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64th International Astronautical Congress, Beijing, China. Copyright ©2013 S. Engelen et al. All rights reserved.
IAC-13,B4,7B,6,x18035
THE ROAD TO OLFAR - A ROADMAP TO INTERFEROMETRIC LONG-WAVELENGTH
RADIO ASTRONOMY USING MINIATURIZED DISTRIBUTED SPACE SYSTEMS
Steven Engelen
Delft University of Technology, The Netherlands, [email protected]
Kevin A. Quillien, Dr. Chris Verhoeven, Dr. Arash Noroozi, Prem Sundaramoorthy,
Prof. Dr. Alle-Jan van der Veen
Delft University of Technology, The Netherlands, [email protected], [email protected],
[email protected], [email protected], [email protected]
Raj Thilak Rajan, Dr. Albert-Jan Boonstra
ASTRON, The Netherlands, [email protected], [email protected]
Dr. Mark Bentum, Dr. Arjan Meijerink, Alex Budianu
University of Twente, The Netherlands, [email protected], [email protected], [email protected]
ABSTRACT
The Orbiting Low Frequency Antennas for Radio Astronomy (OLFAR) project aims to develop a space-based low
frequency radio telescope that will explore the universe's so-called dark ages, map the interstellar medium, and
discover planetary and solar bursts in other solar systems. The telescope, composed of a swarm of at least fifty
satellites working as a single instrument, will be sent to a location far from Earth in order to avoid the high Radio
Frequency Interference (RFI) found at frequencies below 30 MHz, originating from Earth. The OLFAR telescope is
a novel and complex system, requiring not-yet proven technologies and systems, therefore, a number of key
technologies are still to be developed and proven. Most of these can be tested on Earth, but four aspects in particular
require in-space verification. Those are (1) the satellite's propulsion and attitude control systems, and (2) their
interactions with the large science antennas, as well as the (3) payload system itself and finally (4) the in-space
interferometry and 3D-imaging. Furthermore, the RFI environment in the intended target orbits is mostly unknown.
Indeed, only three satellites missions have previously been launched into orbit shedding light on the RFI
environment, but sufficiently detailed measurements allowing for the creation of a usable RFI model have never been
performed. To carry out both the hardware qualification and RFI measurements, a few pathfinder missions are
deemed in order. This paper describes these pathfinders in detail; outlining the scientific objective, the technologies
being demonstrated as well as the missions' roadmap which revolves around a novel systems engineering approach.
This approach resembles those used in certain fast-paced industries where development is heavily parallelised and
products are launched as soon as opportunities arise. This will be combined with in-space upgrading of mission
firmware to allow for high flexibility within the limited time and budget constraints of these pathfinders.
I.
INTRODUCTION
The ultra-low frequency regime of 0.1- 30 MHz is
one of the last unexplored bands in radio astronomy.
Earth-based radio interferometry is severely limited in
sensitivity at these low frequencies due to man-made
Radio Frequency Interference (RFI) and ionospheric
scintillation. The ionosphere scintillates below ~30
MHz and is completely opaque below ~10 MHz [1]. A
space-based array of satellites, deployed above the
Earths’ atmosphere and far away from Earth itself, will
be less hampered by these limitations and thus will open
up this unexplored frequency regime.
The feasibility of a space-based array for long
wavelength astronomy has been investigated in the past
few decades via numerous studies, however all such
endeavours were limited by technology (e.g. [2], [3])
IAC-13,B4,7B,6,x18035
The only successful mission to have investigated these
long wavelengths is the RAE-B (Radio Astronomy
Explorer – B) lunar orbiter in 1973, a single satellite
which made astronomical measurements in 25 kHz to
13 MHz spectrum. Subsequently, this mission also
provides us the only sky maps we have at these long
wavelengths.
Since then, technology has advanced significantly.
Previous studies, in particular the DARIS study [4], [2]
have shown that with a modest budget of <500 M€, a
formation of 9 satellites can be launched and operated,
mapping the sky at frequencies ranging from 100 kHz to
10 MHz. The DARIS satellites were designed using
readily available space grade technologies.
