Due to different design parameters, no single radar set has been produced that can perform all of the radar functions required by combatant ships. As a result, the modern warship has several radar sets, each performing a specific function. A shipboard radar installation may include surface search, navigation radar, air search radar, a height finding radar, and various fire control radars. Ship's radars can perform a variety of functions. For example, most height finding radars can be used as secondary air search radars, and in emergencies, fire control radars have served as surface search radars.

The continuous wave (CW) method uses the Doppler effect to detect a target. The frequency of a radar echo changes when the target is moving toward or away from the radar transmitter. This change in frequency is known as the Doppler effect. It is similar to the effect at audible frequencies when the sound from the whistle of an approaching train appears to increase in pitch. The opposite effect (a decrease in pitch) occurs when the train is moving away from the listener. The radar application of this effect involves measuring the difference in frequency between the transmitted and reflected radar beams to determine both the presence and speed of the moving target. This method works well with fast-moving targets, but not well with those that are slow moving or stationary.

In Frequency Modulation [FM] the transmitted frequency is varied continuously and periodically over a specified band of frequencies. At any given instant, the frequency of energy radiated by the transmitting antenna differs from the frequency reflected from the target. This frequency difference can be used to determine range. Moving targets, however, produce an additional frequency shift in the returned signal because of the Doppler effect. This additional frequency shift affects the accuracy of range measurement. Thus, this method works better with stationary or slow moving targets than with fast-moving targets.

Radars using pulse modulation transmit energy in short pulses that vary in duration from less than 1 to 200 mseconds, depending on the type of radar. Echoes are amplified and applied to an indicator that measures the time interval between transmission of the pulse and reception of the echo. Half the time interval then becomes a measure of the distance to the target. Since this method does not depend on the relative frequency of the returned signal or on the motion of the target, difficulties experienced with the CW and FM methods are not present. The pulse modulated method is used almost universally in military and naval applications. Therefore, it is the only method discussed in detail in this text.

In general, the maximum range that can be measured on an indicator is limited by the pulse repetition rate (PRR). This is because with each transmitted pulse the indicator is reset to zero range. Therefore, if the time between transmitted pulses is shorter than the time it takes the transmitted pulse to reach the target and return, the indicator will have been reset and started as a new sweep; thus indicating a false range upon reception of the echo. Pulse width (PW) also affects maximum detection range. The wider the pulse, the greater the average power out, resulting in a greater detection range of small targets. Air search radars usually have a much greater PW than surface search radars. The more sensitive the receiver, the weaker the echo required to produce a target indication. As the receiver sensitivity is increased, which is reflected in a higher minimum discernable signal (MDS), the range at which a particular target can be detected is increased. Target size also affects maximum range. Generally, the larger a target, the greater the range at which it can be detected.

The successful use of pulse modulated radar systems depends primarily on the ability to measure distance in terms of time. Radio frequency energy

Answered by Anju Singh , an ibibo Master, at 6:54 AM on October 24, 2008

A tracking radar system having: an antenna with at least one boresight; means for producing signals indicating an angular difference between the direction of a target and the said one boresight; means for changing said boresight in response to the said signals so as to tend to reduce the difference; control means for preventing said boresight responding to signals indicating that a function of the velocity of the target is greater than a threshold; and means for varying the threshold thereby tending to keep constant the proposition of a time period during which the threshold is exceeded.

2. A tracking radar system according to claim 1 wherein said control means includes means responsive to the threshold being exceeded, for causing the boresight to continue to move in a direction in which it was moving before the threshold was exceeded.

3. A tracking radar according to claim 1 or 2 wherein said control means includes circuit means for determining angular acceleration as said function of the velocity.

4. A tracking radar according to claim 1 or 2 wherein said control means includes circuit means for determining angular velocity as said function of the velocity.

