Phase methods are based on measuring the phase difference sine wave oscillations emitted and received radio signals. Functional scheme of the simplest phase range meter shown in Figure
Figure 1.3 Functional scheme of the simplest phase meter range
The generator creates a undamped oscillations of frequency w0, radiated into space. Phase emitted oscillations:
(1.5)
The phase of the received signal:
(1.6)
Here 
- phase shift, associated with the reflection of radio waves from the target;
ψr - phase shift in the chain, which can be considered as known, because it measurable and can be taken into account.
Received oscillations compared with the high-frequency oscillations of the generator; a phase difference proportional to target range.
(1.7)
or
(1.8)
This measurement method is practically not used for two reasons. Firstly, a very small range of unambiguous measurement and secondly, in the formula includes an unknown quantity ψomp.
The ambiguity of the measurement is determined by what phase-measurement device allows to determine the phase shifts only in the range from 0 to 2π
These shortcomings of the simplest phase range meter eliminated by using of more complex schemes that use at least two frequencies.
Figure 1.4 Functional scheme of the phase meter range with the modulator
In Figure we can obtain a functional diagram of a phase range meter using low frequency W, at which the measurement of the phase shift, and high w0, playing the role of a carrier of information.
The modulated oscillations are emitted into space. The received signals are detected after amplification, and released their envelope, phase of which is compared with the phase of the oscillations of modulator.
The phase of the envelope of the received signals depends on the distance of the target.
(1.9)
In the formula is not added the phase shift of the envelope oscillations in reflection ψomp, which is negligibly small.
The phase shift in chains can be measured and taken into account when grading of phase-measuring device. The phase difference between the low frequency oscillations:
(1.10)
allows to determine target range:
(1.11)
This distance measuring device has an advantages:
- it requires low power radiation, becouse generates undamped oscillations;
- measurement accuracy range is virtually independent of the Doppler shift of the reflected signal;
- a measuring device is simple.
Disadvantages:
- no range resolution, since the presence of two simultaneous targets, their signals can not be observed separately;
- receiver sensitivity decreases due to the leakage of radiation transmitter;
- requires two antennas or decoupling system of transmite and receive oscillations.
