Once a target has been designated, acquired, and tracked by a radar system, the final stage in target engagement is to guide a missile or projectile to destroy the target. There are three basic requirements for successful missile guidance:
There are three distinct phases in any missile intercept: boost, mid-course, and terminal.
Nearly all missiles are unguided during the initial boost phase. During the boost phase, the missile electrical and hydraulic systems are activated and are coming up to operating parameters. The missile is gathering speed and normally will be in an unguided mode of flight.
During the mid-course phase, the missile is actively being guided to the target using some type of guidance signal. Guidance signals deflect the control vanes of the missile to change its direction. These vanes change the roll, pitch, and yaw, in some combination, to control the missile flight path. Normally a gas grain generator powers a small hydraulic pump that deflects the control vanes in response to guidance signals. Each missile carries a limited supply of hydraulic fluid for maneuvering. The fluid is expended through vents with every control surface activation. The limited quantity of hydraulic fluid can be a significant factor during a long-range missile intercept.
The final phase of an intercept is the terminal phase. During this phase, the missile attempts to pass close enough to the target to detonate the fuse while the target is within the lethal radius of the warhead. Modern missiles employ both a contact fuse and some type of proximity fuse. Proximity fuses range from command detonation for command-guided missiles, fractional Doppler gates for semi-active guided missiles, to active laser fuses for IR-guided missiles.
Command guidance uses a fire control computer to constantly send course correction commands to the missile throughout its flight. These commands are a series of electrical missile guidance pulses called doublets or triplets. These pulses provide steering commands to the missile by varying the spacing between each guidance pulse. Each pulse, or pulse combination, relays some roll, pitch, and yaw command to the missile. These inputs are constantly corrected for the spatial relationship between the missile and the target's present position and rate of motion. Guidance commands are passed to the missile by specialized antennas on the TTR and an antenna installed on the missile, called a missile beacon. The beacon is a special radio receiver and transmitter that is attached to the rear of the missile. It acts like a transponder in that the TTR tracks and receives guidance commands. The guidance frequency may be widely separated from the target tracking radar frequency to minimize interference. This beacon is usually masked until missile booster separation. These results in the missile being launched unguided for the first 2-3 seconds. This type of delay is one of the reasons that all command-guided missile systems have a minimum launch range.
Command-guided missiles will generally fly a rectified (full or half) or threepoint pursuit geometry during the mid-course portion of the intercept. However, a command-guided missile may transition to pure pursuit geometry during the terminal phase of the intercept. Rectified geometry involves the prediction of where the target and the missile will be at some point in the future.
The target's direction and rate of movement is tracked and predicted. The missile is then launched, pulls lead on the target, and is guided to the point in the sky where the intercept is predicted to take place. This profile requires the constant update of both the target and missile positions.
Three-point pursuit geometry is often used when there is incomplete range tracking data on the target. In this case, it will be impossible to predict exactly where the target will be at some point in the future. In this profile, the target tracking radar constantly tracks the target. The missile location will be updated by the missile beacon. The fire control computer will direct the missile to fly directly down the tracking radar beam toward the target. In this geometry, the missile may start out on a direct intercept course and, depending on the target's direction and rate of movement, transition to a pure pursuit intercept. The three points in three-point missile geometry are depicted in Figure. Point one is the target tracking radar, point two is the missile itself, and point three is the target.
By keeping all three points always in a line, the missile will intercept the target at some point, although the range of the target is unknown.
Command guidance techniques have many advantages. First, command guided missiles can adjust their flight geometry throughout an intercept profile.
Second, the missiles are uncomplicated since they do not carry onboard computers or target tracking equipment. The fire control computer associated with the TTR accomplishes all intercept calculations. Third, the primary intercept profile, a full- or half-rectified intercept, is the fastest and most fuel-efficient intercept. Fourth, command guidance is difficult to jam since the missile beacon antenna is at the rear of the missile and can be relatively high-powered. And finally, an intercept is possible even without accurate range information by using the three point intercept profile.
Command guidance has several disadvantages. First, the use of a missile beacon delays the capture of the missile by the tracking radar. This can cause a large dead zone which equates to a larger minimum engagement range. Second, the accuracy of the intercept geometry is only as good as the tracking information provided by the target tracking radar. Jamming, interference, or loss of signal will adversely affect the intercept accuracy. In addition, normal radar characteristics could produce sufficient errors to cause the missile to miss the target, especially at longer ranges. Third, with insufficient range information, the three-point intercept profile is very slow and could result in the missile running out of energy before it gets to the target. Fourth, command guidance is reactive. The fire control computer constantly updates the intercept geometry based on target maneuvering. This results in missile maneuvering lagging target maneuvers.
Semi-active guidance is significantly different from command guidance, but only after launch. The first requirement is still for the target tracking radar to maintain a solid target track, with the tracking data being supplied to the fire control computer. The fire control computer then directs a target illumination antenna to point at the target and illuminates it with CW energy. The missile then passively homes on the reflected CW energy.
The missile used by a threat system that uses CW homing is vastly different from the missile being guided by a command guidance signal. The missile that homes on CW energy must be equipped with a seeker section composed of an antenna and an internal receiver. The seeker section processes and computes the necessary course corrections as it flies toward the target. It can do this by knowing the zero boresight line of the antenna within the missile. As the reflected CW energy is received by the seeker, there is normally some deviation from the zero reference position. The onboard computer then directs the control surfaces to change the flight path to reduce the reference errors in the antenna to zero, if possible. When the error between the antenna position and the boresight position is zero, the missile is pointed directly at the target.
