General principle of range measurement

The range of the target can be measured either by measurement of the time of flight of the signal or by measurement of the phase shift of the incoming w.r.t. the outgoing signal. This requires recording in the first case the time and in the second case the phase both at transmission and reception. The relationship between the time-of-flight, t ₂ ? t ₁, of an electro-magnetic wave and the travelled distance, r,is given by the speed of light c:

and the relationship between the shift of phase, ? ₂ ? ? ₁, and the travelled distance, r,is

where ? is the wavelength and f is the frequency of the signal. In the general case these time or phase differences can be measured only when the transmitted signal is reflected by the target and the reflected signal is received again at the location of the transmitter (for an exception see section 7.3.2). In all practical cases, a modulated RF signal is generated by a transmitter and is transmitted by an antenna toward a target. Part of the power of the signal is reflected by the target back in the direction of the transmitter, or the signal is re-transmitted by a transponder on the target, and is received by an antenna at the transmitter location. Transmitting and receiving antenna may be the same piece of hardware if there is a...