Jyh-Ching Juang
Department of Electrical Engineering, National Cheng Kung University, TAIWAN
Taiwan’s independently developed TRITON satellite was launched in October 2023. Its main payload, the Global Navigation Satellite System Reflectometry (GNSS-R) instrument, has been operational ever since. The GNSS-R payload has been continuously operating for nearly a year and ten months, accumulating a wealth of data that can be accessed and analyzed through the CWA (Central Weather Administration) website. This data is valuable for research on sea surface wind speed and significant wave height.
This report primarily explains the procedure for generating the Delay Doppler Map (DDM), the main observable produced by the GNSS-R payload. The GNSS-R payload’s primary function is to receive and process navigation satellite signals that have been reflected off the Earth’s surface. These signals from navigation satellites are inherently very weak, and the signals that scatter off the surface and reach the low-orbit satellite’s receiver are even weaker. Therefore, a major design consideration for the GNSS-R receiver is to ensure it has sufficient sensitivity to detect these faint signals.
During this process, the receiver generates samples with different time delays and Doppler shifts. These samples are then correlated with the received signals to obtain a distribution of reflected power at various time delays and Doppler shifts, which is the Delay Doppler Map. The power near the peak of the DDM primarily comes from signals reflected near the specular point. This signal power is influenced by several factors: the navigation satellite’s transmission power, its transmitting antenna gain, path loss, the characteristics of the reflecting surface, the receiving antenna gain, and the receiver’s processing procedures. By analyzing the intensity of the DDM signal, it’s possible to retrieve properties of the reflecting surface, such as sea surface wind speed, significant wave height, and soil moisture content. This report will detail the working principles of the GNSS-R receiver and the functions and performance of the TRITON GNSS-R payload.
