The NSIDC Web site and data services are currently having intermittent problems and may be unavailable. We are working to restore these services as soon as possible and apologize for any inconvenience this may cause. Please contact NSIDC User Services for assistance.
A radar altimeter, which is carried aboard satellites, collects elevation signals from land and ocean surfaces. These data are used to determine topographical features.
The radar altimeter is an instrument on a satellite that measures the distance from the satellite to the Earth's surface using a radar signal. The instrument transmits an electronic pulse in the microwave frequency to the Earth's surface. The microwave pulse reflects off the surface and returns to the sensor. Altitude is determined from the pulse travel time (from transmit to receive) and from the waveform of the returned pulse.
The radar altimeter measures the altitude of the Earth's surface to determine ice sheet topography and small-scale ocean roughness.
Satellite orbit, tropospheric/ionospheric signals, and geoid model.
The radar altimeter sends electromagnetic pulses at a microwave frequency to a land or ocean surface, then detects the reflected signals. The waveform data are then transmitted to a ground receiving station.
The two major subsystems of the radar altimeter are: A peak power (RF) section and a signal processor.
*GEOSAT radar altimeter
*The GEOSAT and SEASAT radar altimeters share mechanical, thermal, and electrical interface characteristics.
|Satellite||Frequency (Ghz)||Bandwidth||Wavelength (m)||Range Resolution (m)||Pulse Compression||Wave Height|
|Geos-3||13.90||80.00||0.02||0.50||30.00||±25% (4-10 m)|
|GEOSAT||13.50||320.00||0.02||0.10||30000.00||±10% (1-20 m)|
|GEOSAT Follow-On||13.50||0.02||0.018||58000.00||.035 m|
|SEASAT||13.50||320.00||0.02||0.10||1000.00||± 10% (1-20 m)|
This section is not applicable.
The GEOSAT radar altimeter contained an onboard calibration mode that was invoked twice daily to track waveform sample gain and attitude, wave height, automatic gain control, and height.
For waveform sample gain correction, the onboard tracker operated on a set of 60 waveform samples in the power spectrum outputs of a digital filter bank. Effects, such as in-band ripple and band-edge rolloff of anti-aliasing low-pass filters in the altimeter receiver were removed by individual waveform sample gain correction factors.
The correction processes of the attitude determination (and related corrections) started with computation of a voltage proportional to attitude (VATT) based on the amplitude of the last eight waveform samples.
MacArthur, John L., Paul C. Marth, Jr., and Joseph G. Wall. 1987. The Geosat radar altimeter. Johns Hopkins APL Technical Digest 8(2): 176-81.
Moody, Thomas B. 1996. Radar altimetry, Internet home page, University of Texas at Austin, Texas. http://www.ae.utexas.edu/courses/ase389/sensors/alt/alt.html.
February 3, 1998
February 2, 1998