Satellite Autonomy using Crosslinks and Autonomous PNT Reference
Chuck H. Frey, Lockheed Martin Space Systems Company; J.P. Laine, Avinash Agrawal, Lucas Benney, Gregory Blasche, Adam Kelsey, Tony McPherson, Dorri Poppe, Jan Anszperger, DRAPER
Location: Ballroom A
Date/Time: Monday, Jun. 12, 2:30 p.m.
Traditional satellite autonomy over extended periods consists of propagating the last ground-uploaded position or Global Positioning Receiver (GPS) - receiver provided Position, Velocity, and Time (PVT) estimates forward, for use in Mission Operations. The limitation of the accuracy of the initial ground estimates or denial of GPS causes degradation over time. It is even more necessary for GPS satellites to have sustained autonomy given the large number of user equipment and services that depend on the GPS system. With the advent of optical crosslinks and other positioning sensors, the interoperability of large, cooperative constellations allows for autonomous positioning without the use of periodic state updates. One source of error is Earth Orientation Parameter Predictions (EOPP) which degrade over time. EOPP error degrades metric observations and earth positioning of the satellite. For the GPS constellation, the focus of this paper, this directly translates to User Range Error (URE). This paper investigates how optical crosslinks combined with Draper’s Autonomous PNT Reference (APR) system reduces GPS constellation EOPP error. APR systems on individual satellites provide completely independent absolute positioning. Crosslink sharing of timing and APR states between satellites enables GPS constellation independence from other ground aiding. PNT accuracy will be analyzed for the GPS constellation but is applicable to any orbital regime.
Analysis results will show how the GPS constellation currently performs in Autonomy using clock and ephemeris propagation only using a ground and uplink ephemeris. In the event that ground contact is lost, ephemeris accuracy during autonomous operations will slowly degrade over time from propagation only. Improvements to PNT accuracy during autonomy will be shown with the use of crosslink ranging and timing data and the addition of the APR sensor. The APR sensor is an independent sensor that uses celestial and ground based references to provide updates to the SpaceNav kalman filter which is providing ephemeris updates to the PNT broadcast solution.