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Military


Radar

In today's life style, many U.S. citizens are assisted in their personal life by information garnered from land-based, airborne, shipborne, and spaceborne radars. We hear daily weather broadcasts from television and radio stations where the weather radar is mentioned, providing significant detailed information of the weather situation. Air travelers are generally aware that air traffic control radars help with the safe and efficient movement of commercial and military aircraft, although the radars are not always visible to the traveler. Many automobile drivers know of radar speed guns and their use by law enforcement units to help curb speeding drivers. Many ocean-going boats and ships employ maritime surveillance and weather radars to help in piloting during adverse weather conditions.

Historically, the military is primarily credited with developing the radar. The term RADAR is derived from the description of its first primary role as a RAdio Detection And Ranging system. Originally, it was developed as a means of detecting approaching aircraft at long ranges to enable military defenses to react in sufficient time to counter incoming threats. Most natural and man-made objects reflect radio frequency waves and, depending on the radar's purpose, information can be obtained from objects such as aircraft, ships, ground vehicles, terrain features, weather phenomenon, and even insects. The determination of the object's position, velocity and other characteristics, or the obtaining of information relating to these parameters by the transmission of radio waves and reception of their return is sometimes referred to as radiodetermination.

In most cases, a basic radar operates by generating pulses of radio frequency energy and transmitting these pulses via a directional antenna. When a pulse impinges on a object in its path, a small portion of the energy is reflected back to the antenna. The radar is in the receive mode in between the transmitted pulses, and receives the reflected pulse if it is strong enough. The radar indicates the range to the object as a function of the elapsed time of the pulse traveling to the object and returning. The radar indicates the direction of the object by the direction of the antenna at the time the reflected pulse was received.

The "radar equation" mathematically describes the process and may be used to determine maximum range as a function of the pulse width (PW) and the pulse repetition rate (PRR). In most cases, narrow pulses with a high PRR are used for short-range, high-resolution systems, while wide PW's with a low PRR may be used for long-range search.

In general, a higher gain (larger aperture) antenna will give better angular resolution, and a narrower pulse width will give better range resolution. Changing the parameters of radars to satisfy a particular mission requires radar designers to have a variety of frequencies to choose from so that the system can be optimized for the mission and the radar platform.

Radar as a means of detection has been around for over 60 years, and although technology has become immensely more sophisticated than it was in the 1930's, the basic requirement remains the same--to measure the range, bearing, and other attributes of a target. Regardless of whether the system is land-based, shipborne, airborne, or spaceborne, this remains true since whatever the target may be, aircraft, ship, land vehicles, pedestrians, land masses, precipitation, oceans--all provide returns of the transmitted radar energy. What has changed dramatically is the system design, the method and speed of processing the return radar signals, the amount of information which can be obtained, and the way that the information is displayed to the operator. The key to modern radar systems is the digital computer and its data processing capability which can extract a vast amount of information from the raw radar signals and present this information in a variety of graphic and alphanumeric ways on displays as well as feeding it direct to weapon systems. It also enables the systems to carry out many more tasks such as target tracking and identification. In addition, modern signal processors provide adaptive operation by matching the waveform to the environment in which the radar is operating.

Much of the development effort over the past 50 years has been aimed at a number of operational requirements: improvements in the extraction of return signals from the background of noise, provision of more information to the operator, improvement of displays, and increased automation. Other developments have responded to the increasing operational requirements for radars to operate in a hostile electromagnetic environment. It is no longer enough to provide only bearing and range information; to this must be added altitude information, the ability to track a large number of moving targets, including airborne targets at supersonic and hypersonic speeds and to carry out normal surveillance at the same time. The latest shipborne surveillance and tracking radars, and some land-based systems, are designed to allocate the threat priority to incoming targets and guide weapons against them on this basis. Many radars are specifically designed for fire-control of missiles and guns, and also for use in missile guidance and homing systems, which entails packing the system into a very small space. In the airborne role, systems have to be packaged into a relatively small space with units sometimes scattered around the airborne vehicle.

In electronically steered phased array antennas, the forming and shaping of the radiated and/or received beam is performed by an array of discrete antenna elements in conjunction with phase shifters which insert a specified amount of phase shift into the signal being radiated from and received by each antenna element. The amount of phase shift to be introduced for each discrete antenna element is a function of the desired beam pointing angle and the desired beam shape. Individual phase shift amounts for each phase shifter are calculated by a microcomputer and are communicated to the individual phase shifters.

