[time-nuts] How does sawtooth compensation work?

Bob Camp kb8tq at n1k.org
Tue Jul 19 17:58:33 EDT 2016


The reason people do not routinely jump to number 3 on the list is cost. The only 
new GPS modules that I am aware of in category 3 are well over $2K each. That 
is in comparison to Mark’s favorite $5 modules. You can buy eBay surplus older
versions of the fancy boards. So far I have not seen one with the “right options” 
enabled for under $800. 

There is another tangent and that is ionospheric correction. It is normally done
before you get to option 3. You run some combination of L1/L2/L5 to let the 
delta frequency enable a real time guesstimate of the ionospheric delay. The
cost for that part of it can be in the ~$300 for an older receiver. You also need
an L1/L2 antenna. Getting an accurate PPS in or out of a $300 receiver may range from
“exciting” to “impossible”. 

Lots of choices.


> On Jul 19, 2016, at 12:32 PM, Michael Gray <mikenet213 at comcast.net> wrote:
> 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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