Long Wavelength Radio Astronomy with a
CubeSat Cluster
Bob MacDowall, Bill Farrell
Solar System Exploration, NASA/GSFC, Greenbelt, MD, USA
Dayton Jones, Joseph Lazio
JPL/Caltech University, Pasadena, CA, USA
Introduction
• Below ~20 MHz, radio images of
objects in space don’t exist, due to
lack of the required space-based
observatories
• We will describe various plans to
make such observations, which have
not been developed at this time
• A CubeSat cluster would permit radio
burst imaging aka aperture synthesis
• Here, we focus on a 32 CubeSat
cluster orbiting the moon, which has
advantages and disadvantages
Oct 7, 2014
4th International Lunarcubes Workshop
One arm of the lunar-based
ROLSS concept for radio
imaging of solar radio bursts
(3 arms each with 16 dipole
antennas on Kapton film).
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Angular resolution
• Considering frequencies from 100 kHz – 10 MHz, corresponding
to wavelengths of 3 km – 30 m
• Angular resolution (radians) ~ wavelength/diameter of aperture
• Optical (500 nm, Keck) ~ 5e-8 radians
• Radio (300 MHz, VLA) ~ 1 m / 1 km ~ 0.001 radians ~ 0.003 deg~ 10 arcsec
• Radio (10 MHz, ROLSS) ~ 30 m / 1 km ~ 0.03 rad ~ 1.7 deg
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“Low frequencies”/ionospheric cutoff
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Science Targets
• Solar bursts – type II, type III
• Planets – Jupiter, Saturn, etc.
• No radio images at long
wavelengths to date
• Exoplanets – detect magnetospheres
• Cosmology – detect Dark Ages (50150 MHz); requires low noise
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http://swaves.gsfc.nasa.gov/cgibin/wimp.py?date=20130305&do=New+Plot&p
4th International Lunarcubes
lot=ws
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Previous LF radio observatory cluster proposals
• ALFA – MIDEX proposals
submitted by JPL (Jones et
al., 1996, 1998)
• SIRA – planned MIDEX
proposal led by NASA/GSFC
(no more MIDEX AOs)
• PARIS – concept (Oberoi)
• LFSA, etc.
Astronomical
Low Frequency
Array (1996)
Solar Imaging Radio Array
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ALFA/SIRA MIDEX Small Sat cost/issues
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ALFA 1 MIDEX – astrophysics-oriented (JPL-lead)
ALFA 2 – astrophysics + solar physics (JPL-lead)
SIRA – planned to be primarily solar physics oriented (GSFC-led)
– Focused on imaging of solar radio bursts (astrophysics secondary)
– Mission cost estimate (GSFC IMDC, Price-H model):
• First sat = $69 M; includes all development
• 12 sats = $137 M; provides 12*11 = 132 baselines
• 16 sats = $159 M; desired for coverage of U-V plane and
allowance for loss of ~10% of small sats
• Does not include launch vehicle cost
– MIDEX cost cap (2003) was $150 M
• GSFC “Partnership opportunity” selected Orbital Sciences
• No heliophysics MIDEX AOs after 2003;
determined SMEX funding was insufficient
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Consider a CubeSat cluster
• Number of CubeSats needed/desired
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Compared to SIRA; difficult to implement four 5-m monopoles
Higher likelihood of failure of individual Cubesats
So, consider 32 CubeSat cluster each with four 3-m monopoles
Maximum extension of cluster ~5 km => ~20 arcmin resolution (10 MHz)
Sensitivity comparable to SIRA ~ 200 Jy in 5 seconds at 3 MHz
• Proposed location: lunar orbit, similar to LWaDi
• Note others have addressed this approach, but not lunar orbit
Google:
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SOLARA, Knapp, MIT
iCubeSat, Cecconi, Meudon
OLFAR, Bentum, Twente
Etc.
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Why lunar orbiting cluster?
