GEOS-3 Altimeter
Sensor/Instrument Document


The Geodynamics Experimental Ocean Satellite (GEOS-3) was launched in April 1975 and, in addition to the usual instrumentation for accurate position fixing, it carried a 13.9 GHz radar altimeter for measuring the vertical distance to the sea surface.

Table of Contents:

1. Sensor/Instrument Overview:

Sensor/Instrument Long Name, Sensor/Instrument Acronym:

Geodynamics Experimental Ocean Satellite (GEOS-3)

Sensor/Instrument Introduction:

NASA's Geophysical Satellite-3 (GEOS-3, 1975-1978) carried the first instrument to yield useful measurements of sea level and its variability with time.

Sensor/Instrument Mission Objectives:

The purpose of the GEOS-3 mission was to perform an in-orbit satellite altimeter experiment to:

  • Determine the feasibility and utility of a spaceborne radar altimeter to map the topography of the ocean surface with an absolute accuracy of ± 5 meters, and a relative accuracy of 1-2 meters.
  • Determine the feasibility of measureing the deflection of the vertical at sea.
  • Determine the feasibility of measureing wave height.
  • Contribute to the technology leading to a future operational altimeter satellite system with a 10 cm measurement capability.

Key Variables:

The GEOS-3 Radar Altimeter is a precision satellite radar developed primarily to measure ocean surface topography and sea state.

Scanning or Data Collection Concept/Principles of Operation:

Aboard GEOS-3 is a radar altimeter for measuring the ocean surface characteristics and the spacecraft to ocean surface distance; frequency of operation is 13.9 GHz, with two modes of operation: global or long pulse (200 nsec) mode, and intensive or short pulse (12 nsec) mode. For trajectory determination there are a coherent C-band transponder, S-band transponders for communication with high altitude satellites, laser retroreflectors, Doppler transmitters, a non-coherent C-band transponder and an S-band transponder for communication with earth tracking stations.

Within battery power constraints, altimeter data can be taken whenever the satellite is within view of a telemetry site, with data always transmitted in near real time. No storage of data is possible on board the spacecraft, although some averaging of data prior to transmission is possible. In addition transmision may also be via an S-band link through ATS-6, so that more than half a revolution of continuous visibility of GEOS-3 is possible for those missions for which ATS-6 is scheduled.

2. Sensor/Instrument Layout, Design, and Measurement Geometry:

List of Sensors:

The GEOS-3 altimeter consists of the following major subsystems:

  • magnetron transmitter for the long pulse (global) mode
  • TWT transmitter for the short pulse (intensive) mode
  • antenna
  • receiver
  • signal processor
  • built-in test/calibration system

Sensor Description:

The GEOS-3 Altimeter operated at 13.9 GHz with a pulse repetition frequency of 100 pulse/s, resulting in a spatial period of 70 m. The effective pulse width was 14-15 ns. [Rufenach and Alpers, 1978]

Measurement Geometry:

The GEOS-3 satellite was launched on April 9, 1975 from Vandenberg, California, into a near nominal 843 km circular orbit with an inclination of 115°. Succesive tracks cross the equator every 101.8 minutes with a precession of about 26° to the west. Thus for any given region the satellite will cover a 1° x 1° grid in approximately one month.

3. Manufacturer of Sensor/Instrument:

The GEOS-3 spacecraft was designed and fabricated by the Applied Physics Laboratory (APL) of the Johns Hopkins University. The GEOS-3 Radar Altimeter was designed, developed, manufactured, tested, and qualified by the General Electric Company Aerospace Electronic Systems Department (AESD) with the Applied Physics Laboratory.

4. Calibration:


The GEOS-3 radar altimeter has two modes of operation (Global and Intensive) and two corresponding self-test/calibration modes for use in on-orbit functional test and instrument calibration.

The major calibrations considered necessary for the built-in test/calibration system were time delay (bias), AGC/gain, and waveform calibration. Internally generated signals are provided for calibration. A 10-second time delay (bias) test is performed in each mode to determine the bias to within the required one meter. In each mode, a two-point AGC/gain calibration is obtained using IF reference pulses. For waveform calibration, controlled video waveforms are used in both modes. [Hofmeister et al., 1976]

Frequency of Calibration:

Each mode has a pre-determined calibration cycle which includes a test for each of the above three functions.

Other Calibration Information:

For more information, refer to "Calibration Results for the GEOS-3 Altimeter" [Martin and Butler, 1977].

5. References:

Agreen, R.W., 1982. "The 3.5-Year GEOS-3 Data Set,", NOAA Technical Memorandum NOS NGS 33, NOAA, Rockville, MD, 8 pp.

Cartwright, D.E. and A.C. Edden, 1973. Corrected tables of tidal harmonics, Geophysical Journal, 33, 253-264.

Hofmeister, E.L., B.N. Keeney, T.W. Godbey, and R.J. Berg, 1976. Data User's Handbook and Design Error Analysis: GEOS-3 Radar Altimeter Volume I, NASA CR-156870, National Aeronautics and Space Administration, 540 pp.

Leitao, C.D., N.E. Huang, and C.G. Parra, 1978. Remote Sensing of Gulf Stream Using GEOS-3 Radar Altimeter, NASA Technical Paper 1209, National Aeronautics and Space Administration, 31 pp.

Martin, C.F., 1977. Altimeter Error Sources at the 10-cm Permormance Level, NASA CR-141420, National Aeronautics and Space Administration, 63 pp.

Martin, C.F. and M.L. Butler, 1977. Calibration Results for the GEOS-3 Altimeter, NASA CR-141430, National Aeronautics and Space Administration, 119 pp.

Rufenach, C.L. and W.R. Alpers, 1978. Measurement of Ocean Wave Heights Using the Geos 3 Altimeter, Journal of Geophysical Research, Vol. 83, C10, 5011-5018.

Schwiderski, E.W., 1980. On charting global ocean tides, Reviews of Geophysics and Space Physics, 18(1), 243-268.

Stanley, H.R. and R.E. Dwyer, 1980. NASA Wallops Flight Center GEOS-3 Altimeter Data Processing Report, NASA Reference Publication 1066, National Aeronautics and Space Administration, 130 pp.

6. Glossary of Terms:

The Geodynamics Experimental Ocean Satellite (GEOS-3) was launched in April 1975 and, in addition to the usual instrumentation for accurate position fixing, it carried a 13.9 GHz radar altimeter for measuring the vertical distance to the sea surface.

See the EOSDIS Glossary for a more general listing of terms related to the Earth Observing System project.

7. List of Acronyms:

8. Document Information:

Document Revision Date:

10 March 1998

Document Review Date:

10 March 1998

Document ID:

Document Curator:

Kelley Case

Document URL: