[time-nuts] GPS: ADEV or MDEV?

Magnus Danielson cfmd at bredband.net
Sat Apr 14 10:07:28 EDT 2007


From: Dr Bruce Griffiths <bruce.griffiths at xtra.co.nz>
Subject: Re: [time-nuts] GPS: ADEV or MDEV?
Date: Sun, 15 Apr 2007 01:17:17 +1200
Message-ID: <4620D45D.6000901 at xtra.co.nz>

> Magnus Danielson wrote:
> > From: bg at lysator.liu.se
> > Subject: Re: [time-nuts] GPS: ADEV or MDEV?
> > Date: Sat, 14 Apr 2007 14:46:42 +0200 (CEST)
> > Message-ID: <1678.125.24.131.37.1176554802.squirrel at webmail.lysator.liu.se>
> >
> >   
> >> On Sat, April 14, 2007 13:32, Magnus Danielson said:
> >>     
> >>> From: Peter Schmelcher <nebula at telus.net>
> >>> Subject: Re: [time-nuts] GPS: ADEV or MDEV?
> >>> Date: Sat, 14 Apr 2007 02:40:57 -0700
> >>> Message-ID: <5.2.1.1.2.20070414001036.02afa7d0 at pop.telus.net>
> >>>
> >>>       
> >>>>> Surely its better to detect the pseudorange instability in software (if
> >>>>> one has access)  rather than attenuating the signal from all SVs.
> >>>>> Another option is to use multiple GPS antennae operating as a phased
> >>>>>           
> >>>> array.
> >>>>         
> >>>>> Bruce
> >>>>>           
> >>>> Of course but I think that approach is part of sub nanosecond time
> >>>> transfer
> >>>> accuracy. Attenuation is simple and the results are descent. The signal
> >>>> is
> >>>> less than 6dB above the receiver noise floor to acquire SV.
> >>>> Unfortunately
> >>>> the SV do not have matched output power so they are acquired and
> >>>> weighted
> >>>> unequally.
> >>>>         
> >>> The difference in path-length and thus space attenuation is compensated by
> >>> the
> >>> antenna pattern such that it emits stronger to the rimb of its earth view
> >>> than
> >>> central. That evens out power-shifts due to space attenuation for the
> >>> viewer.
> >>> Otherwise we would have yeat another problem to deal with.
> >>>       
> >> Do the HP GPSDOs not have a tuneable maskangle?
> >>     
> >
> > Maskangle is indeed setable.
> >
> > Cheers,
> > Magnus
> >
> > _______________________________________________
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> >
> >   
> Magnus

Bruce,

> But what is really needed, in general, is to be able to vary the mask 
> angle with azimuth to account for hills and other obstructions that do 
> not extend all the way around the the site.

I already propsed that some time back. It is fairly easy to do on most GPSes,
since you can explicitly block certain SVNs from being included and you can
map the signal-strength (and other measures of incorrectness, such as
pseudorange deviation) as a function of elevation and azimuth. With such a 2D
plot you can not only have a contour of the horizon, you can also blank out
spots where excessive multipath corrupts the pseudo-ranges. In essence you now
have a 2D mask rather than a 0D/pseudo-1D mask model on a 2D spaceview.
It is really not that much programming effort and it is really just the exact
command set towards various GPS receivers which changes most of the time.

If you like me have many receivers of the same antenna, it is really the same
decission that goes for all of them, so the main processing is a joint effort
for the site rather than per receiver. You can however compare the result
between the receivers.

Cheers,
Magnus



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