[Ovmsdev] How to get a quick and reliable GPS fix

[Michael Balzer]

Hi Arthur,

the SIM908 hardware design doc says:

    /Power down mode: The GPS engine will be set into this mode by
    sending AT command “AT+CGPSPWR=0”. In this mode the internal power
    supply for GPS will be shutdown, and the current consumption is very
    low. The last position, current time and ephemeris data will be
    stored in the GSM host memory./

...and...

    /User can power on GPS engine by sending AT command “AT+CGPSPWR=1”. /

So it seems a GPS reset is not necessary after a software power down / 
sleep state, I'd first try without.

If you find out you need to do a reset, I'd first try to cope with "hot" 
/ "warm". The command manual says:

    /COLD start mode is recommended For first time reset./

So I think this is normally only needed for the first power up.

Regards,
Michael



Am 04.11.2014 um 17:42 schrieb Arthur Hebert:
> Hi All,
>
> Does anyone know the details of how, most reliably and most quickly, 
> to get a location fix from the GPS module after it has been powered up 
> and after it has come out of sleep mode?
>
> I have written my own code using the OVMS hardware. To save power, it 
> turns off the GPS power (AT+CGPSPWR=0) and puts the module to sleep 
> (AT+CSCLK=2) while the vehicle is parked (i.e. no CAN bus activity). 
> Once there is CAN activity again, it comes out of sleep mode 
> (AT+CSCLK=0) and turns on the GPS (AT+CGSPPWR=1). Immediately after 
> turning on the GPS power, I also issue one of the GPS reset commands 
> (AT+CGPSRS=0 or AT+CGPSRST=1).
>
> The module documentation if vague about when to use the 0 reset or the 
> 1 reset.
>
> I know that the GPS module itself isn't made to function while the 
> module is asleep even though the commands may not give an error.
>
> I've read that it takes up to 15 minutes to get a fix on most GPS 
> devices. In practice, sometimes I get a fix within a minute every time 
> I start up, and other times I don't get a fix even after hours of 
> trying with either reset option.
>
> Any information will be greatly appreciated.
>
> Thanks,
> Arthur
>
>
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-- 
Michael Balzer * Paradestr. 8 * D-42107 Wuppertal
Fon 0202 / 272 2201 * Handy 0176 / 206 989 26

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