Satgen125    Anyone for Mars ?      by GM4IHJ            18th Aug 91

The recent announcement by DJ4ZC that spare capacity on the Phase 3D
Ariane launch planned for Oct 95, could be taken up by an interplanetary
probe aimed at Mars, has started quite a buzz in Amsat circles.

How do we track it ? This is not simple . We radio hams kid ourselves that
we track Earth sats. In fact,  NASA and NORAD radars do all the difficult
stuff. We just put their derived Keplerian elements into our computers. So
who will provide our Mars sat Keps ? NASA run a complex deep space network
based on antennas in Spain, USA and Australia. Noting that no one station
sees Mars for more than a few hours per day. Navigation of NASA deep space
vehicles is based on Doppler shift and is therefore quite accurate in
range along the line of sight but is less good in AZ and EL. NASA doppler
requires constant carrier signals so they use phase modulation so as not
to disturb this. We might use doppler or we might use secondary robot
repeater ranging, timing the 2 way round trip earth to sat , sat to earth.

How will we hear it? A few sums suggest a useful 2.5GHz Mars link on CW
using 100 watts and 2.8m dishes at either end  to a 100 kelvin Rx  100Hz
BW. But we won't get a dish on the sat. We might get it locked in earth
sun ecliptic plane beaming in that plane from a 12dB slotted waveguide.
But again we won't get 100 watts  from the sat. We might get 20 if we use
it cycling 90% off, 10% on. Lots of design effort needs to go into these
aspects. We need to win a lot of dBs if ordinary ops are to hear it.

Will it have a normal elliptical orbit ? Most interplanetary flights use a
method called patched conics. As its name suggests this is a system of
approximation, because we cannot solve the problem of interaction of 3
bodies , Earth, Sun , Sat directly. We assume normal orbit inside earth's
1 million km sphere of influence and similar orbit at martian destination
but in between, sat is under Sun's influence and we assume a "half ellipse
leaving earth orbit at its perihelion and reaching Mars orbit at its
aphelion", which "patches " our earth orbit to a martian orbit. To be
sensible we cannot expect to get a perfect track. We have not got NASAs
facility for using onboard cameras to give us visuals as the sat
approaches Mars, permitting last minute course corrections. But if we get
within a million miles of Mars it will be very good.

So as you will gather we cannot just simply pour in a set of Keps and go.
It is in fact going to be much more interesting than that. Which is after
all an excellent reason for doing it. It will be great fun whatever
happens. Congratulations to Amsat DL . I hope they manage to fit it into 
the Phase 3D schedule.
PS After a false start last weekend. Dove seemed to be back on 145.825 and
can be received on an ordinary packet radio terrestrial VHF station Rx.
Controllers say they will now work on implementation of Dove digital voice
downlink. U5MIR is reported to be staying in Mir an extra 6 months to Mar
92. 73 de John GM4IHJ @ GB7SAN