In General:

A satellite radar altimeter is not an imaging device, but continuously records average surface `spot' heights as it transverses over the Earth's surface. Operating at ~13.6GHz, each altimeter emits a series of microwave pulses towards the surface. By noting the two-way time delay between pulse emission and echo reception, the surface height can be deduced. Each returned height value is an average of all surface heights found within the footprint of the altimeter. The diameter of the footprint depends on the surface roughness, but can typically range between 200m (for open pools of water in calm conditions) to a few kilometers (open water with surface waves). Each satellite is placed in a specific repeat orbit, so after a certain number of days the same point (to within 1km), on the Earth's surface is revisited. In this way, time series of surface height changes can be constructed for a particular location along the satellite ground track during the lifetime of the mission.

There have been a number of altimetric satellite missions. Two are currently operational whilst new missions are scheduled for launch in this decade.

Instrument Summary

Satellite	Operational	Repeat Period
SEASAT	1976	17 days
GEOSAT	1986-1989	17 days
ERS-1	1991-1996	35 days (phases C+G)
T/P	1992-present	10 days
ERS-2	1995-present	35 days
ENVISAT	(launch 2001)	35 days
JASON-1	(launch 2001)	10 days

Although their primary objectives are ocean and ice studies, altimeters have had considerable success in the monitoring of inland water bodies. In particular, the ability to remotely detect water surface level changes in lakes and inland seas has been demonstrated (Birkett 1994, Morris and Gill 1994, Birkett 1995a, Dalton and Kite 1995, Cazenave et al. 1997, Birkett et al. 1999). Unhindered by time of day, weather, vegetation or canopy cover, the technique has further been applied to a number of rivers, wetlands and floodplains in several test-case studies (Rapley et al. 1987, Cudlip et al. 1990, Koblinsky et al. 1993, Birkett, 1995b, Birkett 1998, Rosenqvist et al. 1999, Birkett, 2000). In particular, the results demonstrate how submonthly, seasonal,and interannual variations in height can be monitored.

For full details on Satellite Radar Altimetry, see the References secion, refer to the following websites, and note these additional sources:

• Satellite Radar Altimetry - 1
  

• Satellite Radar Altimetry - 2

• Brooks, R.L., Lake Elevations from satellite radar altimetry from a validation area in Canada, report, Geosci. Res. Corp., Salisbury, Maryland, U.S.A., November 1982.
  

• Chelton, D.B., Walsh, E.J., and MacArthur, J.L., "Nuts and Bolts" of Satellite Altimetry, presented at the Altimeter Algorithm Workshop, Corvallis, Oregon, August 1987.
  

• Rapley, C.G., Satellite radar altimeters, in Microwave Remote Sensing for Oceanographic and Marine Weather-Forecast Models, edited by R.A. Vaughan, Kluwer Academic, Norwell, Mass., 1990.