Ultimate GPS and onboard RTC

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dnorbury
 
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Ultimate GPS and onboard RTC

Post by dnorbury »

I'm looking for information about how the Ultimate GPS module interacts with its onboard RTC circuit. Does the GPS time signal get decoded internally and automatically update the RTC time? I'm guessing that might be the case since the module doesn't have SDA and SCL data and clock connections for the I2C interface that would be required to update the RTC externally. Any details would be appreciated.

How does the accuracy of the onboard RTC compare to a Chrono Dot or to your DS1307 breakout RTC?

Thanks,
Dave

1chicagodave
 
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Re: Ultimate GPS and onboard RTC

Post by 1chicagodave »

Yeah, the RTC is internal to the GPS module itself. Only settable through GPS signal.

Accuracy is a good question though, and not a stat I've been able to find. It may be hidden somewhere on G-Top's website though.

One could write a sketch to "test" it....
With RTC battery installed - get GPS fix (so RTC is set), lose GPS fix for a period of time, then measure any non-linear RTC time jump once GPS fix (or satellite communication) is restored. Number of satellites used and a 'yes/no' for GPS fix are available from NMEA sentences. It may require at least a few days without satellite communication in order to calculate any meaningful difference however.

Let us know if you discover anything!

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dnorbury
 
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Re: Ultimate GPS and onboard RTC

Post by dnorbury »

Thanks for the info on the RTC. It would be useful to know how often the NMEA data is used to reset the RTC time. If it is often enough, the relative stability of the RTC would not be a major problem (as long as you were receiving the GPS signal).

To check out the RTC accuracy, I locked up the time using 6 satellites with the battery in, as you suggested. Then I removed the USB cable, thus disconnecting the power. Now the only circuit running is the RTC using battery power. I'll let this run a few days and see how far off the time is when I reconnect the power. When I do that, I'll try to make sure that the module cannot lock up with any satellites. I have another Arduino-based clock that uses WWV as a reference so I can use it to get an accurate error reading.

I'll post data when I get it.

1chicagodave
 
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Re: Ultimate GPS and onboard RTC

Post by 1chicagodave »

I would imagine as long as you get a GPS fix even every once in a while, the clock should stay pretty accurate. That's basically how GPS works.
Each satellite orbiting at 20,000km above the Earth has an atomic clock (and any derivations are pretty constantly compensated for). The satellites send out their time and location data. Then, the module calculates how long it takes that signal (traveling at the speed of light) to reach the module.....that's how it knows its distance from each satellite (distance = time difference * speed of light). After that, it's just simple triangulation (the intersection of fixed distance from many known reference points).

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