††thanks: These authors contributed equally.††thanks: These authors
contributed equally.

# Precision Magnetometers for Aerospace Applications

James S. Bennett  School of Mathematics and Physics, The University of
Queensland, St Lucia, Brisbane Queensland 4072, Australia  Brian E. Vyhnalek
NASA Glenn Research Center, Cleveland, Ohio 44135, The United States of
America  Hamish Greenall  School of Mathematics and Physics, The University of
Queensland, St Lucia, Brisbane Queensland 4072, Australia  Elizabeth M. Bridge
School of Mathematics and Physics, The University of Queensland, St Lucia,
Brisbane Queensland 4072, Australia  Fernando Gotardo  School of Mathematics
and Physics, The University of Queensland, St Lucia, Brisbane Queensland 4072,
Australia  Stefan Forstner  School of Mathematics and Physics, The University
of Queensland, St Lucia, Brisbane Queensland 4072, Australia  Glen I. Harris
School of Mathematics and Physics, The University of Queensland, St Lucia,
Brisbane Queensland 4072, Australia  Félix A. Miranda
<EMAIL_ADDRESS>NASA Glenn Research Center, Cleveland, Ohio 44135,
The United States of America  Warwick P. Bowen<EMAIL_ADDRESS>School of
Mathematics and Physics, The University of Queensland, St Lucia, Brisbane
Queensland 4072, Australia

###### Abstract

Aerospace technologies are crucial for modern civilization; space-based
infrastructure underpins weather forecasting, communications, terrestrial
navigation and logistics, planetary observations, solar monitoring, and other
indispensable capabilities. Extraplanetary exploration – including orbital
surveys and (more recently) roving, flying, or submersible unmanned vehicles –
is also a key scientific and technological frontier, believed by many to be
paramount to the long-term survival and prosperity of humanity. All of these
aerospace applications require reliable control of the craft and the ability
to record high-precision measurements of physical quantities. Magnetometers
deliver on both of these aspects, and have been vital to the success of
numerous missions. In this review paper, we provide an introduction to the
relevant instruments and their applications. We consider past and present
magnetometers, their proven aerospace applications, and emerging uses. We then
look to the future, reviewing recent progress in magnetometer technology. We
particularly focus on magnetometers that use optical readout, including atomic
magnetometers, magnetometers based on quantum defects in diamond, and
optomechanical magnetometers. These optical magnetometers offer a combination
of field sensitivity, size, weight, and power consumption that allows them to
reach performance regimes that are inaccessible with existing techniques. This
promises to enable new applications in areas ranging from unmanned vehicles to
navigation and exploration.

Magnetometer; Aerospace; Magnetic Navigation

††preprint: BLARGH

##  I Introduction

Magnetometers are a key component in space exploration missions, particularly
in those concerning the study of the Earth from space, as well as the study of
the planets in our solar system. The information gathered from these
instruments has been of great benefit in increasing our understanding of the
composition and evolution of the Earth [1, 2, 3, 4, 5], other planets [6, 7,
8, 9], and the interplanetary (heliospheric) magnetic field [10, 11]. They are
also widely used in technical aerospace applications; for instance, allowing
attitude determination [12] and magnetic geological surveying [13, 14].
Extensive overviews of space-based magnetometers have been previously
performed by Acuña [15] in 2002, Díaz-Michelena [16] in 2009, and Balogh [17]
in 2010, detailing the design, operation, and calibration of magnetometers
flown from the Mariner missions of the early 1960s to the Lunar Prospector and
Mars Global Surveyor missions of the turn of the century. This review is
intended to provide an updated synopsis of aerospace magnetometry, including
both extraplanetary applications and those in Earth atmosphere and orbit, as
well as emerging technologies and applications.

A particular focus of the review is on emerging magnetometer technologies that
use optical readout [18, 19, 20], their performance characteristics, and their
potential aerospace applications. This is motivated in part by the exponential
growth in the use of unmanned aerial vehicles, together with proposals to use
magnetometer-equipped drones for extraplanetary exploration [21, 22, 23]. The
optical magnetometers considered in this review include atomic magnetometers
(e.g., [18]), magnetometers based on quantum defects in diamond (e.g., [19]),
and optomechanical magnetometers (e.g., [20]). While each of these kinds of
magnetometer have quite different characteristics, in general, a key
attraction has been that they offer exquisite sensitivity without requiring
cryogenic cooling. In recent years they have also experienced rapid
miniaturization, with a concomitant reduction in power consumption. This
combination of attributes holds promise for new aerospace applications both on
Earth and in extraplanetary missions.

##  II Existing applications

###  II.1 Interplanetary science missions

Precise magnetic field measurements are critical to the fulfilment of the
objectives of many planetary, solar, and interplanetary science missions.
Careful measurements of the magnetic fields associated with celestial bodies
help the scientific community to better understand and familiarize itself with
the laws of space physics at play in the evolution of planets and the solar
system. Thus, magnetometers are essential for science mission applications,
and space exploration – one of the paramount goals of humankind – as a whole.

Magnetometers have been used primarily for field mapping and characterization
[15, 17, 24, 25, 8, 7, 26], but also for the study of planetary atmospheres
and their climatic evolution due to solar wind interactions – both in-orbit
and from the Martian surface [6]; as well as for indirect detection of liquid
water – a critical element for the existence of Earth-like life beyond our
planet [27].

Below, we provide some examples of relatively recent, high-visibility missions
featuring space magnetometers. Table 1 summarizes the various spacecraft
magnetometers’ key specifications. Notably, fluxgate magnetometers (FGMs)
stand out as the tool of choice, due to their long-proven performance and
reliability in the space environment, as well as their ability to comply with
stringent requirements (e.g., weight and power consumption) associated with
space missions. However, missions requiring exploration of planets/celestial
bodies with extreme environments (i.e., high temperatures and/or high
radiation) such as those exhibited by Venus, Europa, Enceladus, etc.; landing
and exploring planetary surfaces (e.g., rovers); as well as missions requiring
multiple observation platforms (e.g., small satellite constellations and swarm
platforms), may require magnetometers with sensing, configuration, and form
factors different from FGMs.

