[time-nuts] GPSDO Component Selection

[Azelio Boriani]

 There is something missing in this sentence:
>The integration period determines
>how much of the very accurate long term GPS information <missing verb?>
with the short
>term, highly stable, clock information. Hence a GPSDO.
That is: how much of the very accurate GPS information leak? are
transferred to? Interact? with the short term...
I expect that the integration time is related to the end of the short term
oscillator variation and the start of the long term GPS steering. This
should help in the proper selection of this time: maybe the time should be
adapted to the temperature variation too.


On Mon, Sep 10, 2012 at 11:56 AM, Michael Perrett <mkperrett at gmail.com>wrote:

> 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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