Satgen 637  AO40 Signal Patterns  by GM4IHJ  (BID SGEN637) 2001-06-09

Almost all satellite downlink signals display fade patterns. Some are
irregular and are invariably due to ionospheric disturbance or
discontinuity resulting in phase or amplitude scintilation. A
particular case in point which affects satellites every day , is
polar auroral zone scintillation, a regular feature on downlinks from
20Mhz to 3Ghz, when signal from a satellite perhaps far from this
zone none the less have to cross the zone to get to your antenna.
Both RS12/13 and the Japanes mode J satellites are prone to this kind
of irregular signal disturbance. But recent correspondence has asked
about two kinds of satellite signal disturbance which have a regular
repetitive pattern. The first of these is very similar to what some
handbooks refer to as " Aeroplane Flutter". The effect you get on
reception of a distant terrestrial stations signal when an aircraft
is present near your station . The aircraft being position so that it
reflects the Dx signal to you sometimes in phase with the direct
signal, sometimes out of phase. Thereby giving you an enhanced signal
one minute and no signal at all the next minute.

A satellite approaching your station can produce a similar effect
particularly if the approach to you is over water or flat land
unobscured by buildings . In this case you get a direct signal and a
ground reflected signal , fading and rising as the two signal
alternate in and out of phase as their relative path length varies.
World War II radar operators were very familiar with this effect as
aircraft approached on paths where their reflected signal came into
the antenna via the direct path and via ground or water reflection.
Indeed it was useful to know just were and when in your antenna beam
these fades would occur noting that it gave clues to the aircrafts
height. Moon bouncer with limited antennas often find that at certain
elevation angles of the Moon they get a ground reflection which adds
in phase to the direct signal thereby enhancing it. If however the
ground near your station has an irregular profile or is cluttered
with buildings , you are unlikely to see enhanced satellite or EME
signals.

A second type of fading(?) , now being reported appears to have
nothing to do with ground reflection or the ionosphere. It is seen on
the AO40 S Band telemetry signal, and is particularly noticeable when
the signal is processed using FFTDSP software and displayed as a
record of its audio spectrum. In appearance this so called fading
takes  the form of a pattern whereby certain elements of the
telemetry pattern remain visible long after most of the pattern has
disappeared completely. Knowing this you can often follow AO40 when
it is far too weak to read. In practice the signal shows peaks
roughly every minute depending on the particular sequence pattern the
telemetry is transmitting. So this cannot be ionospheric or
reflective in origin. It is not really a fade at all, it is simply
that the signal has greater power output/consistency in parts of the
telemetry cycle usually repeating once every minute. At IHJ this has
allowed checking of AO40 activity , and meaningful orbit tracking
even when the satellite is at low elevation or a poor squint angle.