An Insight on Mass Market Receivers Algorithms and their Performance with Galileo OS.

N. Linty, P. Crosta, P.G. Mattos, F. Pisoni

Abstract:	The scope of the work is the development and demonstration of the main GNSS algorithms currently used in mass-market GNSS receivers, by focusing on a GPS and Galileo consumer receiver. Indeed, the global market for commercial GNSS chipsets and devices continue to grow rapidly, especially with the latest development of dual-constellation receivers [1]. By increasing the number of visible satellites, the TTFF can greatly improve in harsh multipath environments as well as the positioning accuracy but at the expense of a greater complexity and power consumption. In the next future, when more constellations are available, the constellations selection may be driven by particular signal characteristics that can result helpful features in terms of improved TTFF, sensitivity and power saving. In particular, the Galileo OS signal shows new features and theoretical potentialities, that this paper aims at quantifying. Thanks to the release of the Galileo ICD in 2010, Galileo capable chips have been developed well in advance and at present there are several consumer devices ready to process Galileo signals with just a firmware update, for example the STM Teseo-2 [2]. Respectively in October 2011 and in October 2012 the four Galileo IOV satellites were successfully launched. The first GPS and Galileo joint solution was computed in December 2011 [3] and in the second quarter of 2013 all the IOV satellites are expected transmitting correct signals with healthy navigation messages, allowing the first Galileo standalone PVT computation. In this time frame, the Teseo-2 evaluation board has been tested with simulated Galileo signals and live IOV signals and the test results used as an early confirmation of the system potentialities. At the same time, an exhaustive survey on existing mass-market signal processing techniques has been carried out; although several patents, held by mass-market producers, describe algorithms for GPS L1 C/A signal, we experienced a lack of documentation concerning Galileo signals processing for consumer devices. For this reason the most promising state-of-the-art algorithms for GPS signals have been analyzed, implemented in a software receiver and extended to E1b and E1c Galileo signals. The performance of these techniques has been verified grabbing both real and simulated Galileo and GPS data. A front-end/bit grabber with a programmable IF frequency, configurable bandwidth (4.2 MHz) and configurable sampling frequency (16.368 MHz) has been exploited. In order to evaluate Galileo OS performance, three main design drivers have been considered: TTFF, sensitivity and power consumption. The TTFF strictly depends on the architecture of the receiver, i.e. to the number of correlators, and to the structure of the navigation message. Some receivers, like the Teseo-2, embed an acquisition engine that can be activated on request and assures a low acquisition time; moreover it implements ephemeris extension [4]. Some results for different C/N0, for hot, warm and cold start, and for different constellation combinations have been computed using the Teseo-2 evaluation board and simulated GNSS data, showing good results for the Galileo signals. On the contrary, other consumer producers exploit a base band configurable processing unit, with thousands of parallel correlators, generating a multi-correlator output, with configurable spacing, depending on the accuracy required [5]. By selecting an appropriate number of correlators, depending on the assistance information available and on the accuracy required, the TTFF varies. These algorithms, along with interpolation techniques, have been extended to the Galileo OS case and implemented and tested in a software receiver. At the same time, since commercial devices are expected to work in low signal power conditions, such as urban canyons or light indoor environments, some tests on the sensitivity have been carried out with a simulated LMS channel [6] and different user dynamics. This issue is usually solved employing longer integration times; in this case the benefits of Galileo are evident, thanks to the presence of the pilot signal. Moreover, according to a literature survey, the open loop solution is usually preferred to closed loop techniques because of its stability when dealing with long integration times, as well as its higher robustness against fading. The final driver of mass-market receivers design is represented by the power consumption, since most of the communication and GNSS chipsets for the consumer market are equipped with limited capacity batteries. It was found out that the main power saving techniques are based on two different receiver states: an active state, in which all the components are activated and a sleep state, where the receiver is not operating at all [7]. Depending on the duration of the active and sleep states and on the operations performed during the states and during the transitions, different performances and power consumption levels can be achieved, with a theoretical power saving up to 98% compared with the full active operation mode. Also power saving techniques have been added to the software receiver and the different configurations have been tested and compared to the standard case (full active state). The theoretical power saving percentages have been confirmed by tests on real and simulated GNSS data. In particular, it has been proved that the Galileo system offers great advantages from this point of view. For example, the presence of the pilot codes allows longer coherent integration times and thus the reduction of the receiver active period. In addition, the different structure of the navigation message lead to improvements in the total average power consumption, since clock and ephemeris related data have a lower repetition rate. The testing activity has been conducted in the ESTEC Navigation Lab and, on-field, using a mobile test-bed vehicle. The analysis of the results spots the performance differences between the same algorithms applied to different constellation signals and provides an early assessment on the suitability of their characteristics, hereby improving the attractiveness of Galileo to the mass-market community. [1] P. Mattos and F. Pisoni, “Multi-constellation - to Receive Everything,” in Proceedings of the 25th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS 2012), Nashville, TN. [2] P. Mattos and F. Pisoni, “Galileo Consumer Receiver – live satellites become available,” abstract submitted to The 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2013), Nashville,TN. [3] G. Falco, A. Favenza, and M. Nicola, “Data decoding of the first Galileo IOV PFM satellite and joint GPS+Galileo positions,” in Localization and GNSS (ICL-GNSS), 2012 International Conference on. IEEE, 2012, pp. 1–7. [4] P. Mattos, “Hotstart Every Time – Compute the Ephemeris on the Mobile,” in Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008). [5] C. Abraham, S. de la Porte, S. Podshivalov, et al., “Method and apparatus for performing signal correlation,” 2007, US Patent 7,190,712. [6] R. Prieto-Cerdeira et al., “Versatile two-state land mobile satellite channel model with first application to DVB-SH analysis,” International Journal of Satellite Communications and Networking, 2010, [7] Z. Jia, S. A. Kurethaya, and C.-S. Wang, “Navigational signal tracking in low power mode,” Dec. 7 2010, US Patent 7,847,726.
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:	2852 - 2861
Cite this article:	Linty, N., Crosta, P., Mattos, P.G., Pisoni, F., "An Insight on Mass Market Receivers Algorithms and their Performance with Galileo OS.," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 2852-2861.
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