Reduced navigation data for a fast first fix

Every GNSS user is interested in having high accuracy PVT solutions. However, for most mass market applications like location based services or road navigation, having a first position fix within few seconds is more important than waiting some time for an accurate solution. One of the biggest contributions to the TTFF is the time needed to retrieve the navigation data coming with the data message. The concept presented in this paper aims at improving the TTFF performance of navigation receivers by defining a set of clock and ephemeris data (CED) with reduced size. This newly defined message types could be added into the transmission schemes of today's and future GNSS to allow for higher CED repetition rates and thus shorter TTFF. Like other parameters, satellite ephemeris and clock corrections are delivered to the users in binary form. The reduction of the size of these data is therefore corresponding to a reduction of the accuracy with which the parameters describing the satellite's orbit and clock drift are represented and delivered to the users. The idea behind the concept of transmitting reduced CED is that mass market users might tolerate an initial degraded accuracy to have the possibility of a faster first position fix. In order to simplify the generation of this new set of data, rather than computing new parameters from scratch, a very straightforward approach is proposed. The algorithm should take the broadcast CED that are already available in the satellites and obtain the new parameters by a simple reduction of the number of bits assigned to them. The concept has been implemented with an experimental design based on the current data of the GPS L1 C/A code signal, aiming at minimizing the number of bits while not exceeding a horizontal position error of 100 m. After achieving a volume reduction of about 52%, the compliance to the requirement was verified by applying the reduction algorithm to the CED issues broadcast by GPS during the last 10 years, and estimating the horizontal user position error for each of them over the four hours of their validity interval. Some application possibilities for the proposed concept are mentioned, like the inclusion of this data into the transmission pattern of todays and future GNSS signals, increasing the CED repetition rates. In particular the suitability for a new kind of acquisition aiding signal has been underlined. Concerning future work on this concept, there are already plans for a detailed design, tailored to specific GNSS messages, like the Galileo I/NAV. Moreover advanced optimization techniques for the implementation of the reduction algorithm will be investigated.