I was teaching about ultrasound the other day. Radar works on a similar principle that a pulse is sent out and that some of it is reflected back. If you look at the trace in the picture, the amplitude of each reflected pulse is much smaller. In the same way as with an ultrasound scan, you use the oscilloscope trace to calculate the time it takes for the pulse to go out and to reflect back. Use distance = speed x time for your calculation. In this case it's an electromagnetic wave so it's the speed of light you need. The distance to the aeroplane is half your calculated distance because the pulse has gone out and back.
