Satgen 427  Satellite Conga Lines? by GM4IHJ (BID SGEN427)   31 May 97

Now that the first of the LEO Mobile Phone satellites are up, (Iridium
1 to 5 launched May 5th 97). It is possible to follow what might one
day be a very difficult future task for amateur radio, if we ever
decide to go for a fleet of LEO mini comsats. Noting that our orbit
placements in the past have sometimes varied greatly between design
and operational reality. 

The Iridium concept would seem to call for 60+ satellites spaced
evenly around the world in groups, each group following an orbit path
differing in right ascension from that of it neighbours , with
individual satellites spaced evenly around each groups orbit circle,
so that overall coverage is worlwide 24 hours a day, with no holes in
the pattern . But if they intend placing the satellites in carefully
designed patterns, that is not going to be easy . On 27th May 97  in
range times at IHJ were -

Ir5   1543 to 1554     then next orbit   1719 to 1733 z
Ir4   1553 to 1604                       1730 to 1743
Ir3   1558 to 1611   no overlap to Ir2   1737 to 1752 no overlap
Ir2   1615 to 1628                       1754 to 1809
Ir1   1625 to 1638                       1805 to 1819

But be careful, this pattern is deceiving. Next day Iridium 4 and 5
were nearly 40 minutes ahead of the rest. Not surprising noting that
their mean motions are roughly 0.32 orbits per day higher than those
of 1, 2 and 3. So are they going for a fixed pattern one day or have
they other plans. The next instalment of this drama may reveal the
answer , but their intentions are not clear at this moment. Although
this present arrangement will provide a wide range of varying inter
satellite test situations.

The Iridium phone downlink frequency band is reported as being 1621.35
to 1626.5 MHz. Intersat links and gateway/main ground station links
are located around 23.28 and 19.5 GHz respectively.

The only other commercial LEO system of note to presently have
anything in orbit , is Orbcoms store and forward system . Two
satellites designated Osc1 and Osc2 were launched in April 1995. But
to date no others have followed. Although Orbcoms internet pages say
they will have 26 satellites plus spares in orbit in 1998. 

At present Osc1 is a very strong signal local afternoon , evening and
early morning time , on 137.708 MHz plus or minus 4 KHz of doppler.
The signal is very loud broad band noise,on an ordinary communication
receiver, with its keying apparently above the audio threshold ( as
with the Uosat store and forward sats) . Just what 26 of these sats
will produce by way of QRM to other sats on 137 MHz remains to be seen
But this could soon become a point of concern for Wefax operators if
the Orbcom sats spread across the band. At the moment nothing has been
heard at IHJ from Osc2 the other Orbcom sat in orbit .