Satgen384   Ao13's Doppler Switch   by GM4IHJ              3 Aug 1996

   The present orbit of Ao13 is highly elliptical, and it is
reasonably common knowledge that Elliptical orbiters do not exhibit
the characteristic distorted lazy '''\,,, shaped doppler frequency
curve followed by ordinary low altitude LEO satellites. With LEO
satellites the curve is always negative, descending in frequency all
the time, (except in very special circumstances which call for
anomalous propagation of the signal). With elliptical orbiters
however, the doppler curve is of quite a different shape and it can
change from decreasing to increasing in frequency, as you listen to
the satellite beacon or follow a qso contact.

   This switch from negative to positive going doppler is usually so
subtle that almost no one notices it. Now however , Ao13 at perigee
has been giving some excellent examples, which even if you both listen
and watch them happen ( on AF9Ys excellent FFTDSP4 spectrum analysis
software), seem to defy logic.

   The difficult thing one has to accept is, that after a rapid
descent in frequency of about 6 Khz ( Beacon +3 to -3 ), as the
satellite goes through perigee near your station. The satellite is
going away from you quickly , but paradoxically, its beacon frequency
starts to rise. The explanation is all in the figures , as the
mathematicians say. Take a recent Ao13 example :-

 UTC   Az  El   Lat  Long   Ma  Slant Range  Range Change  Vel Kms/sec
1352  220  0    43   17     0    1769
                                              -929          + 7.74
1354  192  24   49    5     1     840
                                              +156          - 1.3
1356   99  33   54   352    2     996
                                              +965          - 8.4
1358   75  19   56   336    2    1961
                                             +1047          - 8.72
1400   69  12   57   321    3    3008
                                             +1037          - 8.64
1402   66   8   55   308    4    4045
                                             +1008          - 8.4
1404   65   4   53   297    5    5053
    Satellite then lost behind 4 deg elevation hill

   As the velocity figures show the satellite speed as it pulled away
from the station rose swiftly, after perigee and point of closest
approach to station, which nearly coincided. But after 1400 utc, the
satellite began to slow up. Its speed away from the station dropped ,
so its descending doppler shift abated slowly, gradually becoming less
negative. It did not yet have a positive doppler frequency of
reception, above the transmitted beacon frequency, but its frequency
began to climb , and would do so slowly until , out of sight of the
station it went through apogee and began to approach perigee at about
2300 utc. At which point its doppler would suddenly begin a very
speedy decline as it headed towards perigee and, the start of another
orbit.