Orbiting Low Frequency Antennas for Radio
Astronomy (OLFAR) is a feasibility study which
Page 1 of 7
64th International Astronautical Congress, Beijing, China. Copyright ©2013 S. Engelen et al. All rights reserved.
investigates the challenges involved in designing and
deploying a co-operative cluster of miniaturized
satellites for interferometry at ultra-long wavelengths
[5], [6], [7]. The OLFAR project is an attempt to
develop a similar space-based telescope, using advanced
Commercial Off-The Shelf (COTS) technologies, with a
significant spin-in from mainstream technologies.
Several technological breakthroughs have been
achieved over the course of the project, yet a few issues
remain open. This paper outlines the roadmap still to be
taken in order to reach the final goal of an operational,
high performance low frequency radio-telescope in
space.
II.
SCIENCE CASE
The exploration of the unknown territory that is low
frequency radio astronomy is expected to yield
extremely interesting science in a number of areas [1]
lists a number of possible science cases for a lowfrequency array of telescopes, those for which OLFAR
will be particularly adapted are listed in this section.
Cosmological studies on the early universe, which
include the Epoch of Reonization (EoR) and the dark
ages in the 21-cm spectrum are a possible first case.
Mapping and tomography require large baselines of the
order of 20km whilst a very large number of antennas
are needed to overcome the weak astronomical signals:
two aspects which OLFAR aims to tackle. Another case
is the surveying of galactic and extra-galactic largescale radio sources such as galaxy clusters, radio
galaxies, extremely red-shifted galaxies as well as
planetary and solar bursts. Finally, surveying of galactic
radio sources could provide valuable information
regarding the origin of cosmic rays and surveys of the
solar system neighborhood can be made.
As demonstrated in Table 1, the deployment location
of OLFAR is not yet fixed; if however the Lunar orbit
is used, an additional science case can be added to the
list. This involves using the Moon as a detector for
Ultra-High Energy (UHE) particles using emitted
Cherenkov radiation as described in [8]. This is curently
done from Earth using the LOFAR telescope; in-situ
measurements provided by OLFAR would prove to be
invaluable.
III.
orbiting scenario [9], [10], as the relative speed in that
case can reach values of over 100 m/s. This would limit
the snapshot integration time to 1/100th of a second for
signals with 1 m wavelength (i.e. at 30 MHz), which in
turn would create data-sets too large to process and/or
transmit ( [11], [6], [12]).
Number of antenna nodes
Number of polarisations
Observation frequency
range
Instantaneous bandwidth
Survey sensitivity
Spectral resolution
Snapshot integration time
Maximum baseline
between antenna nodes
≥ 10, scalable
3
0.3-30 MHz
≥ 1 MHz
≤ 65 mJy
1 kHz
1 to 1000 s, limited to
1
� �
 ≤ 10 �

100 km
High Earth orbit, Lunar
orbit, Lunar L2, Earth
leading/trailing
Table 1: OLFAR system requirements, adapted from [6]
Deployment location
Mainstream technologies have been gaining in
capabilities, which currently surpass the capabilities of
space-grade technologies. They are seen as the biggest
enabler of an OLFAR-like mission, although their
space-tolerance, and therefore their applicability in
space will still have to be proven. Should such
technologies be used however, the possibilities are
almost limitless, as even common mobile phone
platforms can achieve wireless transmission speeds in
excess of 100 Mbps [13] and offer gigabytes of data
storage, as well as computing powers in excess of 5.000
DMIPS per SoC [14], all at low power.
The OLFAR radio telescope would consist of a
swarm of satellites [7], [6], [15] , each of which is
equipped with a radio antenna payload and a payload
processing module. The satellites form a swarm, in
order to utilise a swarm’s properties of almost indefinite
expandability, which allows creation of a scalable
system, as well as offer an increased availability ( [16],
[17]) and reduced management overhead.
THE SYSTEM
In order to create any useful scientific instrument,
certain specifications have to be met. The key driving
parameters for OLFAR are listed in Table 1, adapted
from [6]. As can be seen, the snapshot integration time
is defined primarily by the relative speed of the
satellites. This has been shown to be an issue for a lunar
IAC-13,B4,7B,6,x18035
In order to reach the required survey sensitivity,
either a long mission duration, and therefore long perspacecraft operational lifetime, or a large number of
nodes is required. One advantage the use of a satellite
swarm offers is that defunct satellites can also be
replenished at some point during the active mission, in
which case the system lifetime is increased through
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64th International Astronautical Congress, Beijing, China. Copyright ©2013 S. Engelen et al. All rights reserved.
replenishing the system with fresh satellites, thereby
increasing the system lifetime.
made to work alongside available COTS hardware in
any of the precursor mission(s).
