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GPS modules come with extended ephemeris

Posted: 20 Jan 2010 ?? ?Print Version ?Bookmark and Share

Keywords:GPS? assisted-GPS? extended ephemeris? antenna?

A1084-A/B GPS receiver module

In addition to real-time assisted GPS (A-GPS) techniques, a parallel set of assistance techniques have evolved where, instead of relaying the real information from satellites overhead, a synthetic equivalent is predicted and pre-loaded in the device. This prediction information is generally referred to as 'extended ephemeris'.

GPS receiver and telematics hardware solutions provider Vincotech is providing use of 'extended ephemeris' data for its A1084 and A1035-H range of SiRFstarIII modules.

This was enabled due to cooperation with SiRF, which is now part of CSR. This way GPS modules and GPS antenna modules can calculate a first position after long breaks even under bad conditions much faster than before. Today most GPS applications ask for a short time to first fix (TTFF) after the GPS receiver was switched on. The structural conditions of the GPS system require a new download of the orbital data (ephemeris) of the satellites in case the receiver was switched off for more than 4 hours and is now being switched on again. The time needed to do so is between 18 and 36 seconds under good conditions. Therefore the calculation of a first position takes typically 27 or 28 seconds.

The now available ephemeris data have again a validity of two to four hours only. Therefore one will see the same problem with the next longer off/on cycle again. To overcome a new load of the ephemeris data every time such a cycle has passed 'extended ephemeris' models have been introduced.

In principal these models include an extrapolation of the currently valid ephemeris data based on mathematical methods taking the ellipsoid orbits of the satellites and gravitational effects into account. The validity is extended to several days, the calculation itself is done on separate servers.

A1035-H GPS antenna module

With the support of SiRF, Vincotech can now offer these extended ephemeris data for its current SiRFstarIII-based GPS modules A1084-A/B and A1035-H. Vincotech provides these data on its server for its customers. The customer may access these data once a day and use them for his applications. Examples include telematics unitsdevices mainly based on GSM/GPRS and GPS. They are often used in applications such as fleet management or stolen vehicle recovery.

Vincotech's customer (in this case a service provider) transmits the extended ephemeris data then to the units in the field. There a few commands are enough to feed the data into the receiver and store them. If sent to a low power mode, where only the RTC keeps running and SRAM is maintained, and powered on again afterwards maybe the real ephemeris data are invalid (e.g. after a switch off over night). But the extended ephemeris data can still be used. No new loading is necessary and a first position can be calculated within a few seconds (instead of nearly half a minute). The goal of a short TTFF is achieved.

There is an additional positive effect. As long as ephemeris data are availableno matter if real or 'artificial' onesthe GPS receiver can work with lower signal strength. Therefore a position can be calculated in positions even, where it wouldn't be possible without ephemeris data. This is important for example for units inside a vehicle or generally covered installations.

- Colin Holland

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