[time-nuts] Sawtooth Effect in GPS timing

[Tom Van Baak]

> Tom,
> What I want to look at is how interference is affecting the GPS timing
> rcvrs. I have looking on three different recievers NovAtel, M12 and Leica.

You need to quantify "interference"; not just power
level but frequency, direction, modulation, duration,
spectrum, etc. You may find the characteristics of
the interference have more effect than which receiver
you use.

You need to look at the antenna side of the equation.
Many GPS antennas, especially for use in telecom,
have interference filters. You may find the type of
antenna you pick has more effect than which receiver
you pick.


> When I introduce interferenc, it just does not affect the timing of the
> receivers upto a certain level and once it reaches that level of
interfernce
> it just looses lock. What I think is, interference does affect the timing

This makes sense. I think the same thing happens
if you cover a GPS antenna with more and more
shielding: it works, it sees fewer satellites, it stops.


> recievers but that effect hides behind the sawtooth stuff. This is the
> reason why i want to get rid of these sawtooths and see the effect of
> interference. I think this because variation due to the interference would
> be smaller than that due to introduced interference. Although smaller in

I suggest you read more about the GPS signal format,
and about T-RAIM. Take a look at the pseudo-ranges
and timing solution and see if that supports your claim.

The M12 gives you all this information every second
in the @@Hn message. You can watch SV come in
and out of solution; you can see the timing residuals;
you can see TRAIM at work.


> value it still may cause some problems to timing solutions due to the fact
> that sawtooth effect diminishes when averaged over a long period, but I
dont
> think this is also the case with the intentionally introduced
interference.
>
> Any comments?

I think to support some of your claims you will have to
collect a lot of data and look at it statistically.


> Another thing is that, as I said earlier, I have been using an
oscilloscope
> for calculating the delay in the motorola timing pulses from a reference
> (pulses generated by Leica, which I consider to be more stable than M12).
> Most of the people have been using TIC's which I dont have any access to
> :(
>
> Regards
> Faisal

Does your 'scope automatically calculate time interval
of the 1 PPS pulse relative to its internal timebase?

Perhaps you can borrow a TIC. Where are you located?

/tvb