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

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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 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 Oct 7, 2014 4th International Lunarcubes Workshop 3

“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 (50 150 MHz); requires low noise http://swaves.gsfc.nasa.gov/cgi bin/wimp.py?date=20130305&do=New+Plot&p lot=ws 5 Oct 7, 2014

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 Oct 7, 2014 4th International Lunarcubes Workshop 6

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 insufficient Oct 7, 2014 4th International Lunarcubes Workshop 7

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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.

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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 o 65 Oct 7, 2014 4th International Lunarcubes Workshop 9

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

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 Oct 7, 2014 4th International Lunarcubes Workshop 10

LWaDi Thermal Variation - Worst Case Orbit

• • • LWaDi has an IR spec trometer payload HgCdTe detector is cryo-cooled Electronics Radiator Instrument radiator is thermally isolated 2x1 U blue panel (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

• • • • • • Electric field dipole antennas – stacer type deployment – Four 3 m monopoles electrically combined to provide two 6 + “short” dipoles over frequency range m orthogonal dipoles; note 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 – Specific requirement for radio astronomy: EMC clean platform!

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 4th International Lunarcubes Workshop 12

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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 LWaDi bus, John Hudeck, mechanical, Wallops FF Oct 7, 2014 4th International Lunarcubes Workshop 13

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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