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 <[EMAIL PROTECTED]> 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: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] 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 > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.