Distance-determination

The distance is determined from the running time of the high-frequency transmitted signal and the propagation speed C₀. The actual range of a target from the radar is known as slant range. Slant range is the line of sight distance between the radar and the object illuminated while ground range is the horizontal distance between the emitter and its target and its calculation requires knowledge of the target’s elevation. At this this one, however, to and way back must taken into account. Therefore the following formula arises for the slant range:

The Distances are expressed in kilometers or nautical miles.

Derivation of the equation

Range is the distance from the radar site to the target measured along the line of sight.

v =  s in m t s   v = speed s = range t = time	Figure 1: principle of radar
v =  2·R in m t s	The factor of two in the formula comes from the observation that the radar pulse must travel to the target and back before detection, or twice the range.

If the respective running time t is known then can be calculated the distance R with help of this equation between a target and the radar set.

To distinguish a moving target of a fixed object with help of the Doppler frequency, at least two periods of the deflection must be compared with each other.
Since the Doppler- frequency (few Hertz) is small relatively to the transmitted frequency (much Mega-Hertz), therefore a phase comparison is more easily to carry out than a direct frequency comparison technically.
The storage of a deflection is carried out in suitable memory media, in the past in special analogous vacuum memory tubes, later also with a chain of condensers (distance: digital, signal: analogous) and today only in digital memory cells.

Figure 1: functional block circuit diagram of a coherent receiver

Well, a fixed target suppression happens by the phase comparison of the echoes received by several pulse periods (pulse- pair processing). If the phase relationship is always equal, then there isn’t any phase difference and the target will be suppressed. If the target has moved, the phase difference is unequally zero and the target will be shown on the screen.
To get the necessary frequency-reference for the phase-detector, a high correct coherent oscillator (called: „Coho”) is synchronized with the down converted on the IF- frequency transmitting pulse.

The echo signal of a moving target at the output of the phase-detector changes it’s value and also the polarity in every pulse period. A fixed cluttersignal will keep it’s value and polarity in every pulse period.

A pulse period is stored in a memory and than it is subtracted from the following period. On this way the moving target produce an output signal and the fixed clutter don’t do this.

Figure 2:Oscillogram of an output signal of a phase-detector