Abstract
The scattering of GNSS signals over a water surface is studied when the receiver is at a low height, as in GNSS-R coastal altimetry. The precise determination of the local sea level and wave state from the coast will provide useful altimetry and wave information as “dry” tide and wave gauges. An experiment has been conducted at the Canal d'Investigació i Experimentació Marítima (CIEM) wave channel for two simulated “sea” states. The GNSS-reflectometer used is the P(Y) and C/A ReflectOmeter (PYCARO) instrument, a closed-loop receiver with delay and Doppler tracking loops that uses the conventional GNSS-R technique for the GPS C/A code. After retracking of the scattered GPS signals, the coherent and incoherent components have been studied. To reproduce the transmitted GPS signals indoors, a Rohde and Schwarz signal generator is used. It is found that, despite the ratio of the coherent and incoherent components being ~1, the coherent component is strong enough that it can be tracked. The coherent component comes from clusters of points on the surface that approximately satisfy the specular reflection conditions (“roughed facet”). The Pearson’s linear correlation coefficients of the derived “sea” surface height with the wave gauge data are: 0.78, 0.85 and 0.81 for a SWH = 36 cm and 0.34, 0.74, and 0.72 for a SWH = 64 cm, respectively, for transmitter elevation angles of = 60°, 75° and 86°, respectively. Finally, the rms phase of the received signal before the retracking processing is used to estimate the effective rms surface height of the ‘facets’, where the waves get scattered. It is found to be between 2.5- and 4.1-times smaller than the theoretical values corresponding to the half of the coherent reflectivity decaying factor.
Highlights
GNSS (Global Navigation Satellite System) Earth-reflected signals can be used as sources of opportunity for mesoscale ocean altimetry with improved temporal resolution as compared to traditional monostatic radar altimetry [1]
The P(Y) and C/A ReflectOmeter (PYCARO) GNSS reflectometry (GNSS-R) instrument processed the data, and it was connected to a laptop for monitoring the instrument’s parameters and to log the data. Both antennas were placed over the channel at static locations over two movable bridges (Figure 2) at a height of ~3 m above the water (Table 1)
The scattering of electromagnetic waves from the sea is strongly affected by its roughness, being the total scattered field the combination of many electromagnetic waves coming from multiple individual scatterers on the surface
Summary
GNSS (Global Navigation Satellite System) Earth-reflected signals can be used as sources of opportunity for mesoscale ocean altimetry with improved temporal resolution as compared to traditional monostatic radar altimetry [1]. This relatively novel technology is known as GNSS reflectometry (GNSS-R). The first experimental evidence of GPS reflected signals dates back to 1994 [2], and it was stated that, since the Rayleigh criterion was not satisfied, the scattering was diffuse. In 1996, Garrison et al [3] performed three different aircraft experiments over the ocean using a GPS receiver
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