Increasing the Number of Sea Surface Reflected Signals Received by GNSS-Reflectometry Altimetry Satellite Using the Nadir Antenna Observation Capability Optimization Method

Abstract

High spatial resolution Global Navigation Satellite System-Reflectometry (GNSS-R) sea surface altimetry is of great significance for extracting precise information from sea surface topography. The nadir antenna is one of the key payloads for the GNSS-R altimetry satellite to capture and track the sea surface GNSS reflected signal. The observation capability of the nadir antenna directly determines the number of received reflected signals, which, in turn, affects the spatial resolution of the GNSS-R altimetry. The parameters affecting the ability of the nadir antenna to receive the reflected signal mainly include antenna gain, half-power beam width (HPBW), and pointing angle. Thus far, there are rarely studies on the observation capability of GNSS-R satellite nadir antenna. The design of operational satellite antenna does not fully combine the above three parameters to optimize the design of GNSS-R nadir antenna. Therefore, it is necessary to establish a GNSS-R spaceborne nadir antenna observation capability optimization method. This is the key to improving the number of sea surface reflected signals received by the GNSS-R altimeter satellites, thereby increasing the spatial resolution of the altimetry. This paper has carried out the following research on this. Firstly, based on the GNSS-R geometric relationship and signal processing theory, the nadir antenna signal-to-noise ratio model (NASNRM) with the gain and the elevation angle at the specular point (SP) as the main parameters is established. The accuracy of the model was verified using TechDemoSat-1 (TDS-1) observations. Secondly, based on the theory of electromagnetic scattering, considering the influence of HPBW and pointing angle on the antenna footprint size, a specular point filtering algorithm (SPFA) is proposed. Combined with the results obtained by NASNRM, the number of available specular points (SPs) is counted. The results show that as the antenna gain and the nadir-pointing angle increase, the number of SPs can reach a peak and then gradually decrease. Thirdly, combined with NASNRM and SPSA, a nadir antenna observation capability optimization method (NAOCOM) is proposed. The nadir antenna observation capability is characterized through the reflected signal utilization, and the results obtained by the method are used to optimize the combination of nadir antenna parameters. The research shows that when the orbital height of the GNSS-R satellite is 635 km, the optimal combination of nadir antenna parameters is 20.94 dBi for the gain and 32.82 degrees for the nadir-pointing angle, which can increase the observation capability of the TDS-1 satellite nadir antenna by up to 5.38 times.

Relationship Between Altimetric Quality and Along-Track Spatial Resolution for iGNSS-R Sea Surface Altimetry: Example for the Airborne Experiment

“…The reflected waveform retracking is based on comparing measured and modeled waveforms (Li et al, 2019). The modeled waveform corresponding to each measured waveform in this paper is obtained through the Z-V model implemented in the "wavpy" GNSS-R open-source software (Fabra et al, 2017).…”

“…The modeled waveform corresponding to each measured waveform in this paper is obtained through the Z-V model implemented in the "wavpy" GNSS-R open-source software (Fabra et al, 2017). Currently, the waveform retracker mainly includes HALF (the point at a fraction of the peak power), DER (the point with the maximum of waveform's first derivative), and FIT (fitting the waveform to its model) (Li et al, 2019). In this study, we used the DER to estimate the delay of the specular point along the waveform (Cardellach et al, 2014):…”

“…After correcting the delay error of the specular point, the ellipsoidal height of the sea surface above the WGS84 reference ellipsoid can be obtained by the geometric relationship of the airborne iGNSS-R (Li et al, 2019):…”

“…It is worth noting that altimetric quality mainly includes precision and accuracy. Among them, the altimetric precision is mainly affected by zero-mean random error, and the altimetric accuracy is mainly determined by the total absolute measurement error affected by the additional random and deterministic errors (Li et al, 2019). The along-track spatial resolution represents the ability of spatial sampling along the orbit of iGNSS-R altimeter, which can be expressed as…”

A Necessary Model to Quantify the Scanning Loss Effect in Spaceborne iGNSS-R Ocean Altimetry

“…The satellites as mentioned above are equipped with a single or two directional antennas to receive the reflected signals. We proposed the nadir antenna observation capability optimization method to increase the number of sea surface reflected signals received by cGNSS-R satellite [18]. However, this spaceborne GNSS-R missions are mainly used for wind speed retrieval rather than ocean altimetry.…”