[time-nuts] GPS with PPS

Joseph Gray jgray at zianet.com
Wed Sep 30 23:01:15 UTC 2009


Thanks for the explaination. I forgot about ionispheric latency
effects. I'll look for the book you mentioned.


On Wed, Sep 30, 2009 at 4:11 AM, Magnus Danielson
<magnus at rubidium.dyndns.org> wrote:
> 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
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