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Good old fashioned radar sent out a radio wave and then listened to see if it was reflected back by some object “out there”. By timing how long it took the reflected signal to get back the electronics could tell how far it was to the source of the reflection. By knowing which way the rotating transmitting antenna was pointing it was possible to know both the direction and the distance to the object that the radar had detected. In fact that’s where radar got its name, it stood for “RAdio Direction And Range” if I remember correctly.

Modern radar works differently. It still sends out a high frequency radio wave and listens to see if any of the energy is reflected back, but there’s a new twist. A device known as a transponder is part of the equipment in the aircraft or other object that the radar is looking for and looking at. The transponder is a receiver and a radio transmitter, all in one package. It listens for the radar signal sent out from the transmitter. When it receives the signal it responds by sending back a signal of its own, much more powerful than the signal the radar on the ground would have gotten from just the reflected energy.

The transponder does several other things too. It can be coded by the pilot to return a 4-digit code, like 1-2-0-0 or 0-1-4-6 that the radar will receive and display on the operator’s screen. That way the operator is not looking at just a bunch of dots all over his screen without knowing which dot is which target. Every dot that represents a target that’s using a transponder is uniquely identified by the 4-digit code. Some codes are always the same, 1-2-0-0 is the code transmitted by aircraft that are lying VFR, using visual flight rules, where they are not necessarily under the control of ATC. Other times the controller tells the pilot what code to use. There are codes for when you have an emergency, when you are being hijacked and when you have lost your radio and can’t talk to them. There are others, but these are the primary ones.

The transponder can also send back information about the flight, such as the altitude the aircraft is at and the speed they are flying.

OK, the problem is that because the transponders make such a bright target on the scope that the operator has to turn its brightness way down to keep them from being just big blobs. When they turn the sensitivity of the scope down they lose the relatively weak targets that are not transponders but rather just the energy being reflected by the object the radar has seen. These returns are considered secondary and provide little information other than the fact there’s an object out there and how far away it is. If the secondary returns from a target are recorded over a period of time you can also tell which way it’s going.

In a case like this they will take the tapes of the area where they think Fossett might have gone and analyze them by looking specifically for secondary, very weak, returns. If they are lucky they will be able to find his radar track. The problem is that he might well not be the only aircraft out there, especially in wide open spaces like that.

I can’t believe he wouldn’t have had a transponder in the aircraft he was flying since a responder is required for entering much of controlled airspace these days. Most pilots turn it on and set it to broadcast, it called ‘squawking’, the 1-2-0-0 code so that even though he’s not under control the ATC guys will see him and be able to warn other pilots that he’s there. It also helps them find you when you don’t get where you are supposed to be going.

Controller airspace is not all the same, there are varying levels of complexity. Really busy areas, like Miami or Chicago, can’t be entered without the availability of a transponder that does all sort of things.

Hope I explained it so that it makes some sense!