Satgen136    Planetary Orbits     by GM4IHJ     3rd Nov 91

Ten years ago , many radio amateurs encountered Keplerian Orbital Elements
for the first time. The reception was less than ecstatic and for years
afterwards the faithful pined for their friendly old Equator Crossings
despite the fact that they simply did not work for the elliptical orbit
satellites then coming forward. Ten years on we are approaching another "
Big step" in orbital dynamics and true to form the Amsat Wind section is
trying to pretend it has'nt happened. 

But like it or not, SARA Oscar 23 FX0SAT or whatever you chose to call it,
is up there ,and it can if you are interested provide a neat example to
bridge the gap as we move from earth orbits to planetary orbits. SARA is
listening to Jupiter . So we need to know where Jupiter is and whether
SARA can see/hear it. But SARA is also going around the Earth , so we must
also take account of when SARA goes behind the Earth , out of sight of
Jupiter.(Sara reports Jupiters natural HF sigs on 145.955MHz FM 300bps) 

You might think at first that we could lock SARA into an orbit face on to
Jupiter, the way we produce a Sun facing orbit by balancing the trade offs
between, Solar Time, Sidereal Space Time, and , perturbation of the orbits
Right Ascension. But there is a snag. The Earth goes round the Sun in a
reasonably regular manner. Jupiter by contrast is one week being overtaken
by the Earth then left far behind the Earth, first when the two are the
same side of the Sun then over the next dozen years the Earth does 12
orbits to Jupiters 1, producing an apparent motion of Jupiter backwards
and forwards like a pendulum but without the latters synchronism.

So we have to accept that Saras orbit will be face on to Jupiter one month
never losing sight of it for 3 weeks or so Dec 91, then roughly 3 months
latter Mar92 Saras orbit will be edge on to Jupiter,causing Sara to be out
of sight of Jupiter behind the Earth for up to 35 minutes of each orbit

This is where we need new software. We can track Sara with standard
Keplerian elements , but we must also introduce a tracking program for
Jupiter and somehow combine the dual motions. So with the likelyhood that
Planetary software must be added to our micro memory banks Sara is a good
point to start to find how to do it. If you are interested in seeing how
its done and getting used to this type of software for either Earth
orbiters watching planets, or the similar Earth stations watching
satellites orbiting non earth planets, software is available for IBM PCs
and clones in GWBASIC/BASICA format on 3.5 inch DD DS 720KB disk from
GM4IHJ QTHR. Just send a formatted disk in a tough disk post package
together with enough postage for its return to you, either stamps or
sterling cheques or money orders for postage (no charge for disk or
software). Remember airmail is 1.10 sterling to USA and more to VK. 
Have been using the software in real time for SARA, whilst simultaneously
listening to Jupiter HF direct via a program (also on this disk) which
tracks any Solar system planet. Very interesting . 73 de GM4IHJ @ GB7SAN