Table 1: Summary of various planetary and interplanetary spacecraft magnetometer specifications (FGM = fluxgate magnetometer, VHM = vector helium magnetometer, SHM = scalar helium magnetometer). Mission | Magnetometer | Dynamic Range (nT) | Resolution (pT) | Mass (kg) | Power (W)
---|---|---|---|---|---
GOES-1–3 | FGM (biaxial) | 50–400 | - | - | -
GOES-4–7 | FGM (biaxial) | $\pm 400$ | 200 | - | -
GOES-I–M | FGM (triaxial) | $\pm 1000$ | 100 | - | -
GOES-N–P | FGM (triaxial) | $\pm 512$ | 30 | - | -
GOES-R | FGM (triaxial) | $\pm 512$ | 16 | 2.5 | 4
MAVEN | FGM (triaxial) | $\pm 512$ | 15 | - | $>1$
|  | $\pm 2048$ | 62 |  | -
Cassini | FGM (triaxial) | $\pm 40$ | $4.9$ | 0.44 (FGM) |  $7.5$ (sleep)
| +V/SHM | $\pm 400$ | $48.8$ |  $0.71$ (V/SHM) |  $11.31$ (FGM+VHM)
|  | $\pm 10,000$ | 1200 | - |  $12.63$ (FGM+SHM)
Juno | FGM (triaxial) |  $\pm 1600$ (nominal) | 48 | 5 | $>4.5$
|  |  $\pm 1,638,400$ (largest) | 5000 |  |

Fluxgate magnetometers [28, 29, 30, 31, 32, 33] consist of a drive coil and a
sense coil wrapped around a magnetically permeable core. A strong alternating
current applied to the drive coil induces an alternating magnetic field in the
core, which periodically drives the core into saturation. When there is no
background magnetic field the sense current matches the drive current; however
the presence of an external magnetic field acts to bias the saturation of the
core in one direction, causing an imbalance between the drive and sense
currents which is proportional to the magnitude of the external magnetic
field. These magnetometers are sensitive to the direction of the external
magnetic field and are therefore classed as vector magnetometers. There are
many variations on this basic design, including double-core devices that null
the sense current in the absence of an external field. This technology
provides a magnetic field sensitivity of around 10 pT/ Hz1/2 , a DC (direct
current, i.e., zero frequency) magnetic field resolution of around 5 pT and a
spatial resolution of about 10 mm [32, 33, 29]. The sensitivity of fluxgate
magnetometry is limited by the Barkhausen noise from the core and $1/f$ noise
at low frequencies [15].

####  II.1.1 Mars Atmosphere and Volatile Evolution (MAVEN)

The MAVEN mission, part of NASA’s Scout program, was launched to Mars on
November 18, 2013, and entered into orbit around the red planet on September
21, 2014. Among the primary goals of the mission was to study the role of
atmospheric escape in changing the climate of Mars through time. Other
objectives of the mission were to assess the Martian upper atmosphere,
ionosphere, and interactions with the solar wind, as well as to determine the
escape rates of neutral gases and ions, and collect data that will determine
the ratios of stable isotopes to better understand the evolution of Mars’
atmosphere [24].

(a)

(b)

Figure 1: (a) Illustration of the MAVEN spacecraft in orbit over Mars. The
magnetometer “boomlets” are located at both ends of the solar array panels.
(b) MAVEN magnetometer sensor assembly [25]. Credits: NASA/Goddard Space
Flight Center.

To facilitate these studies, MAVEN was equipped with a payload of multiple
scientific instruments (the “Particles and Fields Package”), including a pair
of ring-core FGMs [25]. Drawing upon the heritage of the Mars Global Surveyor
mission [15], the MAVEN magnetometers were mounted on “boomlets” at either end
of the deployable solar array panels, approximately $5.6$ m from the body
center, rather than on a dedicated magnetometer boom (Figure 1(a)). For the
Martian field environment the magnetometers have two operating dynamic range
modes, $\pm 512$ nT and $\pm 2048$ nT, with digital resolution of $0.015$ nT
and $0.062$ nT, respectively. Additionally, the magnetometer sensors have a
high dynamic range mode ($65,536$ nT at $2.0$ nT resolution), used for testing
in the Earth field environment without requirements for magnetic shielding. A
detailed overview of the design, calibration procedures, and performance is
given in [25]. The MAVEN Magnetic Fields Investigation plays an important role
in understanding how solar wind interactions – including plasma wave formation
and structures – lead to atmospheric escape. A picture of the MAVEN
magnetometer assembly is shown in Figure 1(b).

####  II.1.2 Cassini

The Cassini–Huygens mission, a U.S.–European space mission to Saturn, was
launched on October 15, 1997, with the goal of detailed spatio-temporal
monitoring of physical processes within the Saturnian system environment,
especially in relation to Titan. The orbiter continued to return various
science data until 2017, when its fuel supply was exhausted. In particular,
magnetometer measurements were made of the internal planetary magnetic field;
three-dimensional magnetospheric mapping was performed; the interplay between
the magnetosphere and the ionosphere was investigated; and electromagnetic
interactions between Saturn, its moons, rings, and the surrounding plasma were
observed.

Figure 2: Diagram of the Cassini spacecraft. Credit: NASA.

The Cassini magnetometer was a dual system comprised of both a three-axis FGM
(three perpendicular ring-core FGMs) and a vector helium magnetometer (VHM),
with an additional scalar helium magnetometer (SHM) mode for precise in situ
absolute calibration of the FGM [8]. As is typical of most spacecraft, the FGM
and V/SHM were mounted on a magnetometer boom or “mag boom”, as shown in
Figure 2. In this case the mag boom was 11 m long, with the V/SHM sensor
mounted on the end and the FGM mounted halfway. This configuration allowed for
more effective deconvolution of stray magnetic fields associated with the
spacecraft from the intended observations. As discussed in [8], the FGM
featured four operating ranges spanning $\pm 40$ nT to $\pm 44,000$ nT at
resolutions of $4.9$ pT and $5.4$ nT, respectively, where the largest range
was primarily intended for ground testing within the Earth’s field. In vector
mode, the V/SHM was capable of $3.9$ pT resolution across a $\pm 32$ nT
dynamic range, and $31.2$ pT resolution at $\pm 256$ nT; in scalar mode it had
a single range of $256$-–$16,384$ nT at $36$ pT resolution.

Helium magnetometers [34, 35, 36, 37, 38] are often used as secondary
magnetometers for calibration of FGMs, which are susceptible to long-term
drift. Typically V/SHMs have lower size, weight, and power (SWaP) requirements
than FGMs. They have a low operation bandwidth and are generally used for DC
measurements. Helium-4 atoms are optically pumped into their $2^{3}$S1
metastable state which contains three Zeeman sub-levels. A radio frequency
(RF) source is used to drive the transition between the Zeeman sub-levels, the
resonant frequency of which is determined by the background magnetic field
$B_{0}$, through the relationship
$f_{\mathrm{RF}}=\gamma_{{}^{4}\mathrm{He}}B_{0}$, where
$\gamma_{{}^{4}\mathrm{He}}$ is the gyromagnetic ratio $\approx 28$ GHz/T. The
amplitude of the resonance signal can be amplified using a population stirring
technique where atoms are selectively pumped from metastable Zeeman sub-levels
to the $2^{3}$P0 state and subsequently decay back to the metastable state for
increased interaction with the incident RF field [38].

