LOGO.049

Goddard Space Flight Center High Earth OrbitGPS Flight Experiment

AMSAT-OSCAR 40 (AO-40)

Frank H. Bauer

NASA Goddard Space Flight Center

November 1, 2001

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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, 2000Vehicle: Ariane 5Orbit: 1000 by 58,800 km, i=6

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GPS Receiver (1 of 2)in Phase 3D’s 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

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a 36286.409 kme 0.796838i 6.04 deg 151.818 deg 335.84 degM 2.00 deg Rate of right asc ~ -0.208 deg/day hp = 1000 km

ha = 58800 km

Period: 19.1 hoursEpoch: doy 278.71185185, GPStime: [1134,493517.0]UTCtime: 2001-10-05, 17:05:04

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AO-40 Orbit with Geosynchronous & GPS Orbits Superimposed

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AO-40 Attitude wrt GPS Orbit

= 5.4 rpm

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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

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Lots of data is being collected

High SNRsduringperipapse 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

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AMSAT Data for 10/06/2001: Predicted (green) Tracked (blue)

Time of Day [hrs]

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altit

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[10

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October 6, 2001 Data Near Apogee

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18 19 20 21 22 23 2435

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AMSAT Data for 10/06/2001, compiled 10/19/2001

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AMSAT Data for 10/06/2001, compiled 10/19/2001

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Spin Rate Effects on SNR

15 15.3 15.6 15.9 16.2 16.5 16.8 17.1 17.4 17.7 18 0

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snr

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U]

AMSAT spin rate present in snr data: PRN 27 tracked from 47000 km altitude

15 15.3 15.6 15.9 16.2 16.5 16.8 17.1 17.4 17.7 18 40

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C/N

0 [dB

-Hz]

Elapsed time in minutes since 10/06/2001 07:00:00 UTC

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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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transmitted boresite angle [deg]

AMSAT Data for 10/05/2001 through 10/09/2001, All PRNs

main lobe

tracking

side lobe trackingsignal 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 opportunity—

could develop s/c & GPS receiver• Would like to partner on follow-on experiment---looking for small

s/c launch opportunity