CoAuthors

Walter Smith (NOAA, USA); Eric Leuliettte (NOAA, USA)

The correlation of consecutive pulses on nadir looking pulse limited altimeters, what is now commonly referred to as low resolution mode (LRM) altimeters, was a significant topic of study during earlier stages of radar satellite altimetry to determine the maximum pulse repetition rate at which statistically independent samples could be obtained from this type of measurements. Seminal works by Walsh, and Rodríguez and Martin, had significant influence in the design and development of radar altimeters, from the Topex/Poseidon Mission to the Jason altimeter series. In order to make the satellite operation efficient at the same time as retrieving the most information out of the altimeter pulses, these altimeters were designed so that their echoes were mostly decorrelated between each other.

In 1998 the introduction of the delay/Doppler altimetry (DDA) concept posed a new paradigm in the field of radar altimetry. This processing technique, based on the coherent processing of radar echoes transmitted in bursts, reduces the altimeter footprint along the flight direction by an order of magnitude with respect to LRM altimeters, and achieves a significant increase in the effective number of looks (ENL) of the final multilooked waveforms, leading to a noise reduction in the estimation of geophysical parameters.

Unlike in conventional altimetry, DDA requires correlation of the echoes within the bursts to apply the coherent along-track processing, which imposes the need of much higher pulse repetition frequencies (PRF). The first satellite mission of this kind is the European Space Agency's (ESA) Cryosat-2 mission launched in 2010. Initially devoted for cryosphere sciences, the SAR mode capability of the Synthetic Aperture Interferometric Radar Altimeter (SIRAL) altimeter has also provided the opportunity of demonstrating and studying the performance of the DDA technique over the ocean.

The SRAL instrument, onboard the recently launched ESA Sentinel-3 mission, is a similar radar altimeter to SIRAL. In SAR mode both instruments operate in closed bursts at a PRF of 18.1818 kHz, which makes them a perfect tool for studying the correlation properties between adjacent pulses of nadir-looking radar altimeters. In this study, we make use of five full years of Cryosat-2 SAR mode data acquired over the ocean. The reason for such an extended dataset is to obtain a statistically representative sample over a wide range of sea state conditions. From the SAR mode pulse echoes at 18 kHz we made pseudo- low-resolution mode (PLRM) waveforms at 18, 9 and 2 kHz, that we later retrack by means of an MLE-4 ocean retracker, to help us understand what are the effects of the partial correlation of radar pulses in the determination of geophysical parameters.

We observe that, as noticed in previous works, the correlation properties of pulse limited radar altimeter echoes depend on the range gate within the tracking window, and we have verified that the correlation properties depend also on sea state, as predicted by previous simulation studies. Most significantly, we have determined that the noise in the estimation of ocean parameters can be reduced by transmitting pulses at a much higher PRF than the decorrelation limit predicted by Walsh and Rodríguez, particularly in rough sea state conditions. In addition, the partial correlation of radar altimeter pulses transmitted at high PRFs has important implications on the estimation of geophysical parameters from LRM waveforms, such as the introduction of significant biases in the determination of sea surface height and significant wave height, which will need to be accounted for and corrected.

These results are particularly relevant for the upcoming Jason-CS/Sentinel-6 mission, a joint endeavor of NASA, ESA, EUMETSAT, NOAA, and CNES. This mission, meant to be the follow-on mission in the Jason series, will be operated in an interleaved mode that will produce simultaneously both SAR and LRM data by transmitting pulses at a PRF of 9 kHz.