NavList:

Peter, you wrote:

"The experiment you are working you way towards carrying out involves observations made over a distance parallel to the ground, close to the ground. Refraction will depend on your atmospheric conditions there, and could be different to the refractive effect from sighting from eye height skywards."

 

That's called terrestrial refraction. Although it is variable, it is typically 0.1 minutes of arc per nautical mile and only rarely more than 0.5 minutes of arc per n.m. Even if they decide to run this test over a range of 500 feet, the refraction will be negligible. This refraction would apply to BOTH beams similarly, of course, and so the difference in refraction would be smaller still. In short, refraction is completely negligible for this test.