MDTS 5705 : Guidance

MDTS 5705 : Guidance
Lecture 2 : Line-of-Sight Guidance
Gerard Leng, MDTS, NUS
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
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MILAN ATGM
Tracker
Missile Launcher
Gerard Leng, MDTS, NUS
2.1.2. Three point guidance
missile
target
LOS
tracker
There are three points of interests
1. the tracker,
2. the missile
3. the target.
Hence LOS guidance is also referred to as “three point guidance”.
Gerard Leng, MDTS, NUS
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)
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2.2 Beam Rider
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.
2.2.2. 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
Gerard Leng, MDTS, NUS
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.
Gerard Leng, MDTS, NUS
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
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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)
Gerard Leng, MDTS, NUS
Types of CLOS guidance
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
i) MCLOS or Manual CLOS
The operator tracks both target & missile, and generates guidance
commands for the missile.
Any problems ?
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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.
Gerard Leng, MDTS, NUS
AT-3 Sagger Guidance Platform
Carrying case doubles as launch
platform
Wire -guided via control stick &
periscope on the case
AT-3 Sagger, an example of a
MCLOS ATGM
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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
Gerard Leng, MDTS, NUS
TOW 2A ATGM
Computer-generated
(SACLOS) guidance
commands are relayed via
wire link.
Rear-mounted coded
IR beacons on the
missile
IR tracker on the launcher
detects deviation from the
LOS
TOW 2A, an example of a SACLOS AGTM
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iii) ACLOS or Automatic CLOS
No operator tracking of target and missile is required.
Sounds great, right ?
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Jernas/Rapier(FSC) SAM
Based on the Rapier Mk2 +
Blindfire tracking radar and
Dagger surveillance radar
Automatic command to
( radar) line of sight
guidance
Jernas/Rapier(FCS), an ACLOS SAM
Gerard Leng, MDTS, NUS
2.4 LOS guidance characteristics
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 ?
4. What are advantages of LOS guidance ?
5. What are the disadvantages of LOS guidance ?
Gerard Leng, MDTS, NUS
Moral
LOS guidance means that missile is “blind” to
the target.
No tracking beam  no guidance  no hit
Gerard Leng, MDTS, NUS
2.5 LOS Guidance Analysis
Most missiles have two planes of symmetry.
yaw plane
(azimuth)
This reduces the
guidance analysis from
3D to 2D.
pitch plane
(elevation)
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LOS Guidance Planar Geometry
at
Y
Vt
gt
T
target
Vm
LOS
Rt
qt
am
M
qm
Rm
O
tracker
gm
missile
X
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Kinematic equations
2.5.1. The dimensional kinematic equations are :
missile
dRm/dt
Rm dqm/dt
Vm dgm/dt
=
=
=
Vm cos(g m - qm )
Vm sin(g m - qm)
am
dRt/dt
Rt dqt/dt
Vt dg t/dt
=
=
=
Vt cos(g t - qt )
Vt sin(g t - qt)
at
target
Gerard Leng, MDTS, NUS
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
=
d( )/d Vm / Ro
Hence
d( )/dt
Substitute this in the kinematic equations and clean up the algebra ...
Gerard Leng, MDTS, NUS
2.5.3 The non dimensional kinematic equations are :
missile
drm/d
dqm/d
dgm/d
=
=
=
cos(g m - qm )
sin(g m - qm) / rm
m
drt /d
dqt/d
dg t/d
=
=
=
 cos(g t - qt )
 sin(g t - qt) / rt
t / 
=
Vt / Vm
target
where the speed ratio 
(0<  <1)
and the non dimensional accelerations are defined as
m
=
am / (Vm2 / Ro )
t
=
at / ( Vm2 / Ro )
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2.6 LOS guidance law design
Question: What should am be to intercept the target ?
Hint : What can be measured ?
LOS
am
D
qt
qm
Vm
Rm
M
O
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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
= KD
= K Rm sin(qt - qm )
Question : What happens if the angular error is small ?
Gerard Leng, MDTS, NUS
2.6.2 Trajectory characteristics
There are a few quirks in LOS guidance. Consider this engagement
scenario for a speed ratio of 0.8.
target
LOS
missile
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2.6.3 Appreciating the kinematics
1. Is the missile aligned with the LOS ?
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 ?
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Body to beam angle
Question
How significant
is a misalignment
of 18o ?
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Exercise : Estimating the operational constraints
ATGM intercepts target at a range of 3 km
tracker
d
LOS
18 o
18o body to beam angle at 3km means a misalignment d of
= 3000 x tan(18 o)
= 975 m at the tracker
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target
Lateral acceleration
Question :
What is the
dominant feature of
the latax behaviour?
Question :
What does a non
dim latatx of 0.4
mean ?
Gerard Leng, MDTS, NUS
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 ?
Gerard Leng, MDTS, NUS
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 ?
Gerard Leng, MDTS, NUS
Anticipating the target motion
LOS
Vm
am
M
Rm
qt
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
O
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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 ?
Gerard Leng, MDTS, NUS
Feedfoward LOS guidance - latax
Comments :
Look at the latax
Any differences
with the earlier LOS
guidance law ?
Gerard Leng, MDTS, NUS
Feedforward LOS guidance - body to beam angle
Question
What is the effect
of the feedforward
terms ?
Gerard Leng, MDTS, NUS