Roger Tippets

Results from an experiment that collected visible-light polarization data using unresolved imagery for classification of geosynchronous satellites

In order to protect critical military and commercial space assets, the United States Space Surveillance Network must have the ability to positively identify and characterize all space objects. Unfortunately, positive identification and characterization of space objects is a manual and labor intensive process today since even large telescopes cannot provide resolved images of most space objects. The objective of this study was to collect and analyze visible-spectrum polarization data from unresolved images of geosynchronous satellites taken over various solar phase angles. Different collection geometries were used to evaluate the polarization contribution of solar arrays, thermal control materials, antennas, and the satellite bus as the solar phase angle changed. Since materials on space objects age due to the space environment, their polarization signature may change enough to allow discrimination of identical satellites launched at different times. Preliminary data suggests this optical signature may lead to positive identification or classification of each satellite by an automated process on a shorter timeline. The instrumentation used in this experiment was a United States Air Force Academy (USAFA) Department of Physics system that consists of a 20-inch Ritchey-Chrétien telescope and a dual focal plane optical train fed with a polarizing beam splitter. Following a rigorous calibration, polarization data was collected during two nights on eight geosynchronous satellites built by various manufacturers and launched several years apart. When Stokes parameters were plotted against time and solar phase angle, the data indicates that a polarization signature from unresolved images may have promise in classifying specific satellites.

Spectral Measurements of Geosynchronous Satellites During Glint Season

Operational communication geosynchronous satellites are typically large structures with solar panels that maintain a stable attitude relative to the Earth and Sun. During the equinox periods, the geometry of the satellite, the sun, and an Earth observer is favorable for a condition to occur called a glint, or a specular reflection off the satellite. It is presumed that the glint is caused by the solar panels because those structures are solar oriented. This phenomenon was previously observed and reported using broadband photometry techniques. In this paper, glint observations from two satellites are presented, Wildblue-1 and DirecTV-12, as measured across the visible spectrum using slitless spectroscopy techniques. It is clear from the results that the wavelength where the glint maximum occurs is shifted toward the blue or shorter wavelength end of the spectrum relative to the time periods before and after the glint. It is also clear from the data that the spectral reflectance during the glint is less lik...

Slitless spectroscopy of geosynchronous satellites

Abstract. A preliminary investigation into the use of slitless spectroscopy for characterization of geosynchronous satellites is described. A 100  line/mm diffraction grating is used as the dispersing device, and the spectral data obtained are compared to a model with good results. A method used to collect and calibrate slitless spectral observations accounting for pixel to wavelength conversion, pixel response as a function of wavelength, and solar features is presented. Observations of several geosynchronous satellites throughout a night reveal reflectance with noticeable and different profiles indicating that slitless spectroscopy offers the potential for another modality for identifying and discriminating satellites.

Calibration of a system to collect visible-light polarization data for classification of geosynchronous satellites

In order to protect critical military and commercial space assets, the United States Space Surveillance Network must have the ability to positively identify and characterize all space objects. Unfortunately, positive identification and characterization of space objects is a manual and labor intensive process today since even large telescopes cannot provide resolved images of most space objects. The objective of this study was to calibrate a system to exploit the optical signature of unresolved geosynchronous satellite images by collecting polarization data in the visible wavelengths for the purpose of revealing discriminating features. These features may lead to positive identification or classification of each satellite. The system was calibrated with an algorithm and process that takes raw observation data from a two-channel polarimeter and converts it to Stokes parameters S0 and S1. This instrumentation is a new asset for the United States Air Force Academy (USAFA) Department of Physics and consists of one 20-inch Ritchey-Chretien telescope and a dual focal plane system fed with a polarizing beam splitter. This study calibrated the system and collected preliminary polarization data on five geosynchronous satellites to validate performance. Preliminary data revealed that each of the five satellites had a different polarization signature that could potentially lead to identification in future studies.

Analysis of stellar radiance contamination in observed satellite spectra

Reflectance spectra of Earth orbiting satellites can be readily observed with small diameter telescopes (D < 1 m) by utilizing a method known as slitless spectroscopy. Satellite spectra can be observed by simply placing a transmission grating within the collimated optical path of the telescope without the need to image through a slit. The simplicity of the slitless spectroscopy design makes it a promising alternative to spatially resolving satellites with larger and more expensive diameter telescopes for applications of space situational awareness. However, accurately observing satellite re ectance spectra without imaging through a slit requires a dark and homogeneous background. This requirement is frequently violated as background stars streak across the image due to the slewing motion of the telescope during satellite tracking. Rather than throwing out all images with noticeable stellar contamination, a principle component analysis of contaminated images from three geostationary satellite observations showed that it may still be possible to assess and identify satellite characteristics depending upon the amount of stellar contamination in the spectral region of interest. Additionally, a simple technique for automatic removal of contaminated frames is proposed based on an outlier analysis using Gaussian statistics and was found to successfully remove all signicantly contaminated frames.