GPS Breakout skips 1PPS and isn't stable in position

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Hedgehog
 
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GPS Breakout skips 1PPS and isn't stable in position

Post by Hedgehog »

I'm using an Ultimate GPS Breakout board in a GPSDO as a 1PPS timing source. Most of the time, it's good enough for my purposes. But, it seems like it can't manage satellites coming into view or going out of view very well. From time to time it will skip at least one of the PPS pulses. Also when satellites come into or go out of view, the reported position will move around causing phase errors as the 1PPS pulse moves with the position. Is there a way to tell the board the minimum elevation below which a satellite will not be used? I couldn't find one in a manual. My antenna is in the attic and SNR is high even at low elevation angles.

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adafruit_support_rick
 
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Re: GPS Breakout skips 1PPS and isn't stable in position

Post by adafruit_support_rick »

The PPS is spec'ed be good to 10ns. Are you seeing more than that?

I know of no way to exclude satellites based on elevation. Perhaps you could set up some physical shields in your attic?

Hedgehog
 
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Re: GPS Breakout skips 1PPS and isn't stable in position

Post by Hedgehog »

From my point of view, it periodically fails to meet the published specs. I have two links below. The first is a plot of what's happening in my GPSDO since 22:00 yesterday. The first half hour is acquiring lock, so ignore that. The cyan smear is what is of interest. It is the difference in nanoseconds between the 1PPS pulse and the output of my OCXO; a Trimble 34310-T. The setpoint for my PLL is 50ns. As you can see, most of the time the cyan trace is between 40 and 60ns with very little correction needed from my GPSDO. I think this is within what I understand the specs for the breakout device to be. However, as you can also see, periodically it greatly exceeds that 10ns. My belief is that these large excursions occur when new satellites are acquired and/or when satellites drop out. I have not done a thorough study, which would require capturing positions, elevation, and signal strength of all satellites during the test period.

http://evoria.net/Adafruit/fll.png

The image below is a screen capture of the program FoxtrotGPS. It is the position plot since about the same time yesterday. I expect some movement, but this seems to be a bit large. The scale is at the bottom of the image. I have two of the ultimate breakouts. I don't see any real difference between the two of them, but I have not gone through the big effort of comparing them side-by-side.

http://evoria.net/Adafruit/GPSWander.png

Bob

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adafruit_support_rick
 
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Re: GPS Breakout skips 1PPS and isn't stable in position

Post by adafruit_support_rick »

I'll investigate the 1PPS jitter.
Meanwhile, I think the 'wander' in FoxTrotGPS is within spec - from what I can see, it is within about +/- 10ft. The spec is 3.0m, which is also about 10 feet.

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adafruit2
 
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Re: GPS Breakout skips 1PPS and isn't stable in position

Post by adafruit2 »

We have very little control over the internal chipset (its from MediaTek and they don't do support for small companies or individuals) - we can send you a new one or refund but if its something inside the chipset then there's not much we can do about it. The specs we have are the specs for the MTK3329

Hedgehog
 
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Re: GPS Breakout skips 1PPS and isn't stable in position

Post by Hedgehog »

It's a nice little board, but I think they've been a bit too optimistic on their specs for the chip, or at least haven't published enough qualifiers. My complaint is not with you, though I had hoped you had some "magic solution", such as entering some command that's not in the datasheet. =) All things considered, I've probably gotten more value out of this device than I expected.

My real concern is the skipped PPS signal. But, I understand that that's not under your control. Maybe I'll just accept that it happens with this chip, and add code to allow it to happen gracefully.

Thanks for your replies.

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