Satgen458  Formation Flying (2)  by GM4IHJ  (BID SGEN458)   3 Jan 98

Satellite constellations with individual satellites spaced at equal
intervals around a particular orbit plane, plus other satellites
similarly placed in other orbit planes. Require complex inter
satellite and, satellite to ground links. The frequencies used for
these links are chosen to suit 2 different tasks.

Up and Down links between the terrestrial mobile units and the
satellites. These paths transit the atmosphere. Which usually calls
for frequencies below 2.5GHz, in order to mitigate the effects of
rain, hail and snow. All the satellites mentioned in Sgen457 , with
the exception of the Teledesic constellation, use link frequencies
below 2.5GHz. The Teledesic satellites use 20/30 GHz links which would
be susceptible to rain attenuation . However these satellites will
only use that part of their ground footprint which is above an
elevation angle of 40 degrees. Thereby providing a very direct near
vertical path down through the atmosphere which hopefully will
overcome the atmospheric attenuation problems in inclement weather
conditions. There are however serious penalties inherent in this
solution. The Teledesic satellite footprints will be less than 7
degrees radius = 13 degrees or so maximum spacing between satellites.
Indeed in practice the teledesic satellites are planned to be only 9.5
degrees apart. Hence the very large number of satellites required for
complete coverage.

Links between satellites, and links satellite to/from fixed ground
stations. In this case the satellite to satellite links are well above
the atmosphere so 30GHz links are possible. While in the satellite
to/from fixed ground links high frequency 20/30 GHz links are proposed
noting that higher power and better antenna siting at the ground
station should compensate for atmospheric absorption losses.

Doppler shift is an important consideration in both the above
situations. The higher the frequencies used, the greater the doppler
shift. It is therefore likely that some systems will use a form of
doppler compensation in the hardware of handsets and feeder links.

General remarks. This whole exercise is very interesting , noting the
varying complexity of the equipment being used. Which includes a
simple store and forward slow data only system, bent pipe systems
using simple transponders in the satellites and relying on ground
station connections to carry calls worldwide, versus systems of far
greater complexity which require very sophisticated mobile handsets
and satellite hardware and software. 

It could be asked whether some of the simpler systems will be out of
date before they become operational. Or, whether some of the complex
systems may be too ambitious . It will be interesting to see which
systems prevail. Although in the long term it would seem that the ever
increasing call for greater bandwidth may be the deciding factor .
With only the 20/30GHz systems surviving for very long. 1998 should be
very interesting.