent $\mu_{\rm ff}$ as expectations of imaging
field of view $\theta_{\rm FOV}$ increases; however, $\mu_{\rm ff}$ does not
capture the relative information density of the Fourier plane, and for many
source morphologies, the importance of Fourier coverage decreases with radius
from the $(u,v)$-coordinate origin. In this paper, we assume
$\theta_{\text{res}}=14$ $\mu$as (i.e., the angular resolution of an Earth-
diameter baseline observing at 345 GHz) unless otherwise specified, but we use
several different fields of view; when quoting FF metric values, we will thus
specify the corresponding assumed of view using the notation
$\mu_{\text{ff}}(\theta_{\text{FOV}})$.

For our quantification of array sensitivity, we use the point source
sensitivity (PSS) metric,

$\text{PSS}=\left(\sum_{i=1}^{N}\frac{1}{\sigma_{i}^{2}}\right)^{-1/2},$ (5)

where $\sigma_{i}$ is the value of the thermal noise on visibility $i$ (see
Equation 2), and the sum is taken over all visibilities in the dataset. The
PSS metric, which has units of flux density, quantifies the sensitivity that
the array could in principle achieve when measuring the flux density of a
point source. It naturally folds in not only the observing bandwidth and
diameter of each telescope in the array, but also the amount of mutual
visibility that each site has with every other as well as the atmospheric
transmission at each site.

###  4.4 Site selection

The stringent atmospheric opacity requirements for observing at millimeter
wavelengths means that only a small number of locations around the globe are
suitable candidates (see Section 4.1). Given that the list of candidate sites
presents a finite number of discrete locations on the globe where telescopes
could be placed, we could in principle evaluate all possible new array
configurations. The ability to confine the site search space to a finite
number of options in this way is fairly unique to high-frequency VLBI, and it
informs our optimization strategies below; the analogous site selection
problem for connected-element arrays (e.g., VLA, ALMA, ngVLA) and low-
frequency VLBI arrays (e.g., VLBA, SKA) presents a qualitatively different
challenge.

In practice, though the number of possible new array configurations is finite,
the space remains large and difficult to search comprehensively; the number of
possible new array configurations that could be made using the 44 sites listed
in Table A1 is approximately $1.8\times 10^{13}$. Additionally, we would like
to ensure that the selected sites enable the ngEHT array to perform well
across all of the following situations:

  * •

in observations of both M87∗ and Sgr A∗;

  * •

during observations that take place throughout the year;

  * •

when observing alongside any subset of the EHT.

The performance of each candidate array must also be evaluated using several
different quality metrics (see Section 4.3) that correspond to the various
scientific goals. All of the above considerations result in multiplicative
factors that further increase the expense of a comprehensive analysis.

Given the difficulty of comprehensively searching all possible combinations of
new stations, we instead partition our site selection efforts into two stages
corresponding to the two anticipated phases of ngEHT development. In the first
stage – corresponding to ngEHT Phase 1– we consider the selection of three new
sites from the pool of candidates. The availability of three 6.1-meter BIMA
dishes for refurbishment and relocation (see Section 2) provides a pathway to
realizing a Phase 1 ngEHT array on a shorter ($\sim$few-year) timescale than
it will take to field a larger array of newly-constructed dishes. Optimizing
for only three new sites at a time also reduces the number of site
combinations to only ${44\choose 3}=13244$. In the second stage of the site
selection analysis – corresponding to ngEHT Phase 2– we then consider the
selection of five new sites from the remaining pool of candidates,
corresponding to ${41\choose 5}=749398$ different site combinations. Dividing
the optimization strategy into two stages in this way, and selecting a
specific target number of new sites in each stage, substantially reduces the
computational cost of optimizing the array configuration.

The sites and frequency configurations corresponding to the selected Phase 1
and Phase 2 ngEHT arrays are listed in Table 2.

Site | EHT | ngEHT Phase 1 | ngEHT Phase 2 | ngEHT Phase 2 (alt.)
---|---|---|---|---
ALMA | 86+230+345 | 86+230+345 | 86+230+345 | 86+230+345
AMT | - | - | - | 86+230+345
APEX |  86+230+345 | 86+230+345 | 86+230+345 | 86+230+345
BOL | - | - | 86+230+345 | -
CNI | - | 86+230+345 | 86+230+345 | 86+230+345
GLT | 86+230+345 | 86+230+345 | 86+230+345 | 86+230+345
HAY | - | 86+230+345 | 86+230+345 | 86+230+345
IRAM | 86+230+345 | 86+230+345 | 86+230+345 | 86+230+345
JCMT | 86+230+345 | 86+230+345 | 86+230+345 | 86+230+345
JELM | - | - | 86+230+345 | 86+230+345
KILI | - | - | 86+230+345 | -
KP | 86+230+345 | 86+230+345 | 86+230+345 | 86+230+345
KVNPC | - | - | - | 86+230+345
KVNYS | - | - | - | 86+230+345
LCO | - | 86+230+345 | 86+230+345 | 86+230+345
LLA | - | - | - | 86+230+345
LMT | 86+230+345 | 86+230+345 | 86+230+345 | 86+230+345
NOEMA | 86+230+345 | 86+230+345 | 86+230+345 | 86+230+345
OVRO | - | 86+230+345 | 86+230+345 | 86+230+345
SGO | - | - | 86+230+345 | -
SMA |  86+230+345 |  86+230+345 |  86+230+345 |  86+230+345
SMT |  86+230+345 | 86+230+345 | 86+230+345 | 86+230+345
SPM | - | 86+230+345 | 86+230+345 | 86+230+345
SPT |  86+230+345 | 86+230+345 | 86+230+345 | 86+230+345
SPX | - | - | 86+230+345 | -
Table 2: Site participation and frequency capabilities for the EHT and both
phases of the ngEHT array. For the first column, EHT sites with existing 86
GHz capability are noted, but the EHT does not currently support 86 GHz
operation; and some of these 86 GHz receivers cannot be used simultaneously
with higher frequency receivers. In each of the three rightmost columns, sites
that do not participate in the specified array are indicated with a “-” sign.
Multi-frequency capabilities are indicated with a “+” sign; e.g., “230+345”
indicates that the station can observe at both 230 GHz and 345 GHz
simultaneously, whereas “230 345” indicates that it can only observe at each
frequency separately. For completeness, we list in the rightmost column an
alternative incarnation of the ngEHT Phase 2 array, in which we forgo the need
to field new telescopes by relying instead on external facilities that are
anticipated to come online in the next few years (see Section 4.4.3). For this
alternate case, the JELM site would be added in Phase 1. Figure 6: A version
of the site map from Figure 2 with the top 1% of all three-station candidate
site combinations from the Phase 1 exploration (see Section 4.4) plotted as
black triangles.

