On 09/09/2012 11:05 PM, Chris Albertson wrote:
On Sun, Sep 9, 2012 at 1:14 PM,<[email protected]> wrote:
True for a cheap oem navigation receiver. Not true for a geodetic quality
receiver, who usually have some options (external frequency input, PPS_in)
to make them the best timing receivers available. However they are much
more expensive than the typical single frequency timing reciver.
I looked at every link and can't see where they give a timing accuracy
spec on the PPS with respect to UTC. Possition accurracy is very
good and we might assume the timing is as good. But they don't say it
is. What's interesting is these GPSes will accept an accurate clock
input in order to give better location data. That is the opposite of
a timing GPS where you tell it accurate location data so that it can
get better timing. Cutting down the unknown in one lets you do
better in the other. I assume these all cost well over $50. You can
get a pretty good timing GPS for $30 and it WILL have the PPS error
specified.
To the OP. None of this matters a lot because PPS is a standard input
signal. It is easy to swap out a GPS receiver later. Same with the
OCXO. From a control point of view they are all pretty much the same.
You can swap them out later
There can be uncompensated delays, that always needs to be calibrated
out. If you go looking around, you will find that they have gone through
fair amount of effort to characterize the delay and it's temperature
dependence among other things.
Also, the PPS generation and it's precision isn't a good quality measure
here, as the PPS is typically generated out of the internal clock and
stepped in that clock. For the higher precision you need to use the data
alongside it.
For most of these, you do not provide external time but a better
reference clock for a stable frequency, which will increase the quality
of both time and position results. There are receivers in which you
provide time as in 5/10 MHz and PPS, but then you get the time
difference and position out of the receivers.
So I agree with Björn here, but it may be beside the point unless you
are eager to go over the steep step and into really good receivers.
There even are rubidiums which learns of time and frequency errors out
of GPS receiver messages.
Cheers,
Magnus
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