SparkFun Forums

[teekay_tk]

hello

can some one tell me when $PSRF104 nmea input message is used. can i complelty switch of the gps for 3-4 hours and then send this command and expect a quick fix?

also where can i find more info about this commands execution and internal working , sirf nmea manul info is very limited

regards

[ckuethe]

You'd use this if the stored fix parameters were invalid. Moving 500km since the last fix, or if the internal clock was wrong, or if it had been a long time since your last fix. The idea is that you are offering your GPS some hints about where to start looking.

If you can give a reasonable hint to what time it is, the receiver will get code lock much sooner than if it has to do a cold start - it knows what code words it should be looking for.

If you can give it an initial position, that helps to bootstrap the navigation solution, and also helps with initial acquisition by limiting the satellites sought to the ones that could actually be visible from your location.

By allowing you to force the ephemeris and almanac to be cleared, you can prevent the receiver from using stale data. After 3 or 4 hours, your ephemeris should still be valid, but your clock might have drifted; you'd be doing either a hot or a warm start. Probably under 10 seconds for a hot start if you have good skyview.

Have you tried testing the effects of sending good data (ie. the last good fix) as well as sending bad data (like time in the past/future or a postion on the other side of the world?) I think you would find that very educational.

[teekay_tk]

thanks for your answer.
my basic attempt was to see if gps will work after being turned of for 3-4 hours. it seems that it will not. i tried many testes where i turned of the gps form 10secs to 4hours time intervals and then tried to get a lock.
it seems that after 1-2 hours the astellites that were in view last time are no longer in the sky and a warm start of 45 seconds is nessasary. which i wanted to avoid. bascilly i was getting less than 7 seconds on hot starts. if i switch on the gps after a sleep of 1 hour.

[des3dhj]

Am having a few difficulties with the ET-301. Put simply, I can make it do everything documented except.... tell me where it is and process the PSRF104,PSRF101 commands. With debug info enabled it reports no errors , but refuses to use the sent parameters. r.e. location data, the antenna has a great view of the sky and the device is reporting correct time and week values. Any thoughts appreciated.