####  4.4.1 Phase 1

To determine the optimal locations for the three new Phase 1 dishes, we carry
out a survey of all possible three-station combinations of the 44 sites listed
in Table A1. For each candidate set of three sites, we explore the performance
of the resulting array (1) for observations of both M87∗ and Sgr A∗, (2) under
weather conditions appropriate for January, April, July, and October, and (3)
when observing alongside four different variants of the existing EHT array
(specified in the top section of Table A2). These pre-existing array variants
include various subsets of the EHT array, as well as the HAY and OVRO dishes
that are expected to be outfitted with ngEHT equipment (see Section 2). We
evaluate each candidate array using the metrics described in Section 4.3 for
100 instantiations of the weather conditions at each site, from which we then
take median values to establish typical performance.

After evaluating all candidate arrays, we determine a “performance score” for
each array according to its average ranking across the full suite of observing
parameters. E.g., if a particular array is ranked first for one set of
observing parameters, ranked third for a second set of observing parameters,
and ranked fifth for a third set of observing parameters, then its performance
score would be $(1+3+5)/3=3$. Arrays with smaller values of the performance
score are those that have performed well across a range of observing
parameters. Figure 6 shows the top 1% of all three-station candidate site
combinations after ranking them by their performance scores, with each set of
sites plotted as a connected three-baseline triangle. We identify six heavily
populated clusters of high-performing site combinations:

  * •

An “eastern cluster” containing two sites in South America and either CNI or,
less frequently, one of the other mainland European sites (BGA, SKS, SPX).

  * •

A “western cluster” containing two sites in South America and a site in North
America, most typically either SPM, PIKE, or FAIR.

  * •

A “northern cluster” containing two sites in South America and GLTS, or less
commonly with BGK.

  * •

A “southern cluster” containing two sites in South America and one of the
Antarctic Dome sites.

  * •

An “equatorial cluster” containing one site in South America, one site in
North America, and CNI.

  * •

A “polar cluster” containing one site in South America, GLTS, and one of the
Antarctic Dome sites.

We see that the most favored sites tend to be those that are able to leverage
simultaneous observability with existing sites. The overrepresentation of
existing sites in the Western hemisphere means that sites in the Eastern
hemisphere – particularly those in Asia and New Zealand – are correspondingly
penalized.

To select from among the top-performing three-site combinations, we impose
additional, more qualitative considerations. We disfavor the northern,
southern, and polar clusters because they contain sites that are unable to
observe either M87∗ (in the case of the Antarctic sites) or Sgr A∗ (in the
case of GLTS and BGK). The eastern and western clusters suffer from a similar
asymmetry, in that they include sites that have little mutual visibility with
existing American and European stations, respectively. The equatorial cluster
provides the most balance in terms of site geography, and it contains the
three most favored regions for a new site: South America, North America, and
CNI. Several of the South American sites are comparably well-represented among
the top site combination candidates, as are a couple of the North American
sites. After additionally accounting for initial site cost estimates and
favoring lower-cost sites, we settle on the three-site combination of CNI,
LCO, and SPM as our fiducial ngEHT Phase 1 additions.

####  4.4.2 Phase 2

In the second stage of our site selection analysis – corresponding to ngEHT
Phase 2– we consider the addition of five new sites to the previous three
determined from the Phase 1 selection. We explore the same observing targets
and weather conditions as for the Phase 1 exploration, but we use updated pre-
existing arrays that include the Phase 1 sites (see the bottom section of
Table A2). We again evaluate each candidate array using the metrics described
in Section 4.3 for 100 instantiations of the weather conditions at each site,
and we use median metric values to establish typical performance.

The selection process for Phase 2 is ongoing, but preliminary results indicate
that the combination of BOL, JELM, KILI, SGO, and SPX would provide a strong
improvement to the array coverage. We thus take these sites to be our fiducial
ngEHT Phase 2 additions for the purposes of this paper.

####  4.4.3 Alternate Staging of New Sites

Several new radio telescopes that could be used for ngEHT observations are
planned to become operational in the coming years. Thus, an alternative
staging approach would be to augment Phase 1 by adding JELM to the three new
sites described in Section 4.4.1, and Phase 2 could then consist solely of the
following planned telescopes: the LLAMA telescope in Argentina, the AMT in
Namibia, the KVNYS telescope near Seoul, Korea, and the KVNPC telescope
(currently under construction) near Pyeongchang, Korea. Together, the four
Phase 1 sites (CNI, JELM, LCO, SPM) combined with OVRO, HAY, and the four
planned telescopes (LLAMA, AMT, KVNYS, KVNPC) would constitute a near-doubling
of the existing EHT array and would achieve comparable $(u,v)$-coverage to
that provided by the array described in Section 4.4.2. This alternate pathway
to a Phase 2 ngEHT would also provide capabilities sufficient to achieve all
ngEHT Key Science Goals.

####  4.4.4 Baseline Coverage

Simulated EHT coverage for the array fielded during the 2023 observing
campaign is shown in Figure 7. The enhanced baseline coverage that will be
provided by the ngEHT Phases 1 and 2 is shown in Figures 8 and 9. In Figure 8,
the JELM station has been added to reflect a Phase 1 array as described in
Section 4.4.3, while Figure 9 shows the array as described in Section 4.4.2.

Figure 7: (Top) Current EHT array (2023). (Bottom) Interferometric coverage
for M87∗ and Sgr A∗ at 230 & 345 GHz, assuming April observing conditions. The
coverage reflects estimated detections made through simulating M87∗ and Sgr A∗
models at both frequencies with the EHT array as fielded in 2023 (see Table 2,
and Section 4.2). Note that for the EHT in 2023, 230 GHz and 345 GHz
observations cannot be made simultaneously, so the coverage shown cannot be
combined to form a full image (as is possible in the ngEHT Phase 1 and Phase 2
arrays). The opacity of each plotted data point is proportional to how
frequently it is expected to be detected. The outer and inner dashed circles
mark baseline lengths corresponding to angular scales of 15 $\mu$as and 30
$\mu$as, respectively.