A state-of-the-art solid state radar transmitter/receiver module (T/R module) combines, on a single integrated circuit board, a phase shifter, a transmit/receive switch (T/R switch), a transmit amplifier, a receive amplifier, and a T/R module controller. An integral antenna element may also reside on or be co-located with the integrated circuit T/R module.

An electronically steered phased array antenna may be constructed with an array of T/R modules and associated antenna elements in which the respective T/R modules are each connected to a data bus which feeds phase delay information to the individual T/R modules.

However, performance of such phased array antennas can be sharply reduced due to unwanted movement, flexure and vibration of the phased array antenna on its platform. This movement, flexure and vibration causes displacement of the antenna elements with respect to one another which in turn causes errors to be introduced into the operation of the antenna array. These errors are particularly pronounced when an antenna array operates at a relatively high microwave frequency such as X-band or higher. Unwanted movement, flexure and vibration causes errors to some degree in all antenna arrays but such errors are most pronounced in antenna arrays having relatively lightweight and flexible back structures, such as where a lightweight antenna array is mounted on an aircraft or other vehicle.

To combat such unwanted movement, flexure and vibration, rigid back structures are presently used to precisely and rigidly support the array of discrete antenna elements and to thereby fix the relative position of each antenna element in order to eliminate flexure across the overall antenna. By rigidly fixing the relative position of each discrete antenna element, the relative position of each antenna element with respect to other elements and with respect to the antenna platform remains constant and need not be compensated for in controlling the phase shift of signals provided to the discrete antenna elements.

However, in modern high resolution radar systems, the antenna flexure tolerances required to maintain acceptable resolution are extremely low. As a result, the back structures required to maintain such low tolerances are quite massive and present numerous design obstacles. For example, these back structures are considerably large and heavy and, in an airborne environment, often require extensive and costly modifications to the host aircraft in order to accommodate them.

Frequency Bands and Radar OperationalPropagation Limitations

LF

30-300 kHz

Allocations are provided in the frequency range but no radar usage or applications have been identified.
MF

300-3000 kHz

Used by continuous wave (CW) radar systems for accurate position location. Very high noise levels are characteristic of this band.
HF

3-30 MHz

Refractive properties of the ionosphere make frequencies in this band attractive for long-range radar observations of areas such as over oceans at ranges of approximately 500-2000 nautical miles. Only a few radar applications occur in this frequency range because its limitations frequently outweigh its advantages: very large system antennas are needed, available bandwidths are narrow, the spectrum is extremely congested with other users, and the external noise (both natural noise and noise due to other transmitters) is high.
VHF

30-300 MHz

For reasons similar to those cited above, this frequency band is not too popular for radar. However, long-range surveillance radars for either aircraft or satellite detection can be built in the VHF band more economically than at higher frequencies. Radar operations at such frequencies are not affected by rain clutter, but auroras and meteors produce large echoes that can interfere with target detection. There have not been many applications of radar in this frequency range because its limitations frequently outweigh its advantages.
UHF

300-3000 MHz

Larger antennas are required at the lower end than at the upper end of the UHF band. As compared to the above bands, obtaining larger bandwidths is less difficult, and external natural noise and weather effects are much less of a problem. At the lower end, long-range surveillance of aircraft, spacecraft, and ballistic missiles is particularly useful. The middle range of this band is used by airborne and spaceborne SAR's. The higher UHF end is well suited for short to medium-range surveillance radars.
SHF

3 GHz-30 GHz

Smaller antennas are generally used in this band than in the above bands. Because of the effects of atmospheric absorption, the lower SHF band is better for medium-range surveillance than the upper portions. This frequency band is better suited than the lower bands for recognition of individual targets and their attributes. In this band, Earth observation efforts employ radars such as SAR's, altimeters, scatterometers, and precipitation radars .
EHF

30-300 GHz

It is difficult to generate high power in this band. Rain clutter and atmospheric attenuation are the main factors in not using this frequency band. However, Earth observation efforts are made in this band employing radars such as altimeters, scatterometers, and cloud profile radars.



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