Both the miniaturization of electronics and other
technology, as well as the increased robustness of a
distributed instrument, and the cost of producing a
larger number of satellites open the doors to the use of
nano-satellite platforms for the OLFAR instrument
nodes. The CubeSat standard with the plethora of
readily available Commercial Off The Shelf (COTS)
harware make adhering to this standard a valuable
interest, especially regarding the design of the precursor
missions discussed later.
The operational band of 0.3-30 MHz as defined by
the key driving requirements requires antennas with
lengths in the order of meters for each instrument node.
Due to the large bandwidth, even these antennas will be
electrically short, which gives rise to the preference of
an omnidirectional receiving pattern for each of the
nodes. The use of three orthogonal short active dipole
antennas allows for the design of wide bandwidth,
limited size antennas capable of achieving the scientific
requirements. A length of 4.8 m for each dipole
element (monopole) was set due to the effect of longer
antennas on the RF properties [18] [19]. An overview is
shown in Figure 1.
Figure 1: Antenna configuration on an OLFAR node
To integrate the monopoles into the nano-satellite
platform of the OLFAR node, a deployment mechanism
was developed. For a long stiff antenna that can be
rolled up in a satellite, a tape-spring type antenna was
designed. Each monopole consists of an extruded
polymer tape-spring cross section with an embedded
conductive element. Three of these wrappable antennas
are stored in a single deployment subsystem, of whom
two will be used per satellite. A deployment subsystem
will be located at each end of the satellite, and deploy
six monopoles orthogonally as depicted in figure (ref:
configuration figure). A prototype of the deployment
subsystem, shown in Figure 2, has been developed,
along with its antennas. The system is CubeSat
compliant (with some exceptions) and can therefore be
IAC-13,B4,7B,6,x18035
Figure 2: OLFAR antenna deployment mechanism
prototype
Besides to the monopoles for radio astronomical
obsevation, antenna systems for the communication
tasks will also be integrated in the CubeSat platform.
Inter-satellite links (ISLs) and the downlink towards
base station will be established by using non-deployable
antennas. Planar radiating elements such as patches
provide a good solution for the CubeSat scenario as they
are characterised by reasonable gain figures while being
small, robust and very easy to integrate.
Furthermore, appropriate algorithms will be
employed to support the physical layer into fulfilling the
communication requirements. The ISLs will use one
antenna on each facet of the CubeSat, and a combining
scheme that maximizes the gain in the direction of
transmission [20]. The swarm-to-Earth communication
will require a smaller number of antennas per satellite as
it will take advantange of the antenna diversity that the
swarm has to compensate for length of the
communication link, and for the random orientation of
the satellites [21].
For accurate radio astronomy imaging, inter-satellite
communcation and collision avoidance , the position
and timing of the OLFAR satellites must be known to a
fraction of the observation wavelength [6] . Moreover,
the relative position and relative synchronization
between the satellite nodes suffice the needs of the
OLFAR system. Given the far away deployment
location (from Earth) and the large number of antennas,
the OLFAR network will be a co-operative network
with minimal communication with Earth based ground
stations. Thus, localisation and synchronisation of the
OLFAR satellites is a boot-strap problem, which must
be dynamically solved by the OLFAR system.
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64th International Astronautical Congress, Beijing, China. Copyright ©2013 S. Engelen et al. All rights reserved.
For accurate timing, all satellites will be equipped with
chip scale Rubidium clocks [22], which enables us to
approximate the clock output as a first order model with
unknown clock offset and clock skew, for a short
duration in time. Thus, assuming this affine clock and
the positional stability of the satellites within desired
accuracies, the unknown pairwise distances and the
clock parameters can be jointly estimated via two-way
ranging [23]. The relative locations can be estimated
from these distances using Multi-Dimensional Scaling.
Furthermore, when the satellites are in motion, the
pairwise distances between the satellites can be
modelled as polynomial in time. In which case, for a
first order distance model [24], second order distance
model [25] and more generally for any order of
approximation, the time varying distances along with
the clock parameters can be estimated using closed form
solutions [26]. A step further, the relative velocity can
also estimated be as shown in [27] [28].
