Radar basic Concepts Chapter 1

Overview Radar Concepts Basic form of Radar system Types of Radar Radar Medium Targets Types Radar Performed functions Overall system considerations Basic Radar Parameters Application of Radar Radar frequencies General Block Radar System display Radar Waveform, Power and energy Radar Waveform equations Pulsed radar system Peak & average Transmitted power Radar Range Range ambiguity Range resolution  R Example CW radar Doppler frequency Doppler effect due to target motion (calculation of Doppler frequency)

Radar concepts Radar is the name of an electronic system used for the detection and location of objects. In the "language" of radar the objects are called targets. The word radar is an acronym for “Radio Detection and Ranging” A radar's function is intimately related to properties and characteristics of electro- magnetic waves as they interface with physical objects (the targets). All early radars used radio waves, but some modern radars today are based on optical waves and the use of lasers (LIDAR).

Radar Concepts The radio waves could be reflected by physical objects. This fundamental fact forms the basis by which radar performs one of its main functions; by sensing the presence of a reflected wave, the radar can determine the existence of a target (the process of detection).

Basic form of Radar Transmitter with transmitting antenna, Receiver with receiving antenna, The Channel In general, the target is part of the propagation, medium (also called the channel) between the transmission and reception locations. The radar can detect the presence of a target by observing the presence of a signal sr(t).

Types of Radar a) Antenna locations Monostatic, bistatic, multistatic Active Radar (monostatic), Passive Radar (bistatic, multistatic) The radar components might be located on land or water (e.g., on a ship), in the earth's atmosphere (on an aircraft, missile, bomb, cannon shell, etc.), in free space (on a satellite or space vehicle), or even on other planets. Clearly there is almost no limitation on where a radar might be located. Its location have an effect on operation because of the medium, or channel, in which the radar's waves must propagate

Types of Radar b) Types of the transmitted waveform St(t) A continuous-wave (CW) type is one that transmits continuously (usually with a constant amplitude); it can contain frequency modulation (FM), or can be constant-frequency (CW Radar, CW-FM Radar). When the transmitted waveform is pulsed, we have a pulsed radar type. c) Radar Functions Detection type, search type, terrain avoidance type, tracking type, and so forth.

Radar Medium The most elementary and simple radar medium is free space. The medium becomes more interesting if some target of interest exists in the free space (perhaps a space vehicle or satellite); this is the next most simple radar medium. The next level of medium complexity would involve addition of unwanted targets, such as returns from a nearby planet's surface when the radar is close to the surface. Next, the medium might contain an atmosphere with all its weather effects (rain, snow, etc.); this case might correspond to a surface-based radar that have interference with unwanted target signals, such as from land, forests, buildings, weather effects, and other propagation effects associated with the atmosphere. As conclusion, the medium has a direct effect on the performance of the radar

Target types Point target (having small dimensions compared to the angular and range resolution of the radar) Extended targets that are too large to be point targets, they can cause spreading in received pulses. Distributed targets. One class of examples includes earth surfaces such as forests, farms, oceans, and mountains. These are also called area targets. Another class of distributed target, also called a volume target includes rain, snow, sleet, hail, clouds, fog, smoke, and chaff.

Target types Moving targets are those having motion relative to the radar. If the radar is stationary on the earth, natural targets such as forests or grassy fields (vegetation in general) tend to have relatively low-speed motions that tend to only slightly spread the spectrum of the received signals. Moving targets such as missiles, jet aircraft, satellites, and cannon shells are often fast enough to shift the spectrum of the received signal by a significant (Doppler) amount in frequency relative to the transmitted signal. Some targets are called active if they radiate energy on their own. All other targets are called passive.

Radar Performed Functions The most important functions that a radar can perform are : 1. Detection: The detection function consists in sensing the presence in the receiver of the reflected signal from some desired target. 2. Measurement : Measurement of target range (as included in the name of Radar). 2. Resolution: radar's ability to separate (resolve) one desired target signal from another and to separate desired from undesired target signals (noise and clutter). However, modern radars commonly measure much more than radial range; they can measure a target's position in three-dimensional space, its velocity vector (speed in three space coordinates), angular direction, and vector angular velocity (angle rates in two angle coordinates). Some more advanced radars may even measure target extent (size), shape, and classification (truck, tank, person, building, aircraft, etc.).

OVERALL SYSTEM CONSIDERATIONS When designers are called on to develop a new radar, most considerations fall into three classes:  system choices,  the transmitting end,  the receiving end

Basic Radar Parameters Range: distance, Angular, Velocity. Measurement accuracy Range resolution Velocity resolution Angular resolution Carrier Frequency, pulse repetition frequency, pulse length, power of the transmitting wave, etc.

Applications of radar General applications On ground : Detection, location, and tracking of aircraft and space targets. In the air : Detection of other aircraft, ships, or land vehicles; mapping of land; storm avoidance, terrain avoidance, and navigation. On the sea : Navigation aid and safety device to locate buoys, shore lines, other ships, and for observation of aircraft. Some specific applications are as follows: Air traffic control : Controlling of air traffic in the vicinity of airports; and also for automated landing. Aircraft navigation : Weather avoidance to indicate regions of severe precipitation; terrain following/terrain avoidance (TF/TA); radio altimeter and doppler navigator are also radars. Ship safety : Collision avoidance; detection of navigation buoys. Space : Rendezvous and docking; landing on the moon and other planets; detection and tracking of satellites. Remote sensing: Sensing of geophysical object, or the ”environment” like weather, cloud cover, earth resources, water resources, agriculture, forests, geological formation, etc. This is usually done from aircraft or satellites. Law enforcement : To monitor speed of vehicles in traffic. Military : Surveillance and navigation; control and guidance of weapons (The largest use of radars occurs here).

