Radar Altimetry Reveals the Smoothness of the Surface: the Case of Salar de Uyuni, Bolivia

Salar de Uyuni is a salt desert in Bolivia, spanning approximately 10,000 km². During the wet season a thin layer of rainfall water covers the salt flats, making its surface mirror-like and earning it the title of “the largest natural mirror in the world”. The surface reflects the sky like a mirror, and attracts tourists who document this effect only from its outer perimeter. No evidence is documented in the interior, accessible only during the dry season. The only frequent observations of the Salar surface are from satellites, particularly altimetric radars, which are specifically designed to measure topography. Originally developed to measure sea level [1], they have recently been used, with a different metrics, to describe how emitted radar pulses are reflected by the surface, measuring the intensity of the reflected echo, and thus the Radar Cross Section (RCS) of the surface [2]: higher RCSs correspond to smoother surfaces. RCS was initially estimated in [1] with a an approximate method. Later EUMETSAT shared a better estimate by solving the radar equation with satellite parameters that were previously unknown to us [3]. In this study we used Sentinel-3A and 3B RCS measurements over the Salar flats, along six ground tracks, to describe for the first time the evolution of the Salar surface smoothness in space and time. A field campaign (16^th - 20^th of February 2024) was also conducted to validate the interpretation of radar measurements during the Sentinel-3A overpass on the track 167. At the field site, in a water depth of 1.8 cm (horizontal wind 4.5-3.4 ms^-1), we measured a null vertical surface displacement to within ±0.5 mm, which classifies the surface as electromagnetically smooth at the radar frequency. The RCS values near the site were around 120 dBsm, as expected for radar return from a smooth surface. Three peaks are observed on the statistical distribution of the RCS: 87 (dry), 101 (intermediate) and 120 dBsm (wet season).The wet season, characterized by values above 101 dBsm, begins in December, peaking from late January to early March. February thus ensures the highest chance to observe mirror-like effects. Rainfall climatology from Uyuni city meteorological station reflects such statistics. The spatial and temporal evolution of RCS over the Salar, however, do not describe this place like a uniform mirror at the radar frequency, and so it is unlikely to observe such effect at shorter wavelengths, contrary to what is believed in the literature. Finally, satellites can help tourism stakeholders in programming the most enjoyable experience for travellers.

[1] Vignudelli et all. 10.1007/s10712-019-09569-1

[2] Abileah and Vignudelli, 10.1016/J.Rse.2021.112580

[3] Dinardo and Lucas, EUM/RSP/TEN/23/1376566