Cell Towers Can Double as Cheap Radar Systems for Ports and Harbors (2014)

(either directly, or by bouncing a radar signal off the ionosphere and receiving it again)

You should still show up as a shadow?

The radar absorbing compounds of stealth aircraft are highly optimized for specific wavelengths (usually X-band) and fall off heavily outside that frequency band. Similarly, the radar cross section of stealthy aircraft is highly optimized for specific purposes (usually evading GBAD in the forward direction) and rapidly falls off in other scenarios. Most "stealth" aircraft are actually fairly visible from other directions.

That said, multistatic radar with transmitters-of-opportunity like cell towers and civil radio stations has always been in strong competition with fusion power as "the tech that is forever 10 years in the future". The transmitters are often not very powerful compared to dedicated radar systems and worse, they transmit energy in the horizontal plane rather than upwards where the planes are. The frequencies involved are much lower, which inherently leads to less radial accuracy unless you use VERY large antennas. Unlike a dedicated radar system the signals they send out are typically not shaped optimally for radar purposes, so signal processing like pulse compression becomes much harder. Because the signals are inherently not as predictable as normal radar signals you need MUCH more computing power. Finally, atmospheric conditions become fiendishly tricky for long range, because signal delays between each transmitter-target-receiver triple will be different. This means resolution goes way down if there's too many clouds or ionospheric interference, often to the point of uselessness.

Many of those problems are mostly terrible when trying to detect aircraft at long range though, and largely go away for short range surface use like in port. I'm still not entirely sure why for a port, which is stationary and requires tons of infrastructure investment anyway, this system would be preferable to a normal civilian type radar system. You can get a conventional one for at most a few tens of thousands, while this system apparently requires a trailer full of RF signal processing equipment. That is likely to cost at least in the order of magnitude more, while probably being less accurate.

Which they don't.

Then the B-2s fly in in unopposed.

The key to the B-2s is dropping the F-35s. Which seems to be hard.

Also, the APQ-181 is a LPI radar, which means it’s specifically designed to avoid correlation of signals such that you can track by the signals emitted. There are presumably some downsides to working in LPI, but the upside is that the signal is designed to be indistinguishable from an increased noise floor.

The stealth bombers were just the most convenient vehicle for carrying the massive bomb.

This is the most banal passive radar you can make. There's also one that doesn't require "over the horizon", but does require two receivers, you need two directional antennas at the same wavelength (two identical yagis will do, or if you're clever, two 9wl:0.25wl off-center fed dipoles), one aimed toward a radio source, and the other aimed at your desired "radar area", you can correlate signals on the radio-side to the radar-side.

So because i typed this, does that mean black helicopters later for me?

^"over the radio horizon" for VHF/UHF is a function of transmitting antenna height, relative to your location, and is usually "line of sight, plus 10%", assuming no tropospheric ducting. VHF/UHF are not like lower frequencies that are reflected by the ionosphere (sometimes) and the "ground" (sometimes), their range is drastically limited.

so in essence, if you know of a station in a nearby county or whatever, but you have never received it at your location, even with sensitive radios and good isolation (>=15dBd), and there's no physical barriers between those two points, and you aim a sensitive antenna and receiver at that transmitter, if you do receive "snippets" of signal - something is reflecting it.

this stuff is on various websites, archive.org, probably wikipedia.

If you have a VHF receiver of any sort, that allows external antennas, you can measure out nine wavelengths of wire, as straight as possible, aimed slightly (a degree or two, depending) off center from your target area; and 1/4th wavelength of wire in line with the other, and attach the short one to "ground" and the long one to "antenna", you now have a ridiculously cheap antenna. It's easier to make and set up than a beverage antenna, as well.

note: mods, delete this if i violated any rules, i don't see how, but i'm no law-thing