Satgen 437  Orbital Basics 2    by GM4IHJ   (BID SGEN437)   9 Aug 97

Problem . You have 5 low earth orbit satellites going aloft on the
same rocket. You want them placed in the same orbit plane, 30 degrees
apart, period 100 minutes. At first sight, the simplest method would
seem to be - carry them aloft on a common bus and release them at
suitable intervals. But, consider what this means. Your bus is
travelling at 7500 metres/sec. Each release every 8.33 minutes gives
the individual satellite a push of 1 metre/sec. So when all are
released , the whole group from first to last are following closely
behind the bus , all having retained roughly the same velocity.

To get 30 degree separation around the orbit, all going at the same
speed, one of several different techniques can be used. EG the initial
launch is to an orbit below the final one. the satellites are dropped
from the bus and remain in a loose group. Then, one at a time in a
strict sequence they use their onboard engines to raise their orbits. 
In this higher orbit their orbit period is longer and they lag behind
the lower sats. Then as separation reaches 30 degrees, another of the
lower orbit group is pushed up to the higher orbit. This process being
repeated until the satellite which was raised last in the group of 5,
is now 120 degrees ahead of the first to be raised up.

The above description has been drastically simplified for clarity. So
no one should imagine this kind of orbital formation flying is easy,
to achieve or maintain. Worse still, Murphy usually intervenes, as it
has with the latest Iridium Mobile Phone sats. One of which presently
refuses all orders to take up station. So sometime in the future a
special accurate placement launch may be required to fill this gap.

At around 770 kms altitude the Iridium sats have a footprint angle of
around 27 degrees. Which at 27 + 27 , easily links two adjacent sats
in the same orbit plane. With 6 orbit planes, ascending and descending
planes are roughly 30 degrees apart in longitude. So again inter sat
connection of sats in adjacent orbit planes near the Equator is quite
adequate, even allowing for considerable overlap, so that connection
is possible with satellites well above one anothers horizons. In this
way smooth tranfer from a satellite departing your horizon to one
entering your horizon should be assured, and satellites in adjacent
orbit planes should stay in mutual contact for reasonable periods of
time, even near the Equator, where maximum spread between orbit planes
occurs. At higher latitudes as satellites climb to their 86 degrees
highest north or south, they get closer and closer together, so there
is no problem there . Indeed as used to happen with RS5 to 8 , polar
ground stations were spoilt for choice and, in the case of the mobile
phone sats some priority system may be necessary when a ground station
may perhaps "see" up to 6 satellites at any one time.

It is therefore, going to be of continuing interest , to observe the
unfolding pattern of events, as this Iridium constellation gets into
,and maintains formation.