Satgen143      Hearing A Mars Probe           By GM4IHJ        22nd Dec 91

Amsat may send a Mars probe on the Phase 3D Ariane launch planned for 1995
Several mentions of this have already been published but I have not yet
seen one which discusses what the ordinary radio amateur might need to
hear it. Looking at a 95 launch to a very high earth parking orbit. Then a
boost from that orbit to an economic Mars transfer orbit in say Sept 96 ,
the probe could be expected to take about 260 days to arrive near Mars
when the Earth Mars separation is minimal around 100+ million kms approx.
Given these figures , the path between similar stations using 100 watts CW
, and 100Hz Rx bandwidth for 3 dBs sig to noise ,would require :-

Frequency GHz    Dish at each end    Dish Gain       Remarks
1.3 GHz             2m                26 dB      Freq may not be available
2.4 GHz             1.5m              27.5 dB    Good Rx gear exists
5.0 GHz             0.9m              32 dB
10  GHz             0.7m              35 dB      Possible adapted LNB Rx
24  GHz             0.4m              37.5 dB

Eg at 1.3 GHz we need a 2m dish at either end of the path. While at 24 GHz
we need 40cm dishes at either end for strong CW reception . Wideband
signals such as voice or FSK would need much bigger dishes.

But we probably cannot put a dish on the probe. It will be difficult to
stabilise to point at Earth. Perhaps we could get some gain by fitting say
a stacked slot waveguide antenna on the probe, gain at most perhaps ,
about 16 dBs. So the 1.3 GHz case would be one dish minus 16 dB short in
gain = 10 dB. We might improve this by using a bigger dish on the Earth
with 36 dB gain = 6m diameter. The 24 GHz case would be one dish minus 16
dB short = 21.5 dB. To regain this loss we need to increase our Earth
station 24 GHz dish to one of about 6m. So we seem to require a dish of
about 6m diameter whatever microwave frequency we send from the probe.

We could possibly improve the situation if operators are good enough to
copy CW below noise level. Some super Ops have been tested at -6dB and
some stations may use Digital Signal Processing to extract signal from the
noise . This might permit use of a 3m dish, or even 2m if DSP is very good

How do we get 100 watts on a small probe ?. I suggest by having a transmit
cycle as low as 1 minute in 30. But what about the controllers who need
much more from this bird if they are to check any telemetry and
navigation. I suggest here we can compromise by having a second Tx period
where a wideband signal is sent. Naturally this will require reception
dishes with perhaps 20 db more gain Eg at least 8 times bigger than the
ones suggested for CW reception. Such dishes may be available at the Deep
Space Exploration Society site on Table Mesa near Boulder , Col, USA.

So if this project goes ahead, we might best use a dual approach . Complex
data for very big dish owners, and, simple CW and/or DSP for those of us
who might manage a 2, 3 or 6m dish. RSGB Space Radio Handbook p235 has a
worked example of maths for this type of path. 73 de John GM4IHJ@GB7SAN