Satgen137    Moon and Planet Orbits   by GM4IHJ                10 Nov 91

Last weeks Satgen introduced, the problem of tracking Sara Oscar 23 as it
watched Jupiter, and reported that software was available ex GM4IHJ QTHR
on receipt of a formatted 3.5 inch 720 Kb disc in a re usable tough post
packet plus return postage. In view of the demand it is hoped to short
circuit this long supply route by putting these software files on Uosat3.
Meanwhile W and VE amateurs can if they wish send GM4IHJ a formatted disk
+ packaging and he will provide postage for return if the applicants agree
to make a suitable donation to AMSAT NA Po Box 27 Washington DC 20044
    Its now obvious from the volume of answers on this subject that more
explanation is required about Solar system orbits. So here goes. Simple
Keplerian element programs only work for near earth satellites. There is
no way to get 1 degree accuracy using Earth keps for Moons or planets.
BECAUSE THEY DO NOT ORBIT THE EARTH. THEY ORBIT THE SUN. So we first
relate everything to the Sun, working out Helio(Sun)centric coordinates
for each planet using well established EPHEMERIS data .Ephemeris data is
similar to but not same as Kep Elements, and it differs firstly because
the focus at the centre of any orbit ( the Sun ) has a mass 333432 times
that of the Earth and therefore exerts a much greater accelleration due to
its gravity ( like the G0 term in Kep programs but much bigger ). It also
differs because mean motions are much smaller ( Eg earth = 1/365.25 orbs
/day) and, we must consider the way the planets perturb one anothers
orbits. So these calculations have a long time scale and it is
inconvenient to work in days or earth years ( Jupiters orbit takes 12
years, imagine sorting out the leap year problems on that ). This problem
is overcome by using the JULIAN DAY COUNT which started at MIDDAY on 1st
Jan 4713 BC. Note this is not the same as Day of the Year which both Amsat
NA and Amsat UK confuse with true Julian Day count. A typical Ephemeris
will have an Epoch start date of say 0.5 Jan 1900 or becoming popular 0.5
Jan 2000. It is then easy to work out Julian days since then ( 365.25 per
year , no leap years., and apply this day count to the data in the
Ephemeris. We work out where our target moon or planet is with respect to
the Sun. We work out were the Earth is in respect of the Sun, and we
extrapolate from Heliocentric to Geocentric coordinates before calculating
the targets Azimuth and Elevation from our Earth station. The second
program on the disc  P.bas does this for any of the planets , while the
first program does it for Jupiter then extrapolates to SARA Oscar 23 to
see if SARA has a clear view of Jupiter , whether it sees Jupiter through
the Earths ionosphere ( restricting reception by SARA of Jupiters natural
2 to 25MHz signals), or whether SARA is eclipsed behind the Earth out of
direct line of sight to Jupiter.
     Since offering the above I have been asked why cant this be done for
the Moon. It can. I will put a Moontrack program on the disc - I simply
did not realise that anyone out there, lacked Moon tracking. Amsat UK sold
lots of my MOONTRACK and MOONTALK programs in 83/84 . So I thought you all
had them. Thanks for letters with discs.Your response shows how miguided
the IARU campaign against SARA really is. Contrary to some reports AMSAT
UK committee have not unanimously banned SARA. That report does not in
fact represent the view of the whole committee. Several Amsat UK members
are deeply concerned that we do not loose access to upcoming Solar Sail
Race, Mars probes etc because of the destructive views of a noisy " Ban
Happy" minority . Thanks for putting me right gents. 73 de GM4IHJ@GB7SAN