What is Crankshaft angular position/RPM sensor in vehicle?

In order to determine the spark timing, value timing, etc the position of the piston w.r.t TDC and BDC is necessary. However, one full engine cycle from intake through exhaust requires two complete revolutions of the crankshaft. One complete engine cycle corresponds to the crankshaft angular position going from zero to 720 degree. During each cycle, it is important to measure the crankshaft position with reference to TDC for each cylinder. The camshaft rotational speed is one-half that of the crankshaft, so the camshaft angular position goes from zero to 360 degree for one complete engine cycle.