[time-nuts] Mounting GPS Antenna on Steel Roof

[Didier Juges]

My experience is that when severe multipath occurs, even when short distances are involved, the GPS receiver seems to get utterly confused and won't provide a solution.

When I had my  GPS antenna only about 2 feet away (above and to the side) from the top of a signal generator on the top of a cabinet (offering a 19x24" reflective surface to the South-East of the antenna), the Thunderbolt receiver had a hard time performing a self survey and had extended hold-over periods. When I moved the antenna up and aside to the West by another 2 feet, the problem went completely away. There was no other significant metal object in the area.

The problem could be reproduced, so it was not a one time event.

This was when I was using an inexpensive GPS antenna (not mag-mount, but similarly priced), I have not verified that the Symmetricom or the Trimble Bullet antenna in the same location would have the same problem.

Didier KO4BB

---- Keith Payea <kpayea at bryantlabs.net> wrote: 
> I hope we aren't running Matthew around in circles here for nothing.
> 
> Multipath in a GPS situation is a problem where the differences in path
> length are significant relative to the time accuracy desired.  If you are
> trying to make a measurement on a city sidewalk with large reflective
> objects above you, the differences in path lengths could be hundreds of
> meters.  
> 
> An antenna mounted less than a meter above a reflective ridge cap should be
> no problem.  I suspect the time errors introduced, if any, will be much,
> much less than the other errors in the system.  Also remember that you could
> solve one problem (potential multipath error of a few nanoseconds from a
> nearby reflector) but add another (differences in propagation speed of coax
> over temperature) by moving the antenna far away.
> 
> Regards,
> 
> 	Keith
> 
> 
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Matthew Smith
> Sent: Tuesday, March 04, 2008 1:36 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Mounting GPS Antenna on Steel Roof
> 
> Quoth Rob Kimberley at 2008-03-04 19:29...
> > Height of antenna is not important, but being clear of obstructions and
> > large metallic objects are. Can you mount it on a pole at the end of the
> > garden? 
> 
> I can, but the end of the garden is some 50 metres from the house, which
> brings back my concern of excessive cable lengths.
> 
> We're in a bit of a weird situation - it's a rural location, but there
> are sheds, water tanks and all sorts dotted around and cables can only
> really be run along (steel) fences as vehicular access is required all
> around.  (Think of a farm and you wouldn't be far off.)
> 
> I had thought about putting the time server in the big shed and running
> an underground Ethernet cable (this is a fairly reasonable proposition)
> until the reflection issue came up - the shed (also steel) has a North
> to South roof, so the problem would be even worse.
> 
> Cheers
> 
> M
> 
> -- 
> Matthew Smith
> Smiffytech - Technology Consulting & Web Application Development
> Business: http://www.smiffytech.com/
> Personal: http://www.smiffysplace.com/
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> 
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