Site elevations

Allen Thomson (
Wed, 9 Jun 1999 12:01:17 -0500

>     I am curious why do we need to specify elevation in addition  to
> longitude and latitude. Does this affect the precise apparent
> positioning fixing of satellite? What is the error involved comparing say
> Sea-level vs someone high up in Colorado..

>I did a quick experiment using quicksat with my latitude/longitude but
>for two different elevations: 0 and 10,560 feet (= 2 miles = 3.22 km).
>For one night's worth of predictions (of "LEO" objects), the largest
>difference in culmination point (basically, the highest part of a pass)
>seemed to be about 0.1 degrees in declination and/or 1 minute of Right
>Ascension. So unless you are into precise positional measurements,
>the elevation probably isn't worth worrying about.

Correct. You can also do a BOTE (back of the envelope) upper-bound
calculation to come to a similar conclusion:

Most people live at between 0 and 3 km altitude, and most satellites have
perigees of more than 200 km. So the maximum paralax between sea level and 3
km when
viewing satellites would be 3/200 =  0.015 radians = 0.86 degrees, 50 arc
minutes. Actually, it's considerably less, as the satellite would be
directly overhead at 200 km range, and in that geometry observer elevation
differences would be along the line of sight and wouldn't matter at all.

So, unless you're trying to point a telescope or fine-tune orbital elements
by differential correction (both of which some people do, and can be
important, I hasten to say), observer elevation isn't that big a deal.