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Dual-Frequency Topex Radar Altimeter-ALT

NSSDC ID: 1992-052A-01
Mission Name: Topex/Poseidon
Principal Investigator: Mr. Lee-Lueng Fu


The NASA-provided TOPEX dual-frequency altimeter (ALT) was the primary instrument on the NASA/CNES TOPEX/Poseidon oceanographic spacecraft. The objectives of the ALT were to measure the satellite-to-sea-surface height, wave height, provide ionospheric corrections, and measure wind speed directly beneath the spacecraft. The NASA ALT was similar to the radar altimeter previously flown on the GEOS 3 and Seasat spacecraft. The primary measurement frequency was 13.6 GHz (Ku-Band), and the secondary frequency was 5.3 GHz (C-Band). The subsequent range difference measured at these two frequencies provided a first-order correction for the influence of the ionosphere. Corrected accuracy was expected to be 2.4 cm in altitude. The ALT consisted of a signal processor, an RF section, and an antenna assembly. The signal processor was mounted separately on the altimeter base plate. The signal processor unit was essentially the same as the unit flown with the Seasat radar altimeter in 1978. The ALT shared the antenna with the CNES Poseidon radar altimeter. The antenna used a multifrequency feed horn which provided waveguide connections for the two NASA ALT frequencies and the single CNES frequency. The aft portion of the feed horn was an orthomode coupler providing signal paths to the NASA and CNES altimeter electronics. The altimeters were not operated simultaneously. The NASA ALT operated 88% of the time, with the CNES experiment operating approximately 12% of the time. The altimeter had a pulse command interface and a data command interface with the spacecraft. The ALT field-of-view at nadir was 3 degrees. The ALT operated in transmit and receive mode; mode selection was either automatic or manual and was performed through the adaptive tracker unit (ATU). In transmit mode, a 5 MHz reference frequency entered and exited the altimeter through the interface control unit (ICU), was converted to 80 MHz and then converted to a 250 MHz signal. The signal was chirped producing a 260 MHz FM pulse. The FM pulse was multiplied and converted to 13.6 GHz at a pulse repetition frequency of 4000. The 13.6 GHz signal drove the Ku-band traveling wave tube amplifier (TWTA) producing the transmit RF power which was then directed through the antenna and transmitted to the ocean surface. The other C-band (5.3 GHz) frequency was produced by a down converter from the 13.6 GHz signal. In the receive mode, the reflected signals were routed through the microwave transmission unit (MTU), converted to intermediate frequencies (500 MHz), fed through the hybrid coupler to the receivers which further reduced the signals. The signals were passed through a digital filter bank where analog-to-digital conversion was done and a Fourier transform applied to each channel. The data telemetered to the ground consisted of the height and waveform data. See Zieglar,A.R.,D.W.Hancock,G.S.Hayne,and C.L.Purdy,"NASA Radar Altimeter for the TOPEX/POSEIDON Project",Proc.IEEE,Vol.79,No.6,June 1991.

Facts in Brief

Mass: 206.0 kg
Power (avg): 237.0 W
Bit rate (avg): 8.192 bps

Funding Agency

  • National Aeronautics and Space Administration (United States)


  • Earth Science: Atmospheric Dynamics
  • Earth Science: Geophysics/Geodesy
  • Earth Science: Physical Oceanography

Additional Information

Questions or comments about this experiment can be directed to: Dr. Dieter K. Bilitza.



NameRoleOriginal AffiliationE-mail
Mr. Lee-Lueng FuTeam LeaderNASA Jet Propulsion

Selected References

Zieger, A. R., et al., NASA radar altimeter for the Topex/Poseidon project, IEEE Proc., 79, No. 6, 810-826, June 1991.

Born, G. H., et al., Topex: Observing the oceans from space, EOS, 65, No. 28, 433-434, July 1984.

Topex/Poseidon mission description, In--1633-5, Rev. B, JPL D-601, Rev. B, JPL/NASA, Dec. 1985.

TOPEX/Poseidon Sci Work Team, Topex/Poseidon science investigations plan, In--JPL/NASA, Sep. 1991.

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