Missile systems that use semi-active guidance normally use velocity as the primary target discriminator during the intercept. The missile seeker locks onto a reference Doppler signal provided by the fire control computer before launch. This Doppler signal establishes a tracking gate around the velocity of the target.
After the missile is launched, it initially compares the reference Doppler to the target Doppler signal.
The mid-course phase for a semi-active missile is also different from that of a command-guided missile. A semi-active guided missile follows the reflected CW energy during the mid-course phase of the intercept and normally attempts to fly a lead pursuit profile to the target. If the target maneuvers, however, the missile may transition to a pure pursuit flight path. Unlike a command-guided missile, a semi-active guided missile does not use a missile beacon. The fire control computer does not need to know where the missile is to compute course corrections since all that is necessary is to illuminate the target with the CW illuminator. This also means that the missile can begin to track and guide when it is launched and locked on to the reference Doppler gate. Semi-active guidance is the primary mode of guidance for many surface-to-air missiles, and radar-guided air-to-air missiles.
As the missile enters the terminal phase of the intercept, there is no change in the guidance mode used by a CW homing missile. The missile may complete the terminal phase of the intercept geometry by going to a pure pursuit flight path, if necessary. The missile continues to home in on the reflected CW signal until it passes close enough for the fuse to function.
Semi-active missile guidance has many advantages. First, a semi-active guided missile is resistant to electronic jamming that may be used to deny range information. Second, a semi-active missile can be guided almost immediately after launch. This gives it a very small minimum range since it can maneuver almost as soon as it clears the launch rail. Third, it computes its own course corrections as necessary. This allows for a much quicker reaction to target maneuvers compared to a command-guided missile. Fourth, during a long-range intercept, a CW missile can be more accurate than a command-guided missile.
This is accomplished by taking the inherent long-range radar tracking errors out of the equation. The target tracking radar only has to keep the target illuminated so that it can point the CW antenna at the target.
Although semi-active missile guidance is generally considered an excellent guidance technique, it does have some disadvantages. First, a semi-active guided missile normally requires reference Doppler values to be entered into the missile computer before launch. Without this reference, a semi-active missile cannot be launched. Second, a semi-active homing missile must maintain a lock onto the target Doppler. The use of chaff and beam maneuvers, which result in a near zero target Doppler, may cause a missile or radar to break lock. Third, if a break-lock occurs, a CW homing missile normally cannot regain target track and complete the intercept.
This specialized guidance mode is only active during the terminal phase of flight. The mid-course phase usually employs semi-active or command guidance. The range at which the missile goes “active” is dependent on the intercept geometry. High-aspect angle intercepts allow the activation of active guidance sooner than beam or tail-aspect intercepts. Missiles that employ active guidance carry a complete miniature radar system and fire control computer within the missile. As the missile nears the target, its internal radar system turns on and locks onto the target. The internal fire control computer directs control inputs to complete the intercept
Active-guided missiles have many advantages. First, active-guided missiles are very accurate at long ranges. This is because they do not rely on the target tracking radar once their internal radar takes over the intercept. Second, an active missile is extremely difficult to jam. It uses a narrow beam and its relative power is constantly increasing as it nears the target. Third, an active-guided missile is a fire-and-forget weapon. Command or semi-active missile guidance requires the target tracking radar to maintain lock-on until the intercept is completed. In an air-to-air engagement, this means the interceptor is predictable until the missile hits the target, and vulnerable to an enemy missile attack. An interceptor with an active missile, however, may launch the missile and, once it goes “active,” can then turn around or maneuver defensively.
Active-guided missiles have a few disadvantages as well. First, the active homing missile is a complex missile integrating both command and active guidance modes. Second, the missile may still be susceptible to electronic jamming during the mid-course phase of flight. Remember, during the mid-course phase, the missile relies on command or semi-active guidance. Jamming the target tracking radar may affect the missile's ability to “see” the target near the terminal phase.
SEEKER-AIDED GROUND GUIDANCE/TRACK-VIA-MISSILE GUIDANCE
In seeker-aided ground guidance (SAGG) and track-via-missile (TVM) guidance, the target is illuminated by the ground-based radar and the missile receives reflected energy from the target. Unlike conventional semi-active homing, the missile does not generate its own guidance commands. Instead, the missile transmits raw engagement data to the ground-based fire control system (FCS) in order to generate uplink guidance commands. TVM is similar to SAGG; however, additional processing is done on-board the missile prior to transmitting the engagement data to the ground-based FCS.
Track-via-missile and seeker-aided ground guidance are two relatively new missile guidance techniques with similar advantages. First, they are extremely accurate at long ranges where the inherent radar tracking errors may be large enough to cause a miss. Second, they can respond very quickly to any actions taken by the target since the missile seeker can track these changes and transmit the new position to the TTR fire control computer. Third, TVM and SAGG can be used with a large and capable fire control computer since most computations are accomplished by the TTR. Fourth, the integration of phased array radar and the powerful TTR fire control computer allows the missile system to engage multiple targets. The Patriot missile battery, for example, can track and engage at least four targets simultaneously.
The major disadvantage of track-via-missile and seeker-aided ground guidance is that they are the most complex forms of missile guidance. They require the use of sophisticated computers to combine radar tracking data and data received from the missile. This required hardware is expensive and demands greater maintenance and logistical support. In addition, the missile itself needs to be large enough to store the appropriate hardware for computations and data transfer.