The charge qubit is typically read-out by electrostatically coupling the island to an extremely sensitive electrometer such as the radio-frequency single-electron transistor.
For a long time, bubble chambers were used for this purpose, but with the improvement of electronics, it became desirable to have a detector with fast electronic read-out.
None of the instruments had a read-out display of any type, so transposing, changing tone, and other functions generally required counting button-clicks.
The transponder then transmits an answerback signal from the data read-out from its internal memory, which signal may be interpreted at the base station.
Navigators, occupying the observer seat (devoid of instrumentation), stood unsecured on final approach to oversee the pilots' instruments and read-out indications (despite voice synthesizers being fitted).