Continuous-wave radar system is a radar system where a known stable frequency continuous wave radio energy is transmitted and

CONTINEOUS WAVE RADAR SYSTEM DOPPLER RADARSTUDENT NAME : DINESH KUMAR*, ROLL NO. : R702A05, REG. NO. : 10804713, M.PHIL PHYSICS(REGULAR),LOVELY PROFESSIONAL UNIVERSITY,LSTS,PHAGWARA.

AbstractCw radar is a radar system where a known stable frequency of continuous wave radio energy is transmitted and then received from any reflecting objects .The return frequencies are shifted away from the transmitted frequency based on the doppler effect if they are moving .Types of continuous wave radar are pulsed Doppler radar ,Fmcw radar, Fmcw radar is overcome the limitation of ranging of simple cw doppler radar.Computers now played large role in improving radar performances,by varying key parameters during operation and optimizing the receiver so that it has the best opportunity to detect targets reflection correctly.INTRODUCTION : A number of radar system are sufficiently unlike those treated so far to be dealt with separately.They include first of all Cw radar which make extensive use of the Doppler effect for target speed measurements.Another type of Cw radar is frequency modulated to provide range as well as velocity.A simple Doppler radar sends out continuous sine waves rather than pulses It uses the Doppler effect to detect the frequency change caused by moving target and display this as relative velocity.Cw radar is capable of giving accurate measurements of relative velocities using low transmitting powers and low power consumption and equipment whose size is much smaller than that of comparable pulsed equipment.Cw Doppler radar has some disadvantages one of the main disadvantage of Doppler radar is incapable indicating the range of the target.The greatest limitation of Doppler radar i.e its inability to measure range ,may be overcome if the transmitted carrier is frequency modulated . Cw Doppler radar has large number of applications firstly is the aircraft navigation for speed measurement and Radar speed meters used by police.*corresponding student.Tel.:01612632045

E-mail address: dinesh alagh @ yahoo.comContinuous Wave Radar

Continuous-wave radar system is a radar system where a known stable frequency continuous wave radio energy is transmitted and then received from any reflecting objects. The return frequencies are shifted away from the transmitted frequency based on the Doppler effect if they are moving.

The main advantage of the CW radars is that they are not pulsed and simple to manufacture. They have no minimum or maximum range (although the broadcast power level imposes a practical limit on range) and maximize power on a target because they are always broadcasting. However they also have the disadvantage of only detecting moving targets, as stationary targets (along the line of sight) will not cause a Doppler shift and the reflected signals will be filtered out. CW radar systems are used at both ends of the range spectrum; e.g., as radio-altimeters at the close-range end (where the range may be a few feet), and early warning radars at long range.

CW radars also have a disadvantage because they cannot measure range. Range is normally measured by timing the delay between a pulse being sent and received, but as CW radars are always broadcasting, there is no delay to measure. Ranging can be implemented, however, through a technique known as "chirping, or frequency modulated continuous-wave radar. In this system the signal is not a continuous fixed frequency, but varies up and down over a fixed period of time. By comparing the frequency of the received signal to the one currently being sent, the difference in frequency can be accurately measured, and from that the time-of-flight can be calculated.

The military uses continuous-wave radar to guide semi-active radar homing (SARH) air-to-air missiles such as the U.S. AIM-7 Sparrow The launch aircraft illuminates the target with a CW radar signal, and the missile homes in on the reflected radar waves. Since the missile is moving at high velocities relative to the aircraft, there is almost always a strong return. Most modern air combat radars, even pulse Doppler sets, have a CW function for missile guidance purposes.

Principle of Operation

As opposed to pulsed radar systems, continuous wave (CW) radar systems emit electromagnetic radiation at all times. Conventional CW radar cannot measure range because there is no basis for the measurementof the time delay. Recall that the basic radar system created pulses and used the time interval between transmission and reception to determine the target's range. If the energy is transmitted continuously then this will not be possible.

