Goddard Space Flight Center High Earth Orbit GPS Flight Experiment AMSAT-OSCAR 40 (AO-40) Frank H. Bauer NASA Goddard Space Flight Center November 1, 2001

Goddard Space Flight Center AMSAT Phase 3D (AO-40) AMSAT Phase 3D in Kourou Experiment Objectives Long term, real time attitude and orbit determination experiment Mapping the GPS constellation antenna patterns above the constellation Understanding the robustness and limitations of using GPS above the constellation Team AMSAT, NASA GSFC GPS Hardware 2 Trimble Tans Vector Receivers 4 patch antennas on perigee side of spacecraft 4 high gain (10 dB) antennas on apogee side of spacecraft Launch: November 16, 2000 Vehicle: Ariane 5 Orbit: 1000 by 58,800 km, i=6

Goddard Space Flight Center GPS Receiver (1 of 2) in Phase 3D’s Equipment Bay Perigee Antenna (1 of 4)Apogee Antenna (1 of 4)

Goddard Space Flight Center Apogee Side of Spacecraft

Goddard Space Flight Center AO-40 Orbit a km e i6.04 deg  deg  deg M2.00 deg Rate of right asc ~ deg/day h p = 1000 km h a = km Period: 19.1 hours Epoch: doy , GPStime: [1134, ] UTCtime: , 17:05:04

Goddard Space Flight Center AO-40 Orbit with Geosynchronous & GPS Orbits Superimposed

Goddard Space Flight Center AO-40 Attitude wrt GPS Orbit  = 5.4 rpm

Goddard Space Flight Center Current Status First data retrieved on Sept 25, 2001 AO-40 represents first long-term HEO GPS experiment—daily results Producing more data than expected—highly likely tracking more satellites than shown due to gaps in data files Expect to continue to retrieve first phase of data through November Experiment will revolutionize the use of spaceborne GPS---new facet to support autonomous navigation and formation flying in HEO/GEO

Goddard Space Flight Center Lots of data is being collected High SNRs during peripapse passes Numerous contacts between high SNR regions indicate high alititude contacts High density of contacts near perigee passes enhance likelihood of fitting an accurate orbit to the data

Goddard Space Flight Center predicted and actual GPS tracks AMSAT Data for 10/06/2001: Predicted (green) Tracked (blue) Time of Day [hrs] altitude [10 3 km] Time of Day [hrs] October 6, 2001 Data Near Apogee

Goddard Space Flight Center

Spin Rate Effects on SNR

Goddard Space Flight Center Point solutions may be possible when 3-axis stabilized Four or more simultaneous GPS satellites

Goddard Space Flight Center Close-up of multiple track overlap window Signals are fading as fourth PRN is acquired Approximately 5 minutes of four satellite tracking

Goddard Space Flight Center Orbit Determination Some cases near perigee, four satellites were tracked simultaneously. Point solution computed using the data collected during perigee pass on Oct 5th. Difference between this point solution and the state computed from two line elements is 120 km primarily in the along track direction. This level of error could simply be the error in the TLE-based ephemeris itself. Point solution being used as initial guess to process the complete data set in Microcosm (a batch filter) and GEODE (a sequential filter). The Microcosm and GEODE orbits will be differenced against each other and the TLE ephemeris.

main lobe tracking side lobe trackingsignal obstructed by the Earth

Goddard Space Flight Center Future Steps Data returned from AO-40 will enable further characterization of GPS signal characteristics at high altitudes, and provides an opportunity to demonstrate orbit determination in a highly eccentric orbit using only observations from a low-cost GPS sensor Lessons learned from AO-40 GPS experiment are guiding future HEO GPS efforts at GSFC: –PiVoT GPS receiver incorporates optimized algorithms and satellite selection logic for HEO –PiVoT will be capable of tracking all satellites in view and computing a real-time orbit solution in the AMSAT orbit GSFC & AMSAT interested in follow-on experiment opportunity—could develop s/c & GPS receiver Would like to partner on follow-on experiment---looking for small s/c launch opportunity