The Key to ’Smart’ Weapons
These photodetectors are ubiquitous components in today’s arsenal,
guiding bombs and missiles with pinpoint accuracy.
A decade after the Gulf War, the
US is again involved in a military campaign, with “smart” weapons playing
a key role. In fact, the political success of this effort has depended in part on
the ability to keep collateral damage to an absolute minimum while effectively targeting
specific groups and installations. Thus, it is easy to argue that smart weapons
have never been more important.
Most smart weapons rely on some type of photonics
technology to provide target guidance or acquisition. Photodiodes are the photodetectors
of choice in many of these weapons because they offer numerous important advantages,
such as compact, lightweight packaging, custom configurations, rugged reliability
and solid-state efficiency. There are several diverse ways in which photodiodes
are used to enable smart weapons.
The goal of an offensive smart weapon is to actively
guide a bomb or a missile to its designated target. Because all onboard systems
are destroyed