A Phase-Altimetric Simulator: Studying the Sensitivity of Earth-Reflected GNSS Signals to Ocean Topography

Abstract

This paper presents a simulation study on Global Navigation Satellite System (GNSS) reflections focusing on a phase altimetric method for ocean topography retrieval. It examines carrier phase residuals of Earth-reflected GNSS signals in preparation for the GNSS Reflectometry Radio Occultation and Scatterometry experiment aboard the International Space Station (GEROS-ISS). The residuals' sensitivity to ocean topography (maximum of 2-m amplitude variation of global sea level) is shown. A trigonometric approach to determine the specular reflection point is proposed. Reflection events are simulated assuming different low Earth orbit receivers and GNSS-type transmitters. Suitable events for phase altimetry are assumed between 5 • and 30 • elevation lasting between 10 and 15 min with ground tracks length of > 3000 km. Typical along-track footprints (1 s integration time) have a length of about 5 km. Within the assumed elevation range the coherent footprint ellipse has a major axis between 1 and 6 km. A Master-Slave sampling is proposed to approximate large-scale delay and Doppler variations of the reflected signal (Slave channel) relative to the direct signal (Master channel). Slave residuals of an example event are simulated to retrieve a small-scale phase delay for ocean topography inversion. The signal-to-noise ratio restricts the quality of the topography results. Height precision on sub-decimeter level for 30-dB SNR is degraded up to a meter level for 20-dB SNR. Ionosphere-free linear combination allows keeping the precision level. Troposphere refraction degrades precision particularly at the low elevation limit. Precision improves toward higher elevations. The tolerance to ocean roughness decreases in the same way.

First spaceborne phase altimetry over sea ice using TechDemoSat‐1 GNSS‐R signals

“…The processing of the raw samples consists of a closed-loop tracking of the direct signal and an open-loop tracking of the reflected signal, also known as the master-slave sampling [Semmling et al, 2016] in GNSS-R phase altimetry. Through the raw samples processing, the power and phase of the reflected signal are extracted.…”

“…According to Semmling et al [2016], the SNR of the reflected waveform must be high enough (∼30 dB) to mitigate unwrap errors. For this reason, a coherent integration time of 20 ms is used to estimate the residual phase of the reflected signal r (t) after open-loop tracking.…”

Lake Level and Surface Topography Measured With Spaceborne GNSS‐Reflectometry From CYGNSS Mission: Example for the Lake Qinghai

“…To increase the signal‐to‐noise ratio, the 1‐ms complex waveforms are integrated coherently to generate the coherently integrated waveform . This is particularly important for phase altimetry as the signal‐to‐noise ratio of the waveform should be high enough (∼30 dB) to mitigate unwrap errors (Semmling et al, ). For this reason, a coherent integration time of 10 ms is used.…”

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

“…Therefore, a conventional satellite radar altimeter only observes a nadir track. A spaceborne observer of specular GNSS reflections has a significantly larger field of view for ocean altimetry, as recently shown by investigating spaceborne altimetry using GNSS reflection (Semmling et al, ; Stosius et al, ; Wickert et al, ). Even grazing observations with incidence angles >80° are possible and can contain altimetric information, as it was reported by early coastal experiments (Anderson, ) and spaceborne studies (Beyerle et al, ; Cardellach et al, ).…”