Re: definition of "LEO" (was Globalstar)
Sean Sullivan (tssulliv@unix.amherst.edu)
Mon, 16 Feb 1998 06:01:13 -0500 (EST)
I have never seen a "definitive" definition of LEO, and don't know that
such a thing exists. Here's my interpretation: There is a sharp tail-off
in the number of spacecraft per unit altitude when you get above 1000 km,
dropping very low by 1500 km, and virtually zero at 1800 km. As you go
through altitudes corresponding to more intense regions of radiation in
the magnetic fields, the s/c count stays close to zero - no one really
wants to put their spacecraft there unless you have a really good reason.
Then the count goes up dramatically as you get near GEO.
In between LEO and GEO, there are a few spikes (such as GPS and Glonass
altitudes), but these are after a "null zone" that falls above LEO. Then,
of course, there are also the Molniya-orbit (plus a few GTO) spacecraft
that cycle through this region -- I'm leaving highly eccentric sats out
of this for now.
So, if someone comes along and asks "where's LEO?" how does one answer?
The basic idea is that it's the area high enough to permit an orbit and
low enough that it's below the "null zone" where there aren't any
spacecraft. Just where you put the numerical upper bound depends on
how close to zero you want the "spacecraft per 100 km" statistic to be.
Speaking casually, I'd say 1000 km; since there's the whole gaggle of
Russian s/c near this altitude, anything lower would be silly. But
then there's EGP and the apx 1600 km Russian comsats, and knowing that
this region of space is used, I'd put the limit higher if I'm not just
trying to explain the idea of where LEO is to a novice. Offhand I
don't know of anything much above 1800 km, so that seems like a safe
value, but maybe I'm forgetting something and will change my mind :)
Not that the upper bound of LEO can be pushed higher and higher by
single-case exceptions. The meaning, I think, is the region with a
significant number of spacecraft below the altitude that is generally
considered uninhabitable for non-hardened electronics.
-- Sean Sullivan
On Mon, 16 Feb 1998, Ed Cannon wrote:
(*snip*)
> 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?