[time-nuts] GPSDO Component Selection

[bg at lysator.liu.se]

> On Sun, Sep 9, 2012 at 1:14 PM,  <bg at lysator.liu.se> 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.

Here is an older quote for geodetic quality time receivers.

   http://www.bipm.org/static/gpst/mail/13Jul99.1

           Table 5.  Inferred RMS day-boundary clock
	   variations attributable to individual receivers
           ===============================================
                     USNJ (Javad)         216 ps
                     USNO (AOA TR)        630 ps
                     USNB (Ashtech)       574 ps
           ===============================================

and some other links.

http://tycho.usno.navy.mil/ptti/ptti2003/paper7.pdf
http://tycho.usno.navy.mil/ptti/ptti2007/paper29.pdf

https://goby.nrl.navy.mil/IGStime/index.php

> 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.

You got it wrong, the external frequency input is primarily there to let
you compare accurate clocks. There are few applications that really
require an OCXO or better for position uses.

>  Cutting down the unknown in one lets you do
> better in the other.   I assume these all cost well over $50.

That I said already... ;-) But then you will find a gem from time to time
on the big epay river.

--

   Björn