Case in point: electronics. This story at DefenseTech points out that
The computer which runs the F-22 is an absolute design marvel for its time, for example: 700 MIPS (Millions of Instructions per Second), approximately 300 Megabytes of memory, and some 20 billion DSP [digital signal processing] style operations.This has more relevance than simply comparing hardware. Stealthy aircraft like the F-117 scatter radar signals away from the receivers. (Other aircraft, like the B2, either absorb or are transparent to Radar.) If you've got multiple receivers, and decent computing power, you can still put a signal back together - a trick the Chinese are said to have taught some countries. But it gets much, much worse (if you've invested heavily in stealth technologies):
Yet its time was the late 80s and early 90s, when much of the hardware was finalized. Today, a Playstation 3 meets or exceeds this performance, for $600 instead of perhaps $30,000,000. (Of course, the F22's avionics are considerably more robust and presumably more reliable.)
There are a few prerequisites for multipath radar. The broadcasters, although simple, need to transmit an identifier as part of their signals, and be at known locations. The receivers, on the other hand, need to be very sophisticated. This requires sophisticated radio antennas and, more importantly, "serious DSP magic," which, when networked together, can compute a cohesive picture of the defender's airspace.Most modern countries are awash in radio signals, and it would not be difficult for a well-organized insurgent group - much less a government - to get precise coordinates for things like FM towers and cell phone towers - and disperse the receivers throughout the countryside. With modern technology, putting them on the backs of the ubiquitous Toyota 4x4 shouldn't be hard.
Yet the hardware to perform such DSP operations is becoming commonplace and commercially prevalent. GNU radar and other designs can receive the signals, and conventional computers and DSPs can then process the results, extract the features, and create an overall picture. There have been prototypes built in the United Kingdom, able to track commercial aircraft by observing the reflected signals from cell-phone towers.
Most importantly, the vast majority of militarily-useful targets are not very stealthy. Even if this technology didn't let you see F-22s and F-35s, you could certainly spot the flotillas of heavy-lift cargo planes that fly in and out of Iraq every day. Once you've got a bearing and a vector, putting a guy in place with a decent chance of firing a shoulder-mounted SAM is much easier.
(This would allow insurgents to fire on incoming aircraft well away from the airports themselves, which is what the Iraqis have been doing. This makes controlling the airspace one more task that falls to the army, ironically. It also makes occupation even more labour-intensive.)
That bit about using local cellphone towers is particularly difficult for someone like the US in Iraq. Your options at that point are probably limited to trying to hunt down every insurgent receiver (and the USAF hasn't had much luck finding SCUDs, remember) or shutting down a country's cellphone service somehow, an action which wouldn't endear them to the locals.
It would be interesting to speculate if some countries might start regulating civilian radio broadcasts to take these kinds of technologies in to consideration. There are large parts of the spectrum that commercial broadcasters use as buffers between channels, after all. A government that wanted to expand it's radar options could mandate a specific time/space code in these buffer zones to link every transmission with a place and time and make it easier to put all these signals together.
If I were Iran, I'd be thinking about it.