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Challenges in grazing altimetry using reflected GNSS signals

The swath of airborne or spaceborne sensors increases going from nadir to grazing observations. Especially radar altimeters relying on nadir observed backscatter signals are limited in swath. The forward scattered signals used in GNSS reflectometry (GNSS-R) allow to broaden the view for altimetry to grazing reflections. The anticipated GNSS Reflectometry, Radio Occultation and Scatterometry (GEROS-ISS) experiment [1] with a receiver setup aboard the International Space Station (ISS) is a main driver to study grazing angle altimetry. An early coastal experiment [2] and studies related to the CHAMP satellite mission [3], [4] revealed signatures of grazing reflection in GNSS observations and demonstrated altimetric application resolving ocean tides and ice sheet topography. These studies also showed that grazing reflection data can be retrieved using either coastal geodetic receivers or a spaceborne radio occultation setup. In both cases the differential delay between the reflected signal and the direct (line of sight) signal as well as the corresponding differential Doppler shift are sufficiently small. This means that the reflected signal is in multipath range to the direct signal and samples contain interferometric fringes that allow an altimetric inversion based on carrier phase precision. This multipath range of differential delay and Doppler applies to altitudes < 50 m of the receiving antenna above the reflecting surface [2] or for grazing geometries with very small elevation angles (< 1°) that occur during radio occultation events [4]. It is of major interest for GNSS-R applications to extend this range. Previous studies reported carrier phase retrieval mainly below 30° elevation in the reflection point [5], [6]. At higher elevations the diffuse part of the sea surface reflection usually prevents such retreivals. Therefore, grazing reflection considered here refer to elevation angles < 30°. Considered altitudes include low earth orbits, 300–700 km above sea surface. It is an instrumental challenge to implement tracking algorithms of the reflected signal that allow carrier phase retrievals in this extended range.