[time-nuts] Nav Receiver Sawtooth Correction?
bob at evoria.net
Sun Mar 23 23:08:43 EDT 2014
Random noise or not, wouldn't a position error in a nav receiver cause a corresponding displacement of the 1PPS pulse?
Also there's a bit more to it than just minor noise. It's probably multipath, or perhaps even jammers passing on the freeway about a mile away. Whatever the cause, take a look at this screen capture of "foxtrotgps" over about 40 minutes of elapsed time. The red is the GPS movement around my house that I periodically mention. I judge it to be about 15 ft on the diagonal. Sometimes it is much much worse. Likewise I would expect the 1PPS to move by that amount.
> From: Chris Albertson <albertson.chris at gmail.com>
>To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com>
>Sent: Sunday, March 23, 2014 9:24 PM
>Subject: Re: [time-nuts] Nav Receiver Sawtooth Correction?
>What you are seeing in position error is a random noise. There is no
>pattern to it and it is not predictable. A sawtooth error is very
>nice and regular looking. It's not "noisy" and can be predicted in
>advance. Possition error is not at all like sawtooth.
>I think what you CAN do is look at the size of the error. Then you
>adjust the gain on the loop control based of the measured error sigma.
>On Sun, Mar 23, 2014 at 6:30 PM, Bob Stewart <bob at evoria.net> wrote:
>> Hi Ignacio,
>> Thanks for the response. I've got a UT+ in the parts box. But that's not the problem I'm trying to solve. I'm trying to make the best GPSDO that I can make using a nav receiver at the moment. Call it an obsession if you like. It's OK if I don't have corrections to the nanosecond for each PPS. But I can see the nav receiver wandering around; especially on cloudy days, (the antenna is in the attic, so that's about the best I can do for that) and it just seems to me that I should be able to do a general correction for nav position errors. Sorry if my naive posts are starting to get on people's nerves.
>>> From: EB4APL <eb4apl at cembreros.jazztel.es>
>>>To: time-nuts at febo.com
>>>Sent: Sunday, March 23, 2014 7:35 PM
>>>Subject: Re: [time-nuts] Nav Receiver Sawtooth Correction?
>>>The sawtooth is generated by the granularity of the GPS receiver clock
>>>not being synchronous with the recovered PPS. The receiver program can
>>>calculate the correction to be applied to the next PPS and outputs it in
>>>a message, bu only in timing receivers, this is not a useful thing in
>>>navigation receivers and I think that it cannot be calculated using the
>>>satellites' position, it is a receiver "defect".
>>>Why don't you buy a timing receiver? An used Motorola Encore M12+
>>>timing receiver can be bought by $35 or less (ebay items 290656401551 or
>>>301131583613. The seller is a known Time Nuts supplier). An UT+ or GT+
>>>even for quite less.
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>Redondo Beach, California
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