On the retrieval of the specular reflection in GNSS carrier observations for ocean altimetry

Abstract

[1] This paper presents an altimetric method for ocean monitoring by remote sensing. It uses carrier observations of reflected GNSS signals. The method is illustrated in a simulation study and applied to a long term experiment yielding an ocean tide spectrum. The altimetric concept is based on residual observations of Doppler frequency. A linear relation between Doppler residuals f 0 and height departures DH from the surface level is derived. In contrast to existing phase-based methods which are constrained by smooth ocean conditions, the frequency-based retrieval here described holds good for rougher ocean conditions. Two retrievals of Doppler residuals are distinguished: Tracking Retrieval and Spectral Retrieval. A simulation study investigates the performance of Spectral Retrieval for a rough ocean surface with a noise-like sea level deviation x(t). Simulation settings were adjusted to reflection events in coastal experiments with an elevation range of [5…15] deg. In this range Tracking Retrieval tolerates a surface standard deviation s x < 5 cm, whereas Spectral Retrieval tolerates s x ≤ 30 cm. These limits correspond to significant wave heights of 20 cm for Tracking Retrieval and 1.2 m for Spectral Retrieval. The simulation results are confirmed by applying the altimetric method to the experimental data. The recovery of continuous phase tracks in experimental data is onerous and Tracking Retrieval only works for a period of smooth ocean conditions (162 events). By contrast, Spectral Retrieval yields altimetric estimates throughout the whole experiment (2607 events). The altimetric time series extends over more than 60 days and results in a tide spectrum that resolves diurnal (K1) and semidiurnal (M2, S2) constituents. The formal precision for these estimates lies in the decimeter range.

Analysis of Grazing GNSS Reflections Observed at the Zeppelin Mountain Station, Spitsbergen

“…The simplest case considers a reflection facet parallel to the receiver's horizontal plane, cf. Bilich et al (); Semmling et al (). Then the differential path reads where Δ H is the perpendicular height difference between antenna and horizontal surface facet.…”

“…The simplest case considers a reflection facet parallel to the receiver's horizontal plane, cf. Bilich et al (2008); Semmling et al (2012). Then the differential path reads…”

“…The altimetric use and retrievals to estimate the sea state, sea ice roughness, and concentration have been reported for coastal setups, see, e.g., Fabra et al (2010) and Semmling et al (2011Semmling et al ( , 2012. A mountain setup potentially allows to observe glaciers and ice sheets for remote sensing.…”

Feasibility of Code-Level Altimetry Using Coastal BeiDou Reflection (BeiDou-R) Setups

“…According to the first one, assuming the earth's surface as horizontal without the spherical curvature, it is suitable for a short distance between receiver and specular reflection, the distance is estimated from satellite elevation and the height between receiver and reflection surface. For the second definition, a spherical earth model with the impact of the Earth's curvature is taken for large distances [32], [33]. We selected the first model for geometrical path delay in this paper.…”

“…Then, we can acquire a group of I branch and Q branch of the direct and reflected signals every 1 ms [33]. When squaring and adding I n and Q n I 2 n + Q 2 n , we can acquire a pair of cross-correlation signals with 37 lags every 1 ms.…”