Recent developments in radar altimetry over land and their application to the CryoSat -2 mission

Altimeters are active microwave instruments for the accurate measurement of vertical distances (between the spacecraft and the altimeter footprint). The technology determines the two-way delay of the radar pulse echo from the Earth's surface to a very high precision (to less than a nanosecond). The instrument is also able to measure the power and the shape of the reflected radar pulses. Since the first dedicated altimeter was launched on the Seasat platform in 1978, satellite altimetry has experienced incredible and continuous development as new sensors have been designed and have become operational. The accuracy in range measurements gradually reached values that have produced an extraordinary increase in our knowledge of many aspects of ocean and ice dynamics and variability. Mainly, the evolution of the altimeter transmitter is marked by improvements in pulse compression techniques that have substantially reduced peak power requirements, as reported in [1] and [2], for example. The altimeter missions considered in this paper operate at Ku-band. The choice of frequency is constrained by both the system and the operational requirements. Since a narrow transmitted pulse is required to achieve reasonable range precision, high-frequency operation supports both the large receiver bandwidth and the narrow antenna beamwidth requirements. The upper limit on the operational frequency is constrained by atmospheric attenuation effects that significantly degrade the performance of the altimeter for frequencies > 18 GHz. In some altimetric missions, such as Topex, the radar altimeter instrument includes a C-band transmitter so that ionospheric propagation delays can be accurately measured. Generally, the two-frequency system produces subdecimeter range precision so that very small variations (particularly in the ocean surface) can be detected.