Direct the radar device at the approaching body and activate the emitter. Ensure that there are no objects between the emitter and the approaching body that could block the radiation. Record the emitter frequency.
Use the echo function of the radar device to detect and record the frequency of the radiation reflected off the surface of the approaching body.
Calculate the beat frequency. The beat frequency is the difference between the emitted frequency and the echo or reflected frequency. For example, if the emitter frequency is 5,391,950 hertz (Hz) and the echo frequency is 5,391,951 Hz, then the beat frequency is 1 Hz = 5,391,951 Hz -- 5,391,950 Hz = echo frequency -- emitter frequency. Note that some radar devices calculate the frequency difference automatically. Check the manufacturer's manual for the device to determine if the device automatically calculates the beat frequency.
Multiply the beat frequency by half the speed of the emitted radiation. The radiation or waves emitted by a radar device move at the speed of light: 299,792,458 meters per second or approximately 186,282 miles per second. For example, if the speed of the waves from the emitter is 299,792,458 m/s and the beat frequency is 1 Hz, then the (beat frequency) x (speed of waves)/2 = (1 Hz) x (299,792,458 m/s)/2 = 149,896,229 (Hz)(m/s).
Divide the result from Step 4 by the frequency of the emitter to obtain the approach speed of the moving body. For example, is the emitter frequency is 5,391,950 Hz and the beat frequency is 1 Hz, then the speed of the moving body is = (result from step 4)/(emitter frequency) = [(beat frequency) x (speed of waves)/2]/[emitter frequency] = [(1 Hz) x (299,792,458 m/s)/2]/[5,391,950 Hz] = [149,896,229 (Hz)(m/s)]/[5,391,950 Hz] = 27.8 m/s. The units for the speed of the moving body are the same as those used in Step 4. For example, if the speed of the emitter radiation is given in meters per second, then the speed of the approaching body will also be given in meters per second.