RE: Site information why elevation..

Ted Molczan (
Thu, 10 Jun 1999 12:06:35 -0400

I wrote:

> >A skilled visual observer can measure satellite positions to a
> precision
> >approaching 0.01 deg. At a range of 500 km, that is 87
> metres. For that
> >precision to be useful, the observer's position must be known to the
> same
> >precision, which makes elevation relevant.

Dale responded:

> This all presumes the elements and predictions have that kind of
> accuracy...

> I mean do your common week old elements for a LEO satellite calculated
> with SGP4 give you a positional accuracy around 100 meters,
> or 1 meter,
> or 1000 meters?? That may determine if knowing your elevation is even
> relevant.

We are talking about two different issues. My comment pertained solely to
making precise positional observations, not predictions.

The purpose of positional observations is to produce data of sufficient
precision to enable the satellite's position to be determined in
three-dimensional space. Since the measurements are relative to the position
of the observer, his/her position in three dimensional space must also be
known, and to an accuracy equivalent to that of the observation. Since 100 m
cross-track positional accuracy is readily achievable, we need to ensure at
least that accuracy in our elevation.

The observer's precise altitude rarely is an issue in making predictions of
sufficient accuracy to acquire an object. The only time I ever found it
important, was in predicting a shuttle pass that I hoped to observe from a
Boeing 767 I on which I was a passenger. Turned out that it was going to
enter eclipse before overtaking the plane :(  And the pilot was not amenable
to turning the plane around briefly so I could get a better view :((

Ted Molczan