####  II.1.3 Juno

The Juno spacecraft, in orbit around Jupiter since 2016, has the primary
mission goals of characterizing Jupiter’s planetary magnetic field and
magnetosphere. Juno’s magnetometers have been used for three-dimensional
mapping of Jupiter’s magnetic environment, and play an important role in the
investigation of the formation and evolution of Jupiter, particularly by
allowing scientists to study how the planet’s powerful magnetic field (20,000
times stronger than Earth’s) is generated [39]. Similar to previous missions,
the Juno spacecraft’s magnetic field instrumentation utilizes two independent
triaxial ring-core FGM sensors, along with co-located non-magnetic imaging
sensors (i.e., star trackers), to provide accurate attitude information near
the point of magnetic field measurement [7]. The FGMs and star trackers were
mounted on vibration-isolated carbon–silicon-carbide platforms on a $4$ m
boom, nominally $11$ m from the spacecraft body. In terms of sensitivity, the
magnetometers are capable of six different ranges, extending from a minimum
range of $\pm 1600$ nT to a maximum range of $\pm 1,638,400$ nT, with $0.0488$
nT resolution in the nominally most sensitive range [7].

###  II.2 Future interplanetary missions

Magnetometers form an integral part of planetary missions being planned or
currently under development for future space exploration. While some upcoming
high-profile missions – such as NASA’s Psyche Discovery mission or the
European Space Agency’s (ESA) JUpiter ICy moons Explorer (JUICE) – will
continue to use improved heritage instrumentation, particularly the widely-
used fluxgate magnetometer, there is also an emerging set of potential
applications in view of the trend towards smaller platforms and probes (e.g.,
CubeSats, NanoSats, PocketQubes, etc.), in addition to rovers and rotorcraft.

The Psyche Discovery mission aims to study the 16-Psyche asteroid, an asteroid
orbiting the sun between Mars and Jupiter, and unique in that it is made
almost entirely of nickel-iron metal, unlike the rocky, icy, or gas-covered
worlds explored by all other previous space missions. Magnetometry plays a
significant role in the mission; the first objective of the mission is to
detect and measure a magnetic field, which would confirm that Psyche is the
core of a planetesimal [9]. Typical of past interplanetary missions, the
Psyche magnetometer consists of two identical fluxgate sensors in a
gradiometer configuration located at the middle and outer end of a mag boom.
Drawing heritage from the Magnetospheric Multiscale Mission [40], the Psyche
magnetometers have two selectable dynamic ranges of $\pm 1000$ and $\pm
10^{5}$ nT, with resolutions of $\pm 0.1$ and $\pm 10$ pT respectively.

The JUpiter ICy moons Explorer (JUICE), a mission being developed by the
European Space Agency (ESA), will have a payload consisting of ten state-of-
the-art instruments to carry remote sensing and geophysical studies of the
Jovian system. The JUICE spacecraft is scheduled for launch in June 2022, and
is set to arrive in orbit around Jupiter in 2030. There, it will perform
continuous observations of Jupiter’s atmosphere and magnetosphere over a
2.5-year period [41]. Among the instruments of the payload is a magnetometer
intended for the characterization of the Jovian magnetic field, its
interaction with the internal magnetic field of Ganymede, and the study of the
subsurface oceans in the icy moons. The magnetometer is of the fluxgate type,
using fluxgate inbound and outbound sensors mounted on a boom [41].

The Europa Clipper Mission being developed by NASA, which will be launched in
the 2020’s (specific launch date is not yet declared), will conduct studies of
Jupiter’s moon Europa to determine if the moon could harbor the necessary
conditions for the existence of life. Nine scientific instruments will
comprise the Europa payload, including cameras, spectrometers, ice-penetration
radars, and a triaxial fluxgate magnetometer, among others. The magnetometer
will be used to measure the strength and direction of Europa’s magnetic field,
allowing scientists to determine the depth and salinity of its ocean [42].

Further, CubeSats (satellites built at the scale of $10$ cm cubed) continue to
gain traction as a suitable platform for breaking new ground in planetary
science and exploration. For example, a CubeSat-based distributed fluxgate
magnetometer network has been proposed for characterizing Europa’s deep ocean
[43].

Alongside orbital surveys, a suite of unmanned rovers with terrestrial,
atmospheric, and/or oceanic capabilities have been proposed for investigations
of extraplanetary bodies, particularly the large moons of Jupiter and Saturn.
Airborne extraterrestrial vehicles, as first demonstrated by NASA’s Ingenuity
flights on Mars [44], have excellent potential for targeted planetary
operations. For example, NASA’s Dragonfly mission – due for launch in 2026 and
projected to arrive at Titan in 2034 – will study the moon’s atmospheric and
surface properties, along with prebiotic chemistry in its subsurface oceans
[21]. Magnetometers are not included in Dragonfly’s payload due to size and
weight restrictions, highlighting the need for miniaturized and efficient
magnetometers for extraterrestrial drones. Other proposed drone missions, such
as those submitted to the ESA’s ‘Voyage 2050’ long-term planning process,
include magnetometer-equipped missions to both Enceladus [23] and Titan [22]
designed to launch within the next thirty years.

###  II.3 Applications in Earth atmosphere and orbit

[33, 29, 32, 45, 46, 31, 28, 30, 38, 36, 34]
---
[47, 48, 49, 50, 51, 52, 53, 54, 55, 56]
---
[20, 57, 58, 59, 60]
---
[61, 62, 63, 64, 65, 19, 66, 67, 68]
---
[69, 70, 71, 72, 73, 74, 75]
---
[76, 77, 78, 79, 80, 81, 82, 83, 84, 85]
---
[86, 87, 88, 89, 90, 91, 92, 93]
---
Figure 3:  Sensitivities for available and emerging magnetometers. The colored
regions indicate typical parameter regimes for each category of device, with
icons showing representative examples from the scientific literature. The dark
gray region (bordered by a white line) contains the majority of traditional
magnetometers (fluxgate, search coil, proton precession, Overhauser) and chip-
scale magnetometers with electrical readout (magnetoresistive, Lorentz-force-
actuated MEMS, etc.). The light gray region shows the parameter regime that
has not yet been explored. Approximate performance limits to some
magnetometers are shown as diagonal lines; the thermal limit to optomechanical
sensing (solid line, [94]; see also [95, 96]), the atom shot noise limit to
SERF sensing (dotted, [97]), and the NV quantum projection noise limit
(dashed, [65]).

