[time-nuts] GPSDO Component Selection

[Michael Perrett]

First - I realize 95% of the folks reading this are well aware of what I am
going to say.

No matter how good your equipment (receiver/antenna) is, the short term
accuracy of GPS time is defined in *GLOBAL POSITIONING SYSTEM STANDARD
POSITIONING SERVICE PERFORMANCE STANDARD (2008)*. This document can be
found at http://www.gps.gov/technical/ps/#spsps.

The guaranteed timing accuracy (short term) is found in Table 3.8-3. SPS
Position/Time Accuracy Standards. The time domain transfer accuracy is
defined as "≤ 40 nsec time transfer error 95% of time
(SIS only)". In order to achieve this the "(SIS only)" comment means you
have a perfect receiver that introduces no errors (good luck with that
one). Most commercial users set their probability at around 30 nS, and
experience virtually no estimates out of that boundary.

The 30 nS error can be reduced to a better number if position is accurately
known and the receiver "knows" that it is stationary - but still 5% of the
time you can get noisy / degraded time and still be "in spec.". I am not
sure over what time span the 95% number is used.

Now: The real answer is to take the relatively noisy GPS timing information
and use it to discipline a device that is (extremely) stable over a short
time such as a OCXO or Rubidium standard. The integration period determines
how much of the very accurate long term GPS information with the short
term, highly stable, clock information. Hence a GPSDO.

Michael / K7HIL



On Sun, Sep 9, 2012 at 3:10 PM, Jerry <jsternmd at att.net> wrote:

> Sounds like the Heisenberg Uncertainty principle at work :-)
>
> jerry
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Bob Camp
> Sent: Sunday, September 09, 2012 5:53 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] GPSDO Component Selection
>
> Hi
>
> Position accuracy and timing accuracy are two very different things.
> Firmware is optimized to improve either one. "Position" firmware is often
> pretty poor for timing.
>
> Bob
>
> On Sep 9, 2012, at 5:05 PM, Chris Albertson <albertson.chris at gmail.com>
> wrote:
>
> > 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.   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
> >
> >
> > Chris Albertson
> > Redondo Beach, California
> >
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