Satgen 541 Satellite Inclination 3 by GM4IHJ  (BID SGEN541) 1999-08-07

As described in previous satgens , it is possible to use retrograde
orbits ( inclination greater than 90 degs), such that a satellite
passes over the same land mass at roughly the same time of day, every
day , for years. In what is called a Sun Synchronous orbit.

There is another type of orbit, usually refered to by its Russian name
MOLNIYA, whereby a satellite placed in an orbit inclined at about 63
degs, will follow the same track over the earth , staying almost
stationary in the sky for several hours each day, for years. These
MOLNIYA orbits are of little value for low altitude satellites. But a
MOLNIYA with an orbit period of about 12 hours, can be arranged to
appear to hover above the earth for several hours whilst near its
apogee orbit high point. So that from the view point of a ground
observer it appears to be almost stationary, fixed in the sky.

For the second half of the day the next 12 hour orbit causes the
satellite to hover above a longitude 180 degrees around the earth from
the earlier orbit. Between these two stationary interludes, the
satellite descends rapidly from its 36000 km apogee high point ,
rounding the other lower hemisphere of the earth at high speed with a
perigee low point altitude of 300 to 700 kms. Whilst at apogee the
satellite sees a very extensive footprint of about 75 degs useful
radius. By contrast its perigee passage footprint is much smaller 
around 25 degrees radius. 

Two amateur radio satellites were originally planned for MOLNIYA type
orbits ( Ao10 and Ao13 ). But launch problems/ changes of plans and a
desire to give coverage in both the northern and the southern
hemispheres of the earth, eventually meant that neither satellite
achieved a true MOLNIYA orbit. So they wandered from one hemisphere to
the other in the years that they were fully operational. By contrast
Russian EKRAN UHF TV satellites in true MOLNIYA orbits have provided
superb TV coverage to large areas of Asia , day after day for many
years. Noting that the true MOLNIYA orbit has the extra advantage that
the satellite transits the dangerous Van Allen Radiation Belts very
quickly in its apogee to perigee, perigee to apogee dashes , then
hovers over latitude 63 N a region where the Van Allen Belt radiation
effects are greatly attenuated. 

While equally important, from the point of view of satellite life
expectancy. The almost geostationary aspect of the MOLNIYAs slow
passage through apogee , allows excellent repetitive pointing of the
satellites solar arrays for receipt of maximum solar insolation. This
has the effect of giving the satellite a very regular battery
charge/discharge cycle. A factor which is conducive to long battery
life. Whereas by contrast, the low earth near circular orbiters which
adopt the Sun Synchronous retrograde orbit have a much less attractive
battery usage cycle. In which the battery gets little use whilst the
satellite is Sun Synchronous, but, the moment when as must inevitably
happen after several years, the orbit slips out of sunsynch, the
battery experiences a sudden change in work cycle ,cannot take it, and
dies.  Usually rendering the satellite inoperative.