RadarLab

A radar is a system for detecting the presence, direction, distance, speed of aircrafts, ships and other objects, by sending out pulses of radio waves which are reflected off the object back (backscatter) to the source.
The term RADAR was coined in 1940 by the United States Navy as an acronym for RAdio Detection And Ranging or RAdio Direction And Ranging.

The implementation and operation of primary radar systems involve a wide range of disciplines such as building works, heavy mechanical and electrical engineering, high power microwave engineering, advanced high speed signal and data processing techniques. Some laws of nature have a relevant importance.

Synthetically, the operation of the radar is quite simple. In fact, thanks to the properties of the electromagnetic waves, that are reflected if they meet an electrically conductive surface and knowing the constant speed of the light that travels through the air $c=299792458\;m/s$, in practice it will be assumed $c\simeq300\;m/ \mu s$, we can compute the distance between the radar and the target. Obviously it's also necessary to know the elapsed time interval between the transmitted signal and the received echo.
The energy transmitted by the radar normally travels in a straight line, and will vary only slightly because of atmospheric and weather conditions. Actually the effects produced by the atmospheric and weather condition are relevant and this will be investigated later, but for determining the range and the direction these aspects will be ignored.
The relative signal that goes from the target to the radar, called “ECHO”, often is similar to the signal transmitted but with a smaller energy due to the losses during the propagation and also due to the capability of the target to reflect the electromagnetic waves (Radar Cross Section). The “ECHO” is picked up by the received antenna, that often coincides with the transmitting antenna, and then it is processed in order to obtain information abut the target.
A typical block diagram of a radar (monostatic configuration) is shown in Figure 2.

This is the most used radar configuration, where the antenna used for transmission and reception is the same. As we can see after the antenna there is a de-coupling device between the TX and RX subsystems. This device called Duplexer is able to switch between the two functionalities of the antenna. This switching is necessary because the high-power pulses of the transmitter would destroy the receiver if energy was allowed to enter in the receiver. The transmitter is able to produce pulses of high-power, that will be propagated in the direction pointed by the antenna.
The received RF-signal is processed through amplification, demodulation and other techniques that we will see later. Finally the obtained data are shown to the user.

Using a radar has many advantages compared to the visual observation, for example a radar can:

• operate at a broad coverage, it is possible to observe the whole hemisphere;
• operate both during day and night;
• operate in any weather condition, e.g. rain, snow and fog;
• detect and track moving objects;
• produce high resolution imaging;
• operate unmanned;

The radar has also some limitations and cannot see everything, there are some techniques to be invisible to the radar called Stealth Technology that will be seen when we talk about the Radar Cross Section (RCS).

The frequency bands usable by radars are the same ones used since the second world war. They have been defined by the IEEE as a standard for the assignment of the radar bands. The International Telecommunications Union (ITU) assigns the frequency bands usable by the radar, through a series of conferences called WARC (World Administrative Radio Conference). It's important to notice that the military radar may not follow the ITU regulations.
The frequency bands standardized by the IEEE (IEEE Std.521, 1984) are shown in Table 1, and following the main applications for each band will be discussed.

$HF$
Used mainly at the beginning of the radar history, now is only used for the OTH (Over The Horizon) radar. The OTH radar allows to see over the horizon exploiting the ionospheric propagation.

$VHF$ and $UHF$
In these bands the radar range is large, thanks to the use of high power. The systems to reduce the RCS (Stealth technique) are not very effective at these frequencies. Sometimes they are used for the detection and tracking of satellites and ballistic missiles over a long range. They are also used for weather radar-applications e.g. wind profiles, because the electromagnetic waves are not much affected by clouds and rain.

$L$
The most important applications are for the long distance air surveillance (400 Km) like the Air Route Surveillance Radar (ARSR). This band allows good performance for the MTI (Moving Target Indicator) and also the attenuation, due to the rain, is not so high.

$S$
With the same antenna size the S band has a better angular resolution respect to the L band. In this band there are radars for surveillance in the terminal maneuver area, air defense radars, meteor radars, and 3D military radars. The atmospheric attenuation in general is negligible.

$C$
In general it's used for medium and short distance surveillance applications. In this band the influence of bad weather conditions is very high and the use is predetermined for most types of weather radar, used to locate precipitation in temperate zones like Europe.

$X$
Thanks to the short wave length the use of this band allows to realize devices with reduced cost, size and weight, ideal for mobile applications. This frequency band is widely used for maritime civil, military navigation and for airport surface traffic control radars. Very small and cheap antennas with a high rotation speed allow a good accuracy. A long range is not a major requirement for these applications.

$K$,$K_u$ and $K_a$
In these bands the atmospheric absorption and attenuation is high, otherwise the accuracy and the range resolution are increased with respect to the previous cases. They are used for surface movement radars, airport surface detection equipment and for multifunctional avionics radars.

$V$,$W$, and $mm$
The frequencies over 40 GHz suffer a very high attenuation. Radar applications is limited to a short range of a couple of meters. These high frequencies are mainly used in the automotive engineering (e.g parking assistants, blind spot and brake assistants) and for the laboratory equipment.

The applications where radars are mostly used are: Military, Air Traffic Control, Remote sensing, Ground Traffic Control, Space and Automotive.
In Military applications the radar is used for target detection, target recognition and weapon control (directing the weapon to the tracked targets). It has a wide use also for surveillance and identification of enemy locations on a map.

