"heads up" to tell the bomb aimer to begin the run, and then a second signal at the right time to drop it. Using this method there was no need for the two stations to compare measurements or perform any trigonometry to determine an actual location in space, both performed simple range measurements directly off their screen and sent their separate corrections to the aircraft. In practice, ranges were not sent by voice to the aircraft. Instead, a tone generator produced Morse code dots or dashes under the control of the operators. This was similar to the beam systems like Lorenz, which the UK aircrew were already familiar with using as a blind landing aid in the pre-war period. If the aircraft was too close to the station the operator would play the dot signal, and when they were too far, dashes. The two could be mixed so that as they approached the correct range, the dots would fill in the gaps between the dashes and form a steady tone. Periodically the signal would be keyed to send out a letter to indicate how far they were from the correct range, X indicating , Y , and Z . Likewise, the Mouse station sent a series of keyed signals to indicate the approach, S to indicate the approach was starting, and then A, B, C and D as the aircraft approached. Development and testing There were some obvious problems with this approach, however. One of the most obvious is that any given ground station could only track a single aircraft at a time, compared to Gee where any bomber could pick up the signals from the UK and carry out the necessary calculations. This did not immediately eliminate it as a useful system; Y-Gerät had the same limitation, so it was used for target marking by flying one aircraft under control and having it drop flares for the following aircraft to drop on. The British adopted the same solution. A more worrying concern was that the bomber aircraft would have to fly straight and level along a gently curving path while the ground station monitored its range as it flew towards the target. During this time the aircraft would be open to attack, which some considered to be almost suicidal. Lastly, the British had no difficulty jamming the German Y-Gerät system, even before it was widely used. There was no