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Radar

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.

Radar Principles

The antennas of most radars are designed so that they radiate energy in one lobe that is moved by moving the antenna itself. The shape of the lobe is such that the echo signal strength varies more rapidly with a change of bearing on the sides of the lobe than near the axis. Therefore, the echo signal varies in amplitude as the antenna rotates. At one antenna position, the echo is relatively small, but at another position, where the lobe axis is aimed directly at the target, the echo strength is maximum. Thus, the bearing of the target can be obtained by training the antenna to the position at which echo is greatest. In actual practice, manipulation of the antenna in this manner could alert an enemy unit that it has been detected and denies remote indicators full use of the radar for search purposes. However, this technique is widely used in weapons control and guidance radar systems in manual and automatic modes.

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 radiated into space travels at the speed of light; i.e., 186,000 miles per second, 162,000 nautical miles per second, or 328 yards (yds) per microsecond (msec). When it strikes a reflecting object, it is redirected, with no loss in time. The constant velocity of radio frequency energy is used in radar to determine range by measuring the time required for a pulse to travel to a target and return. For example, assume that a 1 msec pulse is transmitted toward an object that is 32,000 yds away. When the pulse reaches the target, it has traveled 32,800 yds at 328 yds/msec. Therefore, 100 mseconds have elapsed. When the pulse arrives at the target it is reflected back to the antenna. Since the return path is also 32,800 yds, it takes 100 mseconds for the pulse to return to the radar antenna. The total elapsed time is 200 mseconds for a distance of twice the actual range of the target. Therefore, velocity is considered to be one half of its true value, or 164 yds/msec; i.e., time x 164 = range or 200 x 164 = 32,800 yards.

Radar Types

AN/SPN-35 Aircraft Control AN/SPN-41 Aircraft Control AN/SPN-42 Aircraft Control AN/SPN-43 Aircraft Control AN/SPN-44 Aircraft Control AN/SPN-45 Aircraft Control AN/SPN-46 Aircraft Control

Carrier Controlled Approach (CCA) / Ground Controlled Approach (GCA) Radars are essentially shipboard and land-based versions of the same radar. A complete GCA facility would include radio communications equipment and a search radar in addition to the precision approach radar equipment. Shipboard CCA radar systems are usually much more sophisticated than GCA systems due to the movements of the ship and more complex landings. Both systems, however, guide aircraft to a safe landing at zero visibility. Aircraft are detected and observed during the final approach and landing sequence. The radar set tracks the approaching aircraft down its line of descent (glidepath to landing) and along its course during the final critical phase of the ground controlled approach. It alternately scans in both the vertical and horizontal planes to track the approaching aircraft's line of descent and course. The controller advises the pilot by radio of any changes in the glidepath or course needed to accomplish a safe landing. Guidance information can be supplied to the pilot as verbal radio instructions, or to the automatic pilot.

AN/BPS-15 Navigation AN/BPS-16 Navigation

Shipborne Navigation Radars are required on ships with large displacements, to enable them to navigate in coastal areas and near docks. The 2900-3100, 5460-5650, and 9300-9500 MHz bands are allocated for this purpose. The 9300-9500 MHz band is the most frequently used band because radars using it can provide a very high resolution. Furthermore, the 9300-9500 MHz radars use interference-rejection circuitry to eliminate harmful interference even though several radars may be operating on or near the same frequency in the same geographical area. The requirement for maritime radionavigation radars is contained in the IMO Safety of Life At Sea (SOLAS) treaty. The radars will continue operating well into the next century; thus, long-range spectrum requirements are expected to continue.

AN/SPQ-9 Surface Search AN/SPS-10 Surface Search AN/SPS-55 Surface Search AN/SPS-60 Surface Search AN/SPS-64 Surface Search AN/SPS-67 Surface Search AN/SPS-73 Surface Search

The primary functions of a surface search-radar are detection and determination of accurate ranges and bearings of surface targets and lowflying aircraft, and maintaining a 360° search for all targets within line-of-sight [LOS] distance from the radar antenna. Since the maximum range requirement of a surface search radar is primarily limited by the radar horizon, higher frequencies are used to permit maximum reflection from small target-reflecting areas such as ship masthead structures and submarine periscopes. Narrow pulse widths are used to permit a high degree of range resolution at short ranges, and to achieve greater range accuracy. High pulse repetition rates are used to permit maximum definition of targets. Medium peak powers can be used to permit detection of small targets at LOS distances. Wide vertical beam widths permit compensation for pitch and roll of ownship and detection of low flying aircraft. A narrow horizontal beam width permits accurate bearing determination and good bearing resolution.

AN/SPS-40 Air Search AN/SPS-48 Air Search AN/SPS-49 Air Search AN/SPS-52 Air Search

The primary function of an air search radar is to detect aircraft targets and to determine their ranges and bearings over relatively large areas while maintaining a complete 360° surveillance from the surface to high altitudes. Air search radars have relatively low radar frequencies, permiting long range transmissions with minimum attenuation. They use wide pulse width and high peak power to aid in detecting small targets at great distances, and low pulse repetition rates to permit greater maximum measurable range. A wide vertical beam width helps ensure detection of targets from the surface to relatively high altitudes, and to compensate for pitch and roll of ship, while a medium horizontal beam width permits fairly accurate bearing resolution while maintaining 360° search coverage.

Height-finding radar uses a very narrow vertical beam, which is moved up and down electronically or mechanically to pinpoint targets. The electronic method, produces a frequency scanning pattern along the vertical plane. Lines originating at the antenna depict the number of beam positions required to ensure complete coverage. Each beam position corresponds to a slightly different radiated frequency, which is set at a specific angle or step in relation to the base of the antenna. When the antenna base is stable, the initial radiated frequency sets up the top beam. A slight change in frequency activates the second beam, and the process continues until the entire plane is covered. When the antenna base is unstable, error signals are introduced by components of the system. A change then results in the transmitted frequency, compensating for ship's pitch and roll and ensuring a complete search of the vertical plane.

