A NEW SDR GNSS RECEIVER PROTOTYPE FOR REFLECTOMETRY APPLICATIONS: IDEAS AND DESIGN Micaela Troglia Gamba (1) , Sabrina Ugazio (2) , Yuekun Pei (2) , Letizia Lo Presti (2) , Riccardo Notarpietro (2) , Marco Pini (1) , Patrizia Savi (2) , (1) Istituto Superiore Mario Boella, via Pier Carlo Boggio 61, 10138 Torino, Italy. Email: lastname@ismb.it (2) Politecnico di Torino, Electronics Department, corso Duca degli Abruzzi 24, 10129 Torino, Italy. Email: name.lastname@polito.it ABSTRACT In this paper the preliminary design of a Global Navigation Satellite System (GNSS) receiver with scatterometric capabilities is presented. The aim is to exploit the reflected GNSS signals to retrieve information about the soil characteristics. The receiver prototype is specifically designed to be small and lightweight, to be mounted on board an Unmanned Aerial Vehicle (UAV). The presented prototype, with respect to the existing devices, is characterized by the use of low-cost equipment, enabling, at the same time, a flexible and reconfigurable solution thanks to the use of a Software Defined Radio (SDR) technology. This paper illustrates the overall system, the proposed approach and the development status of the work. In particular, the hardware platform is described. Then, the signal-processing techniques that can be consequently adopted and the methods to retrieve the soil characteristics are outlined too. Furthermore, some preliminary results obtained during a preparatory experimental campaign in static conditions are provided. 1. INTRODUCTION A GNSS-scatterometer is a device that measures the GNSS signal reflected by the soil, using an antenna system pointing towards the ground. In [1], a general overview on GNSS reflectometry is given. GNSS signals have been used as opportunistic bistatic radar for remote sensing of the Earth’s surface. The wide range of applications, all based on processing of the reflected signal, has been deeply exploited in literature: from altimetry [2][3], water basins detection [4] and wind retrieval [5] to monitor the presence of vegetation [6], ice/snow thickness [7], surface roughness [8], soil moisture and ocean wave height [9]. Currently, the soil moisture retrieval, which consists in measuring the soil dielectric constant (or relative permittivity), is becoming an increasingly popular application, as outlined in [10] or [11], and it is exactly the application on which our attention is focused. The traditional hardware for GNSS bistatic radar presented in literature in the past [3][8] consists of custom receivers, based on Application Specific Integrated Circuit (ASIC) or Field Programmable Gate Array (FPGA) and requiring a portable pc to operate. In [2], an OpenGPS receiver for altimetry purposes is described: it is made up of a 12 channel Coarse Acquisition (C/A) code baseband hardware correlator, the GP2021 chipset [12], controlled by a standard PC. The main drawback of such an approach is that the receiver becomes heavy and bulky and cannot be mounted on smaller platforms, like Unmanned Aerial Vehicles (UAVs) or Remote Control (RC) airplanes. To allow for integration on small platforms, the instrument needs to be redesigned with size and power constraints. In [13][14], as instance, a miniaturized receiver is proposed. The design is composed by two GPS L1 front-ends with a common clock, connected to a USB bridge for high-speed data transfer. Instead of using a laptop, a Nano-ITX Single Board Computer (SBC) is used and the collected samples are stored in a memory for post-processing. In [15] an FPGA-based real-time GPS reflectometer is designed, which computes in real- time the full two-dimensional Delay Doppler Maps (DDM) with update rate of 1 ms and performs the coherent and incoherent averaging. In this paper, we present the design of a prototype of a GNSS-scatterometer, light and compact, employing the SDR paradigm, which offers much more flexibility than FPGA or ASIC-based solutions [2][15]. It is specifically conceived to be small and lightweight, to be mounted on board of a UAV, using low-cost equipment. It enables a flexible and reconfigurable solution thanks to the use of the SDR technology. With respect to the work in [13][14], the aim is not only to store the raw samples, but implement advanced signal processing on board, to save the signal correlation results and reduce the size of data to be saved on memory. In its preliminary version, the prototype will process only the GPS L1 signal. The main scope of this instrument is to monitor soil moisture: therefore subsequently we will refer to the device as GPS-scatterometer. The paper is organized as follows. After this Introduction, there are seven main sections. Section 2 recalls the general device structure and requirements. Section 3 describes the hardware platform, while the signal-processing solutions that could be implemented