[time-nuts] GPS orthodontics: sawteeth & hanging bridges

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Dec 21 16:59:06 EST 2006


Randy Warner wrote:
> Paul is right about the temp sensitivity. I am surprised he managed to
> keep the receiver balancing on one foot for seven minutes, but if you
> try hard enough, I guess anything is possible.
>
> I thought everyone on this thread might be interested in what is causing
> the hanging bridge, so here goes.
>
> All Oncore receivers (M12+, UT+, GT+, M12M) are basically run by an
> internally generated 1KHz clock that is "normally" asynchronous to the
> 1PPS epochs. If you could compare this 1KHz to the 1PPS on a scope you
> would see the 1KHz slowly walking in comparison to the 1PPS. Under
> typical conditions these "time slips" occur about once every 15 seconds
> or so. Every time you see a "hanging bridge" you know that a time slip
> has occurred. Naturally, as temp is changed the 1KHZ will change
> frequency, changing the time between each bridge, but it's usually in
> the 10-20 second range.
>
> I have seen several receivers returned over the years that the customer
> returned because of the sawtooth not working. Naturally, when the
> receiver is fired up in my lab it works perfectly....... After reading
> PHK's post I'll bet even money that the 1KHz became synchronous in the
> affected receiver for whatever reason. 
>
> Those of you who worked with the UT+ receiver may remember a problem
> with the time slips in one version of firmware. Motorola updated the
> firmware to do some minor housecleaning and accidentally messed up the
> 100PPS. The 100PPS worked normally until a time slip occurred at which
> time the 100PPS pulse train was delayed by 1ms. It immediately corrected
> itself on the next second, but having your 1PPS slip 1ms every 15
> seconds or so was a real show-stopper. Luckily, few people use the
> 100PPS function, so it was not a critical problem. I sure got REAL TIRED
> of reflashing those receivers however.....
>
>
> Randy 
> ________________________________________________________________________
> ____________________________
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Poul-Henning Kamp
> Sent: Thursday, December 21, 2006 11:17 AM
> To: Tom Van Baak; Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] GPS orthodontics: sawteeth & hanging bridges
> -theeffect of time averaging
>
> In message <000801c7252f$90dad6a0$0b998843 at computer>, "Tom Van Baak"
> writes:
>
>   
>> 1 second raw samples: 10.40 ns
>> then removing small linear frequency offset,
>> 1 second samples:  9.36 ns
>> 30 second averages: 9.62 ns
>> 300 second averages: 10.0 ns
>>
>> Did I do something wrong? PHK, what do you think about this?
>>     
>
> Given what we know, the saw-tooth term has a box distribution from -A to
> +B nanoseconds where A and B depends on the clock frequency used in the
> receiver.
>
> What we don't know is if A = B or for that matter if A and B are equal
> to the period of the clockfrequency.
>
> It is not inconceiveable that A might be 1 nanosecond larger than B due
> to rounding and timing issues in the firmware and hardware and likewise
> it is not inconceiveable that A and B are slightly larger than the clock
> period for similar reasons.
>
> We think we know that the negative sawtooth is a box distribution with
> equal probability over the -A...+B interval, although the flanks may not
> in fact be vertical, due to the mentioned rounding issues.
>
> The theoretical average therefore must be the average of A and B
> possibly with a minor error by the possibly nonvertical flanks.
>
> The standard deviation must be approached with caution since it is a box
> distribution and not a standard deviation.
>
> So, based on what we think we know, the theoretical average probably is
> zero, but could be a few nanoseconds on either side due to rounding and
> timing.
>
> The final thing we know is that no averaging time is long enough to
> guarantee anything about the negative sawtooths actual average.
>
> If you are unlucky, and your oncore is at a very stable temperature, the
> clock generator may just stubbornly decide to run at an integer Hz for
> an hour and if by pure bad luck the sawtooth value was 20ns at the start
> of that, it will be 20 nsec all the way through.
>
> In practice, the probability for this is quite low, but I have tried
> once to provoke , and it is not only possible, it is disturbingly easy:
>
> Wrap your oncore in a couple of blankets or similar and tweak the
> temperature (I varied the supply voltage so the voltage regulator loss
> changed the temperature.  It takes a couple of hours to get it just
> right.  I saw a hanging bridge of slightly over seven minutes this way.
>
> One interesting result of this is that the PRS10 will run better without
> the 256 exponential filter if you feed it from an oncore, if your oncore
> is in a good enclosure, but if you leave it out where the temperature
> changes, the 256 exponential filter helps.
>
> Poul-Henning
>
>   
Randy

If only one had access to the receiver firmware: the cure for "hanging 
bridges" etc is surely to add sufficient (rms noise ~ PPS positioning 
quantisation error) bandlimited gaussian noise to calculated PPS 
position before quantisation of the PPS position occurs in the hardware.



Bruce



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