Satgen 464  Space Propagation Pt3 by GM4IHJ   (BID SGEN464) 1998-02-14

In satgen 461 the effect of Faraday rotation on a signal traversing
the ionosphere was discussed. In satgen 463 the simple case of a
geostationary satellite signal (not moving with respect to the
station) was illustrated . But the majority of satellites are moving
in low earth (LEO) orbits and in consequence their downlink and uplink
signals are encountering a different part of the ionosphere at each
succesive moment in time. In addition to the above, the path length
through the ionosphere is very long when the satellite is first
accessed at the ascending horizon,the path gets much shorter and
steeper as the satellite passes nearest to your station, then the path
gradually lengthens again being at maximum as the satellite reaches
your far horizon and is lost.

In the experiment recorded here a signal from a low earth orbit
satellite was tracked as it moved from horizon to horizon , and its
150 MHz signal was measured for variation of the strongest received
polarity, in degrees from the station vertical.
Table 1   LEO satellite polarisation angle with respect to vertical
Time    0  0.5  1   1.5   2   2.5   3   3.5   4   4.5   5   5,5   6min
PolDegs 44  -   27  65   31    54   48   76   45   54   69  81    48

Time       6.5  7   7.5   8   8.5   9   9.5   10  10.5  11 minutes
PolDeg     33   43  20   27   20   33   45    64   65   90

Summary. A signal from a LEO vertically polarised antenna produced a
received signal which maximised away from the vertical for the whole
experiment , shifting in an apparently random fashion , sometimes near
vertical, sometimes near horizontal, polarisation.

Please note that at least 3 factors are at play here :-
a. Faraday rotation , varying because the electron density is
constantly changing as the path through the ionosphere changes, due to
the varying nature of the ionosphere from one place to another.
b. Change due to the differences in path length from moment to moment.
Eg long paths at start and finish , short path in the middle of the
pass.
c. The antenna on the satellite is pointing at the centre of the earth
So when satellite is acquired at the horizon its antenna is perhaps 30
degrees tilted clockwise from your station vertical, Alternately when
the satellite is near the end of its pass its antenna may be tilted up
to 30 degrees anticlockwise from your station vertical.

Conditions vary one orbit to another. In an orbit passing at long
range, the misaligment between the satellite vertical and your
vertical is almost constant, and the path is always a long one with a
lengthy almost flat transit of the ionosphere changing very little
during the pass. So factors b. and c. above, are of little consequence
but factor a. still applies. Whereas with an orbit which passes nearly
overhead all three factors apply , and hence the operator has a most
difficult task if he wishes to maximise received polarisation at all
times. Worse still overhead passes feature max doppler rate of change.
So an overhead pass by Mir,can be a very difficult to follow.

Chronological List title - Space Propagation Pt3
Suggested Index references - Faraday : Propagation