TOPEX Outreach
Geophysical Parameters and Associated Oceanographic Products

Through signal processing and interpretation, researchers have identified several different types of geophysical parameters which can be derived from the altimeter data provided by TOPEX/POSEIDON. These same parameters can then be directly associated with various oceanographic products as detailed in chart format below:

Elapsed Time of Return Altitude Marine Geoid, Current Location & Velocity and Ice Topography
Waveform Leading Edge Sea/Surface Height, Distribution & Standard Deviation Significant Wave Hight
Waveform Leading Edge Skewness Dominant Wavelength, Significant Slope and Thermocline Information
Amplitude Bacscatter Coefficient:
Sigma (o)
Sea/Ice Boundaries and Sea Surface Windspeed
Trailing Edge Satellite Pointing and Sea/Surface Tilt Ice Slope

Dynamic Sea Surface Topography - (from Altitude)

A measure of sea level relative to earth's geoid from which oceanographers can calculate the speed and direction of winds. Two very precise distance measurements must be established in order to acquire reliable topographical ocean surface maps. First, the height of the satellite above the reference ellipsoidis measured by tracking the satellite in orbit from a globally distributed network of lasers and/or Doppler stations. The trajectory and height of the satellite are further refined by using orbit dynamic equations.

Second, the height of the satellite above the closest ocean surface is measured with a microwave radar altimeter. The difference between the height above the reference ellipsoid and the altitude above the ocean surface is approximately equal to the geoid height.

Depicted in the image below is the difference between the geoid height and mean sea level, commonly referred to as sea surface topography. This sea surface height has two main components, the geoid height, which reflects the gravitational field of the earth and the dynamic sea surface height, which reflects the ocean currents and tides.

Sea Surface Variability
Illustration of Dynamic Sea Surface Topography via time-dependent anomalies relative to a mean sea surface.

Image is from Jet Propulsion Lab's Physical Oceanography Distributed Active Archive Center
Date of Image: January 20, 1996

Wind Speed - (from Backscatter Coefficient)
Determined from the strength of the return pulse of the radar altimeter; a calm sea is a good reflector and returns a strong pulse, while rough seas scatter the signal thereby returning a weak pulse. In general there is a high degree of correlation between wind speed and wave height. The amount of scattered power can be used to observe the sea-ice extent, the boundaries of currents, oceanic wind speed, and the rate of accumulation of snow on glaciers.

Significant Wave Height - (from Wave Form Leading Edge)
Determined from the shape of the return pulse of the radar altimeter; a calm sea with low waves returns a condensed pulse, while rough seas with high waves return a stretched pulse.

Watervapor in the atmosphere delays the return of the radar altimeter pulse and thereby produces a false reading of sea level. Onboard microwave radiometers measure the content of water vapor and allow for adjustments.

What we have learned from TOPEX/POSEIDON


Last Modified: Sat Jul 17, 1998