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RA-2
The main objectives of the Radar Altimeter 2 (RA-2)are the high-precision measurements of the time delay, power, and shape of the reflected radar pulses for the determination of the satellite height and Earth surface characteristics.
RA-2 transmits radio frequency pulses which propagate at approximately the speed of light. The time elapsed from the transmission of a pulse to the reception of its echo reflected from the Earth's surface is proportional to the satellite's altitude. The magnitude and shape of the echoes contain information on the characteristics of the surface which caused the reflection.
Operating over oceans, the measurements are used to determine the ocean topography, thus supporting the research of ocean circulation, sea floor, and marine geoid characteristics.
The processing of the radar echo power and shape on the ground enables the determination of wind speed and significant wave height in the observed sea area, thus supporting the weather and sea state forecasting.
Furthermore, the RA-2 is able to map and monitor sea ice and polar ice sheets.
As a result, measurements over oceans are carried out with improved accuracy at the highest resolution. Over land, ice, or during transitions from one kind of surface to another, the tracking is maintained, although sometimes a certain degradation of the height resolution occurs. Accurate altitude measurements over the ocean carried out by RA-2 at the main frequency of 13.575 GHz are affected by fluctuations in ionospheric characteristics. Measurements at a second frequency channel of 3.2 GHz enable the error to be corrected.
The RA-2 was developed under the leadership of Alenia Spazio (I).
Trivia
1. The radar altimeter is an instrument which has been flying since 1973 with the launch of the first space-borne active microwave remote sensing spacecraft, Skylab. From 1973 until the present day the technology has evolved in a spectacular way.
2. In the late 80s, two different approaches were taken for the improvement of the performance, giving rise to two different families of satellite altimeters.
The first family, developed by the USA and France, was designed to optimise performance over oceans.
- TOPEX/Poseidon (NASA/CNES)
- Jason (CNES)
The second family, developed in Europe, was optimised for use over all kinds of surfaces.
- ERS-1 and ERS-2 (ESA)
- Envisat (ESA)
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