mdts 5705 : guidance

Gerard Leng, MDTS, NUS

MDTS 5705 : GuidanceLecture 2 : Line-of-Sight Guidance


2. Line of Sight (LOS) Guidance

2.1.1. Definition - In LOS guidance the missile follows the line of

sight (LOS) from an external tracker to the target

View through the

MILAN tracker

Target


MILAN ATGM

Tracker

Missile Launcher


2.1.2. Three point guidance

There are three points of interests

tracker

missiletarget

1. the tracker,

2. the missile

3. the target.

Hence LOS guidance is also referred to as “three point guidance”.

LOS


2.1.3. Type of LOS Guidance

Depending on where the guidance commands are generated,

LOS guidance can be divided into 2 important variants

1. Beam Rider

2. Command to Line Of Sight (CLOS)


2.2.1. Definition - If the missile generates its own guidance

commands to follow the LOS then the LOS guidance is referred

to as beam rider guidance.

2.2 Beam Rider

2.2.2. Deployment

The typical set up for

a beam rider SAM is

as shown

Note that two radar

beams are used to

track the target.

One for each plane

of symmetry in the

missile


A typical set up for an ATGM using a laser beam as the LOS is shown

The laser beam has to be “coded” to enable the missile to sense

its position relative to the LOS.

Note that in LOS guidance the missile is “blind ”. It does not

“see” the target.


2.3 Command to Line of Sight (CLOS)

2.3.1. Definition - If the guidance commands to follow the LOS are

generated externally and transmitted to the missile, then the LOS

guidance is called Command to Line Of Sight (CLOS) guidance.

2.3.2. Deployment

The deployment of a

CLOS ATGM is

shown.

Note the wire

command link

../../movies/ATGM/ATGM%20wirelink.mpg


Features of CLOS systems

Question : What is a typical command link ?

Note

The presence of a command link. means that there is no “intelligence “

on-board the missile. (think R/C models)


i) MCLOS or Manual CLOS

2.3.3 Classification of CLOS

There are 3 types of CLOS guidance depending on whether the

tracking of the target and the missile are automated

Types of CLOS guidance

The operator tracks both target & missile, and generates guidance

commands for the missile.

Any problems ?


AT-3 Sagger (9K11 Malyutka) ATGM

HEAT warhead

can penetrate

over 400 mm of

armour at 500m to

3000 m

AT-3 used by the NVA.


AT-3 Sagger Guidance Platform

Carrying case doubles as launch

platform

AT-3 Sagger, an example of a

MCLOS ATGM

Wire -guided via control stick &

periscope on the case


ii) SACLOS or Semi Automatic CLOS

The operator tracks the target only. Guidance commands are

generated automatically

The guidance kinematics are similar to a beam rider except that

commands are up linked to the missile.

Used in 2nd generation ATGM’s and SAM’s


TOW 2A ATGM

Rear-mounted coded

IR beacons on the

missile

TOW 2A, an example of a SACLOS AGTM

IR tracker on the launcher

detects deviation from the

LOS

Computer-generated

(SACLOS) guidance

commands are relayed via

wire link.

../../movies/ATGM/ATGM%20TOW%20launch.mpg


iii) ACLOS or Automatic CLOS

No operator tracking of target and missile is required.

Sounds great, right ?


Jernas/Rapier(FSC) SAM

Jernas/Rapier(FCS), an ACLOS SAM

Based on the Rapier Mk2 +

Blindfire tracking radar and

Dagger surveillance radar

Automatic command to

( radar) line of sight

guidance


1. How does the missile keeps itself on track ?

2. Should we use a wide or narrow tracking beam ?

3. Does a narrow tracking beam cause any complications ?

2.4 LOS guidance characteristics

4. What are advantages of LOS guidance ?

5. What are the disadvantages of LOS guidance ?


Moral

LOS guidance means that missile is “blind” to

the target.

No tracking beam no guidance no hit


2.5 LOS Guidance Analysis

yaw plane

(azimuth)

pitch plane

(elevation)

Most missiles have two planes of symmetry.

