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*Subject*: [amsat-bb] Re: NASA's American Student Moon Orbiter...*From*: "D. Mynatt" <dave@xxxxxxxxxx>*Date*: Sat, 5 Jul 2008 13:48:08 -0600

Neat stuff to be sure and I appreciate the math. So, as I understand it from my limited knowledge, is that once we know the frequencies up and down, then designing the ground station antenna will be easier. My sense is that a 3m dish, with the right slewing and control will be able to *hear* the orbiter at it's closest point to earth without experiencing too many dropouts. CW with a BW of 500Hz appears to be the mode of choice -that seems reasonable to me, as does PSK- and the craft won't be in peril power-wise to do that. I still don't understand, albeit from a novice viewpoint, why the craft and the radio system can't be tethered making it two separate units in a way. The craft can angle for it's purpose and so can the radio. Is that possible, given the state of craftsmanship or art we have? Dave ----- Original Message ----- From: "i8cvs" <domenico.i8cvs@tin.it> To: "Bob Bruninga" <bruninga@usna.edu>; "'Joe'" <nss@mwt.net>; "'Edward Cole'" <kl7uw@acsalaska.net> Cc: "'AMSAT-BB'" <amsat-bb@amsat.org>; "'G0MRF David Bowman'" <g0mrf@aol.com> Sent: Saturday, July 05, 2008 11:20 AM Subject: [amsat-bb] Re: NASA's American Student Moon Orbiter... > ----- Original Message ----- > From: "Robert Bruninga" <bruninga@usna.edu> > To: "'Joe'" <nss@mwt.net>; "'Edward Cole'" <kl7uw@acsalaska.net> > Cc: "'AMSAT-BB'" <amsat-bb@amsat.org>; "'G0MRF David Bowman'" > <g0mrf@aol.com> > Sent: Saturday, July 05, 2008 4:09 AM > Subject: [amsat-bb] Re: NASA's American Student Moon Orbiter... > >> > The whole part that is confusing me on all this power >> > budget stuff is the to me, the seemingly HIGH budget. >> > I've done moon bounce. And many of these >> > numbers seem to be not too far from Moonbounce >> > numbers, and that is a horrid dead piece of rock >> > reflector. that has a efficiency of a wet sponge. >> > ...And it only reflects 6% of the energy it gets. >> >> My guess is ... That 6% is an awful lot of power considering the >> 3.6 million square miles of surface doing the reflecting. >> Conversly, any amateur transmitter at the moon would have a much >> smaller receiving/transmitting antenna. Though lots more >> concentrated power. >> >> So what you gain in changing from a 1/R^4 to a 1/R^2 path loss >> you lose a lot of it in the loss of signal receive aperture. Or >> something like that maybe. >> >> Bob, WB4APR >> >> > but i would think anything there that is active >> > circutry is a thousand times more efficient at >> > sendinga signal back as compared to the moons >> > surface. >> > or what am I missing? > >> >Joe > > Hi Bob, WB4APR > > Your guess is.......absolutely correct. > > I also did 432 MHz EME from 1977 to 1980 and I will try to demonstrate > in more datails to Joe that your analysis hit the centre of his question. > > Hi Joe > > Suppose to be in the center of a sphere with radius of 380.000 km that is > the average distance from the earth to the moon. > > The internal surface S of the above sphere computed in square meters is: > > 6 2 18 > S= 4 x 3,14 x ( 380 x 10 ) = 1,81 x 10 square meters > > > Suppose now to have in your hand an isotropic antenna radiating all around > and uniformly the power P = 1 watt at 432 MHz > > As soon the wave has reached the internal surface of the above sphere the > full power of 1 watt will be collected on it so that the power density D > collected in each square meter is: > > 1 -19 > D = ------------------------ = 5,52 x 10 watt / square meter > 18 > 1,81 x 10 > > But in one point of the above sphere there is the disc of the moon which > radius is 1735 km =1735000 meters and so the surface S1 of the lunar > disc is: > 2 12 > S1 = 1735000 x 3,14= 9,45 x 10 square meters > > The full power density P1 collected over the disc of the lunar surface > will > be D x S1 and so > > -19 12 > P1= 5,52 x 10 x 9,45 x 10 = 0,0000052164 watt > > Only the 7% of P1 at 432 MHz is reflected back by the lunar surface > and very important the reflected power P2 is reirradiated and scattered > back "isotropically" by the lunar disc and so the reflected power is > > P2=(0,0000052164 / 100) x 7= 0,0000003651 watt > > Now P2 make another trip of 380.000 km from the moon to the earth > but