Characterization of GPS Signals in Urban Environments Using Deeply Integrated GPS/IMU

This research evaluates the quality of GPS signals and their usability for localization in urban environments. GPS signals collected on a Software Defined Radio (SDR) platform in urban canyons in Columbus, Ohio (USA) are processed using a deeply integrated GPS/INS scheme. This architecture allows for coherent signal integration with continuous carrier phase tracking for intervals as long as one second. Results characterize the received signals in terms of signal strength, tracking continuity and multipath influence on signal tracking performance. Results show that signals from 5 to 6 Space Vehicles (SVs) are available for processing, even in dense urban canyons. Deep GPS/INS integration enables continuous carrier phase tracking, thus allowing for accuracies in stationary tests on the cm level in integrated velocity and on the 1 – 2 meter level in relative position. Consistent carrier phase tracking is demonstrated for two or more SVs simultaneously even when all satellite Line of Sight (LOS) vectors are blocked by buildings and Carrier-to-Noise Ratios (CNRs) are as low as 12 dB-Hz. Finally, a photographic method is introduced for visualizing a GPS receiver’s “eye view” in an urban canyon. 1.0 INTRODUCTION Urban environments pose some of the most severe challenges known to developers of Global Navigation Satellite Systems (GNSSs). In the presence of tall and densely packed buildings that block and reflect satellite signals, conventional sequential tracking receiver architectures typically display suboptimal performance. This research attempts to circumvent these limitations by demonstrating an entirely different receiver architecture. In previous efforts at Ohio University, a real time, Software-Defined Radio (SDR) based, batch processing GPS receiver architecture was developed and tested [1-3]. This architecture was extended to include a lowcost Inertial Measurement Unit (IMU) to perform GPS/IMU deep integration, in order to enable the system to process signals with Carrier-to-Noise Ratios (CNRs) down to 15 dB-Hz [4, 5]. The deep integration mode developed provides consistent carrier phase tracking without requiring the knowledge of navigation data bits. Consistent carrier phase tracking at a 15 dB-Hz level was previously demonstrated in real flight environments [6]. The first characterizations of the availability and quality of GPS signals in urban environments using deeply integrated GPS/IMU were presented in [7]; those results are presented again below and are extended to include more recent analyses. GPS Signals in Urban Environments via Deeply Integrated GPS/IMU 17 2 SET-104/RSY21 2.0 DATA ACQUISITION ARCHITECTURE AND PROCESSING The data acquisition system architecture is illustrated in Figures 1 and 2. The system was installed in a Ford Econoline 350 cargo van with roof racks, as shown in Figure 1. Although only one GPS channel was needed for deep integration processing, two channels were available and provided redundancy. GPS receiver(s) for comparison Controller for laser sensor Software radio RF components Software radio digital components Inertial Measurement Unit and circuitry GPS antennas Laser sensor For batch processing For sequential processing For potential augmentation to GPS Figure 1: Photographs of Van-Mounted Data Acquisition System The simplified system block diagram in Figure 2 shows the general data acquisition structure. Two GPS patch antennas, one in the L1 frequency band and the other an L1/L2 active antenna, were present. The first of these antennas provided the signals for deep integration GPS/IMU processing. A SiRF StarIII receiver with its own patch antenna (stated incorrectly in [7] as split from the L1 antenna) was used for comparison. This receiver took the place of the 2 NovAtel OEM-4 receivers shown in the original configuration in Figure 1.