Characterization of Time to First Fix for Standalone and Aided GNSS Multi-constellation Receivers in Challenging Environments

T. Ferreira, F. Pelica, R.Sarnadas, L. Bonardi, P. Crosta, R. Prieto-Cerdeira

Abstract:	The Time To First Fix (TTFF) is a performance indicator often used by GNSS receiver manufacturers. The TTFF accounts for the time elapsed from the receiver switch-on until the provision of a navigation solution within a certain performance (e.g. depending on acceptable accuracy degradation). Historically, discussions on improved TTFF arise from mass market receivers in urban environments since professional receivers were mostly focused on high accuracy (they are traditionally used in benign environments such as open sky or rural), and therefore TTFF was often not a driver for manufacturers. The proliferation of mass market receivers and their use in suburban/ urban environments brought the GNSS receiver community attention to availability. TTFF is a fundamental metric in this context. These discussions led to the development of the concept of Assisted GNSS (AGNSS), where the receiver relies on external data (e.g. using terrestrial networks) to greatly reduce TTFF, mainly avoiding cold start conditions where no a-priori information on the visible satellites is available at the receiver. Today, reducing TTFF for mass market receivers operating in urban canyons can be the difference between providing a reasonable solution and not providing a solution at all. Furthermore, the AGNSS concept together with the advent of new GNSS constellations and improved GNSS navigation messages extended the availability discussions to the high-end market as well. The multi-constellation situation is expected to boost availability in challenging environments and as such to improve TTFF. Most of the theoretical work published regarding TTFF is focused on a single-satellite situation in perfect environment and other works use hardware simulators to measure TTFF in an AWGN environment. In addition, it is not always clear how receiver manufacturers actual measure TTFF (e.g. assumptions on the acceptable accuracy degradation or available data). As such, this work focuses on assessing TTFF performance for GNSS multi-constellation (multi-satellite) receivers in challenging urban environments. One of the main drivers for the TTFF performance indicator is related to the information available a-priori at the receiver. As an example, if valid ephemerides are already available at the receiver (or provided via other means following the AGNSS concept) then the receiver only has to decode information on system time from the navigation message, greatly speeding up the time to read the relevant parameters from the message, hence TTFF. On the other hand, many applications need to work in standalone mode in challenging environments, such as in places where terrestrial networks are not available either by design or in emergency situations. In these situations, TTFF in cold start conditions is a fundamental performance indicator, as it is also linked to receiver resource consumption. Therefore this work is also extended to cover both the assisted and the standalone approaches in what concerns the GNSS multi-constellation receivers in challenging environments. The main drivers for TTFF are: - Navigation message: e.g. the transmission rate of the most relevant parameters such as ephemerides for cold start conditions or system time information for warm start conditions. - Receiver conditions: in challenging environments such as urban/ suburban environments, multipath, shadowing and even ionospheric scintillations can cause signal fading that can ultimately lead to signal loss and unavailability of the navigation message. - Receiver architecture: not only related to the acquisition process (e.g. the strategy that the receiver uses to search a given satellite from all possible Doppler and code delay possibilities) but also to the strategy used to select visible satellites. The main improvement could come from a multi-constellation strategy, where the receiver uses as many visible satellites as possible to compute a navigation solution, which is the aspect covered herein. In the coming months, TTFF for both standalone and assisted GNSS approaches will be extensively assessed via simulations for multi-constellation receivers in challenging environments. The simulations are based on a TTFF simulator that has been developed following a statistical paradigm wherein: - TTFF is computed based on the contributions that come from receiver warm-up time, acquisition process, tracking convergence, navigation message decoding time and navigation solution convergence. The Cumulative Distribution Function (CDF) for TTFF is provided based on the contribution from different satellites and therefore TTFF can be assessed in a statistical sense (e.g. @ 95%); - Standalone versus assisted GNSS approaches: they are simulated by adjusting the starting conditions (cold/ warm) of the visible satellites. Please note that these conditions are set per satellite and therefore it is possible to simulate situations where information is only available for a partial set of satellites; - Multi-Constellation: the simulator covers all (public) navigation messages for GPS, Galileo, COMPASS and GLONASS as per their respective SIS ICDs. The simulator considers satellites from different constellations to derive the CDF for the global TTFF (accounting for the additional variables to solve offsets between system times) - Challenging environments: based on statistical channel models (producing signal time series due to fadings and adjusted from real measurements), the simulator computes probabilities of signal blackouts (depending on satellites elevation angles, environment at hand and user velocity) that will lead to penalty times on both the acquisition time and the time to read the navigation message. ACKNOWLEDGEMENTS The content of the present article reflects solely the authors’ view and by no means represents the official ESA or GALILEO views.
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:	3295 - 3304
Cite this article:	Ferreira, T., Pelica, F., R.Sarnadas,, Bonardi, L., Crosta, P., Prieto-Cerdeira, R., "Characterization of Time to First Fix for Standalone and Aided GNSS Multi-constellation Receivers in Challenging Environments," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 3295-3304.
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