Recent ESA studies on precise altimetry using carrier phase information of GNSS reflected signals: suitability to New Space missions

Abstract

The signals transmitted by the Global Navigation Satellite Systems (GNSS) can be used for other applications beyond navigation and positioning. Earth remote sensing is one of the opportunistic applications of the GNSS, achieved after the signals bounce off the Earth surface as a bi-static radar (GNSS reflectometry, GNSS-R), or after their refraction through the atmosphere in the so called radio occultation technique (GNSS RO). The surface height, for altimetry, is one of the geophysical parameters that can be inferred from GNSS-R signals. It is obtained from the inversion of delay measurements: the time required by the signals to travel from the GNSS transmitter down to the surface and up again to the receiver antenna. This delay is measured as the delay of the overall 'echo' or 'waveform' (group-delay measurement) or through the evolution of the phase of the electromagnetic field that carries the GNSS modulations and information (carrier phase measurements). Given the narrow bandwidth of the GNSS modulations, the group-delay measurements tend to present poor precision compared to those achieved with monostatic and dedicated radar altimeters. Nevertheless, the carrier phase measurements are very precise, at a level of few cm level after a few millisecond integration. GNSS-R Carrier Phase Altimetry (CaPA) has been proved in several experiments from ground-based, airborne and even spaceborne receiving systems [e.g. 1-3] . The drawback is that the reflecting surface must be smooth enough to enable coherent scattering and, only way to preserve the carrier phase information. Earth surfaces such as the ocean - where altimetric measurements are required - tend to scatter GNSS signals in a diffuse regime, thus disabling thewith a loss of carrier phase information. The roughness of the surface must be smooth with respect to the electromagnetic wavelength for the scattering to preserve the carrier phase information: the whole reflecting volume ought to be within the first Fresnel zone. Geometry plays a role: the smaller the incidence angle the thinner the vertical component of the Fresnel zone and the higher the chances that peaks and troughs of the ocean waves do not fit within it, thus resulting in diffuse scattering. As the incidence angle increases, the Fresnel zone thickens in its vertical component and eventually captures the whole sea surface wave structure. At these Grazing Angle (GA) geometries - low elevation angles of observation - the chances of coherent scattering increase. GNSS-R at these geometries for carrier phase altimetry will be called hereafter GA-CaPA, and its applicability to sea surface altimetry from spaceborne GNSS-R payloads was demonstrated in [4], using a limited set of CyGNSS raw data samples. Moreover, as the GA geometries are compatible with GNSS RO payloads, sea ice altimetric GNSS GA-CaPA tracks are regularly acquired by the Spire Global constellation of GNSS RO satellites, after an upgrade in their firmware [5]. In view of this promising new altimetric approach, a set of ESA studies have further investigated different aspects of GA-CaPA: during Q2-Q3 2021, a dedicated coastal experiment was installed and operated at the highest mountain in Majorca (Balearic Islands, Spain), to collect GNSS-R in GA geometries at two GNSS frequency bands and polarization states as sea winds and sea waves conditions changed. Together with the deployment of an oceanographic buoy for sea state monitoring and the Sentinel-3 passes across the observational area for altimetric comparisons, the study aimed to understand the underlying conditions of this type of reflectometry and identify the major limitations. In parallel, another study collected and analysed selected raw intermediate frequency samples of the Spire GNSS RO spaceborne constellation. The acquisitions were programmed when tracks of reflected GA-CaPA were predicted within the GNSS RO signals, and these were co-incident with passes of Sentinel-3 radar altimetry tracks. A third study targets to build the theoretical body of the GNSS GA-CaPA with validation including the former and new Spire raw data acquisitions. The technique can be embedded in small low-consuming payloads.: Tthe ESA CubSat mission PRETTY mission, to be launched in 2022, is designated forhas the goal of producing GNSS-R altimetry at GA geometries, and as mentioned before, Spire Global already implemented this technique into its constellation of GNSS RO nanosatellites. The combined outcome of these three ESA projects, which will be presented, permits to get a more comprehensive understanding of the performance, systematic effects, underlying conditions and limitations of the GA-CaPA, its potential complementarity to the current meta-constellation of dedicated radar altimeters and its potential use in the frame of the New Space paradigm. [1] Cardellach et al., 2004, doi:10.1029/2004GL019775. [2] Semmling et al., 2014, doi:10.1002/2013GL058725. [3] Li et al., 2017, doi:10.1002/2017GL074513. [4] Cardellach et al., 2020, doi:10.1109/JSTARS.2019.2952694. [5] Nguyen et al, 2020, doi:10.1029/2020GL088308.