Turning a Television Into a GNSS Receiver

C. Fernández-Prades, J. Arribas, P. Closas

Abstract:	This paper describes a combination of hardware and software that allows users working with a GNSS software receiver and associated hardware for less than $50, including the antenna. This constitutes one of the cheapest solutions available today and a game changer for amateurs and practitioners of GNSS software receivers. In 2011, authors presented in [1] an open source GNSS software receiver, written in C++, that is able to work either from raw signal samples stored in a file, or in real-time with a radio-frequency front-end as signal source. While GNSS-SDR developers can focus in the implementation of the different algorithms required by a GNSS receiver (acquisition, code and phase tracking, lock detectors, demodulation of the navigation message, and so on), all the flowgraph and the underlying scheduler is managed by GNU Radio, a well established set of open source, expertly-written libraries and signal processing blocks that undertakes the internals of the whole receiver. GNSS-SDR offers access to a wide range of radio-frequency front-ends. The most popular family of products is called the Universal Software Radio Peripheral (USRP) and the accompanying daughterboards commercialized by Ettus Research. In receiving mode, daughterboards are devices in charge of signal amplification, downconversion and filtering, while the USRP performs analog-to-digital conversion, fine downconversion and decimation, and manages the communication with a host computer via USB or Ethernet buses. For receiving GNSS signals, the cheapest combination is a USRP v1 plus a DBSRX daughterboard. Without the antenna, this equipment is about $1000, that could be an access barrier for students or hobbyists with limited budget. In the recent years, there have appeared other possible solutions at lower costs. In this paper, we report the use of extremely cheap (about $25) DVB-T receivers based on the Realtek RTL2832U chipset, that are sold in form of USB dongle for its use as a television receiver for personal computers. Exploiting an undocumented mode of operation of the demodulator chip, the user is able to obtain raw I&Q samples, stream them through USB to a personal computer and then apply the GNSS software processing, turning the DVB-T receiver into a GNSS receiver and delivering position in real-time. The RTL2832U outputs 8-bit I/Q-samples with a baseband sample-rate up to 3.2 Msps, according to the specifications. However, the highest sample-rate without losing samples that has been tested so far is 2.8 Msps, enough for receiving GPS L1 C/A signals. The frequency range is highly dependent of the built-in tuner; Galileo E1 and GPS L1 links are centered at 1575.42 MHz, a band that is covered by Elonics’ E4000 IC tuner, thus making the use of those dongles suitable for GNSS processing. However, it is well known that the crystal oscillator that ships with those devices exhibits limited accuracy for this particular application. We observed frequency deviations on the order of tens of kHz with respect to nominal values, shifting the Doppler acquisition search margin and making a calibration process unavoidable for a proper usage as a GNSS front-end. In order to address this issue, among others, we expanded GNSS-SDR with a Secure User Plane Location (SUPL) client that obtains estimations of the Doppler shifts via Internet and use that information to calibrate the dongle. SUPL is an IP-based protocol developed by the Open Mobile Alliance (OMA) for Assisted GNSS that quickly provides the software receiver with the ephemeris and almanac of GNSS satellites via IP, instead of decoding the low bit rate of the navigation message. This protocol provides to the user plane, over IP networks, with the same location capabilities that mobile wireless networks provide to user terminals via the control plane. Indeed, the SUPL client turns the software receiver into a SUPL Enabled Terminal able to interrogate SUPL Location Platforms and thus obtaining Assisted GNSS data. In a didactic, tutorial-like style, this paper provides both an in-deep technical description and step-by-step instructions for running the whole receiver, from the hardware setup to getting the PVT solution and displaying it in a 3D geographic browser such as Google Earth. A preliminary proof of concept can be already checked at [2]. This paper expands the topic by providing practical tips for actual use (from electrical aspects such as suitable antennas and how to feed them to the techniques and parameters used by the software to compensate the poor-quality built-in oscillator of the DVB-T receiver), as well as performance measures both with synthetic and real GNSS signals. The novelty relies on an algorithm that obtains Doppler shift estimations from mobile wireless networks’ GNSS assistance data and calibrates the local oscillator. The source code developed to produce this paper will be published under the General Public License (GPL) v3, thus securing practical usability, inspection, and continuous improvement by the research community, allowing the discussion based on tangible code and the analysis of results obtained with real signals. Anyone would be able to reproduce at home the experiments reported in this paper with a bill of materials well below the $100. [1] C. Fernández-Prades, J. Arribas, P. Closas, C. Avilés, L. Esteve, “GNSS-SDR: an open source tool for researchers and developers,” in Proceedings of the ION GNSS Conference 2011, September 19-23, 2011, Portland, Oregon (USA). [2] GNSS-SDR website, http://gnss-sdr.org/documentation/gnss-sdr-operation-realtek-rtl2832u-usb-dongle-dvb-t-receiver (Accessed: February 20th, 2013)
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:	1492 - 1507
Cite this article:	Fernández-Prades, C., Arribas, J., Closas, P., "Turning a Television Into a GNSS Receiver," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 1492-1507.
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