5. In a tracking radar system including an antenna with at least one boresight, first means for producing output signals indicating an angular difference between the direction of a target and said one boresight, and second means, connected to the output of said first means and responsive to said output signals, for changing the position of said one boresight so as to tend to reduce said difference; the improvement comprising: third means response to said output signals for determining whether said output signals indicate that a function of the velocity of the target is greater than a threshold value; control means, responsive to an ouput signal from said third means indicating that said threshold value is being exceeded, for preventing said second means from responding to said output signals from said first means; and fourth means for varying said threshold value so as to tend to keep constant the proportion of a time period during which said threshold value is exceeded.

6. A tracking radar system as defined in claim 5, further comprising fifth means, actuated upon receipt by said control means of an output signal from said third means that said threshold value is being exceeded, for supplying a signal to said second means to cause said one boresight to continue to move in the same direction.

7. A tracking radar system as defined in claim 5 or 6 wherein said control means includes a switch means for connecting said second means to the output of said first means, when in a normal first condition and for disconnecting said first and second means when in a second condition.

8. A tracking radar system as defined in claim 7 wherein said fifth means includes storage means, connected to the input of said second means, for storing the output signal of said first means when said switch means is in said first condition and for supplying its stored value to said second means when said switch means is in said second condition.

9. A tracking radar system as defined in claim 5 wherein said third means includes circuit means for determining the angular acceleration of a target from said output signals of said first means, and comparator means for comparing a signal corresponding to said angular acceleration with said threshold value and for producing an output signal whenever said threshold value is being exceeded.

10. A tracking radar system as defined in claim 5 wherein said third means includes circuit means for determining the angular velocity of a target from said output signals of said first means, and comparator means for comparing a signal corresponding to said angular velocity with said threshold value and for producing an output signal whenever said threshold value is being exceeded.

Answered by jaivir , an ibibo Master, at 6:52 AM on October 24, 2008

In a tracking radar system target glint and multipath sometimes produce signals which falsely indicate high acceleration rates of the target. These signals cause the boresight of the antenna to be driven away from the direction in which the target actually lies.

To avoid this problem, received signals from a receiver are fed through a switch which is opened whenever the target appears to have an acceleration or velocity above a threshold value. The threshold value is varied so that the proportion of the time during which the switch is open is kept approximately constant. Thus, if there is a period during which a large number of high acceleration or high velocity signals appear from the receiver (for example is there is a very high noise level) the switch still remains closed for a certain proportion of the time period. This ensures that the tracking system never loses the target while also ensuring that false tracking due to target glint and multipath is minimized.

A circuit is included so that, when the switch is open the antenna is not brought to a complete halt but rather is allowed to continue to move in the direction that it was moving before the switch was operated.

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Answered by Nagendra , an ibibo Master, at 5:53 AM on October 24, 2008

Search radars are systems devoted to the systematic exploration of a large volume of space (for typical air search radars, this is performed over 360° in azimuth and over elevation angles ranging from 20-30° up to almost 90°, processing the echoes over the whole PRI, i.e. over the whole observable range), using different scan techniques.

On the other side, tracking radars remains "locked" on a specific target to provide continuos information about its position and motion. Usually, a "pencil beam" (i.e., narrow in both azimuth and elevation) is used. Only a small "range window" correnponding to the target range and its immediate vicinity is processed.

External designation systems (such as search radars) are normally employed for target initial location.

Tracking radars use "closed loop" (feedback) control systems to keep the target aimed in both angle and range. For angle tracking (azimuth/elevation) different techniques, such as conical scan or monopulse are employed to detect if the target is off the antenna axis, and the direction and amplitude of this deviation. These error signals is used to drive the antenna pointing servomechanism (or the steering control system for electronically-steered antennas) to keep the antenna beam centered on the target.

In the same way, range tracking is performed detecting the position of the echo centre of gravity (or, for some applications, the leading edge) with reference to the observed "range window" (using, for instance the early gate-late gate technique, in which the amplitudes of two samples collected on the leading and trailing edge of the echo - which, after filtering, is approximately a triangle - are compared to generate the correction signal) to generate an error signal which shift in time the processed range window to keep the target centered.

Answered by Gyan Singh , an ibibo Master, at 5:47 AM on October 24, 2008