11-09-2013, 04:33 PM
Military Radar System
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INTRODUCTION
RADAR (Radio Detection and Ranging) is basically a means of gathering information about distant objects by transmitting electromagnetic waves at them and analyzing the echoes. Radar finds a number of applications such as in airport traffic control, military purposes, coastal navigation, meteorology and mapping etc. Military radars have a highly specialized design to be highly mobile and easily transportable, by air as well as ground.
This report discusses about the military radar system. Radar system uses the general antenna properties. Radar is used in early warning, altering along with weapon control functions. This report gives the view of configuration of a typical military radar, data flow in a typical radar system, operating the radar, system functions, various terminal equipments used along with their functions, functional description of radar subsystem, some of the important parts of the radar such as transmitter unit, receiver unit, antenna, AFC (Automatic Frequency Control) etc., advanced features of the radar, advantages and limitations of the military radar system. This report also describes the target tracking, firing control, weapon aiming process using military radar.
BASIC PRINCIPLE OF RADAR
A transmitter generates an electromagnetic signal (such as sine wave) that is radiated into the space by using an antenna. A portion of the transmitted energy is intercepted by the target and reradiated in many directions. The reradiation directed back towards the radar is collected by the radar antenna which delivers it to the receiver. There it is processed to detect the presence of the target and determine is location.
Range to a target
The most common radar signal, or waveform, is a series of short-duration, somewhat rectangular-shaped pulses modulating a sine wave carrier. “The range to a target is determined by the time TR is takes the radar signal to travel to the target and back.”
FIRST USE OF RADAR IN MILITARY
During the 1930s, efforts to use radio echoes for aircraft detection were initiated independently and almost simultaneously in eight countries that were concerned with the prevailing military situation and that already had practical experience with radio technology. The United States, Great Britain, Germany, France, the Soviet Union, Italy, the Netherlands, and Japan all began experimenting with radar within about two years of one another and embarked, with varying degrees of motivation and success, on its development for military purposes. Several of these countries had some form of operational radar equipment in military service at the start of World War II.
The first observation of the radar effect at the U.S. Naval Research Laboratory (NRL) in Washington, D.C., was made in 1922. NRL researchers positioned a radio transmitter on one shore of the Potomac River and a receiver on the other. A ship sailing on the river unexpectedly caused fluctuations in the intensity of the received signals when it passed between the transmitter and receiver.
The first radars developed by the U.S. Army were the SCR-268 (at a frequency of 205 MHz) for controlling antiaircraft gunfire and the SCR-270 (at a frequency of 100 MHz) for detecting aircraft. Both of these radars were available at the start of World War II, as was the navy’s CXAM shipboard surveillance radar (at a frequency of 200 MHz). It was an SCR-270, one of six available in Hawaii at the time, that detected the approach of Japanese warplanes toward Pearl Harbor, near Honolulu, on December 7, 1941; however, the significance of the radar observations was not appreciated until bombs began to fall.
OPERATING THE RADAR
The operator’s main task is to watch the PPI (Plan Position Indicator) display, which presents only moving targets in the normal mode (MTI-MODE). Detected target can be assigned with the joystick controlled order marker to initiate target tracking. Target tracking is started and a track marker appears over the target echo. A label is displayed near the track marker. The system computer in the processor unit processes data on this tracked target. When an aircraft does not respond to the IFF interrogation it is considered to be unknown.
IFF (Identification Friend or Foe):
The IFF interrogator sends a coded challenge in the form of pulse pairs. The selected mode of operation determines the spacing between the pulses. A friendly target’s IFF will automatically reply to the coded challenge with an omni-directional transmission. It sends a different sets of pulses at a slightly different frequency than the interrogator. The IFF interrogator receives the coded reply and process it for display. Recognition of the target is based on PPI display. The coded reply from a friendly normally appears as a dashed line just beyond the target pulse.
Threat Evaluation
The data of the targets received is processed by a threat evaluation program, built in to the TDR. This program places all the targets in a sequence according to their threat priority and displays the result (azimuth angle of four most threatening targets) as an engagement advice.
FUNCTIONAL DESCRIPTION OF RADAR SUBSYSTEM
The detection of air targets is accomplished by the search radar, the video processor and the colour PPI unit. The colour PPI unit provides the presentation of all moving targets down to very low radial speeds on a PPI screen.
The search radar is pulse Doppler radar (also called MTI radar) i.e. it is capable of distinguishing between the echo from a fixed target and that of a moving target. The echoes from fixed target are eliminated, so that the echoes from the moving targets are presented on the screen.
The great advantage of this is that it is possible to distinguish a moving target among a large number of fixed targets, even when the echoes from these fixed targets are much stronger. To achieve this the search radar makes use of the Doppler effect, if the target having a certain radial speed with respect to the search antenna is hit by a series of transmitter pulses from the search radar antenna, the change in range between this target and antenna is expressed by successive echo pulses in phase shifts with respect to the phase of the transmitter pulses.
