[time-nuts] What position is measured?

[Magnus Danielson]

On 09/08/2010 04:51 PM, jimlux wrote:
> Tom Holmes wrote:
>> Thanks, Jim.
>>
>> I assume that neither the satellite nor the receiver knows what the
>> variation in the light time delay is, so it must be small enough to allow
>> the claimed nanosecond accuracy of the PPS edge.
>
>
> Well.. that's the difference between a L1 only and a L1/L2 receiver. If
> you measure the same signal at two different frequencies, you can use
> that to estimate the total electron content (TEC) of the path, which in
> turn can be turned into a delay correction.
>
> The uncertainties are on the order of meters/few ns, so keeping the 1pps
> to within 10ns is doable with a L1 receiver.
>
>
>
>
>> Although one sat is sufficient for time work, would using more improve
>> the
>> PPS accuracy? Seems like having more inputs would help with the light
>> delay
>> and other corrections, but it probably is no different than having
>> multiple
>> Rb's in the lab (the guy with two is never quite sure and all that).
>
>
> One sat works *if* you know where it and you are. In practice, though,
> you look at multiple satellites and solve for position and time offset
> simultaneously. The "secret sauce" in GPS receivers that distinguishes
> one from another is:
> 1) acquisition (how long does it take to find the signal and start
> tracking)
> 2) how do you best form the estimate of position and clock offset.
>
> Typically it's done with some form of Extended Kalman Filter (EKF) so
> you also wind up with estimates of the covariance matrix. Whether or not
> that gets shoved out to the user is another matter.
>
> the timing receivers separate the "where am I" and the "what time is it"
> questions.. you do a survey mode to get a precise position, then lock
> that down, and go to timing only mode, essentially averaging the time
> info from multiple satellites (not true averaging, almost always a
> weighted average)

Missing is the RAIM and in timing context the T-RAIM.

RAIM helps to drop false-tickers from the solution and this process is 
done dynamically for every solution. It is fairly straightforward.

The remaining sources forms an average after the RAIM.

Cheers,
Magnus