Remote Sensing of Soil Based on a Compact and Fully Software GNSS-R Receiver

Y. Pei, R. Notarpietro, S. De Mattia, P. Savi, F. Dovis, and M. Pini

Abstract:	GNSS Reflectometry is an emerging technique which allows to monitor Earth’s surface parameters by applying an ad hoc processing to GNSS signal received after reflection [1,2]. It utilizes a bi-static configuration where the GNSS satellites serve as transmitter and a low orbit satellite or an aircraft acts as the receiver platform. Several applications such as sea wind and altimetry retrieval, ice and soil moisture monitoring are becoming more and more popular based on the GNSS-R technique. In this paper, an analysis on GNSS reflected signals measured on different types of terrain considering an open loop approach for the processing of row data and a portable GNSS-R receiver are introduced. In a previous measurement campaign conducted for rice fields’ soil moisture retrieval in the Vercelli area (Piedmont region, North Italy) [3], the open loop approach was applied for processing the reflected GNSS signal. The receiving system was placed on board a small aircraft in order to track reflections from rice fields. Since rice fields were flooded during that month, they were a perfect scenario to study reflection phenomena. The geometry of reflections was analyzed and all the satellites with elevation lower than 33° were discarded, since below this elevation the specular reflections did not enter inside the half power beamwidth of the LHCP nadir looking antenna. On board the aircraft, a video camera was placed to see which fields were really flooded during the acquisition. The panoramic view extracted from the video was superimposed on Google Maps, together with the specular reflection points. Both direct and reflected GNSS signals were captured by a zenith-pointing Right Hand Circularly Polarized (RHCP) antenna and a nadir-pointing Left Hand Circularly Polarized (LHCP) antenna respectively. Raw signals were sampled and converted into digital values by two radio frequency front-ends connected to the two antennas. Digital binary data were further restored into two commercial PCs separately. The instrument is highly reconfigurable, since it collects raw I and Q IF samples of the incoming signals (both for the direct and the reflected one). A sampling frequency of 8.1838 MHz is used, giving about 8 samples per C/A code chip. Direct received data were processed by a fully software receiver (NGene) developed by the Navigation Signal Analysis and Simulation (the NAVSAS) Group to generate positioning information. The total raw-sampled reflected GNSS signal acquired during the flight was processed into Delay Doppler Maps (DDMs) and Delay Waveforms exploiting a fully open loop scheme, in order to evaluate Signal to Noise Ratios (SNRs) time series, without the necessity to wait for standard GNSS close loop acquisition and tracking. For each available satellite, and with a 0.5 second interval, a SNR value related to the reflected received signal is recorded. Aircraft positions derived by processing the navigation data (using the NGene Software Receiver) were interpolated for each sample time, and the corresponding specular points were computed and georeferenced. Even if only the value of the correlation peak was used to estimate SNR and to derive dielectric permittivity information, this new open-loop approach allowed us to evaluate the entire autocorrelation function, whose knowledge could be used in future for other GNSS-R applications. A retrieval process able to estimate dielectric constant of soil surface from evaluated SNR was applied. The ground surface was considered rather smooth and the non coherent power contribution coming from glistening zone (through rough surface scattering mechanisms) was neglected. During the flight, the signal reflected by a lake surface was also acquired in order to calibrate the bistatic radar constant, after having fixed the dielectric constant characterizing the water reflection area to a reasonable value. After the derivation of the radar constant, more reliable dielectric constant values of the rice fields were obtained. A good correlation between dielectric constant value and the rice fields’ flooding state was obtained. A more compact prototype of GNSS-R receiver based on a single HackBerry A10 Dev Board was also developed [4]. The hardware including the storage subsystem, the clock frequency, the power supply and the USB management were customized in order to better suit the performance of the Radio Frequency Front-End. A customized version of the operating system based on Linux Debian was also built for the processor inside the HackBerry board. The entire device can operate via the SSH protocol or via the standard network port. The board and the front-end were finally integrated into a single box. The new prototype of GNSS-Reflectometry receiver was tested from the roof of Politecnico di Torino collecting reflected signals from the meadowland just in front of the building. Processed SNR time series allows us to validate the functionality of the system and to point out the sensitivity of the reflected GPS signal strength. The open loop approach used for calculating SNR and the derivation procedure from SNR to dielectric constant of the soil were proved to be feasible for soil moisture retrieval. A compact GNSS-R receiver was developed and tested. It is promising for future applications due to its portability. Some other measurement campaigns are foreseen during the forthcoming spring season. The new GNSS-R receiver will be mounted onboard an Unmanned Air Vehicle, which will fly over the same field before and after rainy events in order to validate the capability of GNSS-R to monitor soil moisture changing. References (1) D. Masters, P. Axelrad, and S. Katzberg, S. Initial Results of Land-Reflected GPS Bistatic Radar Measurements in SMEX02. Remote Sensing of Environment, Vol.92, No.4, pp. 507-520, doi:10.1016/j.rse.2004.05.016, 2004. (2) S.T. Gleason, Remote Sensing of Ocean, Ice and Land Surfaces Using Bistatically Scattered GNSS Signals From Low Earth Orbit. PhD thesis. University of Surrey, 2006. (3) Y. Pei, S. Yi, J. Zhou, R. Notarpietro, P. Savi, and M. Pini. GNSS Reflectometry for Earth’s Surface Monitoring Exploiting an Open-Loop Approach. In proceeding of XIX RiNEm, Rome, Italy, 2012. (4) http://rhombus-tech.net/allwinner_a10/hackberry/
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	56 - 61
Cite this article:	Pei, Y., Notarpietro, R., De Mattia, S., Savi, P., Dovis, F., Pini, M., "Remote Sensing of Soil Based on a Compact and Fully Software GNSS-R Receiver," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 56-61.
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