actually the power P3 collected by each square meter over the earth > surface will be only: > > 0,0000003651 -25 > P3= ----------------------- = 2,017 x 10 watt / square meter > 18 > 1,81 x 10 > > Since we have in our hand an isotropic antenna at 432 MHz originally > radiating 1 watt we want to know what actually is the power Pr received > back from the moon into the same isotropic antenna. > > The aperture area A of an isotropic antenna at 432 MHz i.e. at a > wavelenght > of 0,6944 meters is: > > / 2 2 > /\ 0,6944 > A = -------- = ----------- = 0,0383 square meters > 4 x 3,14 4 x 3,14 > > It follow that the power Pr received by the isotropic antenna on the earth > is Pr = P3 x A and so > > -25 -27 > Pr= 2,017 x 10 x 0,0383 = 7,725 x 10 watt > > Consequently the round trip isotropic attenuation (Att) earth-moon-earth > for > 380.000 km at 432 MHz off the moon is P / Pr and so in dB > > 1 > (Att) = 10 log ------------------- = 261 dB > 10 -27 > 7,725 x 10 > > The average of 432 MHz EME active stations are using the following: > > Antenna gain = 30 dBi > Power at the antenna feed = 1000 watt > Overall RX noise figure NF= 0.6 dB = 43 kelvin > BW for CW = 500 Hz > Equivalent antenna temperature Ta when pointed at the cold-sky = 50 kelvin > > With the above data NF, BW and Ta the noise floor of the receiving system > KTB = -182 dBW or -152 dBm > > Link budged calculation 432 MHz: > > TX power at the feed.............................+30 dBW > TX Antenna gain....................................+30 dBi > -------------- > Transmitted EIRP toward the moon.....+60 dBW = 1 Megawatt > Round trip attenuation 380.000 km.. - 261 dB > -------------- > Received power Pr on isotropic > antenna at the earth .............................-201 dBW > RX antenna gain................................. +30 dB > -------------- > - > Available power at RX input............. - 171 dBW > RX noise floor.....................................- 182 dBW > -------------- > > CW signal received with a S/N ratio + 11 dB > > To get a S/N ratio of 11 dB off the moon on CW it was necessary to > 6 > transmit + 60 dBW = 10 watt = 1 Megawatt toward the moon but > calculating the round trip attenuation we remember that transmitting > isotropically 1 watt from the earth the power collected by the lunar > disc was > > P1= 0,0000052164 watt > 6 > If now we multiply P1 by 10 we get the full power Pc collected by the > lunar disc while transmitting on CW toward the moon and so: > 6 > Pc = 0,0000052164 x 10 = 5.21 watt ( an awful lot of power as Bob said) > > Only the 7% of Pc at 432 MHz is reflected back by the lunar surface > and very important the reflected power P is reirradiated and scattered > back "isotropically" by the lunar disc and so the reflected power is > > P = ( 5.21 / 100) x 7= 0, 3651 watt > > If I want to receive a CW signal of 0,3651 watt transmitted isotropically > from the moon and if I want to receive it with a S/N ratio of 11 dB it is > evident that I need a 30 dBi antenna gain and a receiving system with > a noise floor of - 182 dBW.........no way ! > > If instead I want to receive a SSB or CW signal transmitted in 2 meters > by a satellite or from the moon with a power of 10 watt feed into a 10 dBi > antenna gain and using a 2 meters ground station antenna with gain of only > 13 dBi and a receiving system with a noise floor of - 178 dBW then > everyting in SSB and CW becomes very easy as calculation shows. > > 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 10 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. > > I hope this helps > > 73" de > > i8CVS Domenico > > > > > > > > > > _______________________________________________ > Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. > Not an AMSAT-NA member? Join now to support the amateur satellite program! > Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb > _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb

**References**:**Re: NASA's American Student Moon Orbiter...***From:*G0MRF

**Re: NASA's American Student Moon Orbiter...***From:*i8cvs

**Re: NASA's American Student Moon Orbiter...***From:*Edward Cole

**Re: NASA's American Student Moon Orbiter...***From:*Joe

**Re: NASA's American Student Moon Orbiter...***From:*Robert Bruninga

**Re: NASA's American Student Moon Orbiter...***From:*i8cvs

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