Satgen 436   Orbital Basics  by GM4IHJ   (BID SGEN436)     2 Aug 97

Most radio amateurs use sophisticated software, written by someone
else, in order to follow the satellite of their choice. One effect of
this total reliance on other peoples efforts, is that when something
goes wrong, all is lost. Eg as when corrupt Keplerian elements caused
half a country's schools to listen for Mir , when it was elsewhere.

This type of man made blunder, will occur again, but it can be easily
avoided if satellite users are aware of a simple check which starts by
looking at the previous days log of times heard, and comparing it with
times predicted by new ( possibly rogue ) elements Eg :-
A sat heard the previous day at say 1400 ut. Having an orbit period of
say 100 minutes ( you get period from 1440 minutes / N0 ), should
reappear next day 100 minutes x 14 orbits later = 1320 ut approx.
If the new Keplerian elements give a very different solution , this
"back of an envelope" check, should clearly show them to be corrupt.

The only variations on this pattern occur with satellites/space
stations in the really low altitude orbits. They lose altitude rapidly
due to drag in the thick atmosphere. So they have to be pushed back up
40 kms or so every 4 or 8 weeks. But this should not catch you out if
you keep a careful check on roughly how often say, Mir does this. At
present , the solar activity cycle is near minimum. So the upper
atmosphere is very thin, and hence Mir and co, do not need altitude
adjustment more than once every two months or so.But by 1999 or 2000 ,
what ever space stations we have up there are going to need much more
frequent short engine burns to keep them up at a safe altitude.

Up till now, very few satellites orbiting above 400kms altitude , have
carried engines and fuel for orbit adjustment ( the exception to this
general rule is the geostationaries which need precise station keeping
and have thrusters and fuel for this purpose). But times are changing
. The advent of the LEO Low Earth Orbit commercial satellite is
causing this change. Typical of this new class of satellites are the
Iridium system Mobile Phone Sats, which need to keep precise
individual stationing and separation.

Launching constellations of LEOs is not easy. Iridium sats are going
up from USA and Russia in groups of 5 or 7, on a common carrier bus,
which drops them off one at a time , so they all have roughly the same
orbit and speed. But they are not properly spaced in their operational
configuration. Shortly after launch each individual sat is raised to a
higher orbit on its own manoeuvering engine , and "hopefully" placed
in exactly the right separation from its neighbours. Thereby allowing
users to be sure that as one satellite goes below their horizon,
another is already coming into range at the opposite horizon .
Permitting  continuous coverage, and much more difficult , able to
connect from the satellite above their horizon to another satellite
capable of daisy chaining their call to a destination far beyond their
horizon and that of the original satellite. Unfortunately one Iridium
bird is already refusing to accept manoeuvring commands. So there is a
hole in the formation. A problem which could well face proposed
constellations of radio amateur LEO sats such as Picosat.