Description of radar

2.1 Description of radar:  

Radar is an object detection system which uses electromagnetic waves—specifically radio waves to determine the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish, or antenna, transmits pulses of radio waves or microwaves which bounce off any object in their path. The object returns a tiny part of the wave's energy to a dish or antenna which is usually located at the same site as the same area.

A radar system has a transmitter that emits radio waves called radar signals in predetermined directions.

When these come into contact with an object they are usually reflected and/or scattered in many directions. Radar signals are reflected especially well by materials of considerable electrical conductivity especially by most metals, by seawater, by wet land, and by wetlands. Some of these make the use of radar altimeters possible. The radar signals that are reflected back towards the transmitter are the desirable ones that make radar work. If the object is moving either closer or farther away, there is a slight change in the frequency of the radio waves, due to the Doppler Effect.

Speed is the change in distance to an object with respect to time. Thus the existing system for measuring distance, combined with a memory capacity to see where the target last was, is enough to measure speed. At one time the memory consisted of a user making grease-pencil marks on the radar screen, and then calculating the speed using a slide rule. Modern radar systems perform the equivalent operation faster and more accurately using computers. However, if the transmitter's output is coherent (phase synchronized), there is another effect that can be used to make almost instant speed measurements (no memory is required), known as the Doppler Effect. Most modern radar systems use this principle in the pulse-Doppler radar system. Return signals from targets are shifted away from this base frequency via the Doppler Effect enabling the calculation of the speed of the object relative to the radar.

The Doppler Effect is only able to determine the relative speed of the target along the line of sight from the radar to the target. Any component of target velocity perpendicular to the line of sight cannot be determined by using the Doppler Effect alone, but it can be determined by tracking the target's azimuth over time. Additional information of the nature of the Doppler returns may be found in the radar signal characteristics article. It is also possible to make a radar without any pulsing, known as a continuous-wave radar (CW radar), by sending out a very pure signal of a known frequency. CW radar is ideal for determining the radial component of a target's velocity, but it cannot determine the target's range. CW radar is typically used by traffic enforcement to measure vehicle speed quickly and accurately where range is not important.

2.2 Radar unit circuit

2.2.1 Component descriptions:

-sweep oscillator:

The sweep oscillator PM 7022X is designed for sweep frequency measurement at low cost with the highest accuracy. Only necessary functions have been included in order to keep the price low. The result is a compact, reliable instrument, easy to operate giving a minimum output power of 10 mW over the whole frequency range. It has variable sweep speeds for use with both oscilloscopes and pen recorders.

- Coax-waveguide transition:

PM 7328X is a short transition from waveguide to 3 mm coaxial type SMA. The mismatch is kept low over the entire frequency range.

- Ferrite circulator

The circulator PM 7050X has a Y configuration with the ferrite material in the center of the symmetrical junction formed by the three symmetrically spaced waveguides.

2.2.2 Problems of Radar

1. Interference

Radar systems must overcome unwanted signals in order to focus only on the actual targets of interest. These unwanted signals may originate from internal and external sources, both passive and active. The ability of the radar system to overcome these unwanted signals defines its signal-to-noise ratio (SNR). SNR is defined as the ratio of a signal power to the noise power within the desired signal.

2. Noise:

Signal noise is an internal source of random variations in the signal, which is generated by all electronic components. Noise typically appears as random variations superimposed on the desired echo signal received in the radar receiver. The lower the power of the desired signal, the more difficult it is to discern it from the noise (similar to trying to hear a whisper while standing near a busy road). Noise figure is a measure of the noise produced by a receiver compared to an ideal receiver, and this needs to be minimized.

3. Clutter

Clutter refers to radio frequency (RF) echoes returned from targets which are uninteresting to the radar operators. Such targets include natural objects such as ground. Clutter may also be returned from man-made objects such as buildings and, intentionally, by radar countermeasures such as chaff.

4. Jammer:

Radar jamming refers to radio frequency signals originating from sources outside the radar, transmitting in the radar's frequency and thereby masking targets of interest.

Jamming may be intentional, as with an electronic warfare (EW) tactic Jamming is considered an active interference source, since it is initiated by elements outside the radar and in general unrelated to the radar signals.

2.2.3 Doppler radar

Compare Speed of Car with Default speed if the Speed of Car Higher than Default Speed gives a pulse trigger to camera to capture image to the car. It is radar that makes use of the Doppler Effect to produce data about objects at a distance. It does this by beaming a microwave signal towards a desired target and listening for its reflection, then analyzing how the original signal has been altered by the object(s) that reflected it. Variations in the frequency of the signal give direct and highly accurate measurements of a target's velocity relative to the radar source and the direction of the microwave beam.

