Satgen 438 Orbital Basics 3  by GM4IHJ    (BID SGEN438)    16 Aug 97

Correspondence following sgens436/437, questions whether 66 satellites
in Leo orbit 86 degree inclination , 6 orbit planes, is the most
economical way to get continous 24 hour coverage, full earth
communications. Computer simulation at IHJ suggests that if not
optimum, it is not far from it . The wasteful crowding of satellites
near the poles being a necessary evil, if full coverage is to be
obtained at the equator.

But ??, there are some variations on the theme which might apply to a
somewhat less ambitious amsat network. Study of a typical series of
equatorial leo orbits suggests at least one kind of interesting
variation. Eg consider the coverage of satellites orbiting at 0
degrees inclination :
Sat Ht kms   Footprint radius  Reasonable coverage  No of sats?
500                22            18N to 18S           9 or 10
750                26            22N to 22S           8 or 9
1000               28            24N to 24S           8
1500               36            32N to 32S           6
1800               39            36N to 36S           5

For complex digital satellites radiation damage is a problem above
about 800 kms altitude but analog voice and cw sats Eg RS5 to RS8 have
survived up at 1800 kms for years. So while a low altitude
constellation of amsats is probably a non starter, a quintet of
radiation hardened voice/cw relay sats interlinking at microwave
frequencies may be a way to go . Certainly , present evidence suggests
that the Phase 3D route is fraught with enormous expense and launch
doubts . So would a trial system of this type , covering the
equatorial and sub equatorial countries have any appeal ?

To obtain greater coverage up to the higher latitude temperate zones
of earth, calls for many more satellites eg :-
1000 kms altitude 28 deg inc covering 54N to 54S requires about 24
satellites. This excludes only Alaska, Northern Europe (Scotland ,
Scandinavia and the Baltic states). So it might be a good compromise .
Although an even higher orbit altitude suitably radiation hardened
would bring in these northern countries.

But it must not be forgotten that this sort of network requires
precise launching and post launch satellite positioning with long term
station keeping . All of which require manoeuvering engines and fuel,
with the fuel likely to be a service life limiting factor, as well as
a serious in orbit weight penalty. Last but not least this theoretical
combination will require specific narrow tolerance launch facilities.
With amsat usually taking any launch they can get , launching a group
of satellites into a useful formation may be out of the question. 

At present the commercial Iridium fleet is 16 working sats in orbit
,succesfully tested for east west and, in orbit plane, inter satellite
links, with a 17th satellite apparently having failed in orbit. Next 5
sats launch via a Delta rocket on 17 Aug , with 7 sats going up on a
Russian Proton in Sept. Now, the big question coming up with everyone
in the space business suggesting they are going to launch LEO sat
fleets, is where will all the launch rockets be built. Certainly there
is no sign of sufficient building capacity being available at this
time , and as Murphy will attest - No rocket - No Orbit.