Magnetometers also serve a critical role in aerospace applications in Earth’s
atmosphere and orbit, ranging from attitude determination in satellites to
geomagnetic surveys using unmanned aerial vehicles (UAVs). Here, we provide an
updated synopsis of Earth-based aerospace magnetometry while highlighting the
advantages and limitations of existing magnetometers.

[34, 29, 31, 33]
---
[54, 55]
---
[59, 58]
---
[62, 65, 64, 68, 19, 61]
---
, [73, 70, 69, 74, 71, 72]
---
[85, 80, 76, 82, 83, 84, 81, 78]
---
[98, 99, 86, 100, 101, 102]
---
Figure 4:  Sensor power requirements for available and emerging magnetometers,
not including support systems such as cryostats (except as indicated below).
Colored regions indicate typical parameter regimes for different varieties of
magnetometer. Representative examples from the scientific literature are given
as ‘open’ (white with black border) icons. Where available, the total power
use of packaged devices (including support systems, etc.) are shown as ‘solid’
black icons. Note that SQUID magnetometers can have extremely low sensor power
dissipation ($\sim 10$ fW) due to their superconducting nature, hence the blue
region extends beyond the left border of the plotted region; however, their
total power use is typically large due to cryogenic requirements. SERF
---
Atomic vapour cell
---
Fluxgate
---
Hall probe
---
Induction coil (air core)
---
Magnetoresistive
---
Magnetoelectric
---
MEMS
---
NV ensemble
---
Optomechanical
---
SQUID
---
$0$
---
$1$
---
$10$
---
$10^{2}$
---
$10^{3}$
---
$10^{4}$
---
$10^{5}$
---
$10^{6}$
---
$10^{7}$
---
$10^{8}$
---
$10^{9}$
---
$10^{10}$
---
$\times 3$ Hz
---
sub-ELF
---
ELF
---
SLF
---
ULF
---
VLF
---
LF
---
MF
---
HF
---
VHF
---
UHF
---
SHF
---
EHF
---
Figure 5:  Operating frequencies for different varieties of magnetometer
(grouped by International Telecommunication Union frequency designations).

####  II.3.1 Geostationary Operational Environmental Satellites (GOES)

Near-Earth satellites are important platforms for the collection of magnetic
data, both for wide-scale geological and military observations [103, 104, 105,
106, 107], plus geomagnetic and magnetospheric monitoring. The GOES – part of
a series of satellites of the National Oceanic and Atmospheric Administration
(NOAA) that have been in operation since the mid 1970s – are a key example of
the latter. Their payloads have included magnetometers to measure the Earth’s
magnetic field, primarily to provide information about geomagnetic storms,
energetic particle measurements, and magnetospheric and ionospheric effects.
These measurements are particularly important for the characterization of
ionospheric scintillation affecting high-precision location measurements with
GPS (Global Positioning System) [1], as well as effects on the electric power
grid [2], high-frequency radio communications in the 1–30 MHz range [3], and
also satellites in low-Earth orbit (LEO), which can experience extra
atmospheric drag when solar activity is high [108]. Additionally, the GOES
magnetometer data have also been used in real-time support of rocket launch
decisions [109].

The initial GOES series – i.e., GOES-1,-2, and -3 (1975–-1978) – featured
biaxial, closed-loop fluxgate magnetometers (these feature a feedback loop
that nulls the external field at the sensor’s location). These FGMs were
deployed on booms approximately $6.1$ m long, with one sensor aligned parallel
to the spacecraft spin axis and the other perpendicular, with a sensing range
from 50–400 nT. The GOES-4,-5,-6, and -7 (1980–1987) satellites were equipped
with spinning twin-fluxgate magnetometers, mounted on 3 m booms, and had a
range of $\pm 400$ nT with $0.2$ nT resolution. Extending the capabilities of
the GOES 1–7 spacecraft, the GOES-NEXT series (GOES-I(8) through GOES-M(12)),
were launched between 1994 and 2001. This series of spacecraft used two
redundant triaxial FGMs, with an increased range of $\pm 1000$ nT at a
resolution of $0.1$ nT. In this case the electronics were located inside the
body, with the two magnetometers mounted on 3 m deployable booms. The
following installments, GOES-N,-O,-P (13–15), had FGMs of reduced dynamic
range, $\pm 512$ nT, in favor of a 2$\times$ improved resolution of $0.03$ nT,
and were mounted in a gradient configuration on 8.5 m booms [110]. Finally,
the most recent in the set are the GOES-R series (GOES-R/S/T/U) with GOES-R
and -S having been launched in 2016 and 2018, respectively. The magnetometers
featured here are similar to the GOES-N series triaxial-FGM configurations,
but with improved resolution on the order of $0.016$ nT [111]. Figure 6 shows
an artistic rendition of the GOES-R spacecraft, illustrating the location of
the FGM.

Figure 6: GOES-R spacecraft (2016). Credit: NASA

Similar magnetometer-equipped satellite networks have been proposed to
supplement crucial RADAR-based early warning systems for dangerous tectonic
activity [112, 11, 113, 4, 5] through the detection of magnetic anomalies
prior to earthquakes (e.g., GESS, Global Earthquake Satellite System).

####  II.3.2 Magnetometers onboard micro- and nanosatellites

Magnetometers are conventionally used onboard satellites as part of the
attitude determination system for low-earth orbit satellites [12]. However,
magnetometers cannot usually obtain three-axis attitude information with only
a three-axis magnetometer, and the measurement is distorted by magnetized
objects and current loops on board the satellite itself. Hence, these systems
include other sensors that can measure the satellite’s motion with respect to
celestial bodies [114].

These additional sensors are too bulky and power-consuming to be used in
micro- and nanosatellites, such as CubeSats, which consequently have to rely
on attitude determination by magnetometer only. As these small satellites are
starting to be applied to more sophisticated objectives, such as remote-
sensing and astronomy missions, precise attitude determination is becoming a
requirement [114]. The task is additionally complicated by the typically large
magnetic moment of satellites with small inertia, which can then cause
magnetic bias noise due to the interaction of the earth’s magnetic field with
the magnetic moment of the satellite [115].