Doppler radars are used in air defense, air traffic control, sounding satellites, police speed guns and radiology.

2.2.4 Doppler Effect

It is the change in frequency of a wave for an observer moving relative to the source of the waves. It is commonly heard when a vehicle sounding a siren approaches, passes and recedes from an observer. The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession. For waves that propagate in a medium, such as sound waves, the velocity of the observer and of the source is relative to the medium in which the waves are transmitted. The total Doppler Effect may therefore result from motion of the source, motion of the observer, or motion of the medium.

2.2.5 Moving targets

Stationary 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.

                                                         

2.3 Radar equation:

The power Pr returning to the receiving antenna is given by the radar equation:

Where

·      P_t = transmitter power

·      G_t = gain of the transmitting antenna

·      A_r = effective aperture (area) of the receiving antenna

·      £m = radar cross section or scattering coefficient, of the target

·      F = pattern propagation factor

·      R_t = distance from the transmitter to the target

·      R_r = distance from the target to the receiver.

In the common case where the transmitter and the receiver are at the same location, R_t = R_r and the term R_t² R_r² can be replaced by R⁴, where R is the range. These yields:

This shows that the received power declines as the fourth power of the range, which means that the reflected power from distant targets is very, very small. The equation above with F = 1 is a simplification for vacuum without interference. The propagation factor accounts for the effects of multipath and shadowing and depends on the details of the environment. In a real-world situation, path loss effects should also be considered.

2.3.1Analysis:

The frequency of the sounds that the source emits does not actually change. To understand what happens, consider the following analogy. Someone throws one ball every second in a man's direction. Assume that balls travel with constant velocity. If the thrower is stationary, the man will receive one ball every second. However, if the thrower is moving towards the man, he will receive balls more frequently because the balls will be less spaced out. The inverse is true if the thrower is moving away from the man. So it is actually the wavelength which is affected.

As a consequence, the received frequency is also affected. It may also be said that the velocity of the wave remains constant whereas wavelength changes; hence frequency also changes. If the source moving away from the observer is emitting waves through a medium with an actual frequency f₀, then an observer stationary relative to the medium detects waves with a frequency f given by

Where vs. is positive if the source is moving away from the observer, and negative if the source is moving towards the observer. A similar analysis for a moving observer and a stationary source yields the observed frequency (the receiver's velocity being represented as v_r):

Where the similar convention applies: v_r is positive if the observer is moving towards the source and negative if the observer is moving away from the source. These can be generalized into a single equation with both the source and receiver moving.

With a relatively slow moving source, vs,r is small in comparison to v and the equation approximates to

Where

However the limitations mentioned above still apply. When the more complicated exact equation is derived without using any approximations (just assuming that source, receiver, and wave or signal are moving linearly relatively to each other) several interesting and perhaps surprising results are found. For example, as Lord Rayleigh noted in his classic book on sound, by properly moving it would be possible to hear a symphony being played backwards. This is the so-called "time reversal effect" of the Doppler Effect. Other interesting conclusions are that the Doppler effect is time-dependent in general (thus we need to know not only the source and receivers' velocities, but also their positions at a given time), and in some circumstances it is possible to receive two signals or waves from a source, or no signal at all. In addition there are more possibilities than just the receiver approaching the signal and the receiver receding from the signal.

2.3.2 Testing Radar circuit

Connecting the radar parts:

Procedures:

1. Setup the equipment as shown in the figure. Start the GUNN-OSCILLATOR and tune it to approx. 9.0 GHz. The bias should not exceed 9 volts.

2. DC-couple the OSCILLATOR with a vertical sensitivity of 50 mV/cm.

The LO power to the detector will now give us a DC-offset on the screen

3. Set the frequency meter to exactly 9,000 MHz and tune the Gunn-oscillator until a sharp decrease in the DC-offset will occur. The oscillator frequency is now 9.0 GHz.

4. Detune the frequency meter at least 50 MHz

Set the actual target in motional-couple the OSCILLOSCOPE and increase the vertical sensitivity until it is possible to study the sinusoidal trace of the Doppler frequency on the screen. Choose a suitable setting of the time/div. on the oscilloscope and measure the time of the complete periods.

After the testing we have got that the maximum output voltage from the radar when moving target to detect it with the maximum speed is 50mvolt.