[time-nuts] Choke-Ring Antennas for GPSDO use?

[Björn Gabrielsson]

Hi Michael,

On Wed, 2008-01-09 at 10:30 -0500, Michael Baker wrote:
> Hello, All--
> 
> Over the past couple of years I have seen several references
> indicating that the reason all GPS units intended for
> precision survey applications use a choke-ring antenna is
> that a properly designed GPS choke ring antenna will cut
> multi-path reception down very significantly.  Other
> references I have seen indicate that using a choke-ring
> antenna results in a moderate improvement in GPSDO performance.
> 
> Can any list members comment on this?  If a choke-ring
> antenna does improve GPSDO performance, what degree of
> improvement can be expected?  Enough to be worthwhile?
> Would switching to a properly designed choke-ring
> antenna on my Trimble Thunderbolt GPSDO be worthwhile?
> My house is surrounded by large trees and my roof-top
> GPS antenna does not have a foliage free view of the
> sky until about 40-degrees above the horizon.  I have
> set a horizon elevation mask of 30 degrees in my T-bolt
> software but have wondered if multipath through the
> remaining tree-tops may still be a slight problem.

A gps SV will circle the earth in 11h58min. Log your positions
(pseudoranges even better) for a few days. Check if you measurements
correlate at 11h58min or 23h56min. If comparing positions you want to
check that the exact same satellites are used in the positions you
compare. Look for errors larger than the normal noise that repeat at the
above intervals. One or several of the satellites used at those times
are probably affected by multipath.

With 1ns equaling about 30cm position error you can judge if your
position is stable enough.

If it looks to bad, then try to find another siting, try other elevation
masks or try another antenna.

If you are able to map the multipath errors very well, you could
possibly block bad directions. If you can make your own solution the
errors could even be corrected.

> Only very rarely do used GPS choke-ring antennas show
> up on eBay and they typically ask around $600 to $800.

Ebay # 280027627565 is $350 for new (old stock), from a real company.

[ snip]

> So-- can any list members shed any light on the efficacy
> of switching to a choke-ring antenna on a GPSDO unit?

> Mike Baker
> WA4HFR
> Gainesville, Florida

--
   Björn