As we mentioned earlier, missiles have a path control system and an attitude control system. Guidance systems are usually classified according to their path control system, since many missiles use the same type of attitude control. The type of attitude control used in the fleet is inertial. The following is a discussion of the types of path control (guidance) in use in SMS missiles.

INERTIAL GUIDANCE.-An inertial guidance system is one that is designed to fly a predetermined path. The missile is controlled by self-contained automatic devices called accelerometers.

Accelerometers are inertial devices that measure accelerations. In missile control, they measure the vertical, lateral, and longitudinal accelerations of the controlled missile (fig. 1-19). Although there may not be contact between the launching site and the missile after launch, the missile is able to make corrections to its flight path with amazing precision.

During flight, unpredictable outside forces, such as wind, work on the missile, causing changes in speed commands. These commands are transmitted to the missile by varying the characteristics of the missile tracking or guidance beam, or by the use of a separate radio uplink transmitter.

BEAM-RIDER GUIDANCE.-A beam-rider guidance system is a type of command guidance in which the missile seeks out the center of a controlled directional energy beam. Normally, this is a narrow radar beam. The missile's guidance system receives information concerning the position of the missile within the beam. It interprets the information and generates its own correction signals, which keep the missile in the center of the beam. The fire control radar keeps the beam pointed at the target and the missile "rides" the beam to the target.

Figure 1-20 illustrates a simple beam rider guidance system. As the beam spreads out, it is more difficult for the missile to sense and remain in the center of the beam. For this reason, the accuracy of the beam-rider decreases as the range between the missile and the ship increases. If the target is crossing (not heading directly at the firing ship), the missile must follow a continually changing path. This may cause excessive maneuvering, which reduces the missile's speed and range. Beam-riders, therefore, are effective against only short- and medium-range incoming targets.

HOMING GUIDANCE.-Homing guidance systems control the path of the missile by means of a device in the missile that detects and reacts to some distinguishing feature of (or signal from) the target. This may be in the form of light, radio, heat, sound waves, or even a magnetic field. The homing missiles use radar or RF waves to locate the target while air-to-air missiles sometimes use infrared (heat) waves.

Since the system tracks a characteristic of the target or energy reflecting off the target, contact between the missile and target is established and maintained. The missile derives guidance error signals based on its position relative to the target. This makes homing the most accurate type of guidance system, which is of great importance against moving air targets. Homing guidance methods are normally divided into three types:, active homing, semi-active homing, and passive homing (fig. 1-21).

Active Homing.-With active homing, the missile contains both a radar transmitter and a receiver. The transmitter radiates RF energy in the direction of the

Figure 1-19.-Accelerometers in a guided missile.

Figure 1-20.-Simplified command guidance systems: A. Radar/radio command; B. Beam rider.

target (fig. 1-21, view A). The RF energy strikes the target and is reflected back to the missile. (This process is referred to as "illuminating the target.") The missile seeker (receiving) antenna detects the reflected energy and provides it as an input to the missile guidance system. The guidance system processes the input, usually called the homing error signal, and develops target tracking and missile control information. Missile control causes the missile to fly a desired flight path.

The effective range of the missile transmitter is somewhat limited because of its size (power output). For this reason, relatively long-range missiles, such as HARPOON, do not switch to active guidance until after midcourse guidance has positioned the missile so that the transmitter is within its effective range.

Semiactive Homing.-In a semiactive homing system, the target is illuminated by a transmitter (an illuminator) on the launching site (fig. 1-21, view B). As with active homing, the transmitted RF is reflected by the target and picked up by the missile's receiver. The fact that the transmitter's size is not limited, as with active homing, allows a much greater range.

The missile, throughout its flight, is between the target and the radar that illuminates the target. It will receive radiation from the launching ship, as well as reflections from the target. The missile must therefore have some means of distinguishing between the two signals, so that it can home on the target rather than on the launching ship. This can be done in several ways. For example, a highly directional antenna may be mounted in the nose of the missile; or the Doppler principle may be used to distinguish between the transmitter signal and the target echoes. Since the missile is receding from the transmitter and approaching the target, the echo signals will be of a higher frequency. Most SMS missiles use both of these methods.

A drawback of this system is that the shipboard illumination is not free to engage another target while the missile is in flight. STANDARD SM-1 and SEASPARROW all use semi-active homing as their primary guidance; they do not use midcourse guidance. The STANDARD SM-2 uses midcourse guidance, and then semi-active homing only for terminal guidance. As a result, the SM-2 needs illumination from the ship only for the last few seconds of flight.

Figure 1-21.-Homing guidance: A. Active homing; B. Semi-active homing; C. Passive homing.

Passive Homing.-Passive homing requires that the target be a source of radiated energy (fig. 1-21, view C). Typical forms of energy used in passive homing are heat, light, and RF energy. One of the most common uses of passive homing is with air-to-air missiles that use heat-sensing devices. It is also used with missiles that home on RF energy that originates at the target (ships, aircraft, shore-based radar, and so forth). An example of this is the STANDARD ARM (anti-radiation missile) used for both air-to-surface and surface-to-surface engagements. An advantage of this type of homing is that the target cannot detect an attack because the target is not illuminated.

Several missiles that normally use other homing methods (active or semi-active) are capable of switching to the passive home-on-jamming (HOJ) mode in a countermeasure environment. That is, if the target detects that it is being illuminated by an active or semiactive guidance radar and initiates jamming (RF interference), the missile will home on the jamming signal if it is unable to maintain track on the reflected illumination signal.

Tracking Radar/Fire-Control Radar

Radar that provides continuous positional data is called tracking radar. Most tracking radar systems used by the military are also called fire-control radars, the two names being interchangeable. A fire-control tracking radar system produces a very narrow, circular beam.