A key challenge is therefore to compensate for this magnetic bias noise. This
could be achieved by estimating the interaction with the earth’s magnetic
field using a gyroscope and a Kalman filter [114]. The other major challenge
is to achieve full three-axis attitude determination using a magnetometer
only. This can, in principle, be achieved by comparing magnetic field readings
to an accurate model of the earth’s magnetic field. However, this method is
computationally expensive [12]. Finally, disturbances onboard the satellite
itself could be accounted for by either very careful calibration [116], or by
using several magnetometers in different parts of the satellite, which would
require further reduction of SWaP.

Considerable work on further SWaP reduction of fluxgate magnetometers has been
spearheaded by Todd Bonalsky, Efthyia Zesta, et al. from NASA Goddard Space
Flight Center [117, 118]. FGMs of significantly reduced SWaP have been
developed and deployed on the Dellingr spacecraft launched in 2017 and the
Scintillation Prediction Observations Research Task (SPORT) CubeSat that is
expected to be launched from the International Space Station in 2021–2022.
NASA’s Gateway platform, an orbital outpost, which is intended to be
positioned near the Moon as a stepping stone to Mars, will utilize these
miniaturized FGMs as part of its space weather monitoring instrument suite,
Gateway HERMES (Heliophysics Environmental and Radiation Measurement
Experiment). The magnetometers on Gateway HERMES will allow NASA to study the
solar winds and the Earth’s magnetotail for the purposes of understanding and
forecasting solar weather events that will affect astronauts and instruments
operating on or around the Moon. The FGMs will be placed on the end of a boom,
far away from the Gateway’s power and propulsion module. Two magneto-inductive
sensors, which have significantly lower SWaP than the FGMs, will be mounted on
the Gateway HERMES platform to detect and subtract magnetic noise generated by
the power and propulsion module.

Magneto-inductive sensors [46, 45] contain a solenoidal-geometry coil wrapped
around a high-permeability magnetic core that forms the inductive element of
an LR relaxation oscillation circuit. The effective inductance of the coil is
proportional to the magnitude of the magnetic field parallel to the axial
direction of the coil. The oscillation frequency of the circuit will vary with
the magnetic field at the coil. Commercially available magneto-inductive
sensors, such as the PNI RM3100, use comparison with an internal clock to
measure the oscillation period of the circuit, and hence the magnitude of the
magnetic field. Such sensors are small ($\sim$ 15 mm3), have a low operating
power ($\sim$ 0.1 W), a resolution of around 20 nT, a dynamic range of -800
$\upmu$T to +800 $\upmu$T, and a sample rate of $>400$ Hz.

NASA’s Goddard team are also working on developing self-calibrating hybrid
devices to overcome the drift experienced by fluxgate magnetometers. These
hybrid devices contain a vector fluxgate magnetometer paired with a scalar
atomic magnetometer. Their small SWaP makes them suitable for deployment in
constellation-type missions where multiple CubeSats simultaneously gather
multi-point observations [117].

An alternative to FGMs onboard micro satellites are magnetoresistive
magnetometers. These are based on either giant magnetoresistance (GMR) or
anisotropic magnetoresistance (AMR). GMR is an effect observed in thin films
comprised of sandwiched ferromagnetic and diamagnetic (‘non-magnetic’) layers
(such as Cu). In the presence of a magnetic field, the magnetic moments of the
two ferromagnetic layers become aligned and the interlayer resistance
decreases drastically [119]. AMR makes use of permalloy (Ni 80%, Fe 20%) that
has electrical resistivity that varies as a function of the strength and
orientation of the external magnetic field [119]. These techniques have been
reported to achieve sensitivity of about 1 nT/ Hz1/2 at micrometer scale
resolution and under ambient operating conditions; thus, they have seen
diverse applications as sensors in biomedicine [120], consumer electronic
products such as smart phones [121], and as precision sensors in aerospace
applications for low-field magnetic sensing. While AMR magnetometers have
historically exhibited hysteresis and stability issues [15], Brown et al. have
reported on the development of a compact, dual-sensor vector AMR magnetometer
for applications on very small spacecraft [122]. The instrument, called MAGIC
(MAGnetometer from Imperial College), exhibits sensitivities of $3$ nT in a
$0$–$10$ Hz band within a measurement range of $\pm 57,500$ nT, at a total
mass of only $104$ grams, and power consumption in the range of $0.14$–$0.5$ W
(depending on the mode of operation). These very low SWaP requirements make
magnetoresistive magnetometers suitable for applications in attitude orbit
control systems of small satellites – they have already been launched in the
TRIO-CINEMA CubeSat space weather mission [123] – as well as planetary
landers. Further discussion of AMR/GMR magnetometers (along with
microelectromechanical MEMS magnetometers [56, 55, 54, 53], which will not be
discussed in detail here) can be found in [16].

####  II.3.3 Navigation in the Earth’s atmosphere

Magnetometers have been used as a part of airplanes’ navigation systems for
many years to provide heading information [124, 125]. Historically,
observations of the local geomagnetic field have been performed using ground-
or aircraft-based proton-precession magnetometers [126, 127, 124, 125]. A
sample of hydrogen-rich material (typically kerosene) is polarised by the
application of a magnetic field; when the field is turned off, the protons
precess around the ambient geomagnetic field at a frequency proportional to
the field strength. This is detected with an induction coil. Scalar and vector
operation is possible [126]. Aerospace applications of proton-precession
magnetometers are primarily hindered by their large power consumption. This is
addressed by Overhauser magnetometers: built around the same precession
phenomena, but leveraging the Overhauser effect [128] to efficiently generate
nuclear magnetic polarisation through RF pumping. The resulting sensitivity
boost and reduction in SWaP has even allowed Overhauser magnetometers to be
flown aboard satellite missions, e.g., the Danish Ørsted satellite
(sensitivity $\sim 20$ pT/ Hz1/2 , $3$ W of power consumption, $1$ kg mass)
[129, 130, 106].

In recent years, it has been proposed to obtain precise position information
by measuring local magnetic field variations and overlapping them with a
detailed map of the Earth’s magnetic field [131]. This proposal relies on the
unique local variations of the Earth’s magnetic field, defined by rock
formations in the Earth’s crust. It will enable navigation in GPS-degraded or
-denied environments, such as in the presence of GPS jamming. It is
impractical to use ground-based proton-precession/Overhauser magnetometers to
obtain the necessary measurements with sufficient spatial resolution and
coverage; aerial surveys are required. Lockheed Martin has recently developed
its Dark Ice technology, which uses a NV-center based vector magnetometer for
this purpose (see also section III.3). Depending on the flight altitude, these
should allow spatial resolution down to $\sim 200$ m, while the small SWaP
could allow operation onboard small UAVs [132].

