RadarLab

A radar is a system for detecting the presence, direction, distance, 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, 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 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 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 was allowed to enter in 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 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 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 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 in Table 1, 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 allows to see over the horizon exploiting the ionospheric propagation.

$VHF$ and $UHF$
In these bands the radar range is big thanks 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 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 radar for surveillance in the terminal maneuver area, radar for the air defense, meteor radar, and 3D military radar. The atmospheric attenuation is general 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.

$X$
Thanks 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 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 respect to the previous cases. They are used for surface movement radar, airport surface detection equipment and for multifunctional avionics radar.

$V$,$W$, and $mm$
The frequencies over 40 GHz are suffering from a very high attenuation. The radar application is 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 use 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 is 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 in the ground traffic control. While in the automotive the radar are used to controlling the movement of vehicles by giving warnings about the presence of other vehicles or any other obstacles behind them, in order to prevent collisions.
In Figure 5 are shown the type of use in automotive radar and also is illustrated 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 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 TX antenna and reflected by the object is collected by the RX antenna and then processed by the 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 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 has 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). The choice of Pulse Repetition Frequency ($PRF$) decides the radar range, sometimes is also used the parameter Pulse Repetition Time, $PRT=1/PRF$. While the duration of the emitted pulse, defined with $\tau$, decides 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.
Pulse 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.

Continuous Wave (CW) radars continuously transmit a high-frequency signal and the reflected energy is received and processed continuously. CW radar transmitting unmodulated signal can measure only the velocity of a target by using the Doppler-effect. It can not 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 is 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.

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 one, $\Delta f$, 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.

Another 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 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 is possible to exploit better the doppler information in order to measure the velocity of the targets.
In Figure 10 is shown 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 frequency 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 is 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 successive 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. How we can see in Figure 11 is used an Automatic Frequency Control (AFC) in order to produce a signal whit 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.