[time-nuts] 1PPS accuracy of commercial GPS receivers

Ulrich Bangert df6jb at ulrich-bangert.de
Thu May 14 10:42:48 UTC 2009


Phil,

as soon as a receiver sees more than 3 sats there is redundant information
available. The receiver can use this redundant information in two ways:

a) to improve the solutions for the position

or 

b) to improve the solutions for the time

but not both at the same time. 

That implies that there are good position receivers available and good
timing receivers but not any that is suited well for both purposes (which
matches the market siuation pretty well). On a not-timing receiver the PPS
is more or less a useless gimick which may have accuracies as bad as 1 us or
even more. Dont't worry: These receivers are not made with a precise PPS in
mind. On the other hand: If a good timing receiver has ended his "site
survey" it's position messages stay constant, even if you move it around.
Obviously that is not what you expect from a navigation system. Keep
navigation and timing receivers clear apart from each other. They are as
different as horse and zebra.

Best regards
Ulrich Bangert

> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Philip Pemberton
> Gesendet: Mittwoch, 13. Mai 2009 23:06
> An: Discussion of precise time and frequency measurement
> Betreff: [time-nuts] 1PPS accuracy of commercial GPS receivers
> 
> 
> Just out of idle curiosity, is there any significant 
> difference in 1PPS 
> accuracy between different GPS modules?
> 
> I've got a pair of Trimble SVeeSix CM3 boards (firmware 4.13 
> if memory serves, 
> have to be reflashed to change the comm protocol, which can 
> be either TSIP or 
> NMEA) and an Axiom Sandpiper (SiRFStar II, RAM only, no 
> onboard NVM, SiRF 
> Binary or NMEA switchable on-the-fly) which are specified at 
> 1us and 40ns 
> accuracy respectively. As a comparison point, I've also been 
> looking at the 
> Fastrax iTrax321 (IT321) which is a solder-down "micro-GPS" 
> module based on 
> the "20-channel" SiRFStar III and is -- like the Trimble -- 
> specced at 1us 
> accuracy. This is one of the newer SiRFStar III based design.
> 
> Am I missing something blindingly obvious here, or is there 
> really that much 
> spread in 1PPS accuracy on commercial receiver boards?
> 
> Is 1us jitter really that good for a GPS module?
> 
> Thanks,
> -- 
> Phil.
> lists at philpem.me.uk
> http://www.philpem.me.uk/
> 
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