This reduces the

guidance analysis from

3D to 2D.


LOS Guidance Planar Geometry

q

g

Rm

Vm

am

X

Y

M

O

m

m

q

g

Rt

Vt

at

T

t

t

tracker

target

missile

LOS


missile

dRm/dt = Vm cos(g m - qm )

Rm dqm/dt = Vm sin(g m - qm)

Vm dgm/dt = am

2.5.1. The dimensional kinematic equations are :

target

dRt/dt = Vt cos(g t - qt )

Rt dqt/dt = Vt sin(g t - qt)

Vt dg t/dt = at

Kinematic equations


2.5.2. We non dimensionalise the equations to reduce the

number of parameters.

Using the initial range Ro and the missile speed Vm, we define

non dimensional time = t Vm / Ro

non dimensional distance r = R/ Ro

Hence d( )/dt = d( )/d Vm / Ro

Substitute this in the kinematic equations and clean up the algebra ...


missile

drm/d = cos(g m - qm )

dqm/d = sin(g m - qm) / rm

dgm/d = m

2.5.3 The non dimensional kinematic equations are :

target

drt /d = cos(g t - qt )

dqt/d = sin(g t - qt) / rt

dg t/d = t /

where the speed ratio = Vt / Vm ( 0 < < 1 )

and the non dimensional accelerations are defined as

m = am / (Vm2 / Ro )

t = at / ( Vm2 / Ro )


2.6 LOS guidance law design

q

M

O

m

qt

LOS

Rm

DVm

am

Question: What should am be to intercept the target ?

Hint : What can be measured ?


LOS Guidance Law

Idea ! The acceleration to keep the missile on the LOS is

proportional to the measured deviation of the missile from

the LOS.

am = K D

= K Rm sin(qt - qm )

Question : What happens if the angular error is small ?


There are a few quirks in LOS guidance. Consider this engagement

scenario for a speed ratio of 0.8.

2.6.2 Trajectory characteristics

target

missile

LOS


2.6.3 Appreciating the kinematics

2. Is this increasing body to beam angle desirable ?

3. Are there any effects on warhead effectiveness ?

4. Just how large can this misalignment get ?

1. Is the missile aligned with the LOS ?


Body to beam angle

Question

How significant

is a misalignment

of 18o ?


Exercise : Estimating the operational constraints

tracker targetLOS

18 od

ATGM intercepts target at a range of 3 km

18o body to beam angle at 3km means a misalignment d of

= 3000 x tan(18 o)

= 975 m at the tracker


Lateral acceleration

Question :

What is the

dominant feature of

the latax behaviour?

Question :

What does a non

dim latatx of 0.4

mean ?


Exercise : Estimating the latax requirements

Consider an ATGM, speed 250 m/s engaging a target at an initial

separation of 2 km

A non-dim latax of 0.4 means that the actual acceleration required is

= 0.4 x 2502/2000

= 12.5 m/s2

Question : Is this large ?

How does this compare with the pickup of a car ?


Question : Can we improve on the performance ?

Answer :

Yes, if we account for the beam motion as well.

Idea

We have used beam angles, what about using beam angle rate and

beam angle acceleration ?


Anticipating the target motion

M

O

qt

LOS

Rm

Vm

am

Suppose the missile is on the

LOS and aligned with it.

As the LOS rotates with the

target, the missile must generate

latax to rotate with the LOS


LOS guidance with feedforward latax terms

The required latax can be derived as :

am = 2 Vm dqt/dt + Rm d2qt /dt2

and this serves as an additional command to the guidance system.

Note

1. It is a “feed forward” command because qm is not involved

in the generation of dqt /dt and d2qt /dt2

2. There’s a price to pay. Can you spot the catch ?


Feedfoward LOS guidance - latax

Comments :

Look at the latax

Any differences

with the earlier LOS

guidance law ?


Feedforward LOS guidance - body to beam angle

Question

What is the effect

of the feedforward

terms ?

mdts 5705 : guidance

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