Height-finding radar provides the two mathematical components that are used to determine the altitude of an aircraft, angle of elevation, and slant range. The slant range of an aircraft is the distance of the aircraft from the radar antenna, measured along the radar beam. When both the angle of elevation and slant range are known, the altitude of the aircraft can be found. The solution may be by calculation, by reference to a graph, or by a computer built into the radar. To find the altitude by calculation, multiply the slant range by the sine of the angle of elevation. Altitude found in this way is not the true height of the airplane above the earth because the calculation is based on the assumption that the earth is flat. However, most height finding radars have a circuit that computes the error due to the curvature of the earth.

AN/SPG-51D Missile Director AN/SPG-60 AN/SPG-62 Illuminator AN/SPN-35 Aircraft Control

A Fire Control Radar that provides continuous positional data on targets is called a tracking radars. Most tracking radar systems used by the military are also fire control radars; the two names are often used interchangeably. Fire control tracking radar systems usually produce a very narrow, circular beam. Fire control radar must be directed to the general location of the desired target because of the narrow beam pattern. This is called the designation phase of equipment operation. When the radar beam is in the general vicinity of the target, the radar system switches to the acquisition phase. During acquisition, the radar system searches a small volume of space in a prearranged pattern until the target is located. When the target is located, the radar system enters the track phase of operation. Using one of several possible scanning techniques, the radar system automatically follows all target motions. The radar system is said to be locked on to the target during the track phase. The three sequential phases of operation are often referred to as modes and are common to the target-processing sequence of most fire control radars. Typical fire control radar characteristics include a very high PRF, a very narrow pulse width, and a very narrow beam width. These characteristics, while providing extreme accuracy, limit the range and make initial target detection difficult.

A Fire Control Radar that provides information used to guide a missile to a hostile target is called a guidance radar. Missiles use radar to intercept targets in three basic ways. Beamrider missiles follow a beam of radar energy that is kept continuously pointed at the desired target. Homing missiles detect and home on radar energy reflected from the target. The reflected energy is provided by a radar transmitter either in the missile or at the launch point and is detected by a receiver in the missile. Passive homing missiles home on energy that is radiated by the target. Because the target's position must be known at all times, a guidance radar is generally part of, or associated with, a fire control tracking radar. In some instances, three radar beams are required to provide complete guidance for a missile. The beam-riding missile, for example, must be launched into the beam and then must ride the beam to the target. Initially, a wide beam is radiated by a capture radar to gain (capture) control of the missile. After the missile enters the capture beam, a narrow beam is radiated by a guidance radar to guide the missile to the target. During both capture and guidance operations, a tracking radar continues to track the target.

AN/SPY-1 AN/SPY-2 VSR AN/SPY-3 MFR AN/SPY Solid State AMDR DBR HPDR IROS3

A Multifunction Radar (MFR) is equivalent to a suite of radars, sometimes employed for some applications such as air defence. To fulfil its purpose it performs several different functions which previously would have been undertaken by many different, dedicated radars. The exact functions that are undertaken are dependent upon the application, but, as a minimum, the multifunction radar will provide search coverage and concurrent tracking of multiple targets. With the capability to perform multiple functions within a single sensor comes drawbacks. In particular, the total radar time-budget from this single sensor must be shared between each of the functions. This means that radar time is at a premium and in many practical scenarios, less radar time is available than is ideally required. The development of radar techniques and tasks that are efficient in their use of radar time are thus crucial, and form the principal focus for this work.

A Multifunction Radar (MFR) combines the electronic steering of the antenna beam with the capability to use computer control to adaptively vary a range of parameters such as PRF, waveform coding, power and signal processing. This adaptive capability enables the MFR to replace a large number of conventional radars, since it is capable of performing volume surveillance, multiple target tracking, missile communications and aircraft support and navigation.

Thus the MFR attempts to substitute a number of other sensors, each of which dedicates all of its time to performing a specific function. Therefore the issue of time management is crucial if efficient utilisation of the MFR capabilities is to be achieved. This area has attracted intense research and time management is the second of two most crucial factors in MFR design. Choice of radar frequency, high enough to provide the narrow tracking beam, and low enough to permit rejection of clutter driving volume search modes. Budgeting of radar time to provide the dwell times necessary for clutter rejection in both search and tracking modes.

Regarding the optimal height for shipboard radars - the most recent thinking is: low down [16m] S-band Volume Search Radar (VSR; SPY-2) and high up [25m] X-band Multi-Function radar (MFR; SPY-3). This would provide the optimal coverage for ballistic missile targets (SPY-2) and antiship missile targets (SPY-3). Low down favors S-band propagation - that's why you have the S-band jammers in WWII and the PLA Navy ships nearer deck level. Mounting the SPS-48E high up in the NTU ships was an attempt to increase the radar horizon to compensate for a lower data rate than SPY-1. With SPY-1, you'd want to get as much performance out of favorable transmission for optimal long range detection and tracking. Its high data rate compensates for the shorter radar horizon. In other words, the 16m high SPY-1 would first be able to "see" the sea skimmer target at a shorter distance than the SPS-48E high up. But, the faster scan rate enables the SPY-1 to detect and track the target more rapidly and still provide sufficent warning time to engage the target. With a lower data rate, the SPS-48E high up trades off long range performce for high angle targets to have a better chance of detecting the high threat sea skimmer. At the time of its introduction, the processing of SPS-48E actually made it superior to SPY-1A in detection of low angle targets.