Figure 8: (Top) ngEHT Phase 1 array; white sites are current EHT dishes, blue
sites are ngEHT sites. (Bottom) Interferometric coverage for M87∗ and Sgr A∗
at 86 GHz, 230 GHz, and 345 GHz, assuming April observing conditions. The
coverage reflects estimated detections made through simulating M87∗ and Sgr A∗
models at all three frequencies with the ngEHT Phase 1 array (see Table 2 and
Section 4.2). Sites without multi-frequency capabilities are assumed to be
observing only at their highest frequency. The opacity of each plotted data
point is proportional to how frequently it is expected to be detected. The
outer and inner dashed circles mark baseline lengths corresponding to angular
scales of 15 $\mu$as and 30 $\mu$as, respectively.

Figure 9: (Top) ngEHT Phase 2 array; white sites are current EHT dishes, blue
sites are ngEHT sites, and yellow sites are planned or existing facilities
that may join (ng)EHT observations. (Bottom) Interferometric coverage for M87∗
and Sgr A∗ at 86 GHz, 230 GHz, and 345 GHz, assuming April observing
conditions. The coverage reflects estimated detections made through simulating
M87∗ and Sgr A∗ models at all three frequencies with the ngEHT Phase 2 array
(see Table 2 and Section 4.2). Sites without multi-frequency capabilities are
assumed to be observing only at their highest frequency. The opacity of each
plotted data point is proportional to how frequently it is expected to be
detected. The outer and inner dashed circles mark baseline lengths
corresponding to angular scales of 15 $\mu$as and 30 $\mu$as, respectively.

##  5 Operating Modes

Key Science Goals (KSGs) motivate five basic operation modes of the ngEHT,
which enable specific science use cases. Details and constraints of each mode
are defined by cost/benefit analyses and feasibility studies. Factors to be
considered in this analysis include time allocation at various sites, weather,
data throughput with implications for disk inventory and correlation,
reliability and up-time, and maintenance strategy. The following five
subsections provide a narrative summary for each of the five envisaged
operating modes of the ngEHT, which are then summarized with salient
characteristics in table 3.

OpsMode | stations in array | cadence & duration | science case
---|---|---|---
Campaign | 14 to 21 | 1x7 | Sgr A*, M87, blazars, jets
Long term | 5 to 20 | 5 days, | M87∗ & blazar kinematics,
monitoring |  | 3 to 7 mo | Sgr A∗ flares
Target of | 3 to 6 | 1 wk | flares,
Opportunity |  | 3x/yr | gravitational waves
CMF | 14 to 21 |  hours x 2 wks | AGNs, black hole binaries
Beyond | 1 to 10 | dependent on | stellar birth,
ngEHT |  | science case | fast radio bursts
Table 3: The five ngEHT operating modes and selected salient characteristics
of each.

###  5.1 Campaign

This is a single epoch annual multi-day campaign, which is an extension of the
standard annual campaign already executed by the Event Horizon Telescope
(EHT). The 11 EHT sites defined by those used in the 2022 EHT array are
assumed to participate with the ngEHT sites. In this mode, dedicated tracks
are based on clearly defined, community-prioritized science cases, in some
cases led by a principal investigator.

The campaign mode pursues M87 and Sgr A* science cases with enhanced
capability relative to EHT due to improved sensitivity and great uv coverage
from the larger 21 site array. This results in enhanced M87 imaging, snapshot
sensitivity for Sgr A* movies, and studies of blazar jet collimation.

###  5.2 Long Term Monitoring

The long term monitoring mode uses extended duration and more frequent cadence
observations with a smaller subset of the existing EHT sites participating.
The ngEHT sites enable this mode through their purpose-designed flexibility
and dedicated time allocation for VLBI.

Several multi-week observations over the course of the year once again have
dedicated tracks based on clearly defined, community-prioritized science
cases. These science cases are in the broad areas of M87∗ movies, blazar
kinematic studies, and Sgr A* flaring activity monitoring. As an example, to
continuously track changes in the M87∗ appearance (M87∗ movies),
reconstructing images separated by the expected coherence timescale ($\sim
50GM/c^{3}\approx$ 20 days) is needed. A single-year EHT campaign may only
last about a week (Event Horizon Telescope Collaboration et al., 2019a) – too
short for a significant change in the source appearance, while combining
results from separate years only provides uncorrelated source snapshots,
without the ability to track continuous motion of the flow features (Wielgus
et al., 2020). Similarly, in the published EHT analyses of blazar observations
(Kim et al., 2020; Issaoun et al., 2022; Jorstad et al., 2023) short duration
of the EHT campaigns, and the lack of repeated observations on timescales of
weeks or months, has been recognized as the main factor limiting the current
EHT ability to study jet kinematics.

###  5.3 Target of Opportunity

Target of Opportunity (ToO) is an agile operational follow-up by ngEHT to an
unpredictable event observed with another facility. It involves ad-hoc
subarrays of the 11 existing EHT sites - those which are available - while all
of the ngEHT dedicated sites will be made available for suitably
scientifically interesting ToO observations. Broad science areas are expected
to be in the area of flares, gravitational waves, and fast radio burst
counterparts.

###  5.4 Coordinated Multi-Facility

The Coordinated Multi-Facility (CMF) mode is characterized by coordinated,
multi-facility, multi-messenger observations involving multiple ngEHT sites
and at least one other ground or space instrument (e.g., Chandra, the GRAVITY
instrument, and any of various optical/IR facilities). This CMF mode is a
planned continuation of the successful EHT Multi-Wavelength campaigns (see EHT
MWL Science Working Group et al., 2021).

The broad science areas are expected to be multi-wavelength studies of Active
Galactic Nuclei, binary and singular black holes.

###  5.5 Beyond-ngEHT

This single dish mode covers any observation that is performed outside the
core ngEHT science mission, but will still be part of the ngEHT operating
model due to local institutional requirements or synergies with other
communities or facilities.

Science is expected to be in the broad area of star forming regions, fast
radio bursts, and astronomical maser studies of transitions in the ngEHT RF
bands.