For moving targets the phase difference from echo pulse to echo pulse is continually subject to change, whereas for fixed targets this is a constant.
Pulse Unit
The pulse unit comprises of pulse shaping network and pulse transformer.
The pulse discharge of the pulse- shaping network will occur only if the magnetron impedance transformed by the pulse transformer is about equal to the characteristic impedance of the pulse-shaping network.
The thyratron diodes ensure that the remaining negative voltage, caused by the mismatch, on the pulse-forming network is directed to earth.
Magnetron
The magnetron is a self-oscillating RF power generator. It is supplied by the modulator by
high voltage pulses, whereupon it produces band pulses. The generated RF pulses are applied to the receiver unit.
The PRF of the magnetron pulses is determined by the synchronization circuit in the video processor, which applies start pulses to the sub-modulator of the transmitter unit. This
sub-modulator issues start pulses of suitable amplitude to trigger the thyratron in the modulator. On being triggered the modulator, which is supplied by the high tension unit, produces high voltage pulses.
As a magnetron is self oscillating some kind of frequency control is required. The magnetron is provided with a tuning mechanism to adjust the oscillating frequency between certain limits. This tuning mechanism is operated by an electric motor being part of AFC control circuit. Together with circuits in LO+AFC unit, a frequency control loop is created, thus maintaining a frequency difference i.e. the intermediate frequency of the receiver between the output frequency of the SSLO and the magnetron output frequency. The magnetron unit comprises a coaxial tunable magnetron, servo motor driving an adjustable plunger.
Receiver Unit
The receiver unit converts the received RF echo signals to IF level and detects the IF signals. By detecting the IF signals in two different ways, two receiver channels are obtained called MTI channel and linear channel.
The RF signals received by radar antenna are applied to the low noise amplifier. The image rejection mixer mixes the amplified signals with the SSLO signal, to obtain an IF signal. After amplification the IF signal is split into two branches viz. a MTI channel and a linear channel.
CONCLUSION
Military radars are one of the most important requirements during the wartime, which can be used for early detection of ballistic missile and also for accurate target detection and firing. Radar system discussed here has a built in threat evaluation program which automatically
puts the target in a threat sequence, and advises the weapon crew which target can be engaged first. Most essential, the target data is available to the weapon crew in time, so they can prepare themselves to engage the ‘best’ target for their specific weapon location.
A magnetron radar system is relatively simple and reliable. As a consequence, minimum maintenance is required and thus the system life cycle costs can be kept low.
[attachment=57917]
INTRODUCTION
RADAR (Radio Detection and Ranging) is basically a means of gathering information about distant objects by transmitting electromagnetic waves at them and analyzing the echoes. Radar finds a number of applications such as in airport traffic control, military purposes, coastal navigation, meteorology and mapping etc. Military radars have a highly specialized design to be highly mobile and easily transportable, by air as well as ground.
This report discusses about the military radar system. Radar system uses the general antenna properties. Radar is used in early warning, altering along with weapon control functions. This report gives the view of configuration of a typical military radar, data flow in a typical radar system, operating the radar, system functions, various terminal equipments used along with their functions, functional description of radar subsystem, some of the important parts of the radar such as transmitter unit, receiver unit, antenna, AFC (Automatic Frequency Control) etc., advanced features of the radar, advantages and limitations of the military radar system. This report also describes the target tracking, firing control, weapon aiming process using military radar.
BASIC PRINCIPLE OF RADAR
A transmitter generates an electromagnetic signal (such as sine wave) that is radiated into the space by using an antenna. A portion of the transmitted energy is intercepted by the target and reradiated in many directions. The reradiation directed back towards the radar is collected by the radar antenna which delivers it to the receiver. There it is processed to detect the presence of the target and determine is location.
Range to a target
The most common radar signal, or waveform, is a series of short-duration, somewhat rectangular-shaped pulses modulating a sine wave carrier. “The range to a target is determined by the time TR is takes the radar signal to travel to the target and back.”
FIRST USE OF RADAR IN MILITARY
During the 1930s, efforts to use radio echoes for aircraft detection were initiated independently and almost simultaneously in eight countries that were concerned with the prevailing military situation and that already had practical experience with radio technology. The United States, Great Britain, Germany, France, the Soviet Union, Italy, the Netherlands, and Japan all began experimenting with radar within about two years of one another and embarked, with varying degrees of motivation and success, on its development for military purposes. Several of these countries had some form of operational radar equipment in military service at the start of World War II.
The first observation of the radar effect at the U.S. Naval Research Laboratory (NRL) in Washington, D.C., was made in 1922. NRL researchers positioned a radio transmitter on one shore of the Potomac River and a receiver on the other. A ship sailing on the river unexpectedly caused fluctuations in the intensity of the received signals when it passed between the transmitter and receiver.