####  II.3.4 Magnetometers in manned aerial vehicles

Currently, detailed magnetic observations for geological surveys [13, 14],
unexploded ordnance detection [133, 134], magnetic anomaly detection (e.g., of
submarines or sea mines) [135], and other applications [5, 11] are primarily
conducted using magnetometers on manned vehicles, be they land-based [136, 5],
aircraft [137, 138, 134], ships [139, 140], or underwater vehicles [141].
Manned aircraft are relatively large and able to generate significantly higher
power than UAVs, making them suitable platforms for high-sensitivity airborne
magnetometer solutions, such as the SQUID-based tensor magnetic gradient
measurement system UXOMAX [142].

SQUID (Superconducting QUantum Interference Device) magnetometers [86, 87, 88,
89, 90, 91, 92, 93, 143, 144, 145, 146, 147, 148] consist of a superconducting
loop split by one or two Josephson junctions [143, 144]. The current
circulating in the superconducting loop, and the corresponding voltage drop
across the Josephson junction, is sensitive to the magnetic flux threading the
loop. SQUID magnetometers offer high magnetic field sensitivity (sub-fT/ Hz1/2
[145, 91]), high dynamic range (they can operate in the Earth’s magnetic field
[86, 88]), a large range of spatial resolutions (down to the nanometre scale
[149, 146]), and broad bandwidth operation (DC to GHz [147]). To achieve
superconductivity the SQUID needs to be operated in a cryogenic environment;
this incurs large operating costs and is incompatible with low SWaP
applications, although they have been used aboard planes and helicopters [145]
for applications such as nondestructive archaeology and geomagnetic evaluation
[148]. Promising advances are being made towards the creation of ambient-
condition superconducting materials, which would lead to improved SWaP
requirements for SQUIDS in the future [150], but such materials remain highly
speculative and may never eventuate.

####  II.3.5 Magnetometers in unmanned aerial vehicles

Recent field demonstrations of geomagnetic surveys and magnetic anomaly
detection using unmanned aerial vehicles (UAVs) have highlighted several
advantages of low SWaP magnetometers for autonomous or remote-controlled
surveys [151, 152, 153, 154, 155]. UAVs, for instance, allow for exquisite
spatial resolution of a few meters, high sensitivity due to low flight
altitude, and easy access to rugged terrain [156], while saving cost and
operator time. UAV-based surveys are particularly efficient for detecting
small targets, such as unexploded ordnance and landmines [151], and are
predicted to significantly enhance magnetic mapping capabilities which, in
turn, enable improved navigation in GPS-denied environments [157]. However,
the most sensitive magnetometer technology with sufficiently low SWaP to be
used on typical UAVs is fluxgate magnetometry [158], which has sensitivity
several orders of magnitude poorer than techniques such as SQUID magnetometry
[159].

Possible alternative high-sensitivity instruments include miniaturized atomic
magnetometers [160] and ultra-sensitive integrated magnetostrictive
magnetometers [11, 20, 161].

Magnetostrictive magnetometers [162, 163, 11, 164, 20, 165] rely on the strain
induced in a magnetostrictive material (such as galfenol or Terfenol-D) for
detection of the magnitude of applied magnetic fields. Depending on the design
of the magnetometer, applying a magnetic field to the magnetostrictive
material may cause motion, stress, a force or a torque, which can be detected
in a number of ways, but are usually read out electronically or optically. One
such magnetometer, using optical readout, uses a magnetostrictive material
deposited on an fibre optic interferometer to change the relative path length,
and hence the relative phase of laser light, in the two arms of the
interferometer [11]. This integrated device has low SWaP (weight $\approx 110$
g and operating power $<3$ W), a sensitivity of 10 pT/ Hz1/2 over the 1 Hz to
100 Hz frequency range, and a dynamic range of $>100$ $\upmu$T (sufficient to
operate within the magnetic field at the surface of the Earth). An alternative
magnetostrictive magnetometer design uses magnetostrictive material sputter
coated onto a microfabricated optomechanical cavity; these optomechanical
devices are discussed further in section III.

One of the most common off-the-shelf magnetometers for high field applications
are Hall magnetometers [166], which function on the basis of the Hall effect,
i.e., an external magnetic field deflects the current flowing through a
conductor, leading to a voltage difference perpendicular to the current. Hall
magnetometers can measure both AC and DC fields. They are typically used in
high field applications and not in precision sensing as they are less accurate
than other available magnetometers (peaking around $1$ nT/ Hz1/2 [167]). In
aerospace, they typically find applications in safety interlocks, rotation
gauges, and proximity sensors to ensure safe operation of craft, rather than
use as scientific instrumentation.

##  III Emerging magnetometers

Heritage magnetometers – including fluxgate, proton-precession, and optically-
pumped magnetometers – have proven utility in space missions, and will
continue to be used into the future. However, they have limitations. For
instance, FGMs suffer from drifting scale factors and voltage offsets with
both time and temperature, requiring periodic recalibration [71]. Proton-
precession magnetometers and optically pumped magnetometers exhibit excellent
sensitivity (e.g. $10$–$50$ pT RMS), absolute accuracy ($0.1$–$1.0$ nT), and
dynamic range ($1$–$100$ $\upmu$T) [71], but they have considerable mass ($>1$
kg), high power requirements ($>10$ W), and large volume ($>100$ cm3). These
‘workhorse’ scientific instruments are unsuitable for use in many emerging
aerospace applications, particularly in view of the trend towards smaller
platforms and probes, e.g. CubeSats, NanoSats, PocketQubes, etc. To meet the
challenges of sensing on small craft, magnetometers must achieve reductions in
size, weight, and power (‘SWaP’) whilst preserving or even enhancing
performance.

In general, the magnetometers discussed so far are close to the limits of
their applicability. Accordingly, new types of magnetometer need to take their
place to go beyond these limits. Some promising candidates are SQUIDs, atomic,
NV, MEMS and optomechanical magnetometers. These new sensors also have
functionality limits but we are still far from reaching them.

