GNSS Reflected Signal Acquisition with Galileo Signals

INTRODUCTION Since its creation, the potential of Global Navigation Satellite Systems (GNSS) has been continuously demonstrated in a wide range of applications. Typically, most of these applications aim to provide Position, Velocity and Time (PVT). Nevertheless some new applications are targeting objectives far away from the initial GNSS scope. One promising example of these new GNSS application fields is GNSS Reflectometry (GNSS-R). The GNSS-R technique was initially proposed in 1993 [1] where it has been identified that the GNSS signals of the available GPS satellites could be used as opportunistic signals to derive quantities others than the original ones they were designed for, i.e. the PVT determination. GNSS-R applications explore the usually undesired GNSS multipath to derive several geophysical measurements, such as ocean altimetry as suggested in [1]. Basically, the concept behind GNSS-R is the same as the one behind bi-static radar where the receiver and the transmitter are at different locations. The reflected signal, which is altered by the reflection, acquires information about the characteristics of the reflective surface. In the case of GNSS-R, the role of the transmitter is played by the GNSS satellites and the receiver is the GNSS instrument. Although GNSS signals were not designed with their possible use in reflectometry in mind, they gather important features which make them suitable and attractive for this application: i) they are widely available, providing continuous world coverage, ii) they were designed to measure the range between transmitter and receiver, and using them it is possible to estimate the position of both the transmitter and the receiver with a high accuracy. CONTEXT AND MOTIVATION The deployment of additional GNSS constellations (such as Galileo and Compass/BeiDou) and the emergence of new GNSS signals, with increasingly complex modulation and multiplexing schemes, will enable performance enhancements in terms of accuracy and availability. The enhanced performance of the new signals will benefit all GNSS applications, but in this paper we will focus on the improvements that can be achieved in GNSS-R applications. Additionally, the market of remote sensing application is continuously growing. There is an increasing demand for remote system targeting, for instance, high precision altimetry, measurement of wave height, surface winds, surface currents, among other geophysical parameters. GNSS-R can be an attractive potentially lower cost alternative technique to the other sensing systems, such as buoys or dedicated Earth Observation satellites. In the last decade there were several experiments performed to prove the concept of GNSS-R technique. There were space-borne [2], air-borne [3] and ground-based [4] GNSS-R experiments and all of them proved the feasibility of this technique. With all of this in mind, and in the scope of the SARGO project (contract no. 24593 - 2012), funded under the Portuguese QREN SI IDT initiative, DEIMOS Engenharia is developing a complete ground-based GNSS-R system. The SARGO project aims at developing a ground-based GNSS-R system, exploring new algorithms to measure Oceanographic parameters, with particular focus on altimetry. This project will explore the potential use of the upcoming Galileo signals, which have steeper Auto-Correlation Function (ACF) peaks, and will offer lower tracking noise error and more accurate measurements when compared with current GPS signals. Such a system is expected to be competitive in terms of reflection-receiver range resolution when compared with current approaches. SIGNIFICANCE OF WORK In this article the system architecture is presented. The system consists of the receiver instrument and the data processors. The instrument is built around the GNSS receiver developed by DEIMOS and two antennas: one hemispherical antenna for the Line-of-Sight (LOS) signal and another directional high gain antenna for the reflected signal. The GNSS receiver is a Field-Programmable Gate Array (FPGA) based receiver, which has been developed over the past couple years in several activities (the last of these activities was the ENCORE project where an E5 AltBOC capable Galileo receiver prototype was developed [5]). The current receiver version features 200 real correlators that can be rearranged as 25 complex correlators (supporting Galileo E5 AltBOC signals). Using the wideband PCIe interface, the receiver is upgraded to provide correlation outputs in real-time to the processors, which are running in a host computer; precluding, this way, the need for a computational demanding software receiver. The data processors use the information carried in the correlation outputs of both the LOS Signal (used as reference) and the Reflected Signal (distorted due to the reflection), to estimate the receiver-reflection range and, after that, the sea surface level. This article presents also data collected by the instrument. The Instrument’s outputs, which are the correlation values between the incoming signals (LOS and reflected signals) and the local signal, are analysed. The characteristics of the correlation values obtained with the Galileo signals are compared with ones obtained with current GPS L1 signals, highlighting the benefits in terms of range resolution of the Galileo signals. In the end, taking into account the preliminary results obtained by the SARGO project, it is showed the expected benefits of using the new Galileo signals for GNSS-R applications: the steeper ACF peaks enable the estimation of the reflection-receiver range with an higher resolution, and, hence, an accuracy increase of the estimated sea level. REFERENCES [1] Manuel Martin-Neira, “A passive Reflectometry and interferometry system (PARIS): Application to ocean altimetry”, ESA J. Vol 17, pp. 331-355, 1993 [2] Lowe, S., et al: ‘‘First Spaceborne Observation of an Earth-Reflected GPS Signal,’’ Radio Science, Vol. 37, No. 1, 2002. [3] James Garisson, et al.: “Wind Speed Measurement Using Forward Scattered GPS Signals”, IEEE TGRS, Vol 40, No.3, March 2002 [4] Adriano Camp, et al: “New Passive Instruments Developed for Ocean Monitoring at the Remote Sensing Laboratory of Universitat Politecnica de Catalunya”, Sensors, 9, 10171-10189, 2009 [5] Pedro Freire Silva, et al.: “Results of Galileo AltBOC for Precise Positioning”, NAVITEC 2012, December 2012, ESTEC, Noordwijk, The Netherlands.