IV.
OPEN ITEMS
The primary reason for placing the telescope in
space is to avoid man-made and natural emissions at
low frequencies. NASA launched two satellites into
Earth orbit, and later Lunar orbit to asses the presence
of radio astronomical signals at low frequencies. Also
the WAVES instrument, flown aboard the WIND [29]
and STEREO [30] satellites is currently sampling the
low frequency environment. This data however is not
sufficiently detailed to allow extracting RFI mitigation
methods for the OLFAR radio telescope. Moreover, as
the deployment location of OLFAR also depends on the
RFI level, which determines the required sensitivity of
the instrument, a tomographic view of the RFI
environment near Earth is deemed invaluable.
The intention is to place the radio telescope in a
location which is remote enough to avoid RFI, and also
far enough from popular orbits, in order to avoid
collisions. Also debris mitigation is crucial, as
launching a large number of satellites has the potential
of creating a lot of space debris. In order to maximise
the useful data volume, this location is preferably as
close as possible to Earth, hence an optimum can be
established in terms of range. The RFI environment will
therefore act as a lower limit to the range, whilst the
communication link distance will serve as an upper
limit. It is therefore imperative that an accurate picture
of the RFI environment in Earth proximity is formed.
Most of the precursor missions foreseen will therefore
carry at least one instrument designed to achieve this
goal.
IAC-13,B4,7B,6,x18035
One of the most challenging issues on the OLFAR
system is the data processing and distribution. Few
imaging algorithms exist for antennas which are not
coplanar, and of those that do exist, the processing and
transfer bandwidths required are significant [31].
OLFAR will therefore only corellate the data of each
individual antenna in space, and all further imaging will
occur on ground [11]. The fact OLFAR consists of a
swarm will likely result in missing baselines, due to
satellite malfunctions, in which case imaging could
perhaps be salvaged using compressed sensing methods
[32]. No 3D imaging has, to the best of the author’s
knowledge, been performed to date, and this area will
therefore require a significant amount of research. Also
the correllation of the data will still have to be
optimised, and preferably distributed more evenly, in
order to map more optimally onto the swarm
architecture.
Given the cost involved in launching satellites,
reducing the per-satellite mass for OLFAR is
paramount. Low mass satellite platforms, such as nanosatellites are only just starting to apply orbit correction
thrusters and attitude and orbit determination systems.
For OLFAR, proving that orbit maintenance for an
invidivual satellite works, and more importantly, is
reliable enough for long-term operations is quite
relevant, especially given that for a reduction in
operation cost, a mostly autonomous system is
considered to be highly desirable. This, to date, has not
been attempted on very small satellites. Also the
interaction of the attitude and orbit control system with
the large science antennas can prove to be problematic,
as the antennas will act as large springs, possibly
destabilising the control mechanisms.
And lastly, no satellite swarms have been flown to
date. Swarm management with Earth-based mobile
platforms is being investigated, yet for space-based
swarms, which require complex and precise orbit
correction manoeuvres which is still unproven.
V.
ROADMAP
In order to prove certain key aspects, required for
OLFAR, quite a few precursor missions are foreseen.
These will fly under the working title eMoth, named
after “electronic moths”, purposefully flying into traps
like the Van Allen radiation belts. The reason for testing
various aspects in different missions is twofold: firstly
to gain experience with each of the aspects whilst
decoupling the various impacts on each other as much
as possible. This is primarily done through taking small
steps in the developments. Secondly, this allows for
improvements in the procedures for manufacturing and
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64th International Astronautical Congress, Beijing, China. Copyright ©2013 S. Engelen et al. All rights reserved.
launching nano-satellite platforms: experiences learnt at
each of the steps will be taken into account in the next
step, thereby improving the chance of success, as well
as reducing the cost through avoiding making the same
mistakes.
eMoth 0: Testing propulsion and orbit control
The first aspects which have to be validated are
(autonomous) orbit control for a nano-satellite. The first
satellites will therefore primarily test orbit control
algorithms and sensors, as well as validating lifetimes
and performances of the thrusters envisaged. This
mission will also test the utility and ease of integration
of the basic platform. The advantage of this satellite is
that it can be launched into any available orbit, as long
as the orbit crosses the primary ground station for
controlling and monitoring of the experiment.
eMoth 1: Getting to know the environment
Secondly, the typical RFI levels and types will have
to be determined, in order to allow creation of (on-line)
RFI mitigation strategies. Also the minimum required
sampling bit depth for OLFAR depends on the RFI
level, as well as the number of bits to be transmitted.
