[time-nuts] Motion effects on accuracy of GPS pps

Daniel Davson ddavson at peralex.com
Fri Jul 2 05:13:22 EDT 2010

Thanks for all the responses.

I've also had a response from uBlox support informing me that the 
TimePulse accuracy will degrade (both due to motion and of course due to 
multipath, weak signals etc. - datasheet accuracy figures are for 
reasonable/optimal conditions). The uBlox module has a T Accuracy filter 
which can impose a limit on the estimated accuracy of the PPS. When the 
limit is exceeded it will stop the PPS. The T accuracy estimate is also 
accessible in navigation strings, which is what I'm now going to 
monitor. The T Accuracy filter takes more into consideration than does 
TDOP - the accuracy estimate includes detected multi-path, weak signals 
as well as the standard factors that TDOP is based on.

The one question I've yet to resolve is whether or not motion can cause 
sustained increase or decrease in the period of the pulses and not just 
increased jitter.


Magnus Danielson wrote:
> <div class="moz-text-flowed" style="font-family: -moz-fixed">On 
> 06/29/2010 10:32 AM, bg at lysator.liu.se wrote:
>> Hi Daniel,
>>> Hi,
>>> I've recently completed a GPSDO using the pps output of a uBlox Antaris
>>> TIM4 GPS module to discipline a 10MHz OCXO. I'm now investigating the
>>> motion effects (acceleration/uniform velocity) on the accuracy of the
>>> time-pulse and hence on my frequency output.
>> Are you moving around in areas with little obstruction for the GPS 
>> antenna
>> towards the sky? Then I would first look at the dynamics inpact on your
>> OCXO.
>>> I've been unable to find any detailed information on the relationship
>>> between the accuracy of the pps output and the effects of having the
>>> receiver in motion (i.e. in a car/boat, at velocities far below the
>>> modules operating limit of 515m/s) - I'd appreciate some input on this
>>> matter. Does motion cause increased jitter?
>> Tilting/accelerating your GPS oscillator might possibly yield an effect.
>> Other than that there is no theoretical advantage keeping your receiver
>> stationary. All its measurements are done against satellites moving 
>> at ca
>> 4km/s.
> I disagree... for some receivers...
> As you line up you pseudo-ranges you now have to solve only the T 
> variable rather than the XYZT position. This means that all 
> pseudo-ranges is available for solving the T solution and reducing the 
> TDOP error. The T-RAIM is also able to drop more false-tickers.
> For receivers not having a T mode only but normal 3D or 2D mode, the 
> benefit of fixed location is of less concern.
> So, for a receiver at a fixed location knowing it has a fixed location 
> it may be a benefit.
> Cheers,
> Magnus
> </div>

Daniel Davson
HW Design Engineer
Peralex Electronics (pty) ltd.
Peralex House, 5 Dreyersdal Rd, 
Bergvliet 7945, South Africa
Tel: +27 21 710 7446

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