Efficient Link-Layer Coding for GNSS Navigation Data Messages

In recent years, GNSS systems like GPS and Galileo have played a growingly important role in every-day life, testified by the ever increasing diffusion of handsets (smartphones, tablets, etc.) equipped with GNSS receivers to provide for location-based services. Despite that, the land mobile satellite (LMS) channel is often subject, especially in urban environments, to shadowing events, whose duration is too long to be counteracted by means of forward error correction (FEC) at the physical-layer. Link-layer coding (LLC) represents a possible countermeasure to avoid service breakdown. Current GNSS systems rely on a primitive way of LLC for certain types of data, i.e., ephemeris, almanac, long term ephemeris (LTE), in the form of carousel transmission [1]. Carousel transmission is optimal when the erasure probability is negligible, since it allows receiving the whole information word in the shortest possible time, no matter when the receiver starts receiving. However, it becomes largely suboptimal when erasure probability is significant, because the receiver must wait for an entire carousel length in order to be able to recover even a single lost page. An application to DVB-S2 satellite services of LLC techniques can be found in [2], where a Low-Density Parity-Check (LDPC) code is designed to cope with the typical disturbances of the railway environment. In [3], instead, a simple LLC scheme is applied to the downlink message of Galileo. In this paper, we investigate the design of LLC schemes for GNSS messages like the almanac and LTE data, which have the following two common properties: i) the carousel duration is large enough to leave room for an improvement in terms of time to retrieval (TTR) through suitable coding; ii) they are transmitted by multiple satellites.