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1. Create software to simulate the aperture synthesis for a radio
interferometer in orbit
2. Add realistic thermal noise to the simulation
3. Add realistic phase noise corresponding to the positional
uncertainty inherent in orbiting spacecraft
4. Simulate how well Relic can reconstruct a given model image of a
DRAGN over time
5. Simulate SunRISE to identify if it can indeed localize transient
radio hotspots with minimal integration time
Constellations of small spacecraft could be used to realize a low-
frequency phased array for either heliophysics or astrophysics
observations. However, there are issues that arise with an orbiting array
that do not occur on the ground, thus rendering much of the existing radio
astronomy software inadequate for data analysis and simulation. In this
work we address these issues and consider the performance 2 different
array concepts. One is Relic, a 32-spacecraft constellation for
astrophysical observations, whose goal would be to observe Double
Radio-source Associated with a Galactic Nucleus sources (DRAGNs) at
low frequencies impossible to see on Earth due to Ionospheric cutoff. The
other is SunRISE, a 6 spacecraft array whose science goal would be to
clarify the basic physics of Coronal Mass Ejections (CMEs) by localizing
radio emission on CMEs to within ¼ of their width.
• Radio imaging uses Interferometry, which yields 1 sample in the
Fourier plane per pair of spacecraft, 496 for Relic, 15 for SunRISE
• Where the sample is depends on the projected distance between 2
receivers, as seen from the source
• Adapted the APSYNSIM software [1] to orbiting arrays, recalculating
the projected difference every time step, using a orbital prediction file
that describes the location of the spacecraft in the EME2000 frame.
• Add thermal noise corresponding to Galactic Temperature at observing
wavelength [2]
• Add phase noise from various sources of uncertainty, can measure in
seconds, causes phase error of 2πντ for uncertainty τ seconds
Uncertainty = positional uncertainty + clock uncertainty
τ = 𝑑𝑧
𝑐 + 𝑑𝑡
This work began at the NASA Jet Propulsion Laboratory Deep Space
Tracking Division, California Institute of Technology while I spent the
summer of 2016 there on fellowship. Sonia Hernandez at JPL supplied
me with the files describing orbits of Relic for me to test.
[1] Marti-Vidal, Ivan 2015, https://launchpad.net/apsynsim
[2] Perley, R., et al. 1989, Astronomical Society of the Pacific
[3] Leblanc, Y., et al. 1998, Solar Physics, 183
[4] McMullin, J. P., et al. 2007, Astronomical Data Analysis Software and Systems XVI, 127
Simulating 3D Spacecraft Constellations for Low Frequency
Radio Imaging Alexander Hegedus1, Nikta Amiri2, Joseph Lazio2, Konstantin Belov2, Justin Kasper1
1Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI 2Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA
Introduction
Objectives
Methods
Simulating Relic
Acknowledgements
References
Figure 2: SOHO image of a
CME
Figure 3: Example of how 3
dimensional structures like this
shape or a cloud of spacecraft
can have different projections
relative to the source of light
Figure 4: Columns of output from Orbital-APSYNSIM simulating the
response of the Relic Array after 1, 30, and 60 minutes on an idealized
DRAGN at 10 MHz with peak intensity 500 Jy, positional uncertainty of 3m
per spacecraft, 5 ns of clock bias error, a thermal noise corresponding to a
galactic temperature of ~300000K, and a noise rms of ~60 Jy. The right
most column is with integration times 12:00 – 12:29 & 17:00-17:29,
showcasing the code’s ability to combine non-contiguous chunks of time,
useful for systems engineers scheduling the operations of the spacecraft.
The upper right graph shows the error between the input and recovered
image with various sources of noise added.
Simulating SunRISE
Science Goal: Localize Radio Emission on CMEs
• Why? CMEs can potentially cause large blackouts; understanding
their basic physics helps with Space Weather prediction
• CMEs evolve quickly, so SunRISE must localize emission with quick
snapshots. No need for Orbital-APSYNSIM for integration, use
industry standard CASA [4]
Conclusions
Future Work
Relic & Orbital-APSYNSIM
• Orbital-APSYNSIM can realistically simulate the response of a Space
Based Interferometer with a particular orbit on a given science target
• It is useful for fine tuning orbits and scheduling system operations
• Can identify point of diminishing returns for integration time, e.g. for
Relic observing a 500 Jy DRAGN it is about 10-15 minutes
SunRISE
• SunRISE can only take ~25 degrees of phase error (equivalent
distance depends on observing wavelength) before shifting the
apparent location of the hotspot to a sidelobe
• With expected noise values, SunRISE can complete its goal of
localizing radio emission to within ¼ the CME’s width
VLBA Applications
Figure 1: Radio image of Cygnus A, one
of the brightest DRAGNs in the sky.
Photo courtesy of the NRAO
Science Goal: Image DRAGNs
• Why? Observing in this new frequency range helps constrain
models of black holes and shock processes that form DRAGNs
• Long Integration Time, needs new software to do Aperture Synthesis
• Created Orbital-APSYNSIM to simulate 3D orbiting array
• How well can Relic reconstruct a model image?
Figure 6: Left: Idealized Radio Hotspot, size informed by electron
density models & observing frequency [3].
Right: Reconstruction by SunRISE with no noise. Requires data
from multiple frequency bands since there are so few spacecraft.
Figure 7: Left: Noiseless reconstructions of hotspots
Right: Noisy reconstructions, top right has positional uncertainty
corresponding to a 20 degree error in phase, bottom right has 40
degree phase error, enough to lose the true position of the hotspot.
Figure 8: Histogram of
difference in degrees
between 100 different
truth Gaussians and the
SunRISE recovered
Gaussians with realistic
noise.
Orbital-APSYNSIM may also be used to conduct Very Long Baseline Array
(VLBA) observations, where antennae are spread across North America,
as well as in GEO. This is usually hard because the antenna do not lie on
a 2D plane. The vast separation enables high resolution images.
Improving & Using Orbital-APSYNSIM
• Add mode to Orbital-APSYNSIM integrating VINT (VLBI in the near-filed
toolkit ) for close range VLBA targets
• Do side by side analysis of ground based data between CASA and
APSYNSIM
• Find ways to tie the codes together, combine useful routines from CASA
in APSYNSIM, e.g. w-projection for large images
• Use Orbital APSYNSIM to fill out the trade space of building space
interferometers using created metrics for complexity, science value, and
cost over number of spacecraft & their proposed capabilities
• Make plots of Complexity vs Science Value & Cost vs Science Value
and look for ‘knees’, those are sweet spots for future missions
SunRISE Analysis
• Repeat analysis on SunRISE with more realistic CME truth models from
MHD simulations
• Define regression problem to describe likelihood that we can identify
CME hotspot given observations over time and frequency
• Use UVMULTIFIT to reconstruct models from simulations of the
baselines we would expect to see at all frequencies simultaneously
Figure 5: 10 VLBA antennae with 2 GEO satellites working together to
form an image.
Download Orbital-APSYNSIM Download this Poster https://github.com/alexhege/Orbital-APSYNSIM