##  6 Data Processing

The next-generation (ngEHT) expands upon the existing 11-station EHT with
around 10 additional small-dish antennas as well as simultaneous 230/345 GHz
observations. In addition to the roughly $\sim$10-fold increase in aggregate
data rate across the entire array, the ngEHT is expected to operate as a full-
season agile observatory as opposed to the $\sim$few observing days per year
of the current EHT. When all participating sites are observing, one night of
ngEHT produces around $\sim$10 PB of raw data (around 0.5 PB per site),
resulting in up to a $\sim$couple hundred PBs per year that must be processed.
An efficient streamlined approach to data processing and management is
required to facilitate media turn-over and to deliver quality assured science-
ready data products in a timely manner.

The large data rates and volumes of the ngEHT motivate continued adoption and
assimilation of new technologies, which has allowed a rough tracking of
Moore’s law over two decades of global mm-VLBI development (Figure 10). On the
timescale of a $\sim$decade, we anticipate a transition from Hard Disk Drives
(HDDs) to Solid State Disks (SSDs) for recording and eventually transport,
which provides high-bandwidth, high-density, and power-efficient I/O. SSDs
carry a gradually narrowing cost premium of 5–10x versus HDDs in $/TB, but
their use would allow ngEHT recording systems to keep up with the ngEHT data
rates while staying within practical power, weight, and space footprints for
efficient media handling, staging, and transport.

GPU’s have become the platform of choice for massively parallel vector/tensor
calculations due to their efficiency and ease of use, and they are being
researched or already adopted for efficient VLBI correlation across several
experiments. The “embarrassingly parallel” nature of VLBI correlation is
suitable for high-throughput computing (HTC) workflows, and the irregular
scheduling of VLBI observations means that on-demand scalable computational
resources are desirable.

Figure 10: EHT/ngEHT data rate per station over two decades, roughly doubling
every two years. The large bandwidths provide the EHT/ngEHT the necessary
continuum sensitivity for ultra-high resolution VLBI imaging at (sub-)1 mm
using a highly heterogeneous network of telescopes. Maintaining this trend has
required the regular adoption of commercial technologies as they became
available.

###  6.1 Data Transport

While observing, the ngEHT will produce an aggregate $\sim$5 Tbps of digital
signal data that must ultimately be transported from remote sites to a central
location for processing. Similar to the EHT, the only currently available
means for moving such a large total volume of data from the (sometimes very-)
remote locations in a reasonable amount of time is by physical transport of
recorded media. VLBI experiments such as the European VLBI
Network333https://www.evlbi.org/ (EVN) are currently able to transport data
electronically, due to considerably lower data rates and more accessible sites
(typically at sea level) that are linked to a high speed internet backbone.
The ngVLA reference design (Selina et al., 2018) also includes real-time data
transport (320 Gbps per antenna) and correlation via ground fiber (both
dedicated and leased commercial), even for the longest baselines spanning the
United States and territories. However because the ngEHT operates a
(comparatively) small number of antennas at remote locations spanning the
globe, shipment of physical media is expected to remain the fastest and most
economical method of transferring 100s PB of data for the foreseeable future.
Consistent array-wide high-speed internet access, such as that provided by
global commercial Satellite RF internet, will nevertheless be extremely useful
for rapid transfer of small amounts ($\sim$1%) of data for interferometric
validation and for obtaining near-realtime results where scientifically
relevant.

The ngEHT is designed to operate full-season, and this motivates a rapid
processing and recycling of recording media to limit costs. Media are expected
to be redeployed approximately once per two months (on average), versus once
per 2-3 years as for the current EHT. As a result, there is less of a focus on
media utility for economical long-term storage, and more toward efficient
recording and transport. Once data are brought to the correlation facility,
they can be offloaded to local HDD-based storage if needed, for example in the
case of experiments including the South Pole Telescope which can incur several
months of shipping delay. A rotating media library of 200 PB would be required
to support bimonthly turn-around of observations totaling 10 PB every three
days while providing ample time for average shipping time and data offload.

###  6.2 Correlation

Correlation is the process of calculating pairwise correlation coefficients
between the signals captured at each antenna. Because this is an operation on
the PB of raw VLBI data, it is both I/O and computationally intensive and
requires carefully matched computing platforms for effective processing.
Correlation coefficients are typically calculated in the frequency domain
using a so-called FX correlation architecture that enables efficient searching
over unknown time delay via Fourier convolution. Frequency domain processing
also allows for convenient matching of signals from partially overlapping
bandwidths as well as the application of linear and non-linear corrections to
align the data. The consequences of an FX architecture is a large up-front
cost to data channelization, scaling linearly with the number of antennas. For
a 20-station network at ngEHT bandwidths, the $O(N)$ cost from data stream
pre-processing and the $O(N^{2})$ cost from calculating all pair-wise
correlations are expected to be roughly comparable.

The current EHT records at 64 Gbps over 11 stations, for an aggregate rate of
0.7 Tbps. Data are correlated at dedicated computing clusters at MIT Haystack
Observatory and the Max Planck Institute for Radioastronomy using the DiFX
software correlator (Deller et al., 2011). In aggregate, $\sim$2.5k cores are
able to process the full EHT bandwidth at about 10% real-time. Scaling
linearly to the aggregate data rate of ngEHT requires $\sim$20k cores to
process ngEHT’s $\sim$5 Tbps at 10% real-time (in comparison, 300 hours of
data per year is a reasonable upper limit for ngEHT data throughput and
reflects a duty-cycle of 3.5%). A quadratic scaling with the number of
stations would imply double the requirement, but this can be balanced against
$\sim$5-10% year-over-year improvements to single-core performance. CPU core
density and efficiency are also increasing at a much faster rate, and GPU
acceleration of both channelization and cross-multiply stages of correlation
are expected to increase efficiency by another factor of $\sim$several. A
detailed description and modeling of VLBI software correlation performance is
presented in Vázquez et al. (2022) alongside several benchmark results
including those from the literature.

Approximately $\sim$60M CPU core-hours would be required to correlate
$\sim$680 PB (300 hours) of raw data. VLBI data are taken non-continuously
throughout the year and sometimes require multiple passes through correlation
to iterate on a proper configuration. Thus is it necessary to over-provision
on-demand computational resources by a factor of $\sim$few in order to avoid
backlogs and ensure regular turn-around of recording media. Around $\sim$100k
on-demand CPU cores would be appropriate to keep up with the largest projected
ngEHT data volumes, which is the size of a large institutional research
cluster or a few medium-sized clusters distributed geographically. Due to the
over-provisioning, the resources are ideally time-shared with other computing
requirements (calibration and imaging, other VLBI correlation, or other
general uses).

