Global Inland Water Monitoring from Satellite Radar Altimetry - What can we really do?
Berry, Philippa A. M.1; Benveniste, Jerome2

The series of satellite radar altimeter missions has provided a huge database of altimeter waveforms over the earth's inland water surfaces; to date, only a small fraction of these have been successfully utilised to produce height measurements. This paper outlines the current measurement capability, and provides quantitative comparison of the different altimeter missions. This comprehensive global analysis shows that waveforms from 22,223 targets have been identified from ERS2; just under 50% of these targets are currently producing useable time series. For EnviSat, 25,363 targets have useable waveforms with a higher successful retrieval of inland water heights of 59%, and significantly more high quality time series. This is attributed in part to the dynamic mode-switching capability of the RA-2.
The 10-day repeat cycles of the ocean optimised altimeters (TOPEX, Jason1 and Jason2) produce lower numbers of potential and retrieved targets, with 8124, 7200 and 3912 targets respectively having retrieved waveforms. Currently, height time series are successfully derived for 35%, 32% and 31% of these targets.

The reasons for these retrieval differences are identified and their implications for future monitoring are analysed, with the primary limitation identified as failure to capture the waveform leading edge. This is a factor even in flat terrain, and is shown to be caused by snagging on bright targets, particularly a feature for these missions optimised for ocean operation. The performance of the ESA Near Real Time (NRT) system is also discussed, with just over 5% of the Envisat. targets identified being included in the NRT system.

Using data from CryoSat2, together with the 1800Hz 'burst echo' data from EnviSat, the additional potential for inland water measurement from Sentinel3 is explored. The results clearly demonstrate the huge enhancement in measurement capability offered by the high along track sampling rate of Sentinel3 SAR FBR waveforms; this transforms the measurement technique to a true global monitoring capability for entire river systems.