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Let's imagine we're at ground station trying to receive UHF signal transmitted from moon at 433 / 2400 MHz. Our antenna with 25dB gain (circular polarization) points to the moon. Our LNA has 1dB NF.

What realistic spectral power density of noise we should expect in this situation?

I am interested in realistic estimation - not counting on just temperature of receiver/moon. We have cosmic radio background - solar, galactic, both directly and reflected... We have antenna's side-lobes picking signals from elsewhere... We have hot moon that is NOT covering whole main lobe of the antenna. There must be other tricks which affect the results which you are aware of and I am not.

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    $\begingroup$ Well, for a realistic estimate, the noise-equivalent temperature of your receiver and the noise-equivalent temperature of the moon would be what you consider, so I'm not quite sure what you're asking; the moon is so much warmer than cosmic background that "scatter noise" hardly makes any difference. $\endgroup$
    – Marcus Müller
    Jan 30, 2017 at 16:14
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    $\begingroup$ and how should that estimate be more realistic than if you just based it on the moon temperature if you can't tell us how much the side lobes contribute from where? $\endgroup$
    – Marcus Müller
    Jan 30, 2017 at 16:15
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    $\begingroup$ Are you in Tokyo or 1000 miles from civilization? How's your receiver performance? I think too many variables to give an answer here, since the noise can vary from near the theoretical minimum, to many orders of magnitude more. Easiest and most accurate method would be to simply measure with a spectrum analyzer. $\endgroup$
    – Phil Frost - W8II
    Jan 30, 2017 at 19:33
  • $\begingroup$ There was a paper from NASA discussing background noise versus altitude and frequency. I'll see if I can find it... $\endgroup$
    – AndrejaKo
    Jan 31, 2017 at 16:44
  • $\begingroup$ Not sure if you can estimate this accurately, as environmental factors may be at play (as already commented)... why don't you get a (or a couple of) UHF receivers, a 1dB NF-LNA, a 25dB antenna, some good coax/feeder, and point the antenna to the moon, at various locations, at various times, with various weather conditions, and measure and compare... and keep in mind that what you are measuring is only valid for the setup, and exact environment you are working with/in. $\endgroup$
    – Edwin van Mierlo
    Feb 1, 2017 at 10:02

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It's very difficult to say. Factors like ambient man-made noise around you can vary by orders of magnitude based on your location. Likewise, your radio system's internal noise and ability to reject ambient noise can vary by orders of magnitude. The difference between a \$20 RTL-SDR dongle in an urban environment and a \$2,000,000,000 array of receivers operated by a national space agency is quite large.

Assuming you want to know this in relation to your previous question, the noise floor might be low enough that you can achieve near the theoretical maximum capacity, or you may not be able to communicate with your satellite at all. The environmental factors are just too variable to make a more precise prediction.

You can empirically determine the noise floor with a spectrum analyzer, and that's probably the simplest and most accurate way to answer your question.

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