Abstract
The precision of sea surface altimetry using bistatically reflected signals of the Global Navigation Satellite System (GNSS) is typically one to two orders of magnitude worse than dedicated radar altimeters. However, when the scattering is coherent, the electromagnetic phase of the carrier signal can be tracked, providing precise ranging measurements. Under grazing angle (GA) geometries, the conditions for coherent scattering are maximized, enabling carrier phase-delay altimetric techniques over sea waters. This work presents the first implementation of GA carrier phase sea surface altimetry using data acquired from a spaceborne platform (NASA Cyclone GNSS mission) and transmitted from both GPS and Galileo constellations. The altimetric results show that the measurement system precision is 3/4.1 cm (median/mean) at 20 Hz sampling, cm level at 1 Hz, comparable to dedicated radar altimeters. The combined precision, including systematic errors, is 16/20 cm (median/mean) precision at 50 ms integration (a few cm level at 1 Hz). The wind and wave requirements to enable coherent scattering at GA geometries appear to be below 6 m/s wind and 1.5 m significant wave height, although only 33% of tracks under these conditions present sufficient coherence. Given that this technique could be implemented by firmware updates of existing GNSS radio occultation missions, and given the large number of such missions, the study indicates that the resulting precision and spatio-temporal resolution would contribute to resolving some submesoscale ocean signals.
Highlights
S PACEBORNE altimetry using signals of the Global Navigation Satellite System (GNSS) reflected off the oceans (GNSS-Reflectometry or GNSS-R) has poor precision compared to dedicated radar altimeters, when GNSS-R altimetryManuscript received August 30, 2019; revised October 17, 2019 and November 4, 2019; accepted November 5, 2019
We have reprocessed existing NASA Cyclone Global Navigation Satellite System (CyGNSS) mission raw data samples to search for grazing angle (GA) reflected signals at the edge of the antenna pattern, and assess whether GA carrier phase altimetry (CaPA) is possible from space
We present the analysis of 63 tracks of GNSS signals reflected off the sea surface in GA geometries, captured in raw mode by the NASA’s CyGNSS spacecraft constellation in the Central American region
Summary
S PACEBORNE altimetry using signals of the Global Navigation Satellite System (GNSS) reflected off the oceans (GNSS-Reflectometry or GNSS-R) has poor precision compared to dedicated radar altimeters, when GNSS-R altimetry. In [18], data acquired from 3500 m altitude flights over open sea waters showed it is optimal to obtain coherent scattering of GNSS signals and perform its corresponding phase-delay altimetry in grazing geometries between ∼5◦ and 30◦ elevation angle (60◦–85◦ incidence) These studies are promising, but conducted at slow speeds and low altitude compared to spaceborne scenarios. The main impairment to conduct this type of study is the acquisition of the spaceborne data, as currently the GNSS-R missions in orbit point their antennas to angles closer to nadir, away from the grazing geometries Despite this restriction, we have reprocessed existing NASA CyGNSS mission raw data samples to search for GA reflected signals at the edge of the antenna pattern, and assess whether GA CaPA is possible from space
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