[time-nuts] Adafruit Ultimate GPS timing message arrival times
nsayer at kfu.com
Sat Jul 30 19:12:54 EDT 2016
I’ve used the PA6H so far for all of my GPSDOs. The two reasons I am considering the Venus838LPx-T to replace it are that it has a sawtooth correction message and it has a survey and static solution mode to allow it to tolerate poorer reception.
I’m driven to want to experiment with a static mode GPS receiver from this page on Atilla Kinali’s blog: https://attila.kinali.ch/blog/2016/02/07/gps-disciplined-oscillator, which references some e-mail here from some time ago. My take-away was that navigation mode receivers could achieve good results as long as they had ideal reception, but with poor reception they would do much worse than a receiver in static mode that’s done a good survey.
As for the sawtooth correction, I’m less convinced that it’s required given that I’m using averaging on the phase detector output. I can’t detect hanging bridges with the current setup, but since I haven’t actually looked for them, ignorance is bliss. Tom, if you still have the data, can you speak to whether you saw anything of the sort? In any event, it’s a box I’d like to check if for no other reason than to learn something.
> On Jul 30, 2016, at 2:32 PM, Tom Van Baak <tvb at LeapSecond.com> wrote:
> Hi Brooke,
> That's a reasonable assumption. I haven't ever tried mine at 10 Hz. But note that fast update rates is more meant for navigation and positioning than it is for timing.
> Just in case we have some newcomers to the thread I'd like to point out that this recent series of measurements of RS232 / NMEA have no bearing at all on the quality of the timing output. Timing NMEA is more of a curiosity; something to measure at the hundreds or tens of millisecond level. As anyone knows, the real timing output of these receiver is the 1PPS pulse itself, which is good to the tens of nanoseconds level. So a factor of a million different.
> I actually like the Adafruit GPS receiver, and would recommend it to anyone. I use it for projects around the house more than any other receiver. It's so simple to use -- no configuration needed, no sawtooth correction needed, no survey required, fast acquisition, works fixed or mobile, sensitive antenna included on board, small and low power, just give it 5 volts and out comes a UTC 1PPS.
> Its 1PPS output is superb. Attached is the ADEV/MDEV of the raw 1PPS of the Adafruit GPS board.
> ----- Original Message -----
> From: "Brooke Clarke" <brooke at pacific.net>
> To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
> Sent: Saturday, July 30, 2016 1:50 PM
> Subject: Re: [time-nuts] Adafruit Ultimate GPS timing message arrival times
>> Hi Mark:
>> Isn't this the receiver that hears a very large number of GNSS satellites and also has a 10 Hz update rate?
>> If so, I'd expect that there would a large variation in message lengths. How stable is the 10 PPS or 1 PPS output?
>> Have Fun,
>> Brooke Clarke
>> The lesser of evils is still evil.
>> -------- Original Message --------
>>> A couple of people have asked about the poor message arrival time performance of the popular Adafruit Ultimate GPS receiver. I modified Lady Heather to analyze the message arrival times using a histogram instead of a simple average. When I looked at the histogram data (.01 msec resolution), I was rather shocked... With an hour of data, most receivers have maybe a couple dozen bins hit, with the peak bin several hundred counts above the next lower peak. The Adafruit had over 1800 bins hit, with the peak bin having six hits. Attached is the histogram... you probably don't want to use this receiver to drive a clock based upon message arrival times...
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