Speckle Interferometry Tracking of On-Orbit Servicing in Geostationary Orbit
ABSTRACT
On-orbit servicing missions in geostationary orbit with intersatellite separations less than 1 km pose a problem for ground-based electro-optical space surveillance sensors. The close separations between the objects subtend angles comparable to the size of turbulence (seeing) cells of Earth’s atmosphere. Speckle interferometry using a crossspectrum approach was explored as a means to overcome atmospheric turbulence to enable measurement of relative positions of on-orbit servicing satellites without the use of adaptive optics and to enable unambiguous positioning of the secondary satellite. A test of this approach using collocated geostationary satellites, acting as on-orbit servicing proxies, found that cross-spectrum measurements can obtain in-track and cross-track relative position precisions better than 100 m when the satellites’ apparent angular separations were less than 5 arcseconds. Simulations indicate that the radial position estimates requires 3 h of observation data in order to converge and points to a need for longer observation periods to estimate relative orbits. Brightness differences between the objects did not pose a significant observational limitation as brightnesses of mprimary 10.2, Δm 0.3 and mprimary 9.1, Δm 1.5 were speckled successfully with relative position estimates obtained.
1. INTRODUCTION
THERE are more than 400 active geostationary satellites‡ in geosynchronous equatorial orbit (GEO) providing communications, weather, navigation, early warning, and other services to private, commercial, and government users. The service lifetime of these satellites is fated by finite fuel capacity, battery charge/ discharge cycles, and system redundancies, limiting most modern geostationary satellites to approximately 15 years of service life. During launch and early operations, a deployment failure of a solar array or antenna reflector can have large consequences on the services provided by these satellites. Operators have little to no recourse as repairs are currently impossible in GEO orbit.
A technology change is occurring in which autonomous, robotic on-orbit servicing (OOS) systems are showing technical viability. OOS systems could provide fly-around inspection, refueling, repair, antenna or solar array deployment assistance, or deorbit capabilities for orbiting satellites. OOS has historically been performed by astronauts (e.g., [1,2]); however, autonomous robotic OOS has shown continued technological maturity in the past decade, in which several demonstration missions§ [3,4] were performed. Key OOS robotic operations benchmarks were achieved with the 2007 Orbital Express mission [5] including autonomous proximity operations, satellite captivation, intersatellite refueling, and battery replacement, all with a view toward demonstrating the technologies needed to service satellites in GEO orbit.
These exciting new mission possibilities for geostationary satellite operators will stimulate new considerations for the space surveillance community, which is tasked to track and monitor a new class of formation flight and proximity operations.
II. OOS Space Surveillance Problem
Space surveillance of GEO is normally performed with optical telescopes. Orbiting objects are imaged and tagged with their positional information relative to the background stars (see Fig. 1). In recent years, the study of closely spaced objects (satellites) in GEO orbit has focused on the proper tagging (identification and association) of clustered geostationary satellites (separations of ∼0.1 deg or ∼74 km, which is the size of neighboring geostationary station-keeping boxes). Another closer proximity caseis collocated geostationary satellites, in which two satellites occupy the same geostationary box and tend to have separations of tens of kilometers or less (0.01–0.1 deg of separation when viewed from the ground). In both of these cases, position prediction using general perturbation two-line orbital elements lacks the accuracy needed for simple association between measured and predicted positions of the satellites. While reliably associating satellite identity is a problem for these closely spaced geostationary objects, both of these observation cases are routinely detectable and trackable by ground-based telescopes as separate objects resolved on a charged couple device (CCD) or other detector.
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