Satgen348  Operating Fo20 Mode JA Pt3  by GM4IHJ             25 Nov 95

As discussed in previous satgens, the rapid doppler shift on the signals
of a mode JA satellite is the biggest problem the operator faces. 15 years
ago the first JA satellite Oscar 8 had this same problem, and the only
solution then was to produce a grid ( as in sgen347 ) showing frequency
change. Today we have computers, so we can actually calculate the
frequency change , but even this needs careful operator testing before it
become easy to use.

The first thing to get clear about computer calculation of JA doppler is ,
" THE FREQUENCY ON WHICH WE WILL BASE OUR DOPPLER CALCULATION ". Most
modern software calculates doppler in respect of the satellite beacon
frequency Eg in the Fo20 example this would be 435.795 MHz. But Fo20 uses
subtractive mixing sometimes called inverted mixing in its transponder. So
the frequency we must use for a basis for doppler calculations on mode JA
Fo20 is the difference frequency from this subtractive mixing , which if
we take the centre of the JA transponder response = 435.85 - 145.95 MHz
= 289.9 MHz . So if you want to use this technique you must amend your
computer software Fo20 data to show an imaginary beacon frequency of 289.9
MHz. Then you will get a correct prediction of the doppler shift
experienced by a transponder user.

At IHJ , a test , reading computed doppler , and comparing it with the
actual uplink frequency required to hit a favourite downlink frequency,
produces the following :-      Fixed downlink 435.820 for CW
Dop +4 Khz  +3Khz   +2Khz   +1Khz    0     -1Khz   -2Khz   -3Khz   -4Khz
Up 145.973 145.972 145.971 145.970 145.969 145.968 145.967 145.966 145.965

Fixed downlink 435.840 for CW or SSB
Dop  +4Khz   +3Khz   +2Khz   +1Khz    0     -1Khz   -2Khz   -3Khz   -4Khz
Up 145.953 145.952 145.951 145.950 145.949 145.948 145.947 145.946 145.945

Please note that these settings apply only to GM4IHJs equipment , which
includes a crystal controlled 435 to 28 MHz convertor, which produces
about 7 Khz of standing error in its conversion from 435 down to 28 MHz.
Users of modern Phase Locked Loop equipment should not experience such a
big frequency offset , but most systems will produce some offset.Eg a PLL
user should find that these 145 MHz settings produce a signal on a fixed
downlink frequency perhaps around 435.827 and 435.847. 

Please notice also that these two downlinks are 20 Khz apart , and the
Uplinks are a similar 20 Khz apart but in the opposite sense. So 435.860
would need 145.929 uplink and 435.88 would require 145.909 MHz at zero
doppler mid point of satellite closest approach. Remember also that an
over head pass may produce +5 to -5 doppler while a pass on your horizon
may only produce +1 to -1 Khz of doppler shift . So while the overhead
pass uses all the above table , the horizon pass only uses the middle
figures of the table. Equally important in respect of computer calculated
doppler, is the fact that while Automatic doppler tracking aids are
advertised for the digisats, which send a continuous signal. There is as
yet no simple computer operated automatic doppler corrector for use on the
intermittent CW or SSB signal of a typical JA QSO, though there is no
reason why this cannot be done.