###  III.1 Atomic magnetometers (including SERF)

Atomic magnetometers [69, 70, 71, 72, 73, 74, 168, 18, 169] consist of a vapor
of alkali atoms (usually K, Rb or Cs) enclosed in a glass cell, generally
heated to about $400$ K. When a laser beam passes through the vapor cell, the
spins of the atoms’ unpaired electrons align in the same direction. If a
magnetic field is present, the electrons precess, which leads to a
polarization or amplitude change in the transmitted light. This can be
detected and used to infer the magnetic field. The sensitivities achieved can
be very high, on the order of 160 aT Hz1/2 [168, 69], with spatial resolution
as small as the millimeter scale [69, 70, 73, 74]. Some have a high dynamic
range and can operate in the Earth’s magnetic field [69, 70], while others
have a low dynamic range and require magnetic shielding or close-loop
operation [71, 72, 74]. Operation bandwidths typically range from DC to $\sim$
1 kHz. The atom–light interaction is sensitive to the orientation of the
magnetic field, so this type of magnetometer is suitable for vector
magnetometry. The most sensitive commercially-available magnetometer is based
on atomic magnetometry; these can achieve a sensitivity of 300 fT at 1 Hz
[170].

Recent developments in chip-scale atomic magnetometers – such as magnetometers
fabricated with silicon micromachining techniques as part of the “NIST on a
Chip” program – have demonstrated a significant reduction in SWaP [18], making
them competitive candidates for future CubeSat and UAV projects. The size of
these vapor cells is about that of a grain of rice. It is anticipated that
such a magnetometer could be placed aboard low cost CubeSats used for
detection of the Earth’s magnetic field as well as for measuring the magnetic
fields of other planets.

Other authors, such as Korth et al. [71], have proposed miniaturized atomic
scalar magnetometers based on the 87Rb isotope for space applications. This
magnetometer is based on a vapor cell fabricated using silicon-on-sapphire
(SOS) complementary metal-oxide-semiconductor (CMOS) techniques. The vapor
cell exhibits a volume of only $1$ mm3. The multi-layer SOS-CMOS chip also
hosts the Helmholtz coils and additional circuitry required to control the
atoms, along with heater coils and thermometers used to adjust the Rb vapor
pressure. The overall magnetometer system has a total mass of less than $0.5$
kg, consumes less than $1$ W of power, and demonstrates a sensitivity of $15$
pT/ Hz1/2 at $1$ Hz. This is comparable with high-sensitivity heritage
technologies. Accordingly, these magnetometers address the reduction in SWaP
(and potentially cost) without sacrificing performance. They are a viable
option for integration in SmallSats for space exploration.

Improved absolute sensitivity can be achieved in atomic vapor cell
magnetometers by operating with a dense gas at elevated temperatures. Under
these conditions, collisions between the alkali atoms no longer scramble the
electronic polarisation, improving the sensor’s signal-to-noise ratio. These
“Spin Exchange Relaxation-Free” (SERF) devices sacrifice dynamic range for
sensitivity; SERFs cannot tolerate the $\sim\upmu$T fields that are able to be
sensed with standard vapor cells. As a result, they require magnetic shielding
(which is typically heavy) or active magnetic cancellation (which requires
additional control circuitry); they also have increased power requirements
because of the elevated temperatures involved. Nevertheless, SERFs are
promising candidates for nuclear magnetic resonance sensing [171] – as might
be used to detect extraterrestrial organic compounds in situ – and biomagnetic
sensing. They could also be used to perform magnetocardiography or
magnetoencephalography on astronauts for non-invasive health monitoring [172,
173]; for example, the heart produces a field of approximately 10 pT outside
the body, whilst the brain produces fields of around 1 pT at the scalp [18].

###  III.2 Optomechanical magnetometers

Optomechanical magnetometers are usually optically- and mechanically-resonant
mechanical structures (‘cavities’) that deform when subjected to a magnetic
field [60]. The deformation leads to a change in the optical resonance
frequency, which can be detected with extremely high precision. In most
optomechanical magnetometers [161, 20, 57, 58, 60, 174, 175], the deformation
is due to magnetostrictive coatings or fillings that exert a field-dependent
force (much like the aforementioned magnetostrictive magnetometers). Related
designs [176, 57, 59, 177, 178, 179] respond to the magnetic field gradient
via the dipole force, or enhance the magnetostrictive response using
ferromagnetic resonance [180]. It is notable that rapid progress is occurring
in optomechanical sensing, not limited to magnetic fields, but also of other
aerospace-relevant stimuli such as temperature [181, 182, 183], acoustic
vibrations [184, 185], pressure [186], force [187, 188], and acceleration
[189, 190, 191, 192].

To date, the best field sensitivity demonstrated by an optomechanical
magnetometer is $26$ pT/ Hz1/2 at $10.523$ MHz [20]. This is competitive with
that of SQUIDs of similar size (approximately $100$ $\upmu$m diameter), but
without the requirement for complicated and bulky cryogenics. Furthermore,
they do not require magnetic shielding – with typical dynamic ranges being
$\sim 100$ $\upmu$T – and have low power consumption ($\sim 50\;\upmu$W of
optical power). They are often sensitive at frequencies up to 130 MHz, where
they are limited by quantum phase noise of the optical readout. At
intermediate frequencies, optomechanical magnetometers are limited by
thermomechanical noise, and classical laser phase noise becomes dominant below
approximately $\sim 1$ kHz (depending on the light source).

Translational research is being undertaken to integrate these devices into low
SWaP packages for a range of in-field applications. The chief challenges at
this stage are managing stress in magnetostrictive thin films [161], and
reducing or mitigating the effects of low-frequency laser noise.
Optomechanical magnetometers will become prime candidates for small orbital
platforms – both for scientific and communications purposes [11, 193] – and
applications on extraterrestrial rovers or other unmanned vehicles with
stringent SWaP requirements.

###  III.3 Magnetometers based on atomic defects in solids

Many crystalline materials host defects (substitutions, vacancies, and
combinations thereof) that lead to so-called ‘color centers’, magnetically-
sensitive artificial atoms embedded within the crystal that are addressable by
microwave and/or optical fields. Silicon vacancies in silicon carbide [194,
195] have been used to detect magnetic fields in proof-of-concept experiments
($\sim 100$ nT/ Hz1/2 ); however, the best-developed defect-based sensors at
the current time use nitrogen–vacancy centers (NV) in diamond [65, 19, 64, 63,
61, 62, 66, 67, 68, 196, 197, 198, 199, 200, 201, 202, 203].

