[thedoc (OP)]

Ian Scott asked the Naked Scientists:
   
I am very interested in any details related to the current China Lunar missions. I have an R&D engineering design background in radio communications so any technical detail is especially welcome. I suspect that China will be using S-Band as per NASA Apollo missions (~2.3 GHz) whereas NASA Mars rovers 7.2 GHz X-band for satellite to earth data transfer.

I am currently designing an X-Band to 1200-1300 MHz converter (suits amateur radio enthusiast's equipment) and will have a prototype later this year. I may also make a S-Band converter also. Consequently, any specific frequencies and modulation formats / bandwidths for both China and NASA missions are of great interest to me! I look forward to any information you can discover

Sincerely
Ian Scott ZL4NJ

What do you think?

[evan_au]

The amount of power available on a space orbiter or rover is strictly limited, so the transmitter is necessarily low-power.
The size of the dish you can fit on a space orbiter or rover is also strictly limited, so the signal power is spread out over a diameter much wider than the Earth.

Both of these factors mean that you need a very large receiver dish, and probably a cryogenically cooled receiver, which makes it a challenge for the average amateur. The signals are almost certainly encrypted to prevent listeners from obtaining any useful data.

[Ian Scott ZL4NJ]

Thank you for your reply evan_au

The distance from Mars to Earth ranges from 78.9 Gm (closest , same side as the sun) to 378.9 Gm. At closest approach the free space path loss is ~268.9 dB. A satellite orbiting Mars should gather more than enough solar energy to power a 10-20 Watt (40 - 43 dBm) X band (7.2 - 8.5 GHz) transmitter. At this frequency, even a modest 3 meter diameter parabolic dish antenna will exhibit about 43 dB gain. There is no reason to assume that directional accuracy would be inadequate especially given our local positional satellite system. Therefore a Earth receiver using a similar dish antenna could receive +43 dBm + 43 dB + 43 dB - 268.9 dB = -139.9 dBm of signal energy. Given that thermal noise power is ~ -174 dBm in a 1 Hz bandwidth at room temperature, the estimated signal to noise ratio at a Earth receiver, in this scenario will equal SNR = 34.1 dB in a 1 Hz bandwidth, 24.1 dB in a 10 Hz bandwidth,  14.1 dB in a 100 Hz bandwidth and so on.

I appreciate that cyrogenically cooled amplifiers were once used, as in the early NASA Earth-moon missions, but  these parametric amplifiers rarely achieved better than a NF = 3 dB! .Modern readily available SiGe Bipolars (e.g. BFP740) easily achieve NF < 1 dB at S-band (~2.3 GHz) without any need for cooling, and pHEMPs can offer NF < 0.3 dB at consumer prices, some also quite well suited for X-band. However my question was really aimed at China's use of S-band and any specific frequencies they might use. Given that the Moon is quite close to use, in astronomical terms, receiving these should be straightforward. The Mars transmissions could conceivably be captured using amateur resources, and analyzed to some extend, but decoding scientific data (and then without access to any error correction polynomials etc used) is probably unlikely. However the China Lunar mission will presumably have a high component of voice communications and these might be demodulated. Knowing specific frequencies would be helpful in this goal.