###  6.3 Calibration and Reduction

Output from correlation is at a resolution of $\sim$1 MHz in bandwidth and
$\sim$1 second in time, which is required to capture residual instrumental and
environmental systematics that affect the measured correlation coefficients
such as lines, frequency response, relative delays, and time-varying gains and
atmospheric phase (Event Horizon Telescope Collaboration et al., 2019b, c).
These products are smaller than the recorded VLBI signals by a factor of
$>$103 due to the large amount of accumulation following cross-correlation. A
calibration process then solves for a refined instrument model and folds in
any additional priors on the instrument response.

A key element of the calibration process is “fringe-fitting” where a
parameterized phase model (typically relative delay and delay-rate over a
short time interval) is self-calibrated to the correlator output. The fitting
process verifies that a correlated signal exists in the data, measures the
correlation coefficient, and allows data to be further coherently averaged,
reducing the overall data volume by another factor of $\sim$104. Dedicated
fringe fitting and calibration pipelines (Blackburn et al., 2019; Janssen et
al., 2019) were developed for EHT data to address the heterogeneous nature of
the array and unique data properties. Compared to correlation, the computing
requirements to fit a basic phase calibration model are low. For example the
EHT 2017 campaign data set (5 nights, 8 stations) can be processed through a
multi-stage calibration and reduction pipeline using $\sim$1.5k CPU core-hours
(Blackburn et al., 2019).

This initial stage of calibration and reduction is aimed at reducing the
overall data volume and complexity for downstream data products, while
applying only well-determined calibration solutions. Since data are
manipulated and averaged, it is important to avoid introducing calibration
solution noise or detailed assumptions about the source. In cases where
calibration solutions are under-determined or degenerate with source model
parameters, they must be jointly modeled during analysis. The complexity and
computational cost can increase dramatically due to the high dimensionality of
an instrument model, particularly for the case of formal Bayesian inference
(Broderick et al., 2020; Pesce, 2021).

##  7 Instrumentation Design

In this section we describe the basic elements of the ngEHT system (see Figure
11. These are the result of several internal project reviews, including a
Systems Requirements Review, held on June 9-10, 2022. At this stage of the
project, the ngEHT team has developed initial instrumental requirements
through a process of preliminary trade-off analysis. This process has enabled
development of several prototypes, which have been selected for deployment in
Phase 1 of the project, and these specific elements of the ngEHT system are
described below.

Figure 11:  Functional block diagram of a next-generation EHT station. All
elements shown in the figure are either commercially available (e.g., Hydrogen
Maser), or in advanced prototyping stages, and suitable for deployment at
ngEHT stations. The Timing & Coherence block consists of a Maser and GPS
system, which provides ultra-stable clock signals for the DBE and references
for the dual-polarization receivers and the BDC. A high-speed ethernet switch
routes DBE packets to recorders with modular/removable media for shipment to
the central correlator for interferometric processing. ngEHT Monitor and
Control is handled by local and global systems.

###  7.1 Receiver

In Figure 12 (left), we present a block diagram of a dual frequency receiver
being constructed for ngEHT and to be deployed at the LMT. A single cryostat
will hold two different receivers and the two different frequency bands are
sent to each receiver through a frequency diplexer. Each receiver is dual-
polarized, and features sideband separation mixers (see Table 4). Both bands
illuminate a single beam on sky, and the overall dual-frequency receiver has
eight IF outputs, each of which is 4–12 GHz wide.

Figure 12:  Left: Block Diagram of the proposed dual band SIS receivers. Both
the 1.3mm and 850 µm band receivers will be built inside a single cryostat.
Right Top: Schematic outline of the the 1.3mm frontend receiver block. This
block shows the cold section of the corrugated square feed-horn feeding an
orthomode transducer (OMT) section that separates the input signal into two
polarization channels, one in each of the top and bottom halves of the block.
In each polarization, there is a RF $90^{\circ}$ hybrid followed by LO
couplers, ending in two SIS junctions. The IF outputs of the pair of SIS
junctions passes through IF matching and bias tee to a superconducting IF
$90^{\circ}$ hybrid, which outputs the upper and lower sideband IF signals
from that channel. In all 4 SIS junctions are used in each mixer block, with
Cooper pair tunneling suppresesed by permanent magnets.
Right Bottom: Photo of one half of an assembled 1.3 mm fronted receiver block.  Item | Description
---|---
3mm RF Band | 82 - 116 GHz
1mm RF Band | 210 - 280 GHz
850$\mu$m RF Band | 275 – 375 GHz
Polarizations | Dual pol in each band
Sidebands | 2SB Receivers in each band
IF Frequency | 4 – 12 GHz (1mm,0.85$\mu$m)
| 4 – 8 GHz (3mm)
Receiver Noise |  $<$ 50 K in 3mm band
Temperature | 60 – 70 K in 1mm band
| 70 – 80 K in 850$\mu$m band
Table 4: Specifications of the ngEHT multi-band Frequency Receiver

In an effort to make the design highly modular and scalable to reproduce for
additional new telescopes of the ngEHT array, considerable effort has been
invested into making the mixer block compact and highly integrated. In Figure
12 (right), we show the components of this highly integrated block. Shown is a
photo of the bottom block of a split-block mixer (bottom) and a schematic
diagram of the components (top). A similar design will be employed for the 850
$\mu$m receiver as well. The 4 IF outputs from each of the mixer blocks are
amplified cryogenically using commercially available low-noise amplifiers.

Each of the receiver bands are equipped with independent local-oscillator (LO)
systems. YIG oscillators are lower frequencies (in the 18-30 GHz) range are
multiplied up to the 3mm wavelength band, and subsequently amplified using
W-band power amplifiers. This is then fed through cryogenic triplers to
produce the required LO signal. The drain currents of the last stage of the
W-band power amplifiers can be adjusted to set the appropriate LO power for
the mixers. The whole LO system is phase locked, and fully computer controlled
with no mechanical moving parts.

