Roy Lewallen wrote:

I wasn't aware of any GPS receivers using crossed dipole "turnstile"
type antennas. All the ones I've seen use either quadrifilar helix or
patch antennas. Can you point to a reference or two regarding the choke
rings -- I don't know what these are or what they do, and would like to
learn.

. . .

Roy Lewallen, W7EL


For high performance GPS receivers and measurements, it's important that
the apparent position of the antenna be be independent of the look angle
to the various satellites. Partly this is by making antennas with a
phase center that is look angle independent, partly this is by making
sure you're not receiving a combination of direct and reflected waves.

Remember that for precision GPS, what you're looking at is essentially
the carrier phase within a single chip time (about 100ns). The carrier
phase outside the correlator's time window doesn't contribute to the
observable measurement (because it's got a random 180 degree phase shift
superimposed on it).

So what you're really worried about is interference that causes an
apparent change in phase of the carrier (at 1.5GHz.. call it 20cm
wavelength). In precision GPS, you're talking millimeter scale
measurements, or, say, better than 1 degree of apparent phase shift. A
reflected signal that is 35 dB down is enough to get this sort of error.


The multipath from "distant" reflectors is fairly easily dealt with by
putting the antenna on a pole. Distant, here, means a few meters away..
farther than that, and the multipath signal's code phase is far enough
away that it doesn't contribute to the measurement. The chip length is
about 100 ns, or 30 meters. So, putting the antenna on a pole a few
meters up, with a plate underneath it that cuts off the "view" of the
ground closer than a few meters away guarantees that you won't see any
reflections from something closer than 20 meters path length different.

You also choose an antenna that has very little gain below several
degrees above the horizon.

But, just any old flat plate won't work, because you can have a creeping
wave propagate across the surface AND you don't want the plate itself to
reflect a signal. Solution: make a plate that is a RF "black hole" at
the frequency of interest.. it's a series of grooves that are carefully
designed to attenuate the reflected and evanescent wave propagating
across the surface (just like in a corrugated horn).

The most common design is by Dorne and Margolin, and I guarantee you've
seen these if you've seen surveyors doing GPS measurements. They're
about 2' in diameter with several wide grooves around a small conical or
hemispherical radome covering the actual antenna. Sometimes, the whole
thing is covered by a hemispherical radome.

http://facility.unavco.org/project_s.../antennas.html

Is a photo of a typical geodetic installation (sub-mm accuracies)

http://www.trimble.com/infrastructur...?dtID=overview

is the actual antenna.