Audra Jensen, U.S. Air Force Academy; Michael Plummer, U.S. Air Force Academy; Daniel O’Keefe, U.S. Air Force Academy; Francis Chun, U.S. Air Force Academy; David Strong, Strong EO Imaging, Inc.
Keywords: Polarization, GEO Satellites, Space Situational Awareness, Space Domain Awareness
Abstract:
Now more than ever, space situational awareness (SSA) and space domain awareness (SDA) are top priorities to U.S. national security. Fully characterizing a satellite from the Earths surface based solely on optical data would be a monumental improvement in SDA. A resolved image can provide information about a satellite and its characteristics (e.g., size, shape, orientation, materials), but obtaining resolved images at orbital distances requires either a very large aperture telescope or a very large satellite. For example, to obtain sub-meter resolution at geosynchronous orbit using a ground-based telescope, one would require a primary mirror diameter of at least 22 meters. Since telescope apertures of this size are expensive to build, maintain, and operate, we must use alternate imaging techniques. Additionally, the miniaturization of satellites will only compound the problem, thus resulting in the vast majority of manmade space objects appearing as a point source. Using small telescopes and standard astronomical techniques, there are methods we can use to characterize satellites from their unresolved, point source signature. Namely, we can measure satellite optical signatures using broadband photometric filters to obtain brightness and gross color information as a function of time and phase angle. We can also obtain low resolution satellite spectra by using diffraction gratings to disperse the light. A third method we are developing at the United States Air Force Academy (USAFA) is a new instrument to measure the linear polarization of a satellites unresolved optical signature. The polarimeter is mounted on a 16-inch telescope at the USAFA observatory and consists of four linear polarization filters oriented at 0°, 45°, 90°, and 135° relative to the vertical axis of the cameras focal plane. The polarimeter was calibrated using a uniform, polarized light source allowing us to determine a calibration matrix that accounts for the polarization effects within the telescope optical train. This calibration matrix directly converts camera intensities (i.e. I0, I45, I90, I135) measured through the four polarization filters to three Stokes parameters: S0, S1, and S2, where S0 is the total intensity (I0 + I90), S1 is the preference for linear polarization between the vertical and horizontal directions (I0 – I90), and S2 is the preference for linear polarization between the two diagonal directions (I45 – I135). During the vernal and autumnal equinoxes in 2019, 2020 and 2021, we used the polarimeter to observe a variety of operational communication satellites in geosynchronous orbit. During these periods we can usually observe specular reflections (i.e. glints) off the solar panels from a ground-based telescope. In processing the raw image data, we accounted for thermal noise by subtracting the appropriate dark frame. We also used an aperture photometry tool through the commercial astronomy image processing package Mira® Pro to subtract the background contained within the signal aperture using a background annulus of equal area. Furthermore, we developed a technique to determine whether the polarization data collected on any given night were contaminated by cloud coverage, allowing us to eliminate those data from this study. A first-order analysis of the remaining data shows a distinct difference between a satellites polarization signature during a glint versus its signature outside of the glint period. Finally, we compare the polarization signatures to find similarities between observations.
Date of Conference: September 14-17, 2021
Track: Non-Resolved Object Characterization