When we start reading about radar, we come across various terms which are explained differently. There are various kinds of Radar classified in different ways. This article explains the various radar types in a lucid manner
1. Classification based on specific function
Classification based on the primary function of radar is shown in the following figure
A Primary Radar transmits high-frequency signals toward the targets. The transmitted pulses are reflected by the target and then received by the same radar. The reflected energy or the echoes are further processed to extract target information.
Secondary radar units work with active answer signals. In addition to primary radar, this type of radar uses a transponder on the airborne target/object.
A simple block diagram of secondary radar is shown below
The ground unit, called interrogator, transmits coded pulses (after modulation) towards the target. The transponder on the airborne object receives the pulse, decodes it, induces the coder to prepare the suitable answer, and then transmits the interrogated information back to the ground unit. The interrogator/ground unit demodulates the answer. The information is displayed on the display of the primary radar.
The secondary radar unit transmits and also receives high-frequency impulses, the so called interrogation. This isn't simply reflected, but received by the target by means of a transponder which receives and processes. After this the target answers at another frequency.
Various kinds of information like, the identity of aircraft, position of aircraft, etc. are interrogated using the secondary radar. The type of information required defines the MODE of the secondary radar.
Pulsed radar transmits high power, high-frequency pulses toward the target. Then it waits for the echo of the transmitted signal for sometime before it transmits a new pulse. Choice of pulse repetition frequency decides the range and resolution of the radar.
Target Range and bearings can be determined from the measured antenna position and time-of-arrival of the reflected signal.
Pulse radars can be used to measure target velocities. Two broad categories of pulsed radar employing Doppler shifts are
• MTI (Moving Target Indicator) Radar
The MTI radar uses low pulse repetition frequency (PRF) to avoid range ambiguities, but these radars can have Doppler ambiguities.
• Pulse Doppler Radar
Contrary to MTI radar, pulse Doppler radar uses high PRF to avoid Doppler ambiguities, but it can have numerous range ambiguities.
Doppler Radars make it possible to distinguish moving target in the presence of echoes from the stationary objects. These radars compare the received echoes with those received in previous sweep. The echoes from stationary objects will have same phase and hence will be cancelled, while moving targets will have some phase change.
If the Doppler shifted echo coincides with any of the frequency components in the frequency domain of the received signal, the radar will not be able to measure target velocity. Such velocities are called blind speeds.
Where, fo = radar operating frequency.
Continuous Wave Radar:
CW radars continuously transmit a high-frequency signal and the reflected energy is also received and processed continuously. These radars have to ensure that the transmitted energy doesn’t leak into the receiver (feedback connection). CW radars may be bistatic or monostatic; measures radial velocity of the target using Doppler Effect.
CW radars are of two types
An example of unmodulated CW radar is speed gauges used by the police. The transmitted signal of these equipments is constant in amplitude and frequency. CW radar transmitting unmodulated power can measure the speed only by using the Doppler-effect. It cannot measure a range and it cannot differ between two reflecting objects.
Unmodulated CW radars have the disadvantage that they cannot measure range, because run time measurements is not possible (and necessary) in unmodulated CW-radars. This is achieved in modulated CW radars using the frequency shifting method. In this method, a signal that constantly changes in frequency around a fixed reference is used to detect stationary objects. Frequency is swept repeatedly between f1 and f2. On examining the received reflected frequencies (and with the knowledge of the transmitted frequency), range calculation can be done.
If the target is moving, there is additional Doppler frequency shift which can be used to find if target is approaching or receding.
Frequency-Modulated Continuous Wave radars (FMCWs) are used in Radar Altimeters.