long wavelength radio astronomy with a cubesat cluster bob macdowall, bill farrell solar system...

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).


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


“Low frequencies”/ionospheric cutoff

Oct 7, 2014

5

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

Oct 7, 2014 4th International Lunarcubes Workshop


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


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


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


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


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


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.


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

Oct 7, 2014

Specific requirement for radio astronomy: EMC clean platform!


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


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

long wavelength radio astronomy with a cubesat cluster bob macdowall, bill farrell solar system...

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