Introduction to Naval Weapons Engineering

Title: Guidance and Control
Reading: pp 428 ­ 454

I. Objectives
A. Given a description of the guidance system be able to categorize it in table 15-3
(p 454)
B. Given a the type of guidance law, be able to list the advantages and disadvantages of it (table 15-1).

II. Introduction

A. GUIDED WEAPONS (missiles or projectiles) provide a significant increase in accuracy. This increase was necessary due to the parallel increase in target evasiveness. We'll be discussing some of the principles of guidance and control in light of what we've learned to this point in the course.

B. The keys to guidance control are:
1. knowing what the target is doing
2. knowing what the missile is doing
3. control the missile (internally/externally) into a collision course with the target. We'll cover these methods later on in the hour.

C. Detect/Track the Target: RADAR, ACOUSTICS, OR E­O

D. Track the Weapon: Same means, but more detailed info is available because the weapon doesn't mind sending us information about its status from sensors like accelerometers and internal gyros.

III. GUIDANCE PHASES

e.g. Sub-surface Harpoon

A. Boost
Purpose: Get the missile away from the launch platform and in a position where it can be guided (either in the general direction of the target, or into a capture beam). Usually unguided at this point.

B. Midcourse ­ Longest phase
Purpose: Put the missile in the vicinity of the target.midcourse guidance system/autopilot takes the weapon near the target prior to terminaling homing.

C. Terminal ­ High accuracy, high response rate
Purpose: Successfulyl engage the target.
IV. TYPES OF GUIDANCE SYSTEMS

A. EXTERNAL:
1. Control guidance systems (flight commands originate external to the weapon)
a. Command guidance ­ commands are transmitted to the missile based on missile and target tracks. (Fig 15­6, p. 436)

b. Beam rider ­ missile launched into a capture beam which gets it to the guidance beam. Using conical scan techniques, missile keeps itself inside the beam and "rides" it to the target. Requires only one radar & can control many missiles at once, but accuracy degrades with range. No longer used.

c. Modified beam rider ­ same as BR, but 2 beams are used (one for missile, one for target) and a collision course is computed from that data.

d. Wire guided ­ direction given to weapon through a wire connected to the fire control console. Ex: MK48 torpedo, tow missile

2. Homing guidance systems (weapon reacts to some distinguishing characteristic of the target): MOST ACCURATE
a. Active
b. Semi­active
c. Passive

3. Composite systems ­ improves performance due to redundancy and better EP features.

B. INTERNAL
1. Preset ­ Weapon launches with predetermined path ==> no flexibility after launch (harpoon before seeker comes on)

2. Navigational controls
a. Inertial ­ ASROC, depth charges
b. Celestial ­ ICBM's take star fixes
c. Terrestrial ­ a digital map is stored in the guidance section of the weapon so that the weapon can "take a fix", determine its position, and correct its flight path if necessary. (Tomahawk: TERCOM and DSMAC - explain)
V. FLIGHT PATHS

A. Preset ­ weapon path can not be varied after launch
1. Constant ­ straight line torpedo path (no variance; normally one path)
2. Programmed ­ set in various paths as in mk46 torpedo (snake or circle search, then straight run to target, if lost contact, circle search to reacquire).

B. Variable ­ heading of weapon changes with changes in target course and speed.
1. Pursuit ­ (fig 15­16, p. 449) ­ weapon points at target along the weapon's los at all times: a tail chase.
2. Line of sight ­ (fig 15­19, p. 451) ­ missile guided along the launch platform's LOS to target (beam rider)
3. Constant bearing ­ (fig 15­17, p. 449) ­ a CBDR path is computed.
4. Proportional navigation ­ (fig 15­18, p. 450) ­ A more efficient flight path since it uses the rate at which the bearing is changing (bearing drift). This rate is applied to a fixed or variable navigation ratio to determine steering commands to an intercept point.
5. Constant lead angle path ­ modified beam rider

VI. Questions?

A. For a good review ­ see tables 15­1, 15-2, & 15­3

VII. Homework
Rec: WB 15.2, 3, 6
Req: None