[amsat-bb] Re: NASA's American Student Moon Orbiter...
domenico.i8cvs at tin.it
Fri Jul 4 06:42:47 PDT 2008
----- Original Message -----
From: <G0MRF at aol.com>
To: <amsat-bb at amsat.org>
Sent: Friday, July 04, 2008 3:44 AM
Subject: [amsat-bb] Re: NASA's American Student Moon Orbiter...
> In a message dated 04/07/2008 01:16:33 GMT Standard Time,
> domenico.i8cvs at tin.it writes:
> Hi Ed, KL7UW
> If we put AO40 at a distance of 400.000 km instead of 60.000 km
> from the earth the increase of isotropic attenuation at 2400 MHz is
> about 16 dB etc etc etc.........
> Hi Ed / Dom
> On the other hand, if you were to reduce path loss by using 70cm as the
> uplink band and 2m as the downlink the numbers begin to look quite
Hi David, G0MRF
Decreasing the frequency the absolute value of the isotropic attenuation
decreases but the difference in path loss between 400.000 km and 60.000 km
is the same 16.5 dB at any frequency so that to compensate for the above
attenuation using lower frequencies you need bigger antennas both on the
satellite and at the ground station.
> Also, if the satellite is orbiting the moon, then it's quite likely that
> the attitude will be such that the experimental end of the satellite is
> pointing at the moons surface. This probably also means that the
> communication antennas are not pointing at the earth, so high gain
> will not be possible.
> Maybe 3 or 4dB is the limit.
This is why it does not make sense to put a transponder orbiting around
the moon just for the simple reason that it's very much more simple
and cheap to put it into a HEO earth orbit.
> So how about 10W of 2m on the satellite and a passband that's say
> 5kHz wide? Not good for SSB, but passable for CW or reasonable
> speed coherent BPSK
Only considering the 2 meters downlink suppose to put AO40 at 400.000
km with the antennas pointing at the earth with low squint angle let say
less than 10 degrees.
The gain of the AO40 2 meters antennas was 10 dBi and we put your
10 watt on it.
Suppose that your 2 meter antenna has a gain of 13 dBi and the overall
noise figure of your receiving system is NF= 0,7 dB = 51 kelvin so that
the noise floor into a CW passband of 500 Hz with the antenna looking
at the moon (200 kelvin) is about -178 dBW
Suppose that the station in QSO with you has a 70 cm EIRP capability to
get the full 2 meters 10 watt from the transponder only for you and we
can calculate it later on.
2 meters downlink budged calculation:
Satellite power ................................... + 10 dBW
Satellite antenna gain.......................... + 10 dBi
Satellite EIRP..................................... + 20 dBW (100 W EIRP)
2 m isotr. attenuation 400.000 km.. -188 dB
power density received on a ground
isotropic 2 meters antenna..................-168 dBW
2 m ground station antenna gain.........+ 13 dBi
Power density at 2 m RX input...........- 155 dBW
2 m receiver noise floor......................- 178 dBW
Received CW signal S/N.................... + 23 dB
If we increase the BW to 2500 Hz for a SSB QSO than the noise floor
of the receiving system increases by log (2500/500) = 7 dB i.e.
it becames about -171 dB and the SSB signal will be received with a
S/N ratio = 23-7 = 16 dB wich is a very strong SSB signal.
Be aware that the above figures are based on the assumption that the
satellite antennas are pointig toward the earth wich is not the case with
a moon orbiting satellite.
In addition we assume that the station in QSO with you has a 70 cm
EIRP capability in order to get 10 watt from the 2m transponder only
On the other side if a fixed 10 dBi 2 meters antenna is placed over the
moon and it is oriented toward the earth could easily cover the inclination
X libration window without any adjustement and only from the point of
view of the downlink with 10 watt it can be easily used for a transponder
on the moon.
If you make again the downlink budged calculation considering that
the 2 meter transponder will develope only 2.5 watt for you then you
will realize that the transponder will accomodate 3 more stations if each
one is getting 2.5 watt as well.
In this case your S/N ratio will be still +15.5 dB on CW and +8.5 dB
in SSB and the same is true for the other 3 users.
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