Effective SAR image creation using low cost INS/GPS P. Samczynski, M. Malanowski, D. Gromek, A.Gromek, K. Kulpa Institute of Electronic Systems, Warsaw University of Technology, Warsaw, Poland psamczyn@elka.pw.edu.pl J. Krzonkalla, M. Mordzonek, M. Nowakowski Planes and Helicopters Division Air Force Institute of Technology Warsaw, Poland miroslaw.nowakowski@itwl.pl Abstract— In this paper the recent results of Synthetic Aperture Radar (SAR) experiments conducted at the Warsaw University of Technology are presented. In the experiments an SAR radar was mounted on a lightweight airborne platform. The main goal of this experiment was to verify the possibility of obtaining high quality SAR image using navigation data from a low-cost Inertial Navigation System (INS) and classical commercial Global Positioning System (GPS). Keywords— SAR radar, SAR algorithms, INS, Inertial Navigation System, Global Positioning System, GPS, SAR processing I. INTRODUCTION Synthetic Aperture Radar (SAR) technology used for obtaining high resolution imagery has been known for decades [1,2]. Typically SAR systems have been designed as highly specialized sets of hardware and software for professional use. Nowadays, due to advancements in technology, SAR capabilities are available to a wider range of scientists and engineers. The technological advancements which are of importance from the point of view of the SAR technique are in the fields of array antennas, integrated microwave circuits, digital components and navigation systems. In this paper a concept and implementation of a low-cost high resolution SAR system is presented. The system was assembled using commercial off the shelf (COTS) components including communication antennas, connectorised microwave circuits, software defined radio components and portable computers. One of the problems encountered during the design stage was the impact of the parameters of the components on the final SAR image. This refers especially to the Inertial Navigation System (INS)/Global Positioning System (GPS). Efficient SAR image creation requires precise knowledge of the platform motion parameters. To obtain a fully focused image it is necessary to know the platform position during flight with accuracy better than a quarter of lambda – 8 cm for L band, 1.5 cm for C band and 0.8 cm for X band. The knowledge about a platform’s motion can be obtained from a INS/GPS unit which plays an important role in the whole processing chain. As the accuracy of commercial navigation systems are much lower than the presented requirements, the additional step of image focusing based on the received data is required. This paper focuses on the investigation of the impact of using a low-cost and low accuracy INS/GPS on the quality of the final SAR image. The paper is organized as follows: Firstly, the description of the system’s hardware is presented. Next, the signal processing algorithms are described. In the following section the results of the measured data are presented. The paper ends with a conclusion of the findings. II. HARDWARE DESCRIPTION The SAR radar was built using COTS elements. It was composed of two Wi-Fi antennas (one transmitting and one receiving), an AFE (Analog Front-End), two USRPs (Universal Software Radio Peripheral) [3,4], an INS/GPS Platform and a laptop for signal recording and processing. The presented radar works as a classical Frequency Modulated Continuous-Wave (FMCW) radar. A simplified block diagram of the system is presented in Fig. 1. Figure 1. Block diagram of the radar system. The Analog-Front-End (AFE) is the part of the system between the signal generator, the antennas – both transmitter and receiver – and the analogue to digital (A/D) converter. It includes all the parts in the transmitting and receiving channel such as amplifiers, filters, mixers, etc. needed to process the signal. The USRPs are responsible for chirp signal generation (TX). The generated chirp signal is passed via the frequency multiplier to increase the central signal frequency and the bandwidth. After frequency multiplication the signal is amplified and passed to the Tx antenna. Part of the transmitted signal obtained from the directional coupler is mixed with the received signal, producing a beat frequency signal. This signal is digitized and sent to the PC. The SAR FMCW radar can operate in several different modes, allowing a maximum resolution of up to 30cm to be obtained. The parameters of the used in the experiment with wide range swath and lower range resolution are listed in Table I.