A negatively charged NV- defect has a triplet ground state (3A2), a triplet
excited state (3E), and two intermediate singlet states (1A and 1E). The
energy separation between the sub-levels in the triplet ground state varies
with the magnetic field aligned to the NV quantization axis. When illuminated
with green light the defect undergoes photoluminescence at 637 nm; the
intensity of the emitted light is higher when the $m_{s}=0$ ground state sub-
level is populated and exhibits a dip when the population is transferred to
the $m_{s}=\pm 1$ sub-levels. The Zeeman splitting of the ground state, and
hence the magnetic field the defect is exposed to, can be measured by using a
microwave source to drive the ground state population between the sub-levels
and observe the corresponding dip in photon emission [196]. The atomic scale
of an NV-defect means that NV-magnetometers naturally have a very high spatial
resolution (single-defect magnetometers have been demonstrated by e.g., [67]);
this property has been utilized for demonstrating nanoscale imaging of
biological samples [197, 198]. The NV- defects have four possible orientations
within the carbon crystal lattice, enabling vector magnetometry techniques to
be deployed [199, 200, 201]. Sensitivities as good as 0.9 pT/ Hz1/2 have been
demonstrated in laboratory conditions [65, 19] and operation frequencies vary
from DC up to a few gigahertz [64, 63, 61] (with different sensitivities
across this range).

This is a relatively new technology, only recently integrated for use outside
of the laboratory [68]; as such, there are a limited number of near
commercially-available options at present (e.g. Lockheed Martin’s Dark Ice).

##  IV Brief summary

Having introduced the major technologies, we are now in a position to
summarise some of the typical performance metrics of existing and emerging
magnetometers.

Figure 3 shows the sensitivity of various magnetometers as a function of their
typical length scale (linear dimension). As expected, heritage devices
(fluxgates, helium magnetometers, proton precession magnetometers, etc.) have
very good sensitivity, enabling use in many areas, but they tend to be
relatively large. Chip-scale electronic devices (AMR/GMR, MEMS, etc.) are
appreciably smaller and typically exhibit reduced sensitivity. Notably,
superconducting magnetometers (SQUIDs) and devices with optical readout
(optomechanical, NV diamond, atomic vapor cell, SERF) are almost universally
more sensitive than their conventional/electronic counterparts of comparable
sensor size. Furthermore, these emerging technologies are still far from the
ultimate performance limits that are enforced by their fundamental noise
sources. For example, current optomechanical, NV, and SERF magnetometers are
approximately two to three orders of magnitude above their sensitivity limits,
as shown in Figure 3; even these limits can potentially be manipulated by
leveraging quantum-mechanical effects [58, 204, 205, 206]. In contrast, some
heritage technologies are already at their physical limits, such as air-core
search (induction) coils [207]. Notably, fluxgate magnetometers are not yet at
their limit [208].

Typical sensor power requirements are indicated in Figure 4. Note that the
colored regions do not consider the power drain incurred by support systems
such as electronic processing, cryogenics, heating, etc. It is evident that
heritage magnetometers have similar total power requirements (solid points in
Figure 4) to atomic vapor cells, SERFs and SQUIDs, but the new technologies
have an advantage in terms of absolute sensitivity. For applications requiring
low power consumption and intermediate sensitivities, optomechanical, NV, and
chip-scale electronic sensors are most appropriate. Again, we see that optical
readout is an enabling factor for both high-sensitivity devices (AVC, SERF)
and low-power devices (optomechanical). The power requirements of NV
magnetometers are strongly linked to the light collection efficiency of their
readout optics, which are currently low; thus, NV magnetometer power
requirements are likely to drop significantly in the future (e.g., [209]).

Finally, we consider the usual operating frequencies of magnetometers, as in
Figure 5. Note that these frequency ranges do not indicate the magnetometers’
typical operating bandwidths (which are usually much smaller than the ranges
given here), nor that any one magnetometer in a category can be tuned across
the entire range shown (e.g., SQUID magnetometers come in DC and radio-
frequency varieties [144], with high-performance guaranteed only in a
specified part of the spectrum). Many types of magnetometer are able to
operate down to extremely low frequencies (ELF, $3$–$30$ Hz) or even below,
though various low-frequency noise sources tend to lead to reduced sensitivity
near DC. As already touched upon, these frequency ranges are important for
heliospheric/geomagnetic field mapping, through-water or through-earth
communications, etc. Conversely, radio frequency operations up to the VHF
range ($3\times 10^{8}$ Hz, typical of commercial FM radio) are possible with
optomechanical and SQUID magnetometers, permitting magnetic antennae for
interplanetary or orbital communications, etc.

All in all, there is no ‘one size fits all’ magnetometer technology, nor is
there ever likely to be! Careful consideration of the parameters discussed
above – plus others that we have not focused on, such as bandwidth, drift, and
dynamic range – is required when selecting which magnetometer to deploy for a
given application.

##  V Conclusions

In this review paper we have provided a broad overview of the various
aerospace and space-based applications enabled by precision magnetometry. In
the context of these applications we first discussed existing magnetometry
platforms, highlighting the advantages and limitations of each. No doubt,
these ‘workhorse’ platforms will continue to enjoy use and development well
into the future. However, we have identified some emerging aerospace
applications, particularly those involving smaller platforms and probes (e.g.,
CubeSats, NanoSat, PocketQubes, etc.), that require magnetometers with lower
size, weight, and power (‘SWaP’) needs, and equivalent or even enhanced
performance. Since many of the existing magnetometers are already close to
their physical performance limits, new types of magnetometer must be developed
to take their place in these emerging applications. We have highlighted some
promising magnetometers that are currently under development, alongside the
applications that they may enable.

To compare existing and emerging magnetometers, we provided an overview of
relevant operational parameters – field sensitivity, power consumption,
detection frequency range, sensor size etc. – for different classes of
magnetometers. Finally, we identified that optical readout is a key route
towards improved sensitivity and SWaP for next-generation devices.

## Contributions

Conceptualization, W.B. and F.M.; literature search and writing, all; surveyed
historical missions, B.V. and F.M.; surveyed emerging devices, J.B., H.G.,
E.B., F.G, S.F, G.I, and W.B.; compiled figures, J.B.; compiled tables, B.V.;
project leaders, W.B. and F. M.

## Funding

This research is supported by; the Commonwealth of Australia, as represented
by the Defence Science and Technology Group of the Department of Defence (NGTF
QT30/40); the Commonwealth of Australia, as represented by the Australian
Research Council Centre of Research Excellence for Engineered Quantum Systems
(EQUS, Grant No. CE170100009); and the United States of America, as
represented by the National Aeronautics and Space Administration (NASA) Space
Communications and Navigation (SCaN) program. S.F. acknowledges funding from
the EQUS Translational Research Program (TRL, EQUS). E.B. acknowledges funding
from an EQUS Deborah Jin Fellowship. F.G. acknowledges funding from Orica
Australia, Pty Ltd.

The authors declare no conflict of interest.

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