Detecting Aircraft

The U.S., at least, is pretty good at developing anti-radar technology to help the stealth aspect of its aircraft. But this has limits, because there are other ways one can detect aircraft:

1. Visual spotting on the ground with a good communications network that can tell people where an aircraft is heading.

2. Visually identifying the aircraft with satellites or very high altitude aircraft. 

3. Locating the aircraft based upon the noise it makes. This could be automated, a bit like systems the identify the location of firearm discharges automatically by triangulating the source of the sound from multiple listening devices. One could have a whole national network of noise localizing devices tuned to detect aircraft.

4. Locating the aircraft based upon heat or other aspects of its jet emissions such as chemical signatures or contrails that it might leave.

5. Covertly monitoring takeoff locations and guessing at where the aircraft might be heading, taking into account its range and any fuel drop tanks it is carrying.

6. Getting spies or electronic bugs or using signals intelligence to learn from commanders or lower level personnel where the aircraft are headed and when.

7. Covertly planting tracking chips on aircraft, or personnel or cargo or bombs and missiles on the aircraft which are hard for the force using the aircraft to locate.