High Earth OrbitGPS Flight ExperimentAMSAT-OSCAR 40 (AO-40)Frank H. BauerNASA Goddard Space Flight CenterNovember 1, 2001

AMSAT Phase 3D (AO-40)AMSAT Phase 3D in KourouExperiment ObjectivesLong term, real time attitude and orbit determination experimentMapping the GPS constellation antenna patterns above the constellationUnderstanding the robustness and limitations of using GPS above the constellationTeamAMSAT, NASA GSFCGPS Hardware2 Trimble Tans Vector Receivers4 patch antennas on perigee side of spacecraft4 high gain (10 dB) antennas on apogee side of spacecraft Launch: November 16, 2000Vehicle: Ariane 5Orbit: 1000 by 58,800 km, i=6

GPS Receiver (1 of 2)in Phase 3Ds Equipment BayPerigee Antenna (1 of 4)Apogee Antenna (1 of 4)

Apogee Side of Spacecraft

AO-40 Orbita36286.409 kme0.796838i6.04 degW151.818 degw335.84 degM2.00 degRate of right asc ~ -0.208 deg/dayhp = 1000 kmha = 58800 kmPeriod: 19.1 hoursEpoch: doy 278.71185185, GPStime: [1134,493517.0]UTCtime: 2001-10-05, 17:05:04

AO-40 Orbit with Geosynchronous & GPS Orbits Superimposed

AO-40 Attitude wrt GPS Orbitw= 5.4 rpm

Current StatusFirst data retrieved on Sept 25, 2001AO-40 represents first long-term HEO GPS experimentdaily results Producing more data than expectedhighly likely tracking more satellites than shown due to gaps in data filesExpect to continue to retrieve first phase of data through NovemberExperiment will revolutionize the use of spaceborne GPS---new facet to support autonomous navigation and formation flying in HEO/GEO

Lots of data is being collectedHigh SNRsduringperipapse passesNumerous contacts between high SNR regions indicate high alititude contactsHigh density of contacts near perigee passes enhance likelihood of fitting an accurate orbit to the data

October 6, 2001 Data Near Apogee

Spin Rate Effects on SNR

Point solutions may be possible when 3-axis stabilizedFour or more simultaneous GPS satellites

Close-up of multiple track overlap windowSignals are fading as fourth PRN is acquiredApproximately 5 minutes of four satellite tracking

Orbit DeterminationSome 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 trackingside lobe trackingsignal obstructed by the Earth

Future StepsData 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 sensorLessons 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 HEOPiVoT will be capable of tracking all satellites in view and computing a real-time orbit solution in the AMSAT orbitGSFC & AMSAT interested in follow-on experiment opportunitycould develop s/c & GPS receiverWould like to partner on follow-on experiment---looking for small s/c launch opportunity