Implementation of additional 86GHz capability to enable Frequency Phase
Transfer (FPT) will proceed along multiple paths. For existing sites that
already field 86 GHz receivers, these will be coupled where possible to higher
frequency receivers using dichroics that enable simultaneous operation (e.g.,
GLT, JCMT). At existing sites that do not have 86GHz receivers, or where
existing 86 GHz systems cannot be used, new HEMT-based 86 GHz receivers,
cooled to 20K, will be added and coupled via dichroics. These new 86GHz
receivers will follow existing and proven designs. Finally, for the new ngEHT
sites, a tri-band dewar that incorporates 86, 230 and 345 GHz receivers will
be constructed, following existing designs and prototypes for the ongoing
upgrade of the Submillimeter Array in Hawaii.

###  7.2 Backend

The ngEHT backend consisting of the Block Down Converter (BDC) and the Digital
BackEnd (DBE) will process four times the instantaneous bandwidth (dual
sideband, dual polarization, and dual frequency) of the current EHT.

The BDC performs a frequency translation and signal conditioning of the analog
signal from the receivers. The Intermediate Frequency (IF) signal is converted
to baseband, and output power levels are adjusted to optimally load the Analog
to Digital Converter (ADC). The design of this BDC was initiated and
functionality was implemented in a prototype, constructed by Xmicrowave LLC.
The prototype was manufactured using drop-in PCB (Printed Circuit Board)
modules instead of connectorized components, which is more representative of
the final BDC PCB. A full characterization has been conducted and the results
meet the required specifications. The final BDC will consist of integrated PCB
units instead of discrete drop-ins.

The DBE prototype currently being used for testing and development is a two
board system. This prototype uses a custom circuit board holding four ADCs,
which digitize the analog signal from the BDC. This board sends the digital
data stream to a commercial evaluation board, the VCU128 which houses the
VU37P Field Programmable Gate Array (FPGA) from the Virtex Ultrascale+ family
manufactured by Xilinx. Each 4 bit ADC is clocked at 16.384 GHz. The Nyquist
bandwidth of this system is therefore 8.192 GHz, which is interoperable with
the current EHT. The evaluation board is useful for current tests and
development, and it will be replaced with a custom board design; the design of
this new board is underway with an estimated one-year timeline to completion.
Parts are being acquired to support a build of five units.

In addition to hardware (board) development, the initial firmware command set
has been successfully completed, including an ADC interface module, a
requantization block from 4 bits to 2 bits in the processing module, a
packetization module, a 100 Gb transmission module, a Universal Asynchronous
Receiver Transmitter(UART) monitor and control module, and a timing module.
Further features that will be included in the firmware are channelization, 1
Gb monitor and control, and slope and ripple equalization.

With 2-bit quantization and Nyquist sampling, each DBE can process the full IF
bandwidth (8 GHz) from the 1mm and 0.85$\mu$m band receivers for a total data
throughput of 128 Gb/s. For the 3mm band, a narrower IF bandwidth (4 GHz) is
sufficient to achieve Key Science Goals and Frequency Phase Transfer
calibration. At 3mm, the resulting data throughput is 64 Gb/s.

###  7.3 Recorder

The recorder is expected to be based around a set of commercial off-the-shelf
(COTS) components hosted on a commodity multi-processor computer running a
GNU/Linux operating system with a PCIe 4.0 interconnect. A single recording
unit is matched to one or more streams from the digital back-end system (DBE),
which is designed to output 64 Gbps data streams on 100 GbE interconnect using
the VDIF transport protocol (VTP, Philips et al., 2012) over UDP. Specialized
software on the recording unit provides efficient network capture at the
required rates, simple packet inspection to ensure data continuity and
integrity, distributed writing of VDIF file streams to disk, and an interface
to the VLBI monitor and control system.

The host recording system will buffer the incoming data in system RAM, while
simultaneously draining this data to persistent memory for storage. The
persistent storage is expected to be a set of solid-state drives (SSDs)
attached via PCIe/NVMe (integrated media). The total number, individual
capacity and write performance of the component SSDs in the persistent memory
pool will be selected such that they are sufficient to absorb the total
aggregate data rate and meet the desired overall capacity and cost
constraints. In order to facilitate playback of detachable data modules for
subsequent correlation or transfer, the recorder will maintain a file system
on the media so that may be mounted by separate machine.

Although an SSD-based recorder has several advantages over a HDD-based system
in terms of speed, power, density, weight, and latency, SSDs are anticipated
to carry a significant cost premium to HDDs for the next decade. Moreover a
modular removable disk pack system analogous to the semi-custom Mark6 module
(Whitney et al., 2013) has yet to be designed, which limits the flexibility of
current COTS SSD recorders. For this reason, large volume data storage and
possibly transport may still rely on HDD-based solutions for some time, with
SSD-to-HDD data offload capability at site or at the correlator.

| Mark6 | ngRecorder
---|---|---
rack space | 11U | 2U
disks | 32 HDD | 24 SSD
capacity | 512 TB | 369 TB
interface | 4x 10/25 GbE | 2x 100 GbE
rate | 16/32 Gbps | 128 Gbps
hours at rate | 71.1/35.6 | 6.4
disk modules | yes | no
Table 5: Specifications for a modular VLBI recorder, including those of the
Mark6 (Whitney et al., 2013) currently in use across the EHT. Reference
specifications for a next-generation SSD-based recorder are based on common
currently available COTS SSD storage servers.

###  7.4 Array Monitoring and Control

The operations concept for the ngEHT extends beyond the single annual campaign
of the current EHT:

  * •

60 nights of observing per year

  * •

Up to 21 stations observing simultaneously

  * •

Varied observation cadences and durations throughout the year

  * •

Readiness for VLBI observing in 24h or less to capture ToOs

  * •

Multi-messenger campaigns

  * •

Configurable subarrays

  * •

As much remote operation as possible

This model and its increase in capability has direct impact on the
requirements and subsequent complexity of the M&C system for the ngEHT. As the
M&C system serves as a main user interface point for the array, its design
must be user-centered and have due consideration for human factors concerns.
As well, the operations concept is designed to address an explicit need,
voiced at the ngEHT Operations Workshop (31 Mar 2022), to reduce the burden
(relative to 2022 EHT operations) for on-site monitoring, control, and
maintenance of VLBI equipment. The areas to address include differing methods
of monitoring and control for each station and heavy reliance on local
operations at each site, including the need for VLBI specialists on site.

