MISSILE GUIDANCE, NAVIGATION, AND CONTROL SUBSYSTEMS

We begin our technical discussion by describing the sub­systems that make up a missile system. In addition to a warhead, a missile contains several key supporting sub­systems. These subsystems may include 1) a target-sensing system, 2) a missile-navigation system, 3) a guidance sys­tem, 4) an autopilot or control system, and 5) the physical missile (including airframe and actuation subsystem); see Fig. 1.

Target-Sensing System

The target-sensing system provides target “information” to the missile guidance system, e. g. relative position, velocity, line-of-sight angle, and rate. Target-sensing systems may be based on several sensors, e. g., radar, laser, heat, acoustic, or optical sensors. Optical sensors, for example, may be as simple as a camera for a weapon systems officer (WSO) to visualize the target from a remote location. They may be a sophisticated imaging system (see below). For some applications, target coordinates are known a priori (e. g., via satellite or other intelligence) and a target sensor becomes irrelevant.

Navigation System

A navigation system provides information to the mis­sile guidance system about the missile position in space relative to some inertial frame of reference, e. g., flat — Earth constant-gravity model for short-range flights and rotating-Earth variable-gravity model for long-range flights. To do so, it may use information obtained from a variety of sensors, which may include simple sensors such as accelerometers or a radar altimeter. It may include more sophisticated sensors such as a global positioning system (GPS) receiver or an optical terrain sensor that relies on comparisons between an image of the terrain below with a stored image and a stored desired trajectory. Optical stellar sensors rely on comparisons between an image of the stars above with a stored image and a stored desired trajectory.

Guidance System

Target and missile information are used by the guidance system to compute updated guidance commands, which when issued to the missile autopilot should ideally guide (or steer) the missile toward the target (4, 5). When target coordinates are known a priori, missile coordinates pro­vided by the navigation system (e. g., GPS-based) are pe­riodically compared with the (pre-programmed) target co­ordinates to compute appropriate guidance corrections. In general, the quality of the computed guidance commands depends on the quality of the gathered sensor data and the fidelity of the mathematical models used for the missile and target. Targets may be stationary, mobile, or highly maneu­verable (e. g., silo, ship, fighter aircraft). Physically, guid­ance commands may represent quantities such as desired thrust, desired (pitch/yaw) acceleration, desired speed, de­sired flight path or roll angle, and desired altitude. Guid­ance commands issued by the guidance system to the mis­sile autopilot are analogous to the speed commands is­sued by automobile drivers to the cruise control systems in their cars. In this sense, the missile guidance system is like the automobile driver and the missile autopilot is like the automobile cruise control system. Missile guidance commands are computed in accordance with a guidance al­gorithm. Guidance algorithms and navigational aids will be discussed below.

Autopilot

The primary function of the autopilot—sometimes referred to as the flight control system (FCS) or attitude control sys­tem (ACS)—is to ensure 1) missile attitude stability and 2) that commands issued by the guidance system are fol­lowed as closely as possible (4). The autopilot accomplishes this command-following objective by computing and issu­ing appropriate control commands to the missile’s actu­ators. These actuators may include, for example, rocket thrusters, ramjets, scramjets (for hypersonic missiles), or servomotors that move aerodynamic control surfaces. More specifically, the autopilot compares commands issued by the guidance system with real-time measurements (e. g., acceleration, attitude and attitude rate, and altitude) ob­tained from onboard sensors (e. g., accelerometers, gyro­scopes, and radar altimeters) and/or external tracking sys­tems. This comparison, essentially a subtraction of signals, produces a feedback error signal, which is then used to compute control commands for the missile actuators. This computation, the purpose of the autopilot, maybe based on a and is based on the autopilot design and hence its com­plexity. Autopilot design, however, is based on a very com­plex mathematical model that captures the following dy­namical features: missile airframe, aerodynamics (depend­ing on speed, dynamic pressure, angle-of-attack, slide-slip angle, etc.), actuators, sensors, flexible modes, and uncer­tainty descriptions, e. g., dynamic uncertainty, parametric uncertainty (6, 7), and disturbance/noise bounds. It should be noted that commands that are issued by the guidance system to the autopilot cannot always be followed exactly because of the presence of multiple sources of uncertainty. Sources of uncertainty may include disturbances acting on the missile, sensor noise, unmodeled or uncertain missile airframe, actuator, and sensor dynamics.

Flight Phases

The flight of a missile can be broken into three phases: 1) a launch, separation, or boost phase; 2) a mid-course or cruise phase; and 3) an endgame or terminal phase. During each phase, a missile may use distinct guidance, navigation, and control systems, specifically designed to accommodate the requirements during that phase of the flight. During each phase, the missile may very well use different sets of sen­sors, actuators, and power sources.

Guidance System Performance Terminology

To describe the function and performance ofa guidance sys­tem, some terminology is essential. The imaginary line that connects a missile center-of-gravity (cg) to the target cg is referred to as the line-of-sight (8). The length of this line is called the range. The associated vector from missile to tar­get is referred to as the range vector. The time derivative of the range vector is called the closing velocity. The most im­portant measure of performance for any missile guidance system is the so-called miss distance. Miss distance is de­fined to be the missile-target range at that instant when the two are closest to one another (8). The objective ofmost guidance systems is to minimize the miss distance within an allotted time period. For some applications (hit-to-kill), zero miss distance is essential. For some applications (e. g., to minimize collateral damage), it is essential to impact the target at a specific angle. Because miss distance is sensi­tive to many variables and small variations from missile to missile, other quantities are used to measure performance. One of the most common measures used is circular error probability (cep). The cep for a missile attempts to provide an average miss distance for a class of missile-target en­gagements (i. e., Monte Carlo runs). If a missile has a cep of 10 m, then most of the time, say, 68% of the time, it will detonate within 10 m of the target.

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