Multi-frequency GNSS/RNSS reflectometry receiver design

Abstract

The developments in Global Navigation Satellite Systems (GNSS) and Regional Navigation Satellite Systems(RNSS) have given rise to various application areas beyond the conventional Positioning, Navigation and Timing (PNT) services. The GNSS L-band frequency signals can be used as an inexpensive and a passive remote sensing technique called GNSS Reflectometry (GNSS-R). Several satellite mission, ground-based and spaceborne experiments have demonstrated the practicality of retrieving the Earth’s geophysical parameters using the GNSS-R technique. The application of the GNSS-R technique is expanded to soil moisture estimation, vegetation cover, snow depth measurement, wind speed retrieval, and many more. Over the last few years, the use of GNSS signals in passive bistatic or multi-static radar configurations has been researched for Earth surface remote sensing and hence capitalizing on the availability of freely accessible GNSS signals to observe various Earth-surface characteristics. The existing research on GNSS-R research is primarily focused on the use of GPS, Galileo, BeiDou and GLONASS signals. Expanding upon this foundation, the present research aims to utilize signals from the Indian Regional Navigation Satellite System (NavIC) alongside GPS-L1 signals to advance remote sensing applications. The PhD thesis is focused on developing and assessing the capability of a Software Defined Radio (SDR) based GNSS-R receiver for studying the multi constellation and multi-frequency direct and reflected GNSS and RNSS signals for remote sensing applications. The thesis broadly addresses three research problems related to GNSS-R. Firstly, this thesis addresses the compression of large GNSS-Reflectometry (GNSS-R) datasets which is a critical step to minimize the onboard storage resources required by the payload. Secondly, this thesis explores target tracking using the GNSS-R techniques from the state estimation perspective and the analysis of Doppler frequency bound for NavIC-L5 multipath signals. These calculations are crucial for signal acquisition and the accurate determination of reflection points, enhancing the precision of surface property measurements and other geophysical parameters derived from GNSS-R data. Lastly, this research includes the design of an antenna tailored for GNSS-R applications, as well as the development of a Software-Defined Radio (SDR)-based receiver for GNSS-R reflectometry. The commercial off-the-shelf (COTS) GNSS/RNSS receivers cannot perform the necessary computations and do not provide flexibility in customization for reflectometry as per the user’s needs. Therefore, developing a custom antenna and receiver capable of processing reflected signals is essential. This work is also a proof-of-concept of using NavIC-L5 signals for reflectometry. The proposed receiver’s functionality is demonstrated through field experiments and verified using numerical simulation and a HIL simulator testbed. The field experiments are performed using the designed receiver to obtain the DDM using the GPS-L1 and NavIC-L5 signals. The field experiment results show that the proposed receiver is capable of receiving both direct (RHCP) and reflected (LHCP) GPS-L1 and NavIC-L5 RF signals and processing them into DDMs for performing reflectometry. The proposed receiver has a compact size and low power requirement and hence is suitable for performing remote sensing by using it as an air-borne or spaceborne GNSS-R receiver if radiation hardened.