A Software Defined Radio Solution for Hybrid GPS/Signals of Opportunity Navigation

M.B. Mathews, P.F. MacDoran

Abstract:	Motivated by the need for increased portability and flexibility in RF communication systems, software defined radio (SDR) is finding increasing acceptance and use in a wide variety of applications. In particular, SDR provides significant advantages when dealing with signals of opportunity (SoOps) and the growing positioning services within GPS and other Global Navigation Satellite Systems (GNSS). Modern navigation receivers are becoming vastly more complex given the various types of signals and RF bands supported. SDR is a natural solution for addressing this complexity, making it possible to create receivers that can be dynamically configured to support a variety of navigation applications. Further, in supporting sensor fusion applications for GPS/GNSS challenged environments, SDR provides the flexibility to efficiently transition to becoming SoOps based. This paper introduces Loctronix Advanced Software Radio™ (ASR) platform comprising SDR waveforms and hardware for providing positioning, navigation, and timing (PNT) in GPS/GNSS challenged environments. Designed with platform portability in mind, the ASR provides the Waveform Component Architecture (WCA) framework, which abstracts hardware implementation both in software and programmable logic. The WCA is designed to support migration to other SDR standards including the Software Component Architecture (SCA) and the Software Telecommunication Radio System (STRS), while providing a lightweight implementation framework simplifying waveform development and testing. The paper discusses the WCA framework and ASR components including a multichannel transceiver module ASR-2300 and hybrid GPS/SoOp navigation waveform. The ASR-2300 combines two broadband RF transceivers with a high performance FPGA enabling simultaneous RF transmission and reception in the region between 300 MHz and 3.8 GHz with up to 28 MHz of bandwidth per channel. The device is a portable, low size weight and power (SWaP) module providing USB 3.0 interface with up to 310 MB per second data I/O with a general-purpose CPU. In addition, the module provides inertial, geomagnetic, and barometric sensors providing the core sensors needed to implement SoOp-based sensor fusion navigation. This attribute is especially important in situations of jammed / spoofed GNSS signals. Using the ASR-2300, test results of a hybrid GPS/SoOp waveform are presented demonstrating the capabilities and performance of the ASR components in various operating scenarios. The hybrid waveform combines conventional GPS signal processing methods with Loctronix proprietary algorithms to simultaneously process both the C/A and P(Y) channels of GPS as well as SoOps including DTV, cellular, and potentially other S band signals. The hybrid waveform is being developed as a core implementation for applications in space navigation, robust surface navigation, unmanned aerial systems navigation, and detection and geolocation of GPS/GNSS interference/spoofing sources. Test results will be presented showing the use of the hybrid waveform in both space navigation and indoor positioning applications. Loctronix ASR waveforms are based on patented Spectral Compression Positioning™ (SCP) and Doppler Aided Inertial Navigation™ (DAIN) methods developed for navigating in GPS/GNSS challenged environments. SCP is a novel technique for processing intercepted signals that enable extraction of PNT observables without requiring explicit demodulation of information content. A derivative of codeless GPS developed originally at Caltech JPL, SCP can be used with virtually any SoOp producing one or more useful observables for PNT as well as extracting useful physical characteristics of signals in the band of interest. DAIN utilizes SCP Doppler observables to determine the average speed of a mobile sensor. The Doppler observable is based, in part, on the detected changes in the multipath environment, while a device moves through an environment with significant multipath reflections. This measurement quality, which can be better than 5 cm/sec over 15 second intervals, provides the constraint necessary to calibrate inertial sensor drift as well as provide critical observations to estimate the motion profile (e.g. flight speed, step size, and wheel rotations). In addition to discussing current test results and development of specialized waveforms using the ASR platform, the paper also discusses the migration strategy for adapting ASR limitations to support other SDR standards including SCA and STRS. An example is presented from ongoing work in adapting one waveform for a space navigation receiver compliant with STRS for use with SpaceCube satellites. The ASR provides a core foundation for developing SDR waveforms supporting a variety of PNT applications. The inherent advantages of this technology lies in the flexibility it provides for working with SoOps and multiple GNSS signal sources. The low SWaP characteristics of the ASR-2300 makes rapid prototyping and field testing of new waveforms a possibility not easily achieved with other SDR’s currently available. The ASR platform serves as a starting point for developing new waveforms based on existing components implementing both conventional GPS/GNSS methods and SCP / DAIN methods to provide a unique solution set and performance capabilities not readily available by other approaches. Currently, the ASR platform is prepared for commercial launch in mid-2013, with initial waveforms supporting hybrid GPS/SoOps navigation, indoor navigation, and GPS signal assurance and interference detection. As part of the initial offering, GPS/SoOps signal generation, data recording waveforms, and connectors for integrating with GNU radio and other commercially available signal processing tools will be provided.
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:	1539 - 1548
Cite this article:	Mathews, M.B., MacDoran, P.F., "A Software Defined Radio Solution for Hybrid GPS/Signals of Opportunity Navigation," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 1539-1548.
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