By:Prayag Nao
In this Post I’m
going to write about missile guidance systems . This will be a
high level look to give an appreciation of the concepts and techniques
used.
NOTE: If you work in the defence industry,
please don’t email to remind me it is a little bit more complicated
than I have explained it here!
Before we start, it's worth taking a step back to find out a little more about this subject.
What is a missile?
The first question to answer is what a missile is, and what distinguishes it from a rocket, bomb or other projectile weapon?
The simple answer is that a missile has a guidance system to allow it to steer and change course towards its intended target, and also a propulsion system that self drives it.
Non-Powered
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Powered
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Guided
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Smart-Bombs
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Missiles
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Non-Guided
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Bullets
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Rockets
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Munitions that are not self-powered and have no self-guidance are things
like bullets, arrows, darts, artillery shells and cannon shot. Old
school conventional bombs aka dumb-bombs also fall into this
category. Once fired they are at the mercy of gravity and other
external forces like drag and wind.
Some projectiles can self-propel, such as those pushed by rockets or
even small jet-engines. These might even have inertial or stability
platforms to allow them to fly more predefined paths, but they can’t
change course if their intended targets move.
An additional class of weapons can change direction, but are not
powered. These are typically released from planes, then drop down,
changing course as needed through either GPS or following laser
designated signals. In modern times, ‘smart bomb’ kits have been
retrofitted to conventional dumb ‘iron bombs’. These consist of
steerable fins, and an optional intelligent sensing nose cone. A smart
bomb becomes a (very inefficient) glider and steers itself to the
target.
Missiles
Missiles have both propulsion systems and guidance systems.
Most missiles also contain a payload (typically an explosive warhead),
and also a proximity fuze*. Contrary to what you might expect, a
missile does not have to hit its target to explode. Especially in the
province of an air target. Getting ‘close-enough’ is good enough to make
an effective hit. Once the proximity fuze has triggered, the warhead
explodes showering the target with thousands of pieces of very sharp,
supersonic chunks of shrapnel complete with a pressure wave and lots of
hot expanding gases. Aircraft are comprised of many delicate, balanced,
and critical pieces of highly precision components (and often a couple
of warm fleshy ones), typically protected by a very thin skin. Some of
these components are rotating at high speed, and others can be very
explosive. Some are needed to control and stabilize the aircraft.
Damaging to even a small number of these essential components can
critically harm the target and effectively destroy it.
Not all missiles have proximity fuzes, some are intended
to hit their target. These transfer their kinetic energy directly to
the target, acting more like large guided bullets. These are usually
smaller in size (not needing an explosive warhead), but need to be more
accurate. A very fast moving chunk of metal is going to do serious
damage as it passes through a target, even if it does not explode!
It’s possible to go the other way too and make a large
missile with a larger warhead that has an effective destructive radius
that is much greater. With a larger explosive payload the definition of
“close enough” gets less precise.
Missiles
We've now reached the point of understanding why
missiles are so important in modern warfare: Many targets are dynamic
and move. To take down a random moving target requires an ordnance that
is smart enough to manoeuvre, follow, chase and hopefully get close
enough to cause damage. However, the verbs "follow" and "chase" hide
quite a lot of subtly in implementation, as we will see …
Over the years, more and more sophisticated systems have
been developed to implement guidance control rules. In roughly
chronological order, and complexity are:
- Line of sight
- Pure Pursuit
- Proportional Navigation
Line of sight
This is a very simple control system and relies on the use of a base station that is constantly tracking the target.
In Line of sight guidance, there is a reference
point (in this case depicted by a ground radar station, but it could be a
moving platform). Radiating out from this reference point is a beam
pointing at the target. In LOS guidance, control signals are sent to the missile to keep it on this beam.
If the missile stays on the beam pointed at the target,
and has sufficient fuel and a fast enough relative speed, it will attain
the target.
Course deviations are given to the the missile in the
form of lateral acceleration requests (steering commands), to attempt to
keep the perpendicular distance from the beam to zero.
There are two sub-classes of LOS, and these are CLOS (Command Line Of Sight) and BR (Beam Rider). In CLOS,
the reference point is tracking both the target and the missile.
Command signals are sent directly to the missile through some
communications channel (either radio, or through a paid out thin
umbilical cable that streams out behind the launched missile). These
command signals give the course correction signals for the required
lateral manoeuvres. This was is the control method that the first
generation of guided missiles used. The disadvantages of this system are
obvious: Constant communication between the missile and the ground
station are required, and the reference station has to keep tracking
both the target and the missile. The brains of the missile reside in
the tracking station.
With BR guidance, the tracking station paints the
target with some signal (typically a laser) and the missile uses
onboard sensors (optical in the case of lasers), to ensure it stays
riding along the middle of the beam. A BR missile does not
require signals from the tracking station, and in this configuration,
the tracking station only needs to track the target.
It's not a requirement for the reference point and the
missile launch site to be in the same location. The launched missile
will follow its control law and quickly minimize the perpendicular
distance to the Line of Sight.
