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.
Highlights
The global sea surface height can be used to monitor global climate change, acquire the geoid, and invert the ocean gravity field, and plays an important role in the study of oceanic dynamics, geodesy, and geophysics
Since the TDS-1 observations are used to verify the reliability of the nadir antenna signal-to-noise ratio model (NASNRM), the parameters are as consistent as possible with the TDS-1 when calculating the signal-to-noise ratio (SNR)
As the elevation angle increases gradually, the change gradually becomes gentle. This indicates that SNR is sensitive to the change of the elevation angle in a low angle range, while SNR is less affected by the change of the elevation angle in a medium and high angle range
Summary
The global sea surface height can be used to monitor global climate change, acquire the geoid, and invert the ocean gravity field, and plays an important role in the study of oceanic dynamics, geodesy, and geophysics. Sea surface height can be obtained by the ship survey, tide station, and radar altimeter. Radar altimeter can obtain high precision global sea surface height, its spatial resolution cannot meet the requirements of mesoscale observation [1]. According to the method of obtaining the direct and the reflected signal delay, the GNSS-R altimetry can be divided into the group delay altimetry and the phase delay altimetry [4,5]. The phase delay altimetry has been demonstrated in multiple applications, such as sea ice altimetry, using a ground-based instrument [11], water surface survey on the airborne platform [12], and ice altimetry through GNSS-R satellite [13]. Compared with the group delay altimetry, the phase delay altimetry has higher requirements for the observation environment
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