[time-nuts] ok, newbie questions

[W2HX]

Ahh. Very interesting explanation. So is it somewhat correct to assume (yes,
I know) that for a stationary (non-mobile) environment, these extra sats
don’t make much difference? This seems to be what the explanation is saying.

Ok. So let me see. For a frequency standard for use in lab equipment, it
appears that short term, phase noise and other sources of noise are the
things to be concerned with  to get better results. These seem to really be
accomplished with a good oxco.  However, if I want a very accurate
time-of-day clock for long periods of time, then I need long term stability
which is where the GPS comes in.  Do I have this right?

So if I want a really souped-up freq standard for my lab, then I should
concentrate on finding the best oxco I can (which may be disciplined by the
GPS or manually occasionally calibrated to GPS), and use the best power
supply I can find.  These seem to be what I should concentrate on rather
than more channels. 

I do believe that I read some stuff on the internet that the HP GPS DO's do
seem to have very good power supplies (or converters) which contribute to
low spurs. So it seems the HP's do have a real advantage (not just the
name).

Thanks to everyone for the help (hope I am getting the idea here!)

73 Eugene W2HX


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of bg at lysator.liu.se
Sent: Friday, November 26, 2010 4:48 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] ok, newbie questions

> Yes I agree a newer thunderbolt would surely suffice for me and probably
> also the ocxo in my 8662A synth
>
> But I am still academically curious about the impact of more channels of
> satellites? What is the value of these extra sats?
> Thanks!

In a mobile scenario, you need measurements to four or more satellites to
solve for x,y,z and time. A ground vehicle travelling in areas with lot of
antenna masking - high buildings close to the road or trees - blocking
line of sight from antenna to the satellites, will want to track all
satellites in view. And since especially low elevation satellites will
come and go, you want to pick these up really quick once visable.

Thus you want as many channels (correlators) as possible.

For a stationary timing scenario, (x,y,z) might be known - or averaged
over many measurements (site survey). This gives that in a timing mode the
receiver only needs to track one satellite.

The GPS receiver evolution moved from 1-channel multiplexing, to 6
channels, and 8, 12 and even more. While there might be timing variants of
generic navigation receiver, the core correlator chipsets are shared.

So even if the timing receiver does not benefit that much from 8+
channels, once the navigation receivers got more channels, the timing
versions had to follow.

Modern timing receiver have better hardware to put the 1PPS signal where
intended. Se http://www.leapsecond.com/pages/vp/sawtooth.htm for a
comparison of semi modern receiver and an old one. This might have a
bigger impact than the number of channels.

--

    Björn




_______________________________________________
time-nuts mailing list -- time-nuts at febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.