As the first ngEHT sites are brought online, they will participate in the
annual EHT observing campaign. To facilitate this participation, the M&C
system will be compatible with the EHT operations plans and procedures by
relaying data to the existing VLBI Monitor server, providing remote control of
station subsystems, and providing status, logs, and metadata as required.
Outside of the annual EHT campaign, the ngEHT operations concept calls for an
annual monitoring campaign where the M&C system will be used to operate and
monitor the entire array. It will provide a uniform and cohesive monitoring
and control experience to the array operators while managing a heterogeneous
array of ngEHT stations and stations that use the existing EHT VM&C system and
backend equipment.

Collecting observation metadata from a heterogeneous array of telescopes that
have non-standardized interfaces for M&C and data collection is a significant
design and operational challenge. To take advantage of the opportunity
presented by the ngEHT designing its own telescopes, it is expected that the
M&C component of the telescopes for ngEHT sites will be designed in
conjunction with the overall M&C system to make this interface as common as
possible across the ngEHT sites.

As the number of stations and observations grows, providing on-site VLBI
expertise will become increasingly challenging. The ngEHT design approach
follows an operations model where station operators can remotely perform any
required operations and maintenance, with specialist support being provided
only when necessary. Remote operation is facilitated by the focus on human
factors and user-centered design, and leads to less reliance on manual
operations and analysis. A cloud-based deployment of the array-level M&C
system is envisioned as the way to provide ”operations from anywhere”
capability to the array operations staff. This is expected to include server,
database, and UI components that facilitate operation of the array. M&C
capability at each station is still required to provide the control inputs to
station subsystems and aggregate the local data for relay to the array-level
system. Remote access to both the array- and station-level M&C systems are
provided with appropriate security, authentication, and authorization methods.

To achieve all this, the M&C system architecture is expected to be built from
off-the-shelf software components using open standards, including databases,
message queueing and information exchange methods, and user interface
frameworks. This facilitates development and maintenance over the lifecycle of
the array. A robustly defined software architecture allows isolation of site-
specific dependencies to the smallest and fewest components necessary.

###  7.5 Antennas

Figure 13: Photograph showing the condition of the BIMA antenna dish surface
(from March 2022)

The ngEHT concept adds $\sim$10 new antennas to the existing EHT array. In
Phase 1 the ngEHT program will deploy 3-4 modest diameter antennas for the
most rapid increase in next-generation science (see Section 4.4). To mitigate
risk, the program has identified two possible paths towards this Phase 1
enhancement. The first would use three 6 m diameter antennas from the
decommissioned Berkeley-Illinois-Maryland-Array (BIMA), which would be
transported to the Las Campanas, San Pedro Martir, and Canary Island sites.

The BIMA dishes have a surface accuracy specification of $\sim$40 $\mu$m rms,
sufficient for operation up to 345 GHz. Photogrammetry measurements will allow
re-adjustment the surface to the required accuracy after re-assembly of the
antenna. The panels of all three dishes are in good condition, as shown in
Figure 13.

Design specifications
---
Primary reflector diameter | 9 m
Mount architecture | Alt-Az
Optics | Cassegrain
Sun avoidance zone | None
Operating specifications
Surface accuracy | 30 $\mu$m rms
Frequency range | 86 — 345 GHz
Aperture efficiency | 0.8
Pointing accuracy | 2′′ rms (all sky, blind)
Tracking accuracy | $0.^{\prime\prime}2$
Aperture blockage | $<5\%$
Gain variation with elevation | $<5\%$
Range of motion in azimuth | -180∘ — 360∘
Range of motion in elevation | 3∘ — 90∘
Slew speed | 1∘/s
Environmental specifications
Temperature | -15C—35C operational
| -20C—45C high
| -30C—55C survival
Wind speed | 10 m/s operational
| 15 m/s high
| 50 m/s survival
Table 6: Specifications of the new ngEHT antennas

Figure 1 suggests that a 6 m diameter antenna with an aperture efficiency of
0.8 would allow us to reach the required sensitivity, when paired with a large
collecting area dish such as LMT or phased ALMA. But a larger diameter antenna
will relax the requirement on long distance baselines away from such anchor
stations, and also have two additional advantages: easier calibration for
pointing and focus measurements, and ability to carry out single-dish science
projects while the antennas are not observing for ngEHT in VLBI mode.

Therefore, a second possible Phase 1 implementation path would be to use newly
fabricated dishes of 9 m diameter. The specifications of the new antennas are
summarized in Table 6. The ngEHT team is in discussions with several telescope
vendors and it is clear that dishes with the required specifications can be
procured within a reasonable cost envelope. In this case, Phase 1 would target
four sites: the Mt. Jelm site in Wyoming, in addition to Canary Islands, San
Pedro Martir and Las Campanas.

##  8 Summary and conclusions

The ngEHT, described initially to the Astro2020 decadal survey review
(Doeleman et al., 2019), is a program to plan extensions of the EHT array that
will deliver high dynamic range imaging and movie making capability for black
hole studies on event horizon scales. It does so principally by deploying
modest-diameter radio dishes at optimized geographical locations, which
significantly increases interferometric baseline coverage (Figures 7, 8, 9),
by implementing a simultaneous tri-band (86, 230, 345 GHz) receiver suite, and
increasing the bandwidth of backend systems and data processing pipelines.

The process and initial results of optimizing site selection for ngEHT
telescopes described here indicates two possible paths to achieve a next-
generation EHT array.

In the first path, Phase 1 consists of adding dishes at two existing sites
(OVRO and Haystack) to the current EHT, and available refurbished dishes from
the BIMA array would be relocated to three sites (Las Campanas, Chile; San
Pedro Martir, Mexico; Canary Islands, Spain). Then in Phase 2, additional
sites would be developed; current analysis indicates that the combination of
these locations: La Paz, Bolivia; Wyoming, US; Marangu, Tanzania; Santiago,
Chile; and Bern, Switzerland, constitute an array that can deliver all of the
threshold Key Science Goals. These Phase 2 sites should be considered
possibilities at this stage; more work is required to assess them at all
levels, including thorough consideration of cultural and environmental
aspects.

In an alternate path, Phase 1 would again add both OVRO and Haystack to the
EHT, and four new 9 m diameter dishes would be deployed to the Mt. Jelm site
in Wyoming; Las Campanas, Chile; San Pedro Martir, Mexico; and Canary Islands,
Spain. Then in Phase 2, planned new telescopes are added to the array as they
become available, including the AMT, LLAMA and KVNYS, KVNPC facilities. Either
of these approaches to realizing the ngEHT leads to the increases in global
array capabilities that are required to achieve all ngEHT Key Science Goals.