The first radars developed by the U.S. Army were the SCR-268 (at a frequency of 205 MHz) for controlling antiaircraft gunfire and the SCR-270 (at a frequency of 100 MHz) for detecting aircraft. Both of these radars were available at the start of World War II, as was the navy’s CXAM shipboard surveillance radar (at a frequency of 200 MHz). It was an SCR-270, one of six available in Hawaii at the time, that detected the approach of Japanese warplanes toward Pearl Harbor, near Honolulu, on December 7, 1941; however, the significance of the radar observations was not appreciated until bombs began to fall.
OPERATING THE RADAR
The operator’s main task is to watch the PPI (Plan Position Indicator) display, which presents only moving targets in the normal mode (MTI-MODE). Detected target can be assigned with the joystick controlled order marker to initiate target tracking. Target tracking is started and a track marker appears over the target echo. A label is displayed near the track marker. The system computer in the processor unit processes data on this tracked target. When an aircraft does not respond to the IFF interrogation it is considered to be unknown.
IFF (Identification Friend or Foe):
The IFF interrogator sends a coded challenge in the form of pulse pairs. The selected mode of operation determines the spacing between the pulses. A friendly target’s IFF will automatically reply to the coded challenge with an omni-directional transmission. It sends a different sets of pulses at a slightly different frequency than the interrogator. The IFF interrogator receives the coded reply and process it for display. Recognition of the target is based on PPI display. The coded reply from a friendly normally appears as a dashed line just beyond the target pulse.
Threat Evaluation
The data of the targets received is processed by a threat evaluation program, built in to the TDR. This program places all the targets in a sequence according to their threat priority and displays the result (azimuth angle of four most threatening targets) as an engagement advice.
FUNCTIONAL DESCRIPTION OF RADAR SUBSYSTEM
The detection of air targets is accomplished by the search radar, the video processor and the colour PPI unit. The colour PPI unit provides the presentation of all moving targets down to very low radial speeds on a PPI screen.
The search radar is pulse Doppler radar (also called MTI radar) i.e. it is capable of distinguishing between the echo from a fixed target and that of a moving target. The echoes from fixed target are eliminated, so that the echoes from the moving targets are presented on the screen.
The great advantage of this is that it is possible to distinguish a moving target among a large number of fixed targets, even when the echoes from these fixed targets are much stronger. To achieve this the search radar makes use of the Doppler effect, if the target having a certain radial speed with respect to the search antenna is hit by a series of transmitter pulses from the search radar antenna, the change in range between this target and antenna is expressed by successive echo pulses in phase shifts with respect to the phase of the transmitter pulses.
For moving targets the phase difference from echo pulse to echo pulse is continually subject to change, whereas for fixed targets this is a constant.
Pulse Unit
The pulse unit comprises of pulse shaping network and pulse transformer.
The pulse discharge of the pulse- shaping network will occur only if the magnetron impedance transformed by the pulse transformer is about equal to the characteristic impedance of the pulse-shaping network.
The thyratron diodes ensure that the remaining negative voltage, caused by the mismatch, on the pulse-forming network is directed to earth.
Magnetron
The magnetron is a self-oscillating RF power generator. It is supplied by the modulator by
high voltage pulses, whereupon it produces band pulses. The generated RF pulses are applied to the receiver unit.
The PRF of the magnetron pulses is determined by the synchronization circuit in the video processor, which applies start pulses to the sub-modulator of the transmitter unit. This
sub-modulator issues start pulses of suitable amplitude to trigger the thyratron in the modulator. On being triggered the modulator, which is supplied by the high tension unit, produces high voltage pulses.
As a magnetron is self oscillating some kind of frequency control is required. The magnetron is provided with a tuning mechanism to adjust the oscillating frequency between certain limits. This tuning mechanism is operated by an electric motor being part of AFC control circuit. Together with circuits in LO+AFC unit, a frequency control loop is created, thus maintaining a frequency difference i.e. the intermediate frequency of the receiver between the output frequency of the SSLO and the magnetron output frequency. The magnetron unit comprises a coaxial tunable magnetron, servo motor driving an adjustable plunger.
Receiver Unit
The receiver unit converts the received RF echo signals to IF level and detects the IF signals. By detecting the IF signals in two different ways, two receiver channels are obtained called MTI channel and linear channel.
The RF signals received by radar antenna are applied to the low noise amplifier. The image rejection mixer mixes the amplified signals with the SSLO signal, to obtain an IF signal. After amplification the IF signal is split into two branches viz. a MTI channel and a linear channel.
CONCLUSION
Military radars are one of the most important requirements during the wartime, which can be used for early detection of ballistic missile and also for accurate target detection and firing. Radar system discussed here has a built in threat evaluation program which automatically
puts the target in a threat sequence, and advises the weapon crew which target can be engaged first. Most essential, the target data is available to the weapon crew in time, so they can prepare themselves to engage the ‘best’ target for their specific weapon location.
A magnetron radar system is relatively simple and reliable. As a consequence, minimum maintenance is required and thus the system life cycle costs can be kept low.