It happens from time to time that the splendid flight that you were making ends in drama!
It just takes a thoughtless pilot to turn on his transmitter without first checking the frequency panel. If by misfortune, his frequency is
the same as yours, glitching is assured and the loss of control is unavoidable!!
On-the-fly frequency shifting is for sure an effective answer to this kind of mishap.

With SUPERTEF, the plane in flight has two frequencies available, one called normal (Fn) and the other called safety (Fs).

If interference occurs on one of the frequencies, the receiver commutates automatically to the other. The pilot does not have to do anything ... how could he do it besides!!

At the transmitter, there are two solutions:

            - The simplest: the pilot realizing that he lost contact, changes the frequency manually. In that case, a simple HF
module with frequency synthesis,or with two quartz is needed.

            - The most advanced: the transmitter emits simultaneously on the two frequencies, Fn and Fs. The pilot does not intervene at all. The shifting of frequency is done without his knowledge. To tell the truth, it is the only really effective method, because in the first case, the pilot paralysed by the rupture of connection, does not react sufficiently quickly and generally, does not even change a frequency, wondering what is happening. Alas, that goes very quickly!!

The technology of the SUPERTEF associated with the two-frequency HF module, the HF10, allows in conjunction with the RX19 receiver, implementation the most advance yet very simple method. Of course, you can still use the first method with the HF9-3 module.

Inside the plane, there is no apparent complication. Only one receiver with the servos connected exactly as with an ordinary receiver. This simplicity is to be compared with the worrying complication of some systems suggested by others: two distinct receivers, an electronic commutation module, a set of wires between receiver 1 and the commutation module, another for receiver 2 and finally a third between the commutation module and servos. One ends up wondering whether all that is secure, for which a good principle is essential: the simpler it is and the more reliable it is. 
In addition, let us add that it is almost necessary to add to the transmitter signal a unique, owner SIGNATURE.
Indeed, it is absolutely necessary to prevent the guilty, interfering transmitter " to take control " over your transmitter’s signal and that implies the addition of this signature.
See the PPCM CODING used for the RX19.