Satgen295  Satellite Problem No 8 Orbits   by GM4IHJ         19 Nov 94
BID of this msg is SGEN295  Please use this BID if you retransmit this msg

There is no such thing as a perfect orbit that will satisfy all user
requirements. The LEO Low Earth Orbit has the problem that the easily
accessed height zone 350 to 420 kms ( Mir and Shuttle ), is subject to
atmospheric drag and , in consequence , demands the use of onboard engines
at least once a month in order to maintain height. Equally difficult, the
" footprint" view from this low altitude , sees only a small portion of
the earths surface at one time , which reduces communications times to
ground stations, whilst the 15 or so sunrises and sunsets every day,
severely curtail useful experiment time.
Going to a higher altitude LEO orbit takes a lot more fuel, but once there
orbit decay occurs only very slowly over several years because of the much
reduced atmospheric drag, and, the footprint on the earths surface seen by
the satellite is much bigger than that for Mir. Unfortunately however
modern digital satellites cannot go higher, because their delicate
software is severely challenged by the radiation levels encountered over
the South Atlantic Anomaly. By contrast the sturdier analog satellites can
go to higher altitudes and survive for years at heights up to 1600 kms or
so , from where they can see a very large footprint indeed, thereby
providing communications links to 6000 kms or so , bringing greater
service to a much larger section of the radio amateur population.
Equally as important as orbital altitude , orbit inclination angle has a
marked effect on satellite performance. However it is much more
constrained by the location of the launch site than by any other feature.
Countries with launch sites on the equator get the full launch boost from
earth spin as they launch eastwards from low latitude sites, but sats in
low inclination equatorial orbits see very little of the world excepting
that around the equator. Some countries such as the Russian CIS, are
restricted in launch site latitude because they have no useful land below
latitude 49 north or so, and launches towards the equator rapidly leave
their territory and overfly other nations in the dangerous early part of
the launch. So few Russian satellites launch to inclinations below 50
degrees.
In the USA a slightly different restriction applies to the Cape Canaveral
site. Located at 28 degrees latitude it can lauch down towards the equator
to orbits with a minimum inclination angle of 28 degrees, but launches to
higher inclinations are only possible to inclinations up to 57 degrees.
Above that angle any launches would overfly Washington, New York, and
Boston before they achieved safe altitude.  Naturally the Americans do not
take this risk, and they are further encouraged not to do so because
launches to inclinations above 57 degrees would spend a lot of time over
sub arctic and arctic regions where there are no safe landing sites in the
event of an unscheduled descent from orbit. Landing safety factors do not
apply to unmanned satellites so many of them do go to much higher
inclination angles, Eg the Russian LEO sats to 82 degree inclinations and
the US sats to 92 degree inclinations, where they see the whole world
twice a day and view the important "weather producing polar areas" every
orbit. This might at first seem a good orbit for amateur radio satellites
but it should not be forgotten that polar orbits spend a lot of time over
areas inhabited only by Penguins or Polar Bears, neither of whom are
licensed for amateur radio communications. So polar areas are not all that
useful for amateur communications opportunities. 73 de GM4IHJ @ GB7SAN