CW radar can measure the instantaneous rate-of-change in thetarget's range. This is accomplished by a direct measurement of theDoppler shift of the returned signal. The Doppler shift is a change in the frequency of the electromagnetic wave caused by motion of the transmitter, target or both. For example, if the transmitter is moving, the wavelength is reduced by a fraction proportional to the speed it is moving in the direction of propagation. Since the speed of propagation is a constant, the frequency must increase as the wavelength shortens. The net result is an upwards shift in the transmitted frequency, called the Doppler shift.

Figure 1. Doppler shift from moving transmitter

Likewise, if the receiver is moving opposite to the direction of propagation, there will a increase in the received frequency. Furthermore, a radar target which is moving will act as both a receiver and transmitter, with a resulting Doppler shift for each. The two effects caused by the motion of the transmitter/receiver and target can be combined into a net shift the frequency. The amount of shift will depend of the combined speed of the transmitter/receiver and the target along the line between them, called the line of sight. Figure 2. Calculating the relative speed in the line-of-sight.

The Doppler shift can be calculated with knowledge of the transmitter/receiver and target speeds, here designated as s1 and s2 respectively, and the angles between their direction of motion and the line-if-sight, designated 1 and 2. The combined speed in the line-of-sight is

s = s1 cos1 + s2 cos2 .

This speed can also be interpreted as the instantaneous rate of change in the range, or range rate. As long as the problem is confined to two-dimensions, the angles also have simple interpretations: 1 the relative bearing to the target. The difference between the course of the transmitter/receiver and the true bearing to the target. This follows the old nautical rule:

Relative Bearing = True Bearing - Heading

Due to the characteristics of the cosine function, it makes no difference whether angle is positive or negative (strictly speaking, relative bearings are always positive and range from 0 to 3590). 2 = the target angle (relative bearing of transmitter/receiver from target). Computed in an identical manner as the relative bearing, except that the target's course is substituted for the heading and the reciprocal bearing is used instead of the true bearing to the target. The reciprocal bearing is found by:

Reciprocal Bearing = True Bearing 1800

Again, it does not matter is this result is positive, negative or even beyond 3600, although the proper result would be in the range of 0-3590. Assuming that the range rate is known the shift in returned frequency is

f = 2s/

where is the wavelength of the original signal. As an example, the Doppler shift in an X-band (10 GHz) CW radar will be about 30 Hz for every 1 mph combined speed in the line-of-sight.

Police often use CW radar to measure the speed of cars. What is actually measured is the fraction of the total speed which is towardsthe radar. If there is some difference between the direction of motionand the line-of-sight, there will be error. Fortunately for speeders,the measured speed is always lower than the actual.

CW radar systems are used in military applications where the measuring therange rate is desired. Of course, range rate can be determined from the basic pulsed radar system by measuring the changein the detected range from pulse to pulse. CW systems measure the instantaneous range rate, and maintain continuous contact with the target.

DOPPLER RADAR

A doppler radar is a radar using the doppler effect of the returned echoes from targets to measure their radial velocity. To be more specific the microwave signal sent by the radar antenna's directional beam is reflected toward the radar and compared in frequency, up or down from the original signal, allowing for the direct and highly accurate measurement of target velocity component in the direction of the beam. Doppler radars are used in air defense, air traffic control, sounding satellites, police speed guns and radiologyRecent weather radars process velocities of precipitations by Pulse-Doppler radar technique, on top of their intensities This is a slightly different treatment of Doppler data that has been publicized so much in the United Statesthat the term Doppler radar is often wrongly used by the public to mean weather radar.

Christian Andreas DopplerThe phenomenon known as the Doppler Effect is named after Christian Andreas Doppler Doppler was an Austrian physicist who first described in 1842 how the observed frequency of light and sound waves was affected by the relative motion of the source and the detector.

This is most often demonstrated by the change in the sound wave of a passing train. The sound of the train whistle will become "higher" in pitch as it approaches and "lower" in pitch as it moves away. This is explained as follows: the number of sound waves reaching the ear in a given amount of time (this is called the frequency) determines the tone, or pitch, perceived. The tone remains the same as long as you and the train are not moving relative to each other. As the train moves closer to you the number of sound waves reaching your ear in a given amount of time increases. Thus, the pitch increases. As the train moves away from you the opposite happens.

