Lantronix PNT Series PNT-SG3FS GNSS Module Software User Guide - Page 43

5. Assisted GNSS 5. Assisted GNSS The PNT-SG3FS receiver needs accurate satellite position data from at least 4 satellites to produce a position fix. Accurate satellite data -ephemeris data- is valid for 4 hours only for GPS and 30 minutes only for GLONASS. After that time the PNT-SG3FS must download new ephemeris data. Ephemeris download can take from dozens of seconds to several minutes, hours or can fail. Assisted-GNSS is a mechanism to provide ephemeris assistance from external source, this considerably reduces the time to get a fix especially in critical environments when the ephemeris download time could be very long. The PNT-SG3FS supports three types of Assisted GNSS: ♦ Autonomous AGNSS using locally predicted ephemerides (up to 5 days of prediction) ♦ Predictive AGNSS using a networked assistance server (up to 14 days of prediction) ♦ Real-time AGNSS using a networked assistance server for a fast time to first fix (TTFF) Autonomous AGNSS Autonomous AGNSS, which uses the ST-AGNSS NMEA Interface defined in Table 11, is able to provide predicted ephemerides to the PNT-SG3FS in a timeframe less than the usual time (about 30 seconds) needed to download real ephemeris from the sky. This considerably reduces the time to get a fix especially in critical environments when the ephemeris download time could be very long. The Autonomous AGNSS solution works using the past real ephemeris (downloaded from the sky and stored in its internal database) to extrapolate the parameter of future ephemeris (up to 5 days of prediction). The autonomous performance (in terms of position accuracy using predicted ephemeris) is strictly dependent on the real ephemeris database content. The accuracy performance of Autonomous AGNSS (and Predictive AGNSS) is expected to decline over the predicted period. In normal usage of Autonomous AGNSS, the system automatically uploads the real ephemeris into its database as soon as new ephemerides are downloaded from the sky. This means that the global content of the real ephemeris input database is determined by the history of device running periods in the past. The module must operate in full power mode for at least 5 minutes to collect ephemeris data from visible satellites. Autonomous GNSS can take up to 12 hours to collect ephemeris data for a full constellation. The Autonomous AGNSS subsystem supports the following NMEA interfaces with the Host. For best performance, increase the CPU frequency when the STAGPS engine is enabled. PNT-SG3FS GNSS Module Software User Guide 43