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transcript
Long Wavelength Radio Astronomy with a CubeSat Cluster
Bob MacDowall, Bill FarrellSolar System Exploration, NASA/GSFC, Greenbelt, MD, USA
Dayton Jones, Joseph LazioJPL/Caltech University, Pasadena, CA, USA
4th International Lunarcubes Workshop 2
Introduction
Oct 7, 2014
• 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
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).
4th International Lunarcubes Workshop 3
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 degOct 7, 2014
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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 (50-
150 MHz); requires low noise
http://swaves.gsfc.nasa.gov/cgi-bin/wimp.py?date=20130305&do=New+Plot&plot=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.
Oct 7, 2014
AstronomicalLow FrequencyArray (1996)
Solar Imaging Radio Array
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ALFA/SIRA MIDEX Small Sat cost/issues• 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 insufficientOct 7, 2014
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Consider a CubeSat cluster• Number of CubeSats needed/desired
– 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:
– SOLARA, Knapp, MIT– iCubeSat, Cecconi, Meudon– OLFAR, Bentum, Twente– Etc.
Oct 7, 2014
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65o
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
Oct 7, 2014
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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
Oct 7, 2014
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 spec-trometer payload
• HgCdTe detector is cryo-cooled
• Instrument radiator is thermally isolated 2x1 U blue panel
(Deepak Patel, Thermal, GSFC)
Oct 7, 2014
Electronics Radiator
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• 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
• 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
• Processor board (or dedicated computer) to format 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)• 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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Specific requirement for radio astronomy: EMC clean platform!
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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
Oct 7, 2014
• 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
dataLWaDi 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.
Oct 7, 2014