[TowerTalk] GPS receivers

[Jim Lux]

At 11:04 AM 9/17/2004 -0400, Dick Green WC1M wrote:
>HFTA is a terrain analysis program developed by Dean Straw, N6BV, of ARRL.
>It comes free with the ARRL Antenna Book and includes a program called
>MicroDem that is used to capture and process USGS terrain elevation
>information from the Internet. The theory is that terrain has an effect on
>antenna gain. Most of you are familiar with the fact that the gain of a
>dipole over real ground is increased by about 3 dB over a dipole in free
>space. The difference is caused by part of the wave bouncing off the ground
>and reinforcing the wave at the main angle of radiation.

Be aware that HFTA only works for horizontally polarized signals, not 
vertical. So, it would be inappropriate for use with a 4 square, for instance.


>If the terrain varies a lot, as it does in the hilly country where I live,
>an accurate GPS is necessary in order to get the exact location of the
>tower. In some cases, you can do OK by interpolating from a map, but in my



>I use a
>Garmin GPS V, which has accuracy to 3 meters or less.

The accuracy of most GPS receivers (assuming you have a clear shot at the 
sky) is entirely determined by the geometric configuration of the satellites.

If you have differential corrections or WAAS (in newer receivers), then you 
can get better accuracy.  Differential correction can give you centimeter 
level accuracy, but is not common in consumer units.

>It can average any
>number of readings, which will get accuracy down to three feet or so.

Averaging doesn't improve the accuracy unless done over a very long time, 
because it won't remove systematic errors.

Of more importance is to make sure that the datum you make your measurement 
in matches that of the terrain data you're using.  Topo maps, for instance, 
use the NAD27 datum. GPS uses WGS84, and in my area, they're about 30-40 
meters apart.  (it's in the lower left corner of the topo map).  I don't 
know what datum the MicroDEM datasets use, off hand.

I would download the MicroDEM data, then look for my location on the terrain.


>  It
>cost about $350, and I use it a lot in my car -- it has downloadable maps
>and very sophisticated autorouting.