• Distance from Earth reduces
RFI from ground transmitters
(Wind data at right)
• Earth occulted every orbit
(for orbit in ecliptic)
• LWaDi orbit (shown below) is
relatively stable
• Other options exist, such as
Earth-lunar Lagrange points
65
o
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Challenges of lunar orbit
• Considering orbit like
planned Lunar Water
Distribution (LWaDi)
mission, but with low
inclination
• Thermal environment is
major challenge
• Downlink to Earth is
restrictive (3.8e5 km)
• Lunar orbit insertion has
propulsion requirements,
as do orbit and cluster
maintenance
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LWaDi Orbit Characteristics
• 100 km x 5000 km lunar orbit
• Relatively stable orbit – minor
orbit correction maneuvers
• 65 deg orbit inclination
• Lunar Solar Reflectance load
– IR Planetshine
• Dark Side: 5 W/m2
• Sun Side: 1314 W/m2
• Lunar Albedo - 0.13
• Solar Flux - 1420 W/m2
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LWaDi Thermal Variation - Worst Case Orbit
• LWaDi has an
IR spectrometer
payload
• HgCdTe
detector is
cryo-cooled
• Instrument
radiator is
thermally
isolated 2x1
U blue panel
Electronics
Radiator
(Deepak Patel,
Thermal, GSFC) Thermal profiles shown above are for one 7 hr LWaDi orbit, including
solar eclipse; 11 to 34°C variation. 3x2 U panel is radiator for electronics.
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LF Radio CubeSat Payload
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Electric field dipole antennas – stacer type deployment
– Four 3 m monopoles electrically combined to provide two 6+ m orthogonal dipoles; note
“short” dipoles over frequency range
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Preamps covering freq. range of 100 kHz – 10 MHz
Radio receiver board to select and digitize signals; sample approximately 16
frequencies, possibly frequency-agile
– Likely to be 2-bit Nyquist sampled for bandwidth of 1% of frequency
– Frequency stepping rate of ~ 1 Hz
Specific requirement for radio
Processor board (or dedicated computer) to format astronomy: EMC clean platform!
data for transmission to relay CubeSats
– Data must be time-tagged to < 0.1 sec absolute to permit aperture synthesis
– Phase stability required based on highest observing frequency and longest coherent
integration time
– Includes oscillator that maintains phase-lock with a common reference signal from a
designated CubeSat in the cluster (several CubeSats have this capability for redundancy)
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S-band or ULF transmitter to relay data to the CubeSats that perform Ka band
downlink to ground-stations
Probably storage to hold data, until it is transmitted to relay CubeSat
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LF Radio CubeSat Subsystems
• Because orbit and cluster maintenance will require significant
propulsion & attitude control, we baseline 6U CubeSats, like LWaDi
• Clearly several relay CubeSats will need to be 6U
• If the non-relay CubeSats can be reduced to 3U, that would provide
savings in various ways, but it’s likely that the proposed orbit and
lunar environment will force 6U
• Labeled diagram of LWaDi bus at right
contains most of the systems that we will
require; changes would likely be:
– Payload changes, including E-field dipoles
for all non-relay CubeSats
– Relay CubeSats need
• High gain X or Ka band antennas
• Timing signal sent to cluster
• Computational power to manipulate
data
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LWaDi bus, John Hudeck,
mechanical, Wallops FF
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Key issues to be addressed/Summary
• Flight dynamics – detailed assessment of cluster maintenance resources
and orbit optimization
• Mission profile – understand detailed requirements on the relay CubeSats
• Develop high-fidelity payload model
– Include frequency agile receivers?
• Identify carrier to transport and deploy CubeSats into lunar orbit
• Determine down-link scenario
• Given the above, develop detailed cost model for ~32 6U CubeSats
• The challenges that we addressed include CubeSat cluster inlunar orbit,
cluster maintenance, intra-cluster communication, design of CubeSat radio
astronomy payload, instrument requirements, computing capabilities, and
data downlink to Earth.
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