[time-nuts] How does sawtooth compensation work?

[Tom Van Baak]

Hi Michael,

About #3 below...

There are dozens of technical papers about all this in the PTTI, FCS, UFFC, EFTF journals. Google for words like: GPS carrier-phase dual-frequency time-transfer geodetic-receiver IGS precise point positioning PPP

I don't have a link to a handy 1-page summary, but someone else on the list might. Otherwise skim the first ten papers you find and you'll pick up the concepts of high-precision time transfer.

The basic idea is that high-end geodetic-grade receivers often have an external 10 or 20 MHz clock input (and maybe no internal clock at all). You give it your best lab clock and all then all GPS signal processing and SV measurements are based on your fancy clock. The output of the receiver is a stream of these measurements, not necessarily a physical 1PPS or 10 MHz (as with a GPSDO).

So you can see there's no such thing as sawtooth error here, because you're not transferring some internal clock to some external clock via a TIC; there is only the one clock; your clock.

All this measurement data is then post-processed, hours or days later, so that some of the learned errors in the GPS system can be backed out. This would include SV clock and orbit errors, as well as tropo/ionospheric errors. The goal in cases like this are to find out how good your lab clock is (was), not so much to steer anything in realtime.

These receivers also tend to measure GHz carrier phase instead of (or in addition to) MHz code phase. And they often capture both L1 (1575.42 MHz) and L2 (1227.60 MHz) instead of L1, which not only doubles the effective number of SV received, but also is used to help compensate for speed-of-light variations through the ionosphere. With all this attention to precision, you then sometimes enter the realm of fancy temperature controlled antennas and special RF cables, maybe even temperature controlled receivers. It's all a very slippery slope.

/tvb

----- Original Message ----- 
From: "Michael Gray" <mikenet213 at comcast.net>
To: <time-nuts at febo.com>
Sent: Tuesday, July 19, 2016 9:32 AM
Subject: Re: [time-nuts] How does sawtooth compensation work?


> Alright, may I vote for tangent #3? I've heard this mentioned in passing
> a few times on this list, but never seen it described in detail...so
> much so that I have no idea what it means.
> 
> Are we talking an external, steerable LO/clock (similar to #2, except
> for the location of the oscillator itself)? An external oscillator
> triggering a timestamp against the GPS's internal clock (seems identical
> to PPS error-wise, except you can introduce your own dither)? Something
> else? Why does carrier phase tracking matter, as long as the GPS is
> deriving a time solution through some mechanism that exceeds the
> granularity of its CPU clock?
> 
> Forgive my ignorance here, but I'd love to see more of this tangent,
> since I've never seen anyone jump into it in detail.
> 
> Michael
> 
>> There are some tangents we could go down:
>> 1) There are cases where the inherent dithering you get from sawtooth error is actually hugely beneficial to the design of a GPSDO.
>> 2) One GPSDO design (Trimble Thunderbolt) is unique in that is has no sawtooth problem or TIC or XO or TCXO at all. Instead it directly uses the high-quality OCXO as the receiver's LO. They get away with this clean solution because they are a company that makes their own receiver h/w.
>> 3) Carrier phase receivers with external clock input.
>>
>> /tvb
> 
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