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Jitter on GPS Models 746 & 4279
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Jitter on GPS Models 746 & 4279

by gliderpilot on Thu Aug 20, 2020 10:18 pm

I have two questions regarding the above GPS Models...

1. What is the typical second-to-second time jitter on the PPS outputs of each model?

2. The photos of Model 4279 suggest that PPS and GND share the same plated-thru hole? How is this possible?

Thank you.

RF

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Re: Jitter on GPS Models 746 & 4279

by ChliHug on Fri Aug 21, 2020 2:38 pm

1. The datasheet claims 10 ns typical: "High accuracy 1-PPS timing support for Timing Applications (10ns jitter)". I don't have the equipment to verify that, but it seems Adafruit did some testing in that direction. From the FAQ (https://learn.adafruit.com/adafruit-ultimate-gps/f-a-q#faq-3030050):
Do all GPS modules emit PPS pulses at the same time?
Under ideal conditions, GPS modules emit a PPS signal within 10ns of the beginning of each GPS second. That's only a best-case value though.

In practice, each GPS module's sync to the GPS clock system depends on the quality of the fix, how long the GPS module has had a fix, and the group of satellites the module uses for its fix. We've observed offsets of about 300ns between modules that have just acquired a fix, improving to less than 100ns after the modules have had a good fix (Signal-to-Noise ratio higher than 20 for the satellites the modules use for their fix) for ten minutes.

When two GPS modules used the same group of satellites for their fix, there was less than 30ns of offset between PPS pulses as soon as the modules acquired a fix.

2. This seems to be a very unfortunate placement of the silkscreen text. "PPS on Serial RI" is just for your information that you have to get it on the ring indicator. It does not mean PPS is on that through hole. I guess it would be possible to tap it off the pin from the module on the USB breakout. It's the third from the right on the side with the external antenna connector.

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Re: Jitter on GPS Models 746 & 4279

by adafruit_support_mike on Mon Aug 24, 2020 4:20 am

I did the PPS testing, and can confirm that multiple devices will sync their PPS signals to within 10ns over time. In my testing, it took about 24 hours for devices with mediocre satellite views to sync that well.

The process is statistical. Each GPS module estimates the current time and its location from the satellite signals. Then it predicts when the next signals will arrive, and checks its predictions against the actual signals. It uses any difference between the prediction and the real signal to improve its time and location estimates. Every part of the system has some amount of random noise, so the module has to find the point where the offsets between its predictions and the real signals spend equal amounts of time high and low.

The longer a GPS module has a fix, the better its estimate of GPS time will be.

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Please be positive and constructive with your questions and comments.