Precise inland surface altimetry (PISA) with nadir specular echoes from Sentinel-3: Algorithm and performance assessment

• Published in: Remote sensing of environment 2021-10, Vol.264, p.112580, Article 112580
• Main Authors: Abileah, R., Vignudelli, S.
• Format: Article
• Language: English
• Subjects: Algorithms; Altimeter bursts; Altimeters; Altimetry; Antennas; Beams (radiation); Contamination; Fjords; Global positioning systems; GPS; Inland waters; Interference; Lakes; Land pollution; Performance assessment; Radar; Radar cross section; Radar cross sections; Salar de Uyuni; Satellite radar; Satellite radar altimetry; Sentinel-3; Sidelobes; Specular inland water bodies; Statistical analysis; Synthetic aperture radar; Water bodies; Water level; Waveforms; Windows (intervals)
• ISSN: 0034-4257; 1879-0704
• DOI: 10.1016/j.rse.2021.112580

In the recent years satellite radar altimetry has evolved from pulse-limited low resolution mode (LRM) to a synthetic aperture radar (SAR) high resolution mode. The SAR mode focusses and coherently sums all radar return echoes within the 2-s time window the target surface area is in the antenna beam. In principle the SAR processing improves along-track resolution. Land contamination has been a major concern for inland waters altimetry and SAR can reduce land interference. This paper shows that the physics of specular echoes from smooth inland waters leads to a very different approach which we call precise inland surface altimetry (PISA). PISA uses only echoes within the specular “flash” period, which is approximately the time the satellite nadir crosses over the water body. The processing is four orders of magnitude less than SAR. Land interference is negligible because specular water echoes are usually >50 dB greater than land. Sentinel-3 SRAL dataset on the salar de Uyuni (Bolivia) is used to evaluate PISA ranging precision. During inundation (wet months), echoes are at the theoretical maximum radar cross section (RCS), σ = 129 dBsm, and ranging precision is ~1 mm. In dry months the echoes are quasi-specular, with σ = 70–100 dBsm, and ranging precision is ~1 cm. The precision assessment is made with variate-differences, with pass-to-pass repeatability, and by comparison with GPS measurements. In addition to the salar de Uyuni analysis we gathered σ statistics on five millions Sentinel-3 SRAL Ku-band altimeter bursts (one burst = 64 contiguous echoes) from 52 passes of Sentinel-3A track 167 over South America. We illustrate specular and quasi-specular waters on lakes, a river, and a fjord. Ranging precision is similar to Uyuni, in the 1 mm-1 cm range. Water surface slopes of 1–3 cm/km are detected. We propose a simple rule-set to distinguish specular waveforms (σ >100 dBsm, sidelobes (with Hamming window) of −37 dB or lower) and quasi-specular (σ >70 dBsm, sidelobes lower than −20 dB), and non-specular (sidelobes> − 20 dB). PISA is appropriate to specular and quasi-specular echoes. •Focuses on specular water surfaces.•Describes the PISA algorithm for retrieval of range using Sentinel-3 data.•Illustrates PISA performance for a range of water targets.