[CQ-Contest] Using GPSs and Google to place antennas

[Ethan Miller K8GU]

If you want to do surveying with the GPS, you either need to use
differential GPS or dual-frequency GPS and even then, there are some
(minor) caveats.  It turns out that the ionosphere is one of the
largest uncorrected errors in the GPS.  (A ham, now W1TEC, developed
the internal ionosphere model used with the GPS, by the way.  But,
it's relatively crude by today's standards.)  If you use two GPS
receivers very near each other such that the GPS signals pass through
the same volume of ionosphere, it is possible to compute a more
accurate relative navigation solution.  You can also use the
dispersion characteristics of the plasma (in the high frequency limit)
to do a dual-frequency solution.  Of course, the local multi-path
environment can distort your navigation solution as well!

Both the differential and dual-frequency techniques I mention require
special hardware that's not generally within the reach of the average
consumer.  The dual-frequency units are export-controlled.  If you
have a friend who's a surveyor, go for it, though.

For now, I'll be using a compass and a tape, like K1TTT suggests.

I have also noticed significant (10's or 100's of meters) variations
even between very accurate and well-placed GPS receivers (averaged
over months) and Google Earth.  Even if you have the most accurate
receiver in the world, it depends on the receiver (or model points)
they used to generate the map.  I think to first order, Google Earth
is a great planning tool like you mention.  But, I wouldn't count on
it for a lot of fine work.  I have built models of buildings and
towers in SketchUp and imported them into Google Earth just to see
what they look like.  I'm still a fan of topo maps, too.

GPS is good and getting better.  But, it's hard to get the last few
meters of accuracy!

73,

--Ethan, K8GU/9.  (Who spends most of his days studying ionospheric
effects on the GPS and displaying the results with Google Earth.)
http://www.k8gu.com/