GPS navigation device manufacturers know that a quick time to first fix (TTFF) after a GPS receiver is switched on is a selling point with consumers. Running predictive calculations of satellite positions is one way to speed up the time taken to establish a lock on a few GPS satellites so that navigation can begin. It works by giving the restarted GPS device a head start by knowing where to start looking for GPS satellites. Time saved with this technique makes possible TTFFs of as little as 3.5 to 10 seconds.
GPS Satellite Constellation
Without predictive data, obtaining satellite locks involves downloading the ephemeris (satellite orbital position data) from the GPS satellites in view which takes 30 seconds with good signal level (>28 dB-Hz eg. not in city center area). In a poor GPS signal area the start-up time can be reduced by downloading assistance data over a cellular or wi-fi link (A-GPS) if available. However, this takes longer than if predictive data is used.
Garmin refers to the use of predicted or synthetic ephemeris data as Hotfix while TomTom uses the term QuickGPSfix. The strategy of creating predicted ephemeris data is also sometimes referred to as offline A-GPS.
Ephemeris data obtained has a use by time of only 4 hours after which the process must be repeated. With the robust computing power of modern GPS personal navigation devices (PND), it is possible to calculate predictive ephemeris data which is accurate enough to be used for up to 10 days from the last live data obtained. The extended Ephemeris is a mathematical extrapolation of current valid Ephemeris data taking into account extensive factors such as; the ellipsoid orbits of the satellites, gravitational effects and Earth’s oblate spheroid (fat around equator) shape.
If extended or synthetic ephemeris data is used for a PND start-up, the device will automatically switch to freshly downloaded and more accurate ephemeris data when it is available. A new extended ephemeris data file will also be calculated as a background process. A quick start is then available for up to another ten days.
Categories: GPS Technology