Alan,
Do not worry about that sky-view.
I'd guess your issue is more local, as on your roof.
Still, in timing you can afford to drop a lot of satellites if you only
got a good fixed position.
Cheers,
Magnus
On 08/04/2015 11:02 AM, Alan Ambrose wrote:
p.s. here's the view south taken from about 2m below the antenna:
https://www.dropbox.com/s/zejod2sfi4iogq1/IMG_460371644.JPG?dl=0
Alan
From: Alan Ambrose
Sent: 04 August 2015 9:50 AM
To: '[email protected]' <[email protected]>
Subject: 58503A stats
Hi,
Here is a screen capture from excellent Ulrich's Z38XX program:
https://www.dropbox.com/s/6p5dr8apsftrckf/Z38XX-1.JPG?dl=0
This is the first time that I've seen this output and I have a couple of questions.
I should explain that the antenna is fairly well situated in London UK with a good
view south with no obstacles and ~5m above and away from any other roofs. There are
3 small skyscrapers about ½ mile distance at 140° azimuth and 15° elevation. There
are however obstructions about 4m away to the NW & SW (around 240° and 330°)
limiting the view to 45° elevation. The device is a 58503A (it might be some dodgy
far east 're-manufacture') with tracking of max 6 active sats. I've set the
elevation mask temporarily to 0° to get the fullest map - as you can see that
doesn't make much difference.
+ I was surprised to see such a noisy EFC signal - I assumed that the EFC
changed v. gradually on a slow loop - maybe with temperature and aging. However
there's a lot of high frequency crud there which I don't understand.
+ The 'holdover uncertainty predict' seems to go on a daily loop along with the
EFC from 1.5 to 2.4us - presumably with temperature? If that's normal behaviour
it suggests that the DOCXO isn't that well thermally managed?
+ The holdover uncertainty and the 1 pps variance don't relate at all to the
occasional drop down to 3 sats. So some other effect is at work here - is this
just ionosphere and general short term GPS inaccuracy only?
+ The time stability measures are not credible in their relentless plunge
downward?
+ The azimuth/elevation chart looks about right except for the view to the
south (say 150° to 220°) where I would expect at least as good as the view to
the east - say by another 20° of elevation? (There's a point at 0/0 which I'm
sure is an artefact caused by temporary '---' characters in the El/Az output of
the device.) I don't have many ideas on this as the view is dead good. Any
thoughts?
+ There's v little below 20° elevation - I think the view is clear though for
most of the S azimuth down to ~0°. Is this simply because the device tends to
ignore low elevation sats in its tracking because it has better ones to play
with?
Would all you greybeards out there give the benefit of your hard-won experience
and/or maybe there are other people's charts to compare with?
Regards, Alan
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