Regarding the PPS skew between various GPS receiver units...

Regarding the PPS skew between various GPS receiver units...

Post by jdsando » Tue, 19 Feb 2008 10:06:11



I awoke in a panic one night, and afterwards decided to look into what
the skew was in the PPS outputs of the two GPS units I have on hand.
One unit is the venerable HP 58503A GPS Time and Frequency Reference
Receiver, and the other unit is the ubiquitous Axiom Sandpiper OEM GPS
module. (It should be noted here that the relative cost of these two
specimens is a few orders of magnitude apart).

I was wondering if others have looked into this with other receivers,
and what your findings were.

Regards.


HP = HP 58503A GPS Receiver, >7 days run time.
Axiom = Axiom Sandpiper OEM Module, >7 days run time.

Click here for an example 'scope plot:
http://www.yqcomputer.com/ :8080/public/gps/pps-skew.jpg

Relative time interval (TI) statistics for a 14,400 s (4 hr)
interval...
mean 308 ns
high 349 ns
low 222 ns
std dev 20.5 ns

TI 1 to 2 measurement instrument:
HP 53132A 225MHz counter equipped with
ultra high stability timebase (option -012).
HP GPS module 1pps output feeds channel 1;
Axiom GPS module 1pps output feeds channel 2;
Both channels have LP filter on, DC coupling, 0.2v trigger level.

So, which PPS signal is sliding around?

Individual unit period statistics for 10,800 s (3 hr) interval...

HP:
mean 1.00000000022 s
high 1.0000000009 s
low 0.9999999997 s
std dev .000141 us

Axiom:
mean 0.99999999892 s
high 1.0000000708 s
low 0.9999999277 s
std dev .018836 us

Period measurement instrument:
HP 53132A 225MHz counter equipped with
ultra high stability timebase (option -012).
EUT GPS module 1pps output feeds channel 1;
LP filter on, DC coupling, 0.2v trigger level.

I have not been able to find much published data on the specified or
typical variance of the Axiom Sandpiper's PPS output. A similar Axiom
module, the Avocet, is spec'd at "< 75ns" in the Axiom Avocet/
Sandpiper GPS Evaluation Kit manual.

ps:

Incidentally, I tuned in WWV (@10MHz) on my Icom 703+ while waiting
for the stats to finish. It was interesting to see the LED on the
counter blink in sync with the audible ticks produced by WWV's time
service. I demonstrated this to my wife and she was not impressed.
She seemed distracted by some dust motes or something when I tried to
explain that the LED blinks and the audible ticks were not quite in
*perfect* sync. Oh well. :*)