[time-nuts] Mounting GPS Antenna on Steel Roof

Keith Payea kpayea at bryantlabs.net
Tue Mar 4 11:42:40 EST 2008


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/
LinkedIn: http://www.linkedin.com/in/smiffy

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