Project Title: Photometric Satellite Modeling using High-Performance Computers
Organization: AFRL/DESM (MHPCC)
Location: Maui High Performance Computing Center
Project Description:
As an object orbits the Sun or Earth, it exposes different parts of its surface to
illumination causing brightness variations. Many objects are too small or too far away to
be resolved directly (e.g. asteroids for astronomers), and so light-curve inversion
techniques must be employed. These techniques extract physical and orientation
characteristics about the object using the brightness changes as a function of time and
viewing angles. Kaasalainen, Torppa, and Muinonen1 describe a method of light-curve
inversion for asteroids based on modeling the asteroid as a closed set of facets. Each
facet has a surface area, normal direction, and surface scattering law. Synthetic lightcurves are generated by adjusting the various parameters of the model and knowing the
viewing angles to the Sun and to the observer. The total brightness of the synthesized
object is determined by summing over all visible facets of the model. An observed lightcurve can be compared to these synthetic light-curves and chi-squared minimization of all
configurations can then be used to determine which set of parameters produces a
synthetic light-curve that best matches the observed light-curve. This and other
techniques can be used in non-resolved space object identification (SOI) to possibly
determine shape and orientation characteristics of satellites, rocket bodies and space
debris as well.2
Over the past two years (including cadet and faculty summer research at
MHPCC), USAFA faculty and cadets have created Matlab computer programs to not
only automatically reduce USAFA Observatory images to produce geostationary satellite
(GEOS) light-curves, but also fit these satellite light curves to a possible model using a
modified forward-modeling light-curve inversion technique described above. The
development of these programs are primarily aimed at introducing non-resolved SOI into
USAFA’s “Astronomical Techniques” advanced laboratory course for physics majors.
Further modifications and improvements to the MatLAB codes are planned including:
1. Create GUI interfaces for better implementation into USAFA curriculum.
2. Incorporate an existing two-line element (TLE) code to generate light-curves
for synthetic satellites in all possible observing circumstances. (Currently
individual satellites are “hard-wired” into the forward-modeling code).
1
Kassalainen, M. & J. Torppa, “Optimization Methods for Asteroid Lightcurve Inversion I. Shape
Determination” Icarus: 2001, p. 24-36., Kassalainen, M., J. Torppa, & K. Muinonen, “Optimization
Methods for Asteroid Lightcurve Inversion II. The Complete Inverse Problem” Icarus: 2001, p. 37-51.
Hall, D., J. Africano, P. Kervin, & B. Birge, “Non-imaging Attitude and Shape Determination,” AMOS
Technical paper, 2005
2
3. Add to the physics of the forward-modeling (e.g. Earth’s shadow, color
information, multiple reflections off satellite surfaces).
Further USAFA faculty and cadet research using the MatLAB codes are planned,
particularly:
1. A USAFA cadet will begin making multi-bandpass observations of a select
number of GEOS during the Spring 2007 semester to start creating an actual
“light-curve atlas” for these GEOS over the course of a year.
2. The USAFA forward-modeling code will be used to create a synthetic “lightcurve atlas” for comparison to the observations. This work would be made more
practical with the use of MHPCC’s high-performance computers. The Summer
2006 cadet research project successfully modified the forward-modeling code to
run parallelized on a Cray XD1 Linux computing system (Hoku) realizing a 40x
increase in speed over a PC when using 26 processors3. More importantly, due to
PC memory limitations, some jobs impossible for a PC can only be accomplished
on a high-performance computer.
Background or Skills Required: The faculty member should be an expert in
photometry and have experience in processing and analyzing light curves of various
shaped objects. Computer programming in Matlab is a plus. The cadet should have
computer programming skills in Matlab as well as some basic knowledge of photometry.
Number of Academy Cadets/Midshipmen: 1 (Ideally, the USAFA cadet could be
selected well ahead of time to become familiar with the existing USAFA MatLAB code
and incorporate making the necessary modifications and enhancements to the code as part
of his/her Physics 356 Computational Methods course taken during the Spring 2007
semester).
Number of Academy Faculty: 1 (USAFA faculty member is already identified having
worked on development of the existing codes).
Requested Duration: 4-5 weeks (last week in May to late June).
Security Clearance Required: None.
Sponsor Name: Francis Chun and Lt Col Charles (Jack) Wetterer
Sponsor Phone Number: 808-879-5077x225
Sponsor E-Mail Address: Francis.Chun@maui.afmc.af.mil
3
Wetterer, C.J., Stanley, C. & Stikeleather, J.A., “Lightcurve Inversion Program for Non-resloved Space
Object Identification,” Maui High Performance Computing Center (MHPCC) Application Briefs, pp. 1314, September 2006.