Strategies for ngEHT data transport, correlation, calibration and data
reduction are all developed. Requirements for major instrumental sub-systems
are specified, and details of prototypes to be used are described. In sum,
this work brings the ngEHT project to the point of readiness for
implementation.

Support for this work was provided by the NSF through grants AST-1952099,
AST-1935980, AST-1828513, and AST-1440254, and by the Gordon and Betty Moore
Foundation through grant GBMF-10423. This work has been supported in part by
the Black Hole Initiative at Harvard University, which is funded by grants
from the John Templeton Foundation and the Gordon and Betty Moore Foundation
to Harvard University.

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##  Appendix A Additional site selection details

Table A1 lists the sites considered for the ngEHT array optimization
procedures described in Section 4. This pool of candidate sites has been taken
from Raymond et al. (2021), and they have been selected for their favorable
atmospheric transmission properties at 230 GHz and 345 GHz during the typical
EHT observing season in March and April.

Site code | Location | Latitude | Longitude | Elevation (m)
---|---|---|---|---
ALI | Hotan County, China | 35.963 | 79.338 | 6080
BAN | Alberta, Canada | 51.350 | -116.206 | 3470
BAR | California, US | 37.634 | -118.256 | 4340
BGA | Progled, Bulgaria | 41.695 | 24.738 | 1730
BGK | Westfjords, Iceland | 66.032 | -23.052 | 830
BLDR | Colorado, US | 39.588 | -105.643 | 4340
BMAC | Eastern Cape, South Africa | -31.096 | 27.889 | 2420
BOL | La Paz, Bolivia | -16.351 | -68.131 | 5230
BRZ | Espírito Santo, Brazil | -20.439 | -41.799 | 2850
CAS | Tierra del Fuego, Argentina | -54.790 | -68.415 | 2850
CAT | Río Negro, Argentina | -41.170 | -71.486 | 2100
CNI | La Palma, Canary Islands | 28.299 | -16.509 | 2360
DomeA | Upper ice sheet, Antarctica | -80.367 | 77.351 | 4090
DomeC | Upper ice sheet, Antarctica | -75.101 | 123.342 | 3230
DomeF | Upper ice sheet, Antarctica | -77.317 | 39.702 | 3700
ERB | Khalifan, Iraq | 36.584 | 44.466 | 2110
FAIR | Alaska, US | 64.988 | -147.599 | 620
FLWO | Arizona, US | 31.675 | -110.951 | 1270
FUJI | Fujinomiya & Yamanashi, Japan | 35.367 | 138.730 | 3750
GARS | Trinity Peninsula, Antarctica | -63.320 | -57.895 | 20
GLTS | Ice sheet summit, Greenland | 72.580 | -38.449 | 3230
HAN | Ladakh, India | 32.780 | 78.963 | 4500
JELM | Wyoming, US | 41.097 | -105.977 | 2940
KEN | Meru, Kenya | -0.141 | 37.315 | 4260
KILI | Kilimanjaro, Tanzania | -3.088 | 37.406 | 4430
LCO | Coquimbo, Chile | -29.032 | -70.685 | 2320
LOS | New Mexico, US | 35.880 | -106.675 | 2000
NOB | Nagano, Japan | 35.944 | 138.472 | 1370
NZ | Canterbury, New Zealand | -43.987 | 170.465 | 1010
ORG | Oregon, US | 42.635 | -118.576 | 2970
PAR | Antofagasta, Chile | -24.628 | -70.404 | 2640
PIKE | Colorado, US | 38.841 | -105.041 | 4280
SAN | California, US | 34.099 | -116.825 | 3500
SGO | Santiago, Chile | -33.3346 | -70.270 | 3350
SKS | Crete, Greece | 35.212 | 24.898 | 1740
SPM | Baja California, Mexico | 31.045 | -115.464 | 2800
SPX | Fieschertal, Switzerland | 46.548 | 7.985 | 3510
SUF | Zaamin, Uzbekistan | 39.623 | 68.468 | 2440
TRL | Jutulsessen, Antarctica | -72.010 | 2.540 | 1280
VLA | New Mexico, US | 34.079 | -107.618 | 2120
YAN | Huanca Sancos, Peru | -13.938 | -74.392 | 4230
YBG | Lhasa Tibet, China | 30.006 | 91.027 | 5360
Table A1: List of candidate sites for the ngEHT, updated from Raymond et al.
(2021).

Table A2 specifies the pre-existing arrays assumed during the site selection
procedure described in Section 4. Four different variants of pre-existing
array are explored as parameters in the site selection procedure, and these
variants are enumerated in the table.

Parameter set |  Pre-existing stations from EHT array | Other pre-existing stations assumed
---|---|---
Phase 1 set 1 |  none | HAY, OVRO
Phase 1 set 2 |  LMT | HAY, OVRO
Phase 1 set 3 |  APEX, GLT, JCMT, LMT, SMT | HAY, OVRO
Phase 1 set 4 |  ALMA, APEX, GLT, IRAM, JCMT, KP, LMT, NOEMA, SMA, SMT, SPT | HAY, OVRO
Phase 2 set 1 |  none | CNI, HAY, LCO, OVRO, SPM
Phase 2 set 2 |  LMT | CNI, HAY, LCO, OVRO, SPM
Phase 2 set 3 |  APEX, GLT, JCMT, LMT, SMT | CNI, HAY, LCO, OVRO, SPM
Phase 2 set 4 |  ALMA, APEX, GLT, IRAM, JCMT, KP, LMT, NOEMA, SMA, SMT, SPT | CNI, HAY, LCO, OVRO, SPM
Table A2: The different pre-existing arrays considered as part of the site
selection exploration (Section 4.4). Each of these combinations of stations is
the starting set of sites for which the addition of three sites (for the Phase
1 analysis) or five sites (for the Phase 2 analysis) are explored. These
starting arrays are chosen to generally represent the possible operating modes
shown in Table 3. Set 1, for example, might be a minimal array useful for
Target of Opportunity observations. Sets 2 and 3, with the addition of a large
aperture, could provide flexible long-term monitoring capability. And set 4
includes all possible stations for a full campaign mode. The range of starting
arrays also give some indication of optimal placement in the full campaign
mode in the case where some sites are not available due to weather or
technical issues.