Classification of Radar systems (1)
Depending on the desired information, radar sets must have different qualities and technologies. One reason for these different qualities and techniques radar sets are classified in:
Figure 1: Classification of radar sets (interactive picture)
Imaging Radar / Non-Imaging Radar
An Imaging Radar forms a picture of the observed object or area. Imaging radars have been used to map the Earth, other planets, asteroids, other celestial objects and to categorize targets for military systems.
Typically implementations of a Non-Imaging Radar system are speed gauges and radar altimeters. These are also called scatterometers since they measure the scattering properties of the object or region being observed. Non-Imaging Secondary Radar applications are immobilizer systems in some recent private cars.
A Primary Radar transmits high-frequency signals which are reflected at targets. The arisen echoes are received and evaluated. This means, unlike secondary radar sets a primary radar set receive it's own emitted signals as an echo again.
At these radar sets the airplane must have a transponder (transmitting responder) on board and this transponder responds to interrogation by transmitting a coded reply signal. This response can contain much more information, than a primary radar set is able to acquire (E.g. an altitude, an identification code or also any technical problems on board such as a radiocontact loss ...).
Pulse radar sets transmit a high-frequency impulse signal of high power. After this impulse signal, a longer break follows in which the echoes can be received, before a new transmitted signal is sent out. Direction, distance and sometimes if necessary the height or altitude of the target can be determined from the measured antenna position and propagation time of the pulse-signal.
Continuous- Wave Radar
CW radar sets transmit a high-frequency signal continuously. The echo signal is received and processed. The receiver need not to be mounted at the same place as the transmitter. Every firm civil radio transmitter can work as a radar transmitter at the same time, if a remote receiver compares the propagation times of the direct signal with the reflected one. Tests are known that the correct location of an airplane can be calculated from the evaluation of the signals by three different television stations.
Unmodulated CW- Radar
The transmitted signal of these equipments is constant in amplitude and frequency. These equipment is specialized in speed measurings. Distances cannot be measured. E.g. they are used as speed gauges for police. Newest equipments (LIDAR) work in the laser frequency range and measure not only the speed.
Modulated CW- Radar
The transmitted signal is constant in the amplitude but modulated in the frequency. This one gets possible after the principle of the propagation time measurement with that again. It is an advantage of this equipment that an evaluation is carried out without reception break and the measurement result is therefore continuously available. These radar sets are used where the measuring distance isn't too large and it's necessary a continuous measuring (e.g. an altitude measuring in airplanes or as weather radar/windprofiler).
A similar principle is also used by radar sets whose transmitting impulse is too long to get a well distance resolution. Often this equipment modulate its transmitting pulse to obtain a distance resolution within the transmitting pulse with the help of the pulse compression.
Bistatic Radar Sets
A bistatic radar consists of a separated (by a considerable distance) transmitting and receiving sites.