[time-nuts] GPS with PPS

[Magnus Danielson]

Dear Joe,

Joseph Gray wrote:
> Does anyone know if any of those all-in-one hockey puck GPS receivers
> put out PPS on the serial cable? The type that has the antenna and GPS
> together, with a serial cable hanging off of it. I'm thinking about
> using one for a timing project. The more sensitive, the better, as it
> will be used indoors.
> 
> While I'm on the subject of GPS units, a question comes to mind. I
> know that WAAS enhances position accuracy. Does it do anything for
> time? My first thought would be no, as that comes directly from the
> standard GPS satellites.

It does. It improves the GPS position in [XYZT]. However, it does not 
work its magic in a direct sense to time itself. The fast corrections 
adjust the pseudo-range measures comming out the channels. Ionspheric 
errors is also corrected by transmitting grid-based data and the 
receiver interpolates the correction value for its (coarse) location and 
make individual corrections. SBAS (WAAS is one of several SBAS systems) 
also corrects for satellite orbit parameters, and UTC offsets.

These corrections all aids to improve the GPS position in [XYZT] where 
the improvement in T is for most users a side-effect. A GPS with fixed 
position can (if the receiver supports time-only mode) get a quicker and 
more accurate fix (modulus local multipath) and then produce more 
accurate time value as the SBAS aiding reduces the biasing errors and 
fluctuations of those. Using a LBAS broadcast could improve the state 
further.

Dip your nose into the Kaplan & Hegarty book for instance. That is what 
I used to reinforce my recollection of things. There are better sources.

Cheers,
Magnus