Re: Globalstar flares; definition of "LEO"
B Magnus B{ckstr|m (b@eta.chalmers.se)
Mon, 16 Feb 1998 12:23:30 +0100 (MET)
If I've understood the Globalstar satellites correctly, the satellites
have two flat antenna arrays which both point at nadir, i.e. straight
downwards. If this is the case, any flares off the antennas will
miss the earth. A possibility exists if there are wide margins to the
antenna pointing, say +- 5 degrees. Then a reflected beam of light
could strike the earth at a very low angle; thus, a flare could be seen
when the satellite is low on the horizon in the direction of the sun.
The satellite would of course also be very far away -- about 4500 km.
(distance = cot(asin(ER / (ER + H))) * ER,
where ER = Earth's radius = 6378 km, and
H = satellite height above ground = 1414 km)
On Mon, 16 Feb 1998, Ed Cannon wrote:
[...]
>
> The Globalstar people say "low Earth orbit" is less than 1500 km.
> Another definition I read somewhere said below 1000 km. A third
> one said anything below geosynchronous! What's the definitive
> definition of LEO?
>
I've also wondered about this! A long time ago I heard 1000 km
as the limit, and then took it for granted. It's a reasonable figure.
In SGP4/SDP4 propagation algorithm terminology, low earth orbits are
orbits which have a period of less than (or equal to?) 225 minutes.
This equates to about 5900 km for a circular orbit, which isn't very low,
but ... hmm, well, why not? :-)
Clear skies to all
// Magnus Backstrom