An approach to ground based space surveillance of geostationary on-orbit servicing operations
ABSTRACT
On Orbit Servicing (OOS) is a class of dual-use robotic space missions that could potentially extend the life of orbiting satellites by fuel replenishment, repair, inspection, orbital maintenance or satellite repurposing, and possibly reduce the rate of space debris generation. OOS performed in geostationary orbit poses a unique challenge for the optical space surveillance community. Both satellites would be performing proximity operations in tight formation flight with separations less than 500 m making atmospheric seeing (turbulence) a challenge to resolving a geostationary satellite pair when viewed from the ground. The two objects would appear merged in an image as the resolving power of the telescope and detector, coupled with atmospheric seeing, limits the ability to resolve the two objects. This poses an issue for obtaining orbital data for conjunction flight safety or, in matters pertaining to space security, inferring the intent and trajectory of an unexpected object perched very close to one's satellite asset on orbit. In order to overcome this problem speckle interferometry using a cross spectrum approach is examined as a means to optically resolve the client and servicer's relative positions to enable a means to perform relative orbit determination of the two spacecraft. This paper explores cases where client and servicing satellites are in unforced relative motion flight and examines the observability of the objects. Tools are described that exploit cross-spectrum speckle interferometry to (1) determine the presence of a secondary in the vicinity of the client satellite and (2) estimate the servicing satellite's motion relative to the client. Experimental observations performed with the Mont Mégantic 1.6 m telescope on co-located geostationary satellites (acting as OOS proxy objects) are described. Apparent angular separations between Anik G1 and Anik F1R from 5 to 1 arcsec were observed as the two satellites appeared to graze one another. Data reduction using differential angular measurements derived from speckle images collected by the 1.6 m telescope produced relative orbit estimates with better than 90 m accuracy in the cross-track and in-track directions but exhibited highly variable behavior in the radial component from 50 to 1800 m. Simulations of synthetic tracking data indicated that the radial component requires approximately six hours of tracking data for an Extended Kalman Filter to converge on an relative orbit estimate with less than 100 m overall uncertainty. The cross-spectrum approach takes advantage of the Fast Fourier Transform (FFT) permitting near real-time estimation of the relative orbit of the two satellites. This also enables the use of relatively larger detector arrays (4106 pixels) helping to ease acquisition process to acquire optical angular data.
1. INTRODUCTION
On Orbit Servicing (OOS) is the on-orbit delivery of interventional services to spacecraft. On Orbit Servicing encompasses a broad area of mission types including satellite rendezvous, inspection, captivation, repair, consumables replenishment (such as fuel or cryogens), orbital adjustment/deorbit and possibly the on-orbit construction of or reuse of large, complex space structures.
On Orbit Servicing has historically been performed by the manned spaceflight community on Space Shuttle [1], and International Space Station programs [2]. Subsequently, autonomous systems have recently begun to show technical viability in this space mission class. The Japanese ETS-VII mission [3] demonstrated tele-robotic captivation of a small capture article in 1997. The XSS-series [4] of small satellite missions demonstrated autonomous formation flight with an aim to test servicing technologies. Robotic satellite refueling experiments aboard the International Space Station (ISS) [5] are currently being performed with a view toward testing procedures needed for robotic servicing of satellites in geostationary orbit.
The success of the Orbital Express technology demonstration [6] (2007) exemplified the viability of autonomous, robotic, satellite to satellite OOS. Several key spacebased mission milestones were achieved without operator intervention including autonomous rendezvous and captivation, hydrazine refueling, battery and electronics replacement.
Subsequent to the Orbital Express mission some industrial proposals [7,8] for geostationary satellite refueling were socialized but were not fully financed. The current DARPA Phoenix [9] mission intends to demonstrate derelict satellite re-use by severing antenna dishes from a geosynchronous satellite and affixing them to a smaller electronics package. This is a complex and challenging orbital construction activity.
While autonomous OOS space missions are yet to be routinely flown it appears that technical maturity has been achieved by several nations and makes it likely that this capability will be fielded in the near future. A future capability to remotely observe OOS activities prior to an object's captivation would be of value to the space surveillance community.
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