Limitations of LOS
As mentioned earlier LOS was the control law used
in the first generation of guided missiles, and it works reasonable
well on targets that are not using excessive evading manoeuvres. As we
will see later, however, as the missile gets closer, if the target is
manoeuvring aggressively, high lateral acceleration requests are needed
to keep it pointing at the target. It's possible for these requests to
exceed the limitations of the weapon, and if that happens the missile
will overshoot and miss.
This can also occur when the target is approaching the
reference point directly, as in the diagram below. The missile heads
towards the target, constantly turning and pointing at it. As the
target passes overhead, the missile needs to make tighter and tighter
angular accelerations to continue pointing at the target. Eventually it
may reach its turn limit and cannot turn any faster. If this happens,
because of its speed, it will overshoot the target.
Because of the constant tracking needed by a reference station, LOS
missiles can never reach the "Fire and Forget" sophistication level.
If the reference platform is also moving (for instance if the reference
platform is on another plane), then it will need to keep orientated
vaguely in the direction of the target (until it is destroyed) in order
to continue tracking the missile. This is not desirable in a combat
situation.
Pure Pursuit
Pure Pursuit reduces the number of involved
actors from three (reference point, missile, target) to just two
(missile and target). In Pure Pursuit, the missile autonomously tracks
the target and chases it directly, attempting to point at it the entire
time.
There are actually two variants of this control law, one is Attitude Pursuit, and the other is Velocity Pursuit. In AP, it is the axis of the missile that is kept pointing at the target. In VP,
the missile's velocity vector is the thing aimed at the target. These
two are typically different vectors because the angle of attack of the
missile is not aligned with the axis of the missile as it flies and
skids across the sky. (It is VP that appears to perform better in real life.)
In the head of the missile is some kind of sensor array
that is used to track the target (for instance an Infra Red optical
sensor for a heat seeking missile). This is mounted on a gimbal, and
moves to orient the array with the target. In simple AP law, the
angle the sensor points away from the missile axis is the angle used to
correct the course to point at the target. (In VP a vane on the side of the missile is also used to determine the direction the missile is traveling through the air).
Pure Pursuit guidance laws suffer the same late-stage extreme lateral acceleration request issues that LOS laws experience (especially when pointed at a target that is travelling towards the radar, rather than a tail-chase).
Gimbal limits
With guided missiles, it's not just the physical
limitations of the turn radius of the missile (which can be incredibly
high), it's the limitations of the sensor cradle too. The sensor gimbal
has a maximum arc of sweep over which it can move to track the target,
and also a maximum rate at which it is able swivel to keep the target in
view. If the target can manoeuvre outside the sensor cone, it can
'disappear' from view.
A pure pursuit control law is like a predator chasing a
prey in the animal world. For this reason, it is sometimes described as
a 'hound-hare pursuit' and also as a scopodrome [skopien = to observe, dromos = act of running]. It typically results in a tail chase.
There's a hole branch of mathematics dedicated to
pursuit curves and the loci of their paths. The curves have interesting
patterns depending on the ratio of their relative speeds, the distances
apart, the lateral acceleration limits of each entity, and the style of
the evasive manoeuvres taken by the target.
Proportional Navigation
Ships at sea are constantly worried about collisions
with other ships. It's good to know when there is a potential for this
so that evasive manoeuvres can be taken. Just because the paths of two
ships will cross, it does not mean that a collision will occur. For a
collision, both ships must be in the same location at the same time.
Paths have to cross for a collision to be possible, and the relative
speeds are important to determine if both entities will arrive at the
cross over point at the same time.
There are many ways to think about this. If you imagine
you are on one of the ships, set this as your reference platform, and
subtract your velocity from the other ship, you get the other ships
velocity relative to you. It's as is you are standing still, and the
other ship is moving. If the relative velocity of the other ship brings
it on a vector directly towards you, it means a collision will happen.
Another way to imagine this is two cars racing down
roads to a cross-road junction (or other intersection). Both could be
travelling at different speeds. Will they collide?
Imagine you are the passenger in one of the cars looking
out the window. As you look out the window and spot the other car you
might find that something interesting happens. If the other car keeps
the same position out of the window as you race along, a collision will
happen. If the angle of the other car to you gets tighter over time.
it's going to pass the intersection before you. If it gets wider,
you'll cross before it.
If the angle remains constant, there is trouble ahead!
Real Missiles
In the boost phase,
the missile is accelerating up to speed at which it can be controlled
(this may be performed by an expendable secondary stage). The launcher
might also have some input as to the azimuth and elevation from which
the missile is initially launched. For aircraft launched missiles, an
added complication is to make sure the missile engine, when ignited,
does not damage the launching aircraft!
In the midcourse phase the missile might still be guided by external control (or not in the case of an autonomous missile).
The terminal phase (homing phase) is where the missile tries to get as close as possible to the target.
It's more than just guidance
Finally, this article was just a basic introduction into
guidance systems of missiles. Missiles, however, are very complex
dynamic machines requiring skills in many disciplines.
They are precision devices that need to fly, be stable,
be controlled, be lightweight, have good speed, be extremely
manoeuvrable, not break apart with the stress of flight and launch, be
reliable, have long shelf life, effective at destroying a target, cope
with the stress of hanging off a pylon of a manoeuvring plane, be
function at a wide range of temperature conditions, detect and track
targets, not be spoofed by decoys, chaff and noise …
They also need cool names.
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