[amsat-bb] Re: NASA's American Student Moon Orbiter...
D. Mynatt
dave at mynatt.biz
Fri Jul 4 09:03:23 PDT 2008
Probably a dumb question born of ignorance, but why isn't there more than 1
antenna on a sat, so that it's always pointing towards earth? Is Ion
propulsion for stabilization using solar power too far away to be practical
stabilization system?
----- Original Message -----
From: "i8cvs" <domenico.i8cvs at tin.it>
To: "G0MRF David Bowman" <g0mrf at aol.com>; "AMSAT-BB" <amsat-bb at amsat.org>
Sent: Friday, July 04, 2008 7:42 AM
Subject: [amsat-bb] Re: NASA's American Student Moon Orbiter...
> ----- 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
>> possible.
>
> 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
>>
>> Regards
>>
>> David
>
> 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.
> 10
> 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
> for you.
>
> 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.
>
> 73" de
>
> i8CVS Domenico
>
>
>
>
>
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