FRANCESCA ZANIER AND STEFANO CIONI, ESA/ESTEC
The time-to-first-fix (TTFF) is the time that a GNSS receiver takes to make available for the first time from switch-on an indication of the user position. Smaller TTFFs have become a premium performance metrics for any receiver, particularly as smartphones, tablets and location-based services have expanded so rapidly.
The TTFF depends on a number of factors, some specific to receiver design, others depending on the particular conditions the receiver operates in (urban canyon, indoors, foliage, etc.). Still others depend on GNSS system design. Concentrating on the last factor, when GNSS is the only means to obtain user position, reliable and fast decoding of the navigation data message is mandatory. Robustification of the navigation message is usually achieved through coding/interleaving solutions at the physical layer, but the particularly harsh environments just mentioned may require more.