RadarLab

A radar is a method and a system for detecting the presence, direction, distance, and speed of aircraft, 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 radars systems involve a wide range of disciplines such as building works, heavy mechanical and electrical engineering, high power microwave engineering, and 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 lading surface and knowing the constant speed of the light that travels through the air $c=299792458\;m/s$, in practice 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 the target.
A typical block diagram of a radar (monostatic configuration) is shown below.

This is the most used radar configuration, where the the antenna used for transmission and receiver is the same. How 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 were allowed to enter the receiver. The Transmitter is able to produce pulses of high-power, that will be propagated in the direction where the antenna is pointed.
The received RF-signal is processed through amplification, demodulation and other techniques that we will see later. Finally the data obtained are shown to the user.

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

• operate at a broad coverage, it is possible to observe the whole hemisphere;
• operate both 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 limitation and can not see everything, there are some techniques to be invisible to the radar called Stealth Technology that will be seen when we talk about the Rada 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 the frequency bands usable by the radar, through a series of conferences called WARC (World Administrative Radio Conference). It's important to be noted that the military radar may not follow the ITU regulations.
The frequency bands standardized by the IEEE (IEEE Std.521, 1984) are shown below, and following will be discussed the principal applications for each band.

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

$VHF$ and $UHF$
In these bands the radar range is big tanks to the use of high power. The systems to reduce the RCS (Stealth technique) are not very effective at these frequencies. Sometimes 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 very low 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 allow good performance for the MTI (Moving Target Indicator) and also the attenuation due to the rain is not so high.

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

$C$
In general is used for medium and short distance surveillance application. 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 zone like Europe.

$X$
Tanks to the short wave length the use of this band allow to realize device with reduced cost, size and weight, ideal for mobile applications. This frequency band is wide 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 respect to the previous cases. They are used for surface movement radar, airport surface detection equipment and for multi functional avionics radar.

$V$,$W$, and $mm$
The frequencies over 40 GHz are suffering from a very high attenuation. The radar application are limited for 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 assists) and for the laboratory equipment.

The areas where the radar is used for more applications 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 wild use also for surveillance and identification of enemy locations in map.

In Figure 3 there are some examples of military radar, and how we can see they can be used different types 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 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 observing the weather, the planetary position, monitoring sea ice and the ground. In remote sensing radar application using the Synthetic Aperture Radars (SAR) instruments is 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 radar are used to guide the space vehicle for safe landing on moon, detect and track satellites, monitor the meteors and for radio astronomy.
Radar speed meters are used by policy to to determine speed of the vehicle in the ground traffic control. While in the automotive the radar are used to controlling the movement of vehicles by giving warnings about presence of other vehicles or any other obstacles behind them, in order to prevent collisions.
In Figure 5 are shown the type of uses of the automotive radar and also is illustrated a typical radar for this type of applications. For reason of space the antenna size have to be small, consequently the frequency used is high so we have to take in to 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 was transmitted by the transmitting antenna and reflected by the object is collected and processed by the antenna and reception 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 unit and the receiving unit and usually a greater parallax. This means, 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 radar have the advantages that the receivers are passive, and hence undetectable. The receiving systems are also potentially simple and cheap.

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, the use of an impulsive signal makes it possible to use a single antenna (Monostatic Radar). The choice of Pulse Repetition Frequency ($PRF$) decides the range and resolution of the radar, sometimes is also used the parameter Pulse Repetition Time, $PRT=1/PRF$. While the duration of the emitted pulse is defined with $\tau$. 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. Pulse radars can be used also to measure target velocities. Two broad categories of pulsed radar employing Doppler shifts are Moving Target Indicator (MTI) and Moving Target Detector (MTD), which will be discussed later.

Continuous Wave (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. 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. A typical block diagram of a CW radar is shown in Figure 7, where it's possible to see that the presence of an echo is detected through beat of the received signal and a replica of the transmitted signal.
This type of radar are used for speed measurement by the police and also for alarm systems.

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 is possible to measure the frequency difference between the received signal and the transmitted signal $\Delta f$ through a beat, while knowing the shifting in time of the receiving signal respect to the transmitted one we can derive the target distance from $\Delta t = 2\;R/c$, the multiplication by 2 is justified from the fact that two ways are considered back and forth.