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Detecting the high def TV for the Google Lunar X Prize.
On Jun 17, 11:32*am, Jim Lux wrote:
Jim Lux wrote: Yes. *OP said "near real time," which I take to mean "OK to drop some frames," like the satellite video phones the reporters use from the boondocks. *Thus, high-def can be confined to a lot lower bandwidth if you don't mind seeing compression artifacts as each frame is being built on the screen. Here's a back of the envelope link budget for a 500E3 km link carrying 1 Mbps Let's assume 2GHz for the working frequency (not necessarily a good choice, but somewhere to start) Free Space Path Loss from Moon to earth, between isotropic antennas = 32.44 + 20*log10(500E3) + 20*log10(2E3) = 32.44 + 114 + 66 = about 212 dB Assume an antenna 2m in diameter at one end (moon end) lambda for 2GHz is 15cm, so the antenna is 13 wavelengths in diameter Beamwidth will be about 70/13 = 5 degrees.. OK, because Earth is 2 degrees wide from the moon, so you can just point at the middle of the visible earth. Assume an antenna 10m in diameter at the earth end. *Beamwidth will be 1 degree, twice the visible lunar disc size, so you can just point at the moon, generally. Gains of antennas 2m @ 2GHz = 30dB 10m @ 2GHz = 44dB Preceived at Earth = Ptransmitted +30 - 212 +44dB = Ptransmit -138dB. Assume transmitting with 10 Watts or +40dBm.. Prec = -100dBm Now, let's look at the receiver: Power Spectral density of Noise is kT+NF.. kT is -174dBm/Hz and a decent NF might be 2dB (allowing for some plumbing losses, etc. -172 dBm/Hz Eb (energy/Bit) = -100dBm -60dB (1Mbps) = -160dBmJ So, Eb/No is about +12dB... *If you allow 2dB for implementation loss, that gets you to 10dB, which will get you a BER of 1E-6, which isn't terrible. *Coding will improve it, etc. Take home message: 10W at 2Ghz with reasonably sized antennas at moon and earth can carry 1 Mbps. Scales linearly with data rate.. You want 10Mbps, you need 100W. *Or bigger antennas. Thanks for the info. This at least should be doable with receiving antennas operated by universities. Bob Clark |
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