The Radar Altimeter 2 (RA-2) is an instrument for determining the two-way delay of the radar echo from the Earth’s surface to a very high precision: less than a nanosecond. It also measures the power and shape of the reflected radar pulses. The RA-2 is derived from the ERS-1 and 2 radar altimeters, providing improved measurement performance and new capabilities.

RA2_b

RA-2 during integration on Envisat (CreditsESA)

Function

Operating over oceans, RA-2 measurements are used to determine the ocean topography, thus supporting research into ocean circulation, bathymetry and marine geoid characteristics. Furthermore, RA-2 is able to map and monitor sea ice, polar ice sheets and most land surfaces. Measurement of the radar echo power and shape enables wind speed and significant wave height at sea to be determined, thus supporting weather and sea state forecasting.

Principle

The altimeter emits a radar beam that is reflected back to the antenna from the Earth’s surface (see how altimetry works for details). RA-2 operates at two frequencies (13.57 GHz in the Ku-band and 3.2 GHz in the S-band) to determine atmospheric electron content, which affects the radar signal path delay. These two frequencies can also serve to measure the amount of rain in the atmosphere.

Technical data

Emitted Frequency (GHz) Dual-frequency (Ku, S) – 13.575 and 3.2
Pulse Repetition Frequency (Hz) 1795 (Ku), 449 (S)
Pulse duration (microseconds) 20
Bandwidth (MHz) 320, 80 and 20 (Ku) – 160 (S)
Antenna diameter (m) 1.2
Antenna beamwidth (degrees) 1.29 (Ku), 5.5 (S)
Power (W) 161
Redundancy Yes
Specific features Dual-frequency for ionospheric correction, 3 bandwidths in Ku-band

Further information on the RA2 instrument (ESA website)