When fighter jets are scrambled to intercept a suspicious aircraft, and give orders/warnings to that aircraft using radio, how do they know which frequency/channel the target aircraft’s receiver is using?
Or do the fighters have equipment that lets them simultaneously broadcast on all possible frequencies all at once?
Guard Frequency. See here:
There is no guarantee that the aircraft being intercepted has a radio tuned to 121.5, or in fact that it has a radio at all. A radio is not a legal requirement for flying a private plane, as long as you stay out of controlled airspace where you would need to communicate with ATC, and monitoring 121.5 is not a requirement either, at least for private pilots.
It happens fairly frequently that someone goes for a flight in the middle of Bumfuck, Nowhere, where there is no air traffic control facility for miles and thus no reason to have or use a radio, without realizing that the President happens to be visiting Bumfuck that day and there is a 30-mile no-fly-zone around wherever the President happens to be standing at any given moment. This is especially common during election season.
In this case, there are a set of defined signals that the fighter pilots will use to tell the plane they are intercepting what to do. The FAA has published them here:
Of course, if you’re the kind of pilot to take off without conducting even a cursory pre-flight briefing (which would have informed you of the no-fly zone), you’re probably not going to remember the procedures all that well.
Provided there is enough time, another way to communicate with the offending plane is for the fighters to fly next to the plane, sometimes rather close to it. If it keeps persisting, fire away from the plane (provided the area allows one to safely do so).
Having two war planes flank you and fire communicates effectively in any language and on any frequency.
You have better luck getting this kind of warning if you look like something other than a warplane on radar. If you can be mistaken for a warplane in a region where attacks are expected, well…: USS Vincennes (CG-49) - Wikipedia
As above.
Officially every pilot is encouraged to monitor the emergency frequency at all times. If some pilot isn’t doing that the next step is for the fighters to fly up close so the pilot notices them visually. That may get the offending pilot onto the radio. Failing that the standard visual signals are used.
Failing that the aircraft is followed or attacked as the rest of the scenario seems to warrant. As viewed from the fighters’ HQ’s POV, not the intercepted aircraft’s POV.
What’s even more fun is that in most of the world, including a bunch of the US, fighters don’t even have radios that can talk to civilian aircraft. So the fighter is talking to his HQ on military radio, who is talking on the phone to ATC, who in turn is talking to the intercepted aircraft on civilian radio. When lives hang in the balance this idiot game of telephone is anything but fun(ny).
FTR, F-16’s do have VHF radios that can communicate on 121.5, as well as UHF, which is what the military mostly uses. F-15’s have UHF radios only.