The primary function of a multi-dimensional radar is to compute accurate ranges, bearings, and altitudes of targets detected by an air-search radar. This information is used to direct fighter aircraft during interception of air targets.

The multi-dimensional radar is different from the air-search radar in that it has a higher transmitting frequency, higher output power, and a much narrower vertical beamwidth. In addition, it requires a stabilized antenna for altitude accuracy.

The following are some applications of a multi-dimensional radar:

Obtain range, bearing, and altitude data on enemy aircraft and missiles to assist in the guidance of CAP aircraft.

Provide precise range, bearing, and height information for fast and accurate initial positioning of fire-control tracking radars.

Detect low-flying aircraft.

Determine the range to distant landmasses.

Track aircraft over land.

Detect certain weather phenomena.

Track weather balloons.

The modern warship has several radars. Each radar is designed to fulfill a particular need, but it may also be capable of performing other functions. For example, most multi-dimensional radars can be used as secondary air-search radars; in emergencies, fire-control radars have served as surface-search radars. A multi-dimensional air-search radar is shown in figure 1-15.

MISSILE GUIDANCE RADAR

The purpose of a guidance subsystem is to direct the missile to target intercept regardless of whether or not the target takes deliberate evasive action. The guidance function may be based on information provided by a signal from the target, information sent from the launching ship, or both. Every missile guidance system consists of two separate systems-an attitude control system and a flight path control system. The attitude control system maintains the missile in the desired attitude on the ordered flight path by controlling it in pitch, roll, and yaw (fig. 1- 16). This action, along with the thrust of the rocket motor, keeps the missile in stabilized flight. The flight path control system guides the missile to its designated target. This is done by determining the flight path errors, generating the necessary orders needed to correct these errors, and sending these orders to the missile's control subsystem. The control subsystem exercises control in such a way that a suitable flight path is achieved and maintained. The operation of the guidance and control subsystems is based on the closed-loop or servo principle (fig. 1-17). The control units make corrective adjustments to the missile control surfaces when a guidance error is present. The control units also adjust the wings or fins to stabilize the missile in roll, pitch, and yaw. Guidance and stabilization are two separate processes, although they occur simultaneously.

Phases of Guidance

Missile guidance is generally divided into three phases (fig. 1-18). As indicated in the figure, the three

phases are boost, midcourse, and terminal. STANDARD SM-2 missiles (MR & ER) use all three of these phases. Not all missiles, however, go through the three phases. As shown in figure 1-18, some missiles (STANDARD SM-1, SEASPARROW) do not use midcourse guidance. With that thought in mind, let's examine each phase, beginning with boost.

INITIAL (BOOST) PHASE.-Navy surface-launched missiles are boosted to flight speed by the booster component (which is not always a separate component) of the propulsion system. The boost period lasts from the time the missile leaves the launcher until the booster burns up its fuel. In missiles with separate boosters, the booster drops away from the missile at burnout (fig. 1-18, view A). Discarding the burnt-out booster shell reduces the drag on the missile and enables the missile to travel farther. SMS missiles with separate boosters are the STANDARD (ER) and HARPOON.

The problems of the initial (boost) phase and the methods of solving them vary for different missiles. The method of launch is also a factor. The basic purposes, however, are the same. The missile can be either pre-programmed or physically aimed in a specific direction on orders from the fire control

Figure 1-18.-Guidance phases of missile flight.

computer. This establishes the line of fire (trajectory or flight path) along which the missile must fly during the boosted portion of its flight. At the end of the boost period, the missile must be at a precalculated point.

There are several reasons why the boost phase is important. If the missile is a homing missile, it must "look" in a predetermined direction toward the target. The fire control computer (on the ship) calculates this predicted target position on the basis of where the missile should be at the end of the boost period. Before launch, this information is fed into the missile.

When a beam-riding missile reaches the end of its boosted period, it must be in a position where it can be captured by a radar guidance beam. If the missile does not fly along the prescribed launching trajectory as accurately as possible, it will not be in position to acquire the radar guidance beam and continue its flight to the target. The boost phase guidance system keeps the missile heading exactly as it was at launch. This is primarily a stabilizing function.

During the boost phase of some missiles, the missile's guidance system and the control surfaces are locked in position. The locked control surfaces function in much the same manner as do the tail feathers of a dart or arrow. They provide stability and cause the missile to fly in a straight line.

MIDCOURSE PHASE.-Not all guided missiles have a midcourse phase; but when present, it is often the longest in both time and distance. During this part of flight, changes may be needed to bring the missile onto the desired course and to make certain that it stays on that course. In most cases, midcourse guidance is used to put the missile near the target, where the final phase of guidance can take control. The HARPOON and STANDARD SM-2 missiles use a midcourse phase of guidance.

TERMINAL PHASE.-The terminal or final phase is of great importance. The last phase of missile guidance must have a high degree of accuracy, as well as fast response to guidance signals to ensure an intercept. Near the end of the flight, the missile may be required to maneuver to its maximum capability in order to make the sharp turns needed to overtake and hit a fast-moving, evasive target. In some missiles, maneuvers are limited during the early part of the terminal phase. As the missile gets closer to the target, it becomes more responsive to the detected error signals. In this way, it avoids excessive maneuvers during the first part of terminal phase.