This has an effect on the required data-rates of the
satellites, as in case no RFI compensation can be
performed on-line, all sampled bits will have to be
transferred to Earth for post-processing. This
significantly increases the required data-rate, which in
turn will favour orbits close to Earth. Orbits closer to
Earth are also expected to show stronger RFI levels,
hence an optimum will have to be established.
Therefore, the second mission will test unfolding
science antennas, as well as establish functionality of
the payload receiver chain. Interactions of the antennas
with the local plasma will be investigated, and of course
the background levels will be sampled and transmitted
in full. The intention is to place a single satellite into a
GTO orbit, in which it will stay. GTO is chosen both for
the relatively high availability in terms of piggy-back
launches, as well as the high eccentricity, which would
allow for the creation of a 3D (tomographic) view of the
RFI environment over time. Moreover, payload data
storage and processing in a harsh environment can be
validated, as GTO orbits traverse the Van Allen
radiation belts twice per orbit. This mission will
therefore also act as a technology demonstrator for
operation of the envisaged mainstream electronics in
space.
IAC-13,B4,7B,6,x18035
eMoth 2: Interferometry and swarm management
Once a picture of the RFI environment near Earth
has been established, the payload data processing, as
well as the RFI mitigation strategies can be validated.
Therefore, at least 3 satellites will have to be launched
into a favourable or representative orbit, allowing
preliminary science data collection and interferometry
to take place, while testing RFI mitigation strategies as
well as rudimentary swarm management strategies,
controlling the satellites.
OLFAR 0: The first segment of the OLFAR array
At this point, the first OLFAR satellites can be built
and tested. It is commonly accepted that 5 satellites
would be required to form the first useful images.
Lessons will undoubtedly be learnt during design and
construction of these satellites, as well as throughout
their life cycle, which can be taken into a next iteration
of the design of OLFAR satellites. These first 5
satellites however will allow performing useful radio
astronomy, and will therefore act as the first segment of
the OLFAR telescope.
OLFAR: Gradual build-up
Given the lessons learnt up to this point in time,
revised OLFAR satellites can be built and launched.
Given that OLFAR is a swarm, and swarm management
should be in place at this point, any satellites launched
at any given point in time should seamlessly be able to
join the swarm and augment the science data collection
process. This procedure would also be used to replace
defunct satellites throughout the mission, should the
need arise. From this point on, the array can be
completed, or expanded, depending on the level of
funding available, as well as the available launch
opportunities.
Roadmap
These missions have been aligned in a time
sequence, shown in Figure 3. T0 represents the official
starting date of the project. The intention is to launch a
new stage in the roadmap every 2-3 years, although
intermediate steps are allowed. This roadmap is quite
ambitious, even when using off-the-shelf technologies,
so delays can be expected. When all satellite
developments are ran as parallel projects however, the
delay in one step of the roadmap does not have to have a
significant impact on the next steps. Lessons learnt
however will prove crucial, hence some delays are to be
expected overall.
Page 5 of 7
64th International Astronautical Congress, Beijing, China. Copyright ©2013 S. Engelen et al. All rights reserved.
eMoth 0
eMoth 1
eMoth 2
OLFAR 0
OLFAR
T0 + 9-12 years
T0 + 8-10 years
T0 + 6-8 years
T0 + 4-6 years
T0 + 2-3 years
Figure 3: The Road to OLFAR
VI.
CONCLUSIONS
Low frequency radio astronomy will present a new
milestone in our understanding of the universe.
Missions like OLFAR are required to permit access to
this regime of the spectrum, yet quite some
technological challenges still lie ahead. The road
towards realisation of OLFAR is therefore open, but
certain key milestones will have to be reached prior to
being able to launch OLFAR. This paper has identified
some of the challenges and missing links still ahead, and
proposed a roadmap of precursor missions with the
related research goals to do so.
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