[time-nuts] Allan Deviation -> more data: GPS 1PPS against OCXO/128

Magnus Danielson cfmd at bredband.net
Sat Oct 28 15:44:31 EDT 2006


From: "Poul-Henning Kamp" <phk at phk.freebsd.dk>
Subject: Re: [time-nuts] Allan Deviation -> more data: GPS 1PPS against OCXO/128
Date: Sat, 28 Oct 2006 18:11:23 +0000
Message-ID: <27278.1162059083 at critter.freebsd.dk>

> In message <4543777E.9060104 at ispwest.com>, kd7ts writes:
> >Didier Juges wrote:
> >> There are sudden increases in noise (bursts that last from seconds to 
> >> minutes) on the plots I posted.
> >> 
> >> I believe the sudden and drastic increase in noise at times comes from 
> >> the GPS loosing lock. At the moment, I cannot hook up the computer to 
> >> the GPS and verify, but I will do that later.
> >
> >--------
> >
> >I have a Brooks Shera GPSDO that exhibited similar symptoms. 
> >The phase showed huge jumps around 4:00 - 4:30 every 
> >morning.
> 
> This is very likely the effect of low elevation signal reflections.
> 
> Try setting the mask-angle of the GPS receiver higher.
> 
> The jumps happen when a satellite is added to or removed from the
> ensemble but is affected by a large amount of ionospheric delay.

Most receivers I've seen have the capability to put up a list of blocked SVs.
Monitoring the SS and create a 2D elivation angle. Should not be too hard to
do. After some time should the actual horizon be pretty well defined and an
azimuth oriented elevation angle can be set up.

The more advanced approach would look on pseudoranges and monitor how they
degrade in quality. This is a bit harrier to analyse thought.

The point is to block out SVs before they make serious harm to the quality of
the solution.

Anyone done this? Experience to share? I just haven't had the time.

Cheers,
Magnus



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