Although navigation systems for determining the orbit of the Global Positioning System (GPS) have proven to be very effective, the current issues involve lowering the error in the GPS satellite ephemerides below their current level. An idea being discussed is incorporating two-way laser-ranging measurements into the operational methodology for determining the orbit of the GPS satellites.
Locations of ground stations utilized.
Long description of figure 1.
Range covariance plots for initial longitude 0į east.
Long description of figure 2.
The Satellite Laser Ranging Benefits for Orbit Determination at GPS Orbit Assessment, conducted at the NASA Glenn Research Center, characterized the performance of multiple ground system scenarios. The first ground system consisted of pseudorange and integrated Doppler measurements from the six GPS monitor stations around the world. Additional ground system scenarios started with the same measurements from the current system and included laser-ranging measurements from those same ground stations with additional laser-ranging measurements from various numbers of additional ground stations. Five modified systems were analyzed with between 2 and 16 additional ground stations.
The orbit determination assessment was based on an extended Kalman filter (EKF) covariance analysis for GPS orbit. State parameters included Cartesian position, velocity, clock bias, and clock drift. Nine different initial covariance studies were performed for two different initial longitude conditions for the location of the GPS satellite at the start of the simulation.
Measurements included pseudorange signals originating from the GPS satellite, integrated Doppler measurements originating from the GPS satellite, and two-way laser-ranging measurements originating from the ground stations. The simulation was performed under discrete time and measurement conditions for a duration of 1 day and a step size of 1 sec. Ten noise profiles were used to compare the performance of the EKF since the covariance profile of the EKF depends on real noise values. Results show that an increase in the number of ground stations reduced the steady-state range covariance when the initial covariance of the velocity components was not greater than that of the position components. Results also show that as the number of laser-ranging ground stations was increased from 14 to 18, and then from 18 to 22, there were only small reductions in the range covariance.
Range covariance results comparison by system. CS, current system; MSys1 to MSys5, modified system 1 to modified system 5.
Long description of figure 3.
Most benefits (most percent reductions in range covariance) come from transitions from CS to MSys1, MSys1 to MSys2, and MSys2 to MSys3. Further additions to the number of laser ranging ground stations as part of the simulation do not reduce the range covariance by a percentage comparable to the number of stations being utilized.
The Satellite Laser Ranging Benefits for Orbit Determination at GPS Orbit Assessment effort is managed under the Space Communications and Data Systems Project at Glenn. The work was performed in-house by a member of the Communications Systems Integration Branch in Glennís Communications Technology Division.
Find out more about the research of Glennís Communications Technology Division: http://ctd.grc.nasa.gov
Bryan W. Welch, 216-433-3390, Bryan.W.Welch@nasa.gov
Author: Bryan W. Welch
Headquarters program office: Space Communications Technology Program
Programs/projects: Space Communications and Data Systems Project, Space Communication Architecture Working Group
Last updated: December 14, 2007
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