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"clock" Function In Gpsr's


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I have noticed over many years of selling and using GPSr's that almost all unit's clocks become very inaccurate when the unit hasn't been allowed to locate and reset it's clock from the satelite data for some time.

I have noticed this phenomen with several units that haven't been turned on for just a few days. When they're turned on somewhere where they can't receive GPS signals, such as deep inside a building, the time shown can be off by considerable amounts.

Do GPSr's not have at least a cheap quartz clock chip that keeps fairly decent time with the unit turned off? I thought super accurate time was what GPS was centered around anyway?

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I've never even looked at the "clock" in these things! I know this much...My GPS gets me where I'm going with superb accuracy, and the timex on my wrist shows me the time with superb accuracy, and beyond that, I've never even considered one doing the others job. Why the loss or gain of time with no satellite lock? I couldn't guess...

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Do GPSr's not have at least a cheap quartz clock chip that keeps fairly decent time with the unit turned off? I thought super accurate time was what GPS was centered around anyway?

They do have a cheap quartz crystal clock inside, but its function is just so the unit will know which satellites to look for when it's turned on again. It can perform that function even if it's off by a few minutes so there's no need for it to be particularly well calibrated.

 

Once the unit is receiving signals from enough satellites (at least 3 and preferably 4 or more) it can derive the precise time (and position).

Edited by peter
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There's no brand or model mentioned so far (which is rarely a good idea in this forum) but it's worth a note that every one of hte Magellan handhelds released this century has a clock that will wander very badly - despite having access to some of hte highest precision timekeeping devices we've ever built - and that drift can be only corrected by a full 'clear all memory' on the devices.

 

So Peter's answer is (as usual for him) correct - it can derive the preicse time; it just may not. :-)

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There's no brand or model mentioned so far...

 

I was wondering about that too. In older Garmins the clock circuit monitored the temperature of the quartz oscillator and corrected for frequency drifts due to temperature. So they should be pretty darn accurate (at least once the GPS has been in use for a few days and the self-calibration circuitry has figured out the exact behavior of the quartz). A solid time reference speeds up acquisition quite a bit.

 

I don't know if they're still doing that with the SiRF-based models. The SiRF chip behaves very well after a full cold reset, so I assume it's quite insensitive to the accuracy of the internal clock.

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My Magellan 210 can drift off by 15 seconds or so over a couple months. A full reset will bring it back, but it seems there are other conditions that will reset it as well, and without losing anything in memory. I haven't used it for a month or so, yet it's within 1 second right now (I have other WWVB based time receivers, both consumer clocks and lab equipment). I think the unit uses an ordinary quartz clock, and is no better or worse than such things are. It would be really nice if Magellan made some technical disclosures about how things like the clock work. And averaging, if it does, and file formats, and remote input voltage ranges, and what the error indicator actually indicates...

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In older Garmins the clock circuit monitored the temperature of the quartz oscillator and corrected for frequency drifts due to temperature. So they should be pretty darn accurate (at least once the GPS has been in use for a few days and the self-calibration circuitry has figured out the exact behavior of the quartz).

The temperature compensation is done to have a very stable time reference for evaluating pseudo-range differences between the various satellite signals and only applies when the unit is on. The clock providing the time-of-day to allow for signal acquisition when the unit is first turned on again is separate and doesn't involve the temperature sensor. My Garmins have all shown a drift of at least a few seconds per day when turned off, indicating that this second clock is not of high accuracy.

 

The situation with Magellan receivers is less clear. They clearly determine the time from the satellite signals when they first get a lock after a reset, but then there seems to be some firmware problem that can result in the time getting off by a considerable amount (many minutes in some cases) from which they don't spontaneously recover. OTOH, they usually keep more accurate time than I'd expect if their internal clock were just free running so it appears that normally there is some synchronization based on the satellite signal data.

Edited by peter
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I think we are missing the point. A gpsr clock tells you what time it is no matter where in the world you are. Time zones aren't linear, but the almanac takes care of that, just like letting you know the sunset and sunrise, moonphase, etc. no matter where you are. :rolleyes:

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