On the Design of a GNSS Acquisition Aiding Signal

M. Paonni, M. Bavaro

Abstract:	The structure of current GNSS open service signals is the result of a design trade-off between several performance figures of merit. The most relevant are the position accuracy, the sensitivity and the time to first fix (TTFF). The performance of the signal is generally a compromise within those figures of merit. However, if one concentrates the attention to the signal acquisition phase, the last two assume a much higher relevance. Also, the design of the navigation message has to take into account many needs, and the information transmitted has to serve different services and/or class of users. Again, of all the information present in a typical navigation message, only a portion is relevant to reach the first position fix in a reasonable time. The main motivation of this work is to propose a design of an acquisition aiding signal and to discuss the benefits of adding such a signal to the current GNSS baselines. In order to reach this scope, first the performance of today’s satellite navigation open service signals are discussed, focusing the attention on Galileo and GPS, pointing the light on those performance figures that are specific of the acquisition and first position fix. In particular it is discussed how much the TTFF and the sensitivity performance are strictly interrelated and how the key for the design of an acquisition aiding signal has to be identified right in the trade-off within the two. In order to analyze the performance in terms of TTFF a very comprehensive theoretical model is here used. The employed model is an extension of the one originally presented in [Paonni et al., Performance Assessment of GNSS Signals in terms of Time to First Fix for Cold, Warm and Hot Start, Proceedings of the ION-ITM 2010, San Diego, USA, 2010] and can be potentially applied to any GNSS signal. The original model has been extended in order to cover more scenarios and to better represents the realistic and diversified conditions under which the first position fix can take place. After the essential performance analysis phase, the specifications that have to be satisfied by the design are identified, in order to improve the analyzed figures of merit and obtain the desired performance. A fundamental point that needs to be tackled in this regard is how the newly introduced signal should assist the others already present in the baseline, discussing therefore the necessary synchronization requirements that the acquisition aiding signal has to satisfy. Following the identified specifications and requirements, a design is proposed. In order to provide an exhaustive discussion on the signal design, the different elements of the signal are analyzed and discussed and specific solutions for each of them are introduced. The investigation starts from the spreading modulation and is followed by design of PRN codes and of the navigation message. The aspects related to the message coding are discussed as well. Another essential problem is how the acquisition aiding signal can be transmitted with the legacy signal baseline. Also the issue of multiplexing is therefore discussed and analyzed. The design is realized by means of a trade-off of the identified specifications and solutions. When choosing the spreading modulation one of the funding criteria is the spectral efficiency. The current GNSS are already transmitting quite complex signal baselines, made of various components. An essential requirement that has to be fulfilled by the new signal under discussion is a limited impact in terms of interference to the other signals of the same system (intra-system) and to the ones of other systems (inter-system) transmitted in the same band. Commonly used and widely accepted compatibility criteria are employed in order to weight the various possible spreading modulation candidates and identify the one that best fits with the identified requirements having at the same time the highest spectral efficiency possible. Another factor that plays a fundamental role in the choice of the spreading modulation is the correlation function shape. This must aid the acquisition process and minimize the risk of false locks. With respect to the design of the navigation message and its coding various innovative concepts are used. First, the message design is performed by keeping the message as short and simple as possible. A fundamental criterion that is here identified is the one of minimum coarse accuracy. Many classes of users could be ready to renounce to some degrees of (position) accuracy for a limited time (some seconds) obtaining on the other side a faster position fix. Following this fundamental concept the idea of reduced ephemeris presented in [Anghileri et al., Reduced Navigation Data for a Fast First-Fix, Proceedings of NAVITEC 2012, The Netherlands, 2012] is applied to the design of the acquisition aiding message. Moreover, some parts of current navigation messages are not relevant to the first position fix. The so called First-Fix-Data (FFD) comprises the system time reference and the satellite ephemeris and clock correction parameters. The design of the acquisition aiding message is therefore having those elements as fundamental bricks. Once the full message design is complete, different forward error correction techniques are evaluated to be employed and one of the basic criteria that is used to evaluate them is again the sensitivity (here in terms of data demodulation) that they offer. Current state-of-the-art channel coding techniques that have been developed in the context of digital satellite and wireless communications are considered in order to assess their applicability to protect the data message providing the highest possible sensitivity. Another aspect that is tackled is the design of the secondary (or overlay) code. The secondary code should serve two scopes at the same time. First, depending upon the design of the signal and of the message, a secondary code may be required for the bit synchronization that is needed before demodulating the data. At the same time the idea is to make use of the secondary code to ease the hand-over from the acquisition aiding signal to the other signals of the baseline that this is aiding. The paper is closed with a comprehensive performance analysis of the newly defined signal acting together with the baseline signals. In particular the improved performance in terms of sensitivity and first position fix is highlighted.
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:	1445 - 1456
Cite this article:	Paonni, M., Bavaro, M., "On the Design of a GNSS Acquisition Aiding Signal," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 1445-1456.
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