The inability of the CW radar to measure range is related to the relatively narrow bandwidth of its transmitted waveforms. The problem can be overcome by frequency modulating the transmitted carrier. Use of FM will require bandwidth of the system.

The most common application of a FM CW radar is the airborne altimeter.

A typical altimeter operates in the C-band and uses a transmitter power of about 1 to 2W. The source is frequency modulated and for simplicity, sawtooth modulation is used. If the target is stationary with respect to the plane (in case of an airborne altimeter), a frequency. difference proportional to the height of the plane exists between the received and transmitted signal.

It is due to the fact that, the signal received at a particular moment was sent at a time when instantaneous frequency was different, the time difference between the sent and received signals can be readily calculated, if the change of frequency with time due to frequency modulation is known. From this the height of the air craft can be calculated. The output of mixer gives the frequency difference.

This is amplified, limited and then fed to frequency counter whose output is connected to an indicator, this output can be calibrated in feet or meter.

The transmitter power required is very low so that small antennas can be used. A typical FMCW radar altimeter has a range of up to 10,000 meters or above with an accuracy of more than 95 percent.