Satgen 467  Space Propagation Pt6  by GM4IHJ  (BID SGEN467)   98-03-07

It is snowing here in Scotland . and, a Pegasus rocket has just
launched T1 , Teledesics first Ka band test satellite into a 565 km
altitude 97.7 degree inclination orbit. What is the connection ?
Teledesic plan to operate satellite up and downlinks in the 20 and 30
GHz bands. Frequencies which do not like rain, snow or oxygen. 

Satellite T1 is the precursor of a 288 satellite fleet of Low Earth
orbit (LEO) " fibre like" broad bandwidth access, communications
links, which it is hoped will provide high speed internet connection
anywhere, plus a lot of other facilities (ie video conferencing )
which work best in a wide band environment. T1 will therefore be doing
a great deal of testing to see if its high frequency links can over
come the propagation problems caused by the weather and the state of
the atmosphere.

Like Teledesic , amateur radio will be forced to go higher in
frequency if we want to use wideband systems. Teledesic simply cannot
get the bandwidth required at lower carrier frequencies. So finding a
way to overcome propagation attenuation caused by rain, snow and the
actual molecules in the air, is vitally important.

Molecules.   The principal villains are water vapour and oxygen. At
20GHz water vapour losses peak, decreasing slightly at 30 GHz. Whereas
oxygen molecules cause very little loss at 20 GHz , slight losses at
30 GHz then increasingly higher losses if the frequency is extended up
to 60 GHz. The losses due to oxygen are fixed for a particular
atmospheric transit path, but losses due to water vapour increase
greatly as humidity rises, being highest in summer and lowest on cold
winter nights.

Rain and Snow. Attenuation in rain is highly variable changing from
minute to minute as the density of precipitation along the signal path
changes with rapid shifts in the overall weather pattern. Light
drizzle being far less of a problem than heavy rain. Attenuation in a
snow fall is roughly similar to that in rain and equally variable, but
snow has the added disadvantage that it can collect on the antenna.

The critical feature in all calculations of propagation loss during
transit of the atmosphere, is the length of the actual signal path in
the atmosphere. A satellite high overhead has a short path. By
contrast a satellite at low elevation has a very long path, much of it
through the densest part of the atmosphere. So it is no surprise that
Teledesic plan to overcome atmospheric attenuation, by only using
their operational satellites over that part of their ground footprint
where their signal into ground stations is on a path with an elevation
angle greater than 30 degrees. 

Operators who track T1 will be exploring relatively new territory ,
only previously available via the Olympus geosat. The results will be
very interesting in respect of what they can tell us about the 
possibilities for Amateur radio at these high frequencies.