Lantronix PNT Series PNT-SG3FS GNSS Module Software User Guide - Page 45

5. Assisted GNSS Password Generation As mentioned in the previous section, in order to access the RxNetworks servers, the user has to provide a set ofparameters which are used in generating the HTTP request. Predictive AGNSS and Real-Time AGNSS uses the same password generator and the same NMEA commands as described in the next subsection. Predictive AGNSS Seed Transmission Before sending the binary seed for each constellation, it must be divided into blocks.The first block for each constellation is 171 bits long. It has to be transferred through the $PSTMSTAGPSSEEDBEGIN command. Just after the $PSTMSTAGPSSEEDBEGIN command, the list of the satellites block types for that constellation has to be sent using the $PSTMSTAGPSBLKTYPE command. In the case of the GLONASS constellation, also the slot frequency channels list must be sent after the $PSTMSTAGPSBLKTYPE command. It can be done using the $PSTMSTAGPSSLOTFRQ command. Then the remaining part of the seed must be divided into 155-byte blocks (called packets) and must be sent usingthe $PSTMSTAGPSSEEDPKT command. All the packets for the constellation specified in the previous $PSTMSTAGPSSEEDBEGIN command must be sent before issuing the $PSTMSTAGPSSEEDBEGIN commandagain for a different constellation. When all the first blocks and all subsequent packets have been sent for all the available constellations, then the $PSTMSTAGPSSEEDPROP command must be issued to signal the end of the seed and to start the propagation. The seed propagation is a time-consuming operation. The propagation status can be checked in the message $PSTMAGPSSTATUS in response to $PSTMGETAGPSSTATUS command; the message $PSTMAGPSSTATUS reports the propagation status. When the returned status field is equal to zero, the propagation is completed. Real-time AGNSS Real-time AGNSS (RT-AGNSS) is able to provide the approximate current time, the ephemerides, the almanacs and optionally the approximate position to the GNSS engine in a timeframe less than the usual time (about 30 seconds) needed to download real ephemeris from the sky. This reduces considerably the time to get fixed especially in critical environments when the ephemeris download time could be very long. RT-AGNSS requires a network connection to download assistance data from the server. Assistance data includes the current time (if not available, for instance, from RTC), the ephemerides, the almanacs and optionally the rough position. Refer to the guidelines reported in the ST Microelectronics Application Note "AN5160: RxNetworks Assisted GNSS Server Interface Specification" to access the RxNetworks service. All the assistance data can be injected into the device backup memory using a few NMEA commands. First, inject the current time into the device (if the device has no RTC, or if it is set to a wrong time). Thiscan be done either using the $PSTMINITTIME command or, if also the approximate position is available, then both current time and position can be injected using the $PSTMINITGPS command. Then the ephemerides can be injected into the device using the $PSTMEPHEM command for each satellite(between two consecutive commands there must be at least a 20-millisecond delay). PNT-SG3FS GNSS Module Software User Guide 45