Basic conceptA Doppler radar is a radar that produces a velocity measurement as one of its outputs. Doppler radars may be Coherent Pulsed, Continuous Wave, or Frequency Modulated. A continuous wave (CW) doppler radar is a special case that only provides a velocity output. Early doppler radars were CW, and it quickly led to the development of Frequency Modulated (FM-CW) radar, which sweeps the transmitter frequency to encode and determine range. The CW and FM-CW radars can only process one target normally, which limits their use. With the advent of digital techniques Pulse-Doppler (PD) radars were introduced, and doppler processors for coherent pulse radars were developed at the same time.

The advantage of combining doppler processing to pulse radars is to provide accurate velocity information. This velocity is called Range-Rate. It describes the rate that a target moves towards or away from the radar. A target with no range-rate reflects a frequency near the transmitter frequency, and cannot be detected. The classic zero doppler target is one which is on a heading that is tangential to the radar antenna beam. Basically, any target that is heading 90 degrees in relation to the antenna beam cannot be detected by its velocity (only by its conventional reflectivity).

FM radar was highly developed during World War II for the use by US Navy aircraft. Most used the UHF spectrum, and had a transmit yagi antenna on the port wing, and a receiver yagi antenna on the starboard wing. This allowed bombers to fly an optimum speed when approaching ship targets. Later when magnetrons and microwaves became available, the use of FM radar fell into disuse.

When the Fast Fourier transform became available digitally, it was immediately connected to Coherent Pulsed radars, where velocity information was extracted. This quickly proved useful in both weather and air traffic control radars. The velocity information provided another input to the software tracker, and improved computer tracking. Due to the low pulse repetition frequency (PRF) of most coherent pulsed radars, which maximizes the coverage in range, the amount of doppler processing is limited. The doppler processor can only process velocities up to 1/2 the PRF of the radar. This was not a problem for weather radars.

Specialized radars quickly were mechanized when digital techniques became affordable. Pulse-Doppler radars combine all the benefits of long range, and high velocity capability. Pulse-Doppler radars use a medium to high PRF (on the order of 30 kHz). This high PRF allows for the detection of either high speed targets, or high resolution velocity measurements. Normally it is one or the other, that is, a radar designed for detecting targets from zero to Mach 2, does not have a high resolution in speed, while a radar designed for high resolution velocity measurements does not have a wide range of speeds. Weather radars are high resolution velocity radars, while air defense radars have a large range of velocity detection, but the accuracy in velocity is in the 10's of knots.

Antenna designs for the CW and FM-CW started out as separate transmit and receive antennas before the advent of affordable microwave designs. In the late 1960s traffic radars began being produced which used a single antenna. This was made possible by the use of circular polarization, and a multi-port waveguide section operating at X band. By the late 1970s this changed to linear polarization and the use of ferrite circulators at both X and K bands. PD radars operate at too high a PRF to use a Transmit-Receive gas filled switch, and most use solid-state devices to protect the receiver Low Noise Amplifier when the transmitter is fired.

TYPES OF CW DOPPLER RADARPulse-Doppler radar

Pulse-Doppler is a radar system capable of not only detecting target location (bearing, range, and altitude), but also measuring its radial velocity (range-rate). It uses the Doppler effect to determine the relative velocity of objects; pulses of RF energy returning from the target are processed to measure the frequency shift between carrier cycles in each pulse and the original transmitted frequency. To achieve this, the transmitter frequency source must have very good phase stability and the system is said to be coherent.

The nature of pulsed radar, and the relationship between the carrier frequency and the Pulse Repetition Frequency (PRF) means that the frequency spectrum can be very complex, leading to the possibility of errors and tradeoffs. In general, it is necessary to utilise a very high PRF to avoid aliasing, which can cause side effects such as range ambiguity. To avoid this, multiple PRFs are often used.

principlePulse-Doppler radar is based on the fact that targets moving with a nonzero radial velocity will introduce a frequency shift between the transmitter master oscillator and the carrier component in the returned echoes. This is because the signal is subject to Doppler shift, so echoes from closing targets will show an apparent increase in frequency and echoes from opening targets will show an apparent decrease in frequency. Target velocity can be estimated by determining the average frequency shift of carrier cycles within a pulse packet. This is typically done by means of a 1D fast Fourier transform or using the autocorrelation technique. The transform is performed independently for each sample volume, using data received at the same range from all pulses within a packet or group of pulses. In older systems, a bank of analogue filters were used.