Radar Frequencies

General Block Diagram of pulsed radar system

Radar Displays A radar display is a device for visual presentation of target information to an operator, who may be involved with on-line operation or with maintenance of the unit. Most displays use a cathode-ray tube (CRT) which are typically labeled by names such as A- scope, B-scope and C Scope.

Radar Displays A PPI displays refer to plan-position indicator, can have several variations.

Radar Display (LABO)

RADAR'S WAVEFORM, POWER, AND ENERGY

Radar Waveform equation where w 0 is the transmitted frequency,

Radar waveform equation Note that not all the pulses have the same phases. The first pulse (i=0), starting phase equal to 0 The second pulse occurred at i=1, with starting phase equal to T p *W0 The third pulse occurred at i=2, with starting phase equal to (i-1)*T p *W0, and so on… The last pulse is generated at i=N-1, with phase value equal to w0*T p *(N-1) To obtain same phase W 0 shall be equal to ???

Pulsed Radar system Peak & average Transmitted power Radar Range Range ambiguity Range resolution  R Example

Peak & Average transmitted power

Radar Range IPP: inter pulse period = Tp, it is also named Pulse Repetition Interval PRI

Radar Range The range of a target depends on the round trip transit time Velocity of the wave (c ) RK. This is the case of pulsed radar system only. R is noted as the range of the target, the range of radar (maximum detectable range) is different than the target range. The basic measurement of Pulsed radar is the target range, CW radar can detect the ranging of the target if it is frequency modulated (see later)

Range ambiguity The radar range is the range that if the target goes beyond this range we will have range ambiguity. Thus, the radar range can be obtained by answering to this question: What is the maximal distance of the target to be received without ambiguity? If the target distance makes the distance of round trip high in such away the time of received echo is high… If this time of received echo is < Tp; Tp is the pulse repetition interval, no- ambiguity is occurred (i.e the echo is received before the transmitting of the next pulse) Else distance ambiguity occurred or called also range ambiguity. The maximum detectable range of radar: any range that goes beyond this range results range ambiguity. Now what is the Minimum detectable range ?

Minimum detectable range The minimal measuring range R min (“blind range”) is the minimum distance which the target must have to be detect. Therein, it is necessary that the transmitting pulse leaves the antenna completely and the radar unit must switch on the receiver. The transmitting time τ and the recovery time t recovery should are as short as possible, if targets shall be detected in the local area.(exp for τ =1microsecond, corresponds to a minimum range of about 150 m, which is generally acceptable. Targets at a range equivalent to the pulse width from the radar are not detected.

Range resolution  R The range resolution of a radar is the ability to resolve closely spaced targets along the same line of sight. Two targets along the same line of sight from the radar antenna will produce two distinguishable blips on the display if they are separated by a distance equal to or greater than the range resolution. If, however, the separation is less than the range resolution, the two targets will not be resolved, and will appear as a single blip.

Range resolution  R The degree of range resolution depends on the width of the transmitted pulse, the types and sizes of targets, and the efficiency of the receiver and indicator. Pulse width is the primary factor in range resolution. Two targets along the same line of sight from the radar are resolved if they are separated by a distance equal or greater to  R.

Range resolution  R  R=? This parameter can be deduced from computing the receiving times t1, and t2 from R1 and R2. Moreover, from the distance t1=2R1/c, t2=2R2/c, t2-t1 shall be greater than ,

Range resolution  R

Range resolution

Example Consider a radar with pulse repetition frequency 1000 Hz. (a) Find the time duration between two pulses. (b) Suppose an echo from a distant object is received 20 μ sec after a pulse is transmitted, what is the distance of the object from the radar? (c) Is there ambiguity range occurrence for this object?

CW Radar Doppler frequency Doppler effect due to target motion (calculation of Doppler frequency)

CW Radar In CW radar, the transmitter transmits continuously. CW systems can achieve considerable maximum ranges without the high peak-power levels required in pulse radar. CW radar systems are generally simpler, less costly, and more compact than pulsed radar systems. Although an un-modulated CW radar is unable to measure range, it can easily determine the relative speed of a target using the Doppler effect.

Doppler frequency Radars use Doppler frequency to extract target radial velocity (range rate), as well as to distinguish between moving and stationary targets or objects, such as clutter. The Doppler phenomenon describes the shift in the center frequency of an incident waveform due to the target motion with respect to the source of radiation. Depending on the direction of the target’s motion, this frequency shift may be positive or negative. If range measurement is required (using CW radar), the CW signal needs to be frequency modulated before transmission (FM-CW radar to be explained later).

Doppler frequency

Doppler frequency: equation Consider first the case where the target is fixe, if the transmitted signal is St(t)=Asin(Wot), what is the expression of Sr(t)? Take the case where the target is moving toward the radar station The doppler frequency is given by (Approximative equation): Where  is the angle between the target moving direction and the ligne of sight, V*cos(  ) is the radial velocity of the target,

Doppler effect due to target motion Doppler frequency calculation – second method Understanding what happens to a transmitted wave when it is reflected by a target and returns to the receiver. We follow two pieces of the transmitted wave: one that leaves at t = 0, and the second radiated one carrier cycle later, in time, which is at time To =1/f0=2  /  0. The target is assumed to have a range R0 at t = 0 and is moving away from the transmitter with radial speed v.

Doppler effect due to target motion