In Figure 3 there are some examples of military radar, and as we can see they can be use different kinds of antenna; phased array, parabolic reflector and array of dipoles.
In Air Traffic Control the radars are used to control the traffic near airports, to detect and display the aircraft’s position in the airport terminals and to guide the aircraft to land in bad weather using Precision Approach RADAR. But they can be used not only for aircraft, another application is to scan the airport surface for identify the vehicles positions.
In Remote sensing radars are used to obtain information on the environment. They are used to observe the weather, the planetary position, monitor sea ice and the ground. In remote sensing radar applications using the Synthetic Aperture Radars (SAR) instruments, it's possible to produce images using radio waves. Below in Figure 4 is shown an example of SAR images taken over the Capitol Building of Washington.

In Space the radars are used to guide the space vehicle for safe landing on the moon, detect and track satellites, monitor the meteors and for radio astronomy.
Radar speed meters are used by police to determine the speed of vehicles for ground traffic control. While in the automotive the radar are used to control the movement of vehicles by giving warnings about the presence of other vehicles or any other obstacles behind them, in order to prevent collisions.
Figure 5 shows the type of use in automotive radar and also a typical radar for this type of applications. For space reasons the antenna size have to be small, consequently the frequency used is high so we have to take into account the effects of the high frequencies.

With the therm monostatic Radar we mean all the radars that use the same antenna both to transmit and receive. This is the conventional configuration for a radar, we have already seen the basic operating principle in Figure 1 and the block diagram in Figure 2 for this type of radar.

A bistatic radar consists of separately located, by a considerable distance, transmitting and receiving sites. The energy that is transmitted by the TX antenna and reflected by the object is collected by the RX antenna and then processed by the receiving chain, we can see a simple schema in Figure 6.

In case of a bistatic radar set there is a larger distance between the transmitting and the receiving unit and usually a greater parallax. This means that a signal can also be received when the geometry of the reflecting object reflects very little or no energy (stealth technology) in the direction of the monostatic radar. Bistatic radars have the advantages that the receivers are passive, and hence undetectable. The receiving systems are also potentially simple and cheap.

The Pulsed radar transmits high power, high-frequency pulses toward the target. Then it waits for the echo of the transmitted signal for some time before transmitting a new pulse, the use of an impulsive signal makes it possible to use a single antenna (Monostatic Radar). On the choice of the Pulse Repetition Frequency ($PRF$) depends the radar range, sometimes is also used the parameter Pulse Repetition Time, $PRT=1/PRF$. While on the duration of the emitted pulse, defined as $\tau$, depends the radar resolution. Typical values for an air traffic control system are $\tau=1\;\mu s$ and $PRT=1\;ms$. Target Range can be determined from the measured antenna position and time-of-arrival of the reflected signal. A typical waveform of a pulsed radar with the echo received from a target is shown in Figure 7.
The Pulsed radars can be used also to measure target velocity, two broad categories of pulsed radar employing Doppler shifts are Moving Target Indicator (MTI) and Moving Target Detector (MTD), which will be discussed later.

The Continuous Wave (CW) radar continuously transmit a high-frequency signal and the reflected energy is received and processed continuously. A CW radar transmitting unmodulated signal can measure only the velocity of a target by using the Doppler-effect. It cannot measure a range and it cannot differ between two reflecting objects. A typical block diagram of a CW radar is shown in Figure 8, where it's possible to see that the presence of an echo is detected through the beat of the received signal and a replica of the transmitted one.
CW radars may be bistatic or monostatic.
This type of radar is used for speed measurement by the police and also for alarm systems.

The Frequency-Modulated Continuous Wave (FMCW) radar has the same operating principle of the CW radar, but it is also able to measure the distance as well as the speed of an object. In this type of radar the signal constantly and repeatedly changes the frequency in a given interval. Using a frequency-time chart it's possible to measure the frequency difference between the received signal and the transmitted one, $\Delta f$, through a beat.
While knowing the shifting in time of the receiving signal with respect to the transmitted one we, can derive the target distance from $\Delta t = 2\;R/c$, the multiplication by 2 is justified by the fact that two ways are considered, back and forth.

Other two types of radar architectures are the coherent and non-coherent radar, the main difference between the two architectures is in the use of a phase reference. In the coherent radar all the frequencies produced internally have a defined phase shift. This is possible because all frequencies are derived from a highly stable master oscillator. The master oscillator provides low phase noise, local oscillators and the radar system clocks. In this way it's possible to exploit in a better way the doppler information in order to measure the velocity of the targets.
Figure 10 shows a coherent radar of MOPA type, where the frequency of the STALO (STable Local Oscillator) and the frequency of COHO (COHerent Oscillator) are used both in the transmission and in the reception chain. The resulting signal of the sum of the two frequencies is not impulsive, so it's necessary to use the amplifier in an impulsive regime. In the reception chain the two frequencies are used to bring the signal to an intermediate frequency (IF) and then to extract the doppler frequency.

If the observed targets have zero or very low speeds (e.g. control of ship traffic or of the airport surface) the information doppler is not so important, and it's more convenient to use a non-coherent radar. In the non-coherent radar the transmitter switches on and off as a result of modulation by the rectangular modulating pulse, the starting phase of each pulse is not the same for the different subsequent pulses. The starting phase is a random function related to the start up process of the oscillator. In this type of radar architecture the transmitter is typically a Magnetron which operates directly at the working frequency. As we can see in Figure 11 an Automatic Frequency Control (AFC) is used in order to produce a signal whit a frequency close to the frequency of the transmitted signal. Once an intermediate frequency is obtained, the signal is amplified and then sent to the envelope detector.