Velocity measurements are of course limited to measuring the component of the target velocity that is parallel to the beam (radial), since tangential movement will not affect the received signals. A target is either closing or opening, or it will fall into the clutter notch (a velocity range reserved for non-displayed clutter). Velocity information from a single radar will therefore result in underestimates of target velocity. Complete velocity profiles can only be derived by combining measurements from several radars, situated at different locations.

The radial velocity of the target can easily be calculated based on knowledge of the radar frequency, speed of light, pulse repetition frequency and average phase (frequency) shift.

Pulse repetition frequency

Pulse Repetition Frequency (PRF) is the number of pulses transmitted per second by a radar. The reciprocal of this is called the Pulse Repetition Time (PRT), Pulse Repetition Interval (PRI), or Inter-Pulse Period (IPP), which is the elapsed time from the beginning of one pulse to the beginning of the next pulse. PRF is important since it determines the maximum target range (Rmax) and maximum Doppler velocity (Vmax) that can be accurately determined by the radar.

Range ambiguity:A radar system determines range through the time delay between pulse transmission and reception by the relation:

For accurate range determination a pulse must be transmitted and reflected before the next pulse is transmitted. This gives rise to the maximum range limit:

The maximum range also defines a range ambiguity for all detected targets. Because of the periodic nature of pulsed radar systems, it is impossible for a radar system to determine the difference between targets separated by integer multiples of the maximum range using a single PRF. More sophisticated radar systems avoid this problem through the use of multiple PRFs either simultaneously on different frequencies or on a single frequency with a changing PRT.

Signal demodulationThe resulting receiver video is processed in doppler velocity filters or digital signal processing circuits which are used to determine velocity. Most modern Pulse-Doppler radars demodulate the incoming radio frequency signal down to a center frequency of zero prior to digital sampling. This is done to reduce computational burden, since the demodulated signal can be downsampled heavily to reduce the amount of data needed for storage. The resulting signal is usually referred to as complex demodulated, or IQ-data, where IQ stands for in-phase and quadrature-phase, reflecting the fact that the signal is complex, with a real and imaginary part.

For instance, a modulated signal could be S(t) = cos(0t + (t)), it can then demodulated using:

IH(t) = S(t).cos(0t) and QH(t) = S(t).sin(0t)

Using a low pass filter on both IH(t) and QH(t) allows the following:

I(t) = cos((t) + ) and Q(t) = sin((t) + )

Note that I(t) would not be enough because the sign is lost. Having I(t) and Q(t) then enables the radar to properly map closing (approaching) and opening (leaving) doppler velocities.

Errors and Trade offs CoherencyIn order for Pulse-Doppler radar to work at all, it is essential that the received echoes are coherent with the carrier signal, at least during the time it takes for all echoes to return and be processed. To achieve this, a number of techniques are employed, the most common being that the transmitter signal is derived from a highly stable oscillator, (the COHO) and the received signal is demodulated using an equally stable local oscillator, (known as the STALO), which is phase locked to it. Doppler shift may then be accurately resolved by comparing the frequency components of the returned echo with the frequency components of the transmitted signal.

Maximum range from reflectivity (red) and unambiguous Doppler velocity range (blue) with a fix pulse repetition rate.

AmbiguitiesA fundamental problem associated with Pulse-Doppler radar is velocity ambiguity, since Doppler Shifts crossing the next line in the frequency spectrum will be aliased. This problem can, however, be alleviated by increasing the PRF, which increases the spacing between adjacent lines in the transmitted spectrum allowing greater shifts before aliasing occurs. For military radars intended to detect high speed closing targets, it is common for PRFs of several hundred kilohertz to be employed.

Even so, there is a limit to the amount that the PRF may be increased before range ambiguity occurs. However, high PRFs can be utilised by the transmission of multiple pulse-packets with different PRF-values to resolve this ambiguity, since only the correct velocity stays fixed, while all "ghost velocities" introduced by aliasing change when the PRF is altered.

Application considerations Type of RadarThe maximum velocity that can be unambiguously measured is inherently limited by the PRF, as discussed above. The PRF-value must therefore be chosen carefully, based on a tradeoff between maximum velocity resolution and the reduction of velocity aliasing and range ambiguity problems. This tradeoff is highly application dependent, as e.g. weather radars measure velocities at a totally different scale as compared to radars designed to detect supersonic missiles and aircraft.

Moving targetsStationary targets such as earth ground clutter (land, buildings, etc) will be dominant in the low doppler frequencies, while moving targets will produce much higher doppler shifts. The radar processor can be designed to mask out clutter by the use of doppler filters (digital or analogue) around the main spectral line (called the clutter-notch), which will result in the display of moving targets only (in relation to the radar). If the radar itself is moving, such as on a fighter aircraft, or a surveillance aircraft, then much more processing will be required, as the clutter in the filters will be based on platform speed, terrain under the radar, antenna depression angle, and antenna rotation/steered angle.

Frequency Modulated Continuous Wave (FMCW) Radar

It is also possible to use a CW radar system to measure range instead of range rate by frequency modulation, the systematic variation of the transmitted frequency. What this does in effect is to put a unique "time stamp" on the transmitted wave at every instant. By measuring the frequency of the return signal, the time delay between

transmission and reception can be measure and therefore the range determined as before. Of course, the amount of frequency modulation must be significantly greater than the expected Doppler shift or the results will be affected.

The simplest way to modulate the wave is to linearly increase the frequency. In other words, the transmitted frequency will change at a constant rate.

Figure 3. FMCW theory of operation.

The FMCW system measures the instantaneous difference between the transmitted and received frequencies, f. This difference is directly proportional to the time delay, t, which is takes the radar signal to reach the target and return. From this the range can be found using the usual formula, R = ct/2. The time delay can be found as follows:

t = Tf/(f2-f1) where:

f2 = maximum frequency f1 = minimum frequency T = period of sweep from f1 to f2, and f = the difference between transmitted and received.

There is a slight problem which occurs when the sweep resets the frequency and the frequency difference becomes negative (as shown in the plot of f vs. time). The system uses a discriminator to clip off the negative signal, leaving only the positive part, which is directly proportional to the range. Here is a system diagram:Figure 4. FMCW block diagram.

Combining these equations into a single form for the range

R = 2cTf/(f2 - f1)

where f is the difference between the transmitted and received frequency (when both are from the same sweep, i.e. when it is positive).

Another way to construct a FMCW system, is to compare the phase difference between the transmitted and received signals after they have been demodulated to receiver the sweep information. This system does not have to discriminate the negative values of f. In either case however, the maximum unambiguous range will still be determined by the period, namely

Runamb = cT/2

FMCW systems are often used for radar altimeters, or in radar proximity fuzes for warheads. These systems do not have a minimum range like a pulsed system. However, they are not suitable for long range detection, because the continuous power level they transmit at must be considerably lower than the peak power of a pulsed system. You may recall that the peak and average power in a pulse system were related by the duty cycle,

Pave = DC *Ppeak

For a continuous wave system, the duty cycle is one, or alternatively, the peak power is the same as the average power. In pulsed systems the peak power is many times greater than the average.LIMITATIONS OF SIMPLE DOPPLER CW-RADAR :A major limitation of continuous wave radar (CW radar) is that it lacks the ability to measure distance to a target. CW radar cannot determine target range because it lacks the timing mark necessary to allow the system to time accurately the transmit and receive cycle and convert this into range. In pulse radar, this mark was provided by the pulse itself. Pulse radar transmits a form of amplitude-modulated energy. There are other forms of modulation that provide the necessary mark to allow range information to be calculated. Frequency modulation (FM) can also be used. CW radars making use of FM are called FM CW Radar (FM CW radar or sometimes FMCW radar). In addition to the ranging limitations, the CW radar is unable to detect targets with a zero Doppler shift, including stationary targets and beaming targets. Like pulse radar, FM CW Radar overcomes this limitation.

Application of Doppler Radar:RADAR, which stands for Radio Detection and Ranging, is a common method of detecting moving objects. For instance the police use Doppler radar to detect the speed of a moving vehicle. An Austrian physicist by the name of Doppler, first described the concept in the 19th century. The Doppler effect describes what happens when a wave (sound, radio, etc.) hits a moving object.For example, police use a radar detector to determine the speed of a car as it moves down the highway. Radar waves are transmitted from the police car at a certain frequency. Recall that waves have both amplitude and frequency. When the waves bounce off a moving object their frequency is effected. As the radio waves bounce of a car that is moving toward the detector the frequency of the wave decreases. If the waves bounce of a car moving away from the detector the frequency of the wave increases. The detector uses the difference in the transmitted and received wave frequencies to determine the speed of the car.Radar technology has now been built into a new flashlight sized device that can detect human movement through a door or wall. The device can detect movement due to human respiration from up to three metres away. The device will prove useful for police in detecting criminals in an ambush situation, when doing bed checks in prisons or for determining the location of hostages in a building. The device could also be used to locate the survivors of an earthquake or avalanche.Radar technology may also be used to detect land mines. NATO is spending millions to develop a device to identify and neutralize land mines. The basic technology consists of two antennas that focus radar energy to a point just below ground a few feet in front of the person carrying the antenna. The device is programmed to ignore signals that bounce back from the surface and to make buried objects shine brighter in the radar image. This allows the operator to actually detect the land mines without ever touching the ground.FACTORS AFFECTING RADAR PERFORMANCE: Radar accuracy is a measure of the ability of a radar system to determine the correct range, bearing, and in some cases, altitude of an object. The degree of accuracy is primarily determined by the resolution of the radar system and atmospheric conditions. Range Resolution Range resolution is the ability of a radar to resolve between two targets on the same bearing, but at slightly different ranges. 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. Bearing Resolution Bearing, or azimuth, resolution is the ability of a radar system to separate objects at the same range but at slightly different bearings. The degree of bearing resolution depends on radar beamwidth and the range of the targets. The physical size and shape of the antenna determines beamwidth. Two targets at the same range must be separated by at least one beamwidth to be distinguished as two objects. Earlier in this chapter, we talked about other internal characteristics of radar equipment that affect range performance. But there are also external factors that effect radar performance. Some of those are the skill of the operator; size, composition, angle, and altitude of the target; possible electronic-countermeasure (ECM) activity; readiness of equipment (completed PMS requirements); and weather conditions Atmospheric Conditions Several conditions within the atmosphere can have an adverse effect on radar performance. A few of these are temperature inversion, moisture lapse, water droplets, and dust particles. Either temperature inversion or moisture lapse, alone or in combination, can cause a huge change in the refraction index of the lowest few-hundred feet of atmosphere. The result is a greater bending of the radar waves passing through the abnormal condition. The increased bending in such a situation is referred to as DUCTING,and may greatly affect radar performance. The radar horizon may be extended or reduced, depending on the direction in which the radar waves are bent. The effect of ducting is illustrated. Water droplets and dust particles diffuse radar energy through absorption, reflection, and scattering. This leaves less energy to strike the target so the return echo is smaller. The overall effect is a reduction in usable range. Usable range varies widely with weather conditions. The higher the frequency of the radar system, the more it is affected by weather conditions such as rain or clouds. All radar systems perform the same basic functions of detection, so, logically, they all have the same basic equipment requirements. REFERENCES :

(a)David.G 1949 (MCGRAW HILL NEW YORK) Frequency Modulated Radar 466(b)Davis.B,Kennedy.G 1999 (TATA MCGRAW HILL OF INDIA,NEW DELHI)Electronic Communication Systems 634-638

(c)Schweber.W 2002 (PRENTICE HALL OF INDIA ,NEW DELHI )Electronic communication System 653-663(d)http://www.fas.org/man/dod-101/navy/docs/es310/cwradar/cwradar.htm(e)www.bbc.co.4k/dna/h2g2/A827372/A743807