Ensuring that the Sentinel-3A altimeter provides climate-quality data

Abstract

Sentinel-3A, launched in February 2016, is part of ESA's long-term commitment to climate monitoring from space. Its suite of instruments for measuring surface topography includes a Microwave Radiometer (MWR) and SRAL, the first delay-Doppler instrument to provide global coverage. SRAL promises fine spatial resolution and reduced noise levels that should together lead to improved performance over all Earth surfaces. The Sentinel-3 Mission Performance Centre (S3MPC) has been developing the methodology to evaluate the accuracy of retrievals, monitor any changes and develop solutions to known problems.The S3MPC monitors internal temperatures, path delays and the shape of the generated pulses to assess the instruments health. The MWR records over known reference surfaces are compared with those from other spaceborne instruments. Over the ocean the SRAL's return pulses are analysed to give range to the sea surface, wave height and signal strength (which can be interpreted as wind speed). The metocean data are regularly contrasted with records from in situ measurements and the output from meteorological models, which rapidly highlights the effects of any changes in processing.Range information is used to give surface elevation, which is assessed in three ways. First, flights over a dedicated radar transponder provide an estimate of path delay to within ~10 mm (r.m.s.). Second, measurements are compared to GPSlevelled surfaces near Corsica and over Lake Issyk-kul. Third, there are consistency checks between ascending and descending passes and with other missions. Further waveform analysis techniques are being developed to improve the retrieval of information over sea-ice, land-ice and inland waters.

Assessing altimetry close to the coast

“…However, the assessment as to whether there are biases in the altimeter algorithms, or some dependence on the mix of swell and wind sea, is more challenging to investigate, requiring a large number of validation points. Other work 5 , examining the accuracy of Sentinel-3A wave height retrievals over the open ocean have used a large number of buoys, ships and offshore platforms, allowing robust conclusions. Assessment as to whether S-3A delivers the anticipated improved performance in the coastal zone is challenging because of the smaller quantity of validation data.…”