High Earth Orbit GPS Flight Experiment AMSAT-OSCAR 40 (AO-40)

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High Earth OrbitGPS Flight ExperimentAMSAT-OSCAR
40 (AO-40)

• Frank H. Bauer
• NASA Goddard Space Flight Center
• November 1, 2001

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AMSAT Phase 3D (AO-40)

• 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

AMSAT Phase 3D in Kourou
Launch November 16, 2000 Vehicle Ariane
5 Orbit 1000 by 58,800 km, i6
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GPS Receiver (1 of 2) in Phase 3Ds Equipment Bay
Perigee Antenna (1 of 4)
Apogee Antenna (1 of 4)
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Apogee Side of Spacecraft
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AO-40 Orbit
a 36286.409 km e 0.796838 i 6.04 deg W 151.818
deg w 335.84 deg M 2.00 deg   Rate of right asc
-0.208 deg/day   hp 1000 km ha 58800
km Period 19.1 hours Epoch doy 278.71185185,
GPStime 1134,493517.0 UTCtime 2001-10-05,
170504
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AO-40 Orbit with Geosynchronous GPS Orbits
Superimposed
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AO-40 Attitude wrt GPS Orbit
w 5.4 rpm
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Current Status

• First data retrieved on Sept 25, 2001
• AO-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 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

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Lots of data is being collected
High SNRs during peripapse passes
High density of contacts near perigee passes
enhance likelihood of fitting an accurate orbit
to the data
Numerous contacts between high SNR regions
indicate high alititude contacts
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October 6, 2001 Data Near Apogee
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Spin Rate Effects on SNR
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Point solutions may be possible when 3-axis
stabilized
Four or more simultaneous GPS satellites
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Close-up of multiple track overlap window
Signals are fading as fourth PRN is acquired
Approximately 5 minutes of four satellite tracking
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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.

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main lobe tracking
side lobe tracking
signal obstructed by the Earth
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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
  opportunitycould develop s/c GPS receiver
• Would like to partner on follow-on
  experiment---looking for small s/c launch
  opportunity