Satgen 565  SatSignal Reception 1  by GM4IHJ  (BID SGEN565) 2000-01-22

Watch as an HF satellite signal fluctuates as you record it on an
FFTDSP display, or , catch the changes in tone and pitch as you listen
to it on the old fashioned but extremely discerning ear/brain
biological computer. Back in the 1960s almost all satellites used HF
cw downlinks, and no one worried too much about perfect reception as
Sputnik bleeped and its successors informed us of their status using
slow morse or, the simple pulse length codes favoured by the Soviets. 

By 1970 things started to get more sophisticated. We had Amateur Radio
satellites we could use for voice qsos. But with increasing complexity
came a realisation that the signals often faded badly. Worse still ,
the operator at one end of the qso , could have good reception , while
the man at the other end often struggled. Clearly he was a poor
operator. Or was he ? It took a surprising amount of time for
operators to realise that putting satellites up into the clear blue
yonder, did not always mean perfect reception back on mother earth. 

The signal had to transit an often unforgiving ionosphere on the way
down. It mattered greatly, particularly in years of high solar flux/
sunspot cycle peaks, that receiving the satellite downlink through a
breakfast time early morning thin ionosphere , was much easier than
trying to copy it at midday when the ionospheric density was much
higher. Indeed with the HF downlinks in use at the time the
downlink frequency was often quite close to the maximum usable
terrestrial propagation frequency , and common sense said if the
ionosphere was refracting signal coming from the ground back to earth,
the ionosphere was unlikely to be transparent to signals at the same
frequency coming down from space , and by 1986 almost no commercial or
military satellites used HF downlinks below 22 MHz , and only one or
two used 27/29 MHz downlinks.

Indeed if the ionosphere did not ruin your dowlink, Faraday rotation
would make it fade in and out several times during a satellite pass. 
Unfortunately the lower the signal frequency the greater the rotation
of signal polarisation by Faraday rotation, as the signal comes
through the ionosphere. Faraday is usually corrected by using circular
polarised antennas. But circular polarised HF antennas are very bulky
, so they were rarely used.

Fortunately there were people in Amsat who could explain much of what
was going on. So operators got used to the fact that night time orbit
passes of HF satellites were usually quiet and trouble free, given the
much reduced density of the ionosphere over the darkened earth.  But
most American satellites were sun synchronous coming past at roughly
the same time day after day. Great if you lived in New York and the
good orbit passes occured at breakfast time but bad if you lived in
Scotland were the same pass went through during your early afternoon.
Peace was restored however when the Russian RadioSport satellites
began to appear in the 1980s. They were not sun synchronous, so some
months you got good orbits, others you had to make do with less than
perfect ionospheric downlink passage . At least everyone knew that the
new mode B sats would be free of all troubles. A supposition which
turned out to be wishful thinking.