To determine the location of the GPRS tracker GPS satellites two types of data are required by the GPS receiver: the almanac and the ephemeris. This data is continuously transmitted by the GPRS tracker GPS satellites and your GPRS tracker GPS receiver collects and stores this data.
The almanac contains information about the status of the satellites and approximate orbital information. The GPS tracker GPS receiver uses the almanac to calculate which satellites are currently visible. The almanac is not accurate enough to let the GPS receiver get a fix. If the GPRS tracker GPS receiver is new or has not been used for some time, it may need 15 minutes or so to receive a current almanac. In older GPRS tracker GPS receivers, an almanac is required to acquire the satellites, but many newer models are able to acquire the satellites without waiting for the almanac.
To get a fix, your GPRS tracker GPS receiver requires additional data for each satellite, called the ephemeris. This data gives very precise information about the orbit of each satellite. Your GPS tracker receiver can use the ephemeris data to calculate the location of a satellite with a meter or two. The ephemeris is updated every 2 hours and is usually valid for 4 hours. If your GPRS tracker GPS receiver has been off for a while, it may take up to several minutes to receive the ephemeris data from each satellite, before it can get a fix.
Your GPS tracker GPS will have a screen, like the one on the right, which shows which satellites are in use. The bar graphs show the strength of the satellites that the GPS has acquired. If the bar is hollow, the GPS is still downloading the ephemeris. The circular plot shows the location of the satellites in the sky – the center of the circle is overhead.
This technical note details the anticipated operation of the SiRFstar series of GPRS vehicle tracker, when operating through the next GPRS tracker GPS week number rollover event occurring on April 6th, 2019.
The Global Positioning System (GPS), has its own date and time scale for expressing satellite positions, based on counting weeks, and seconds within a week. To limit the size of the numbers used in the data and calculations, the GPS tracker GPS Week Number is a ten-bit count in the range 0-1023, repeating every 1024 weeks.
Week 0 started at 00:00:00 UTC on Sunday, 6th January 1980, causing the first-week number ‘rollover’ from 1023 to 0 to occur at 23:59:47 UTC on Saturday, 21st August 1999. The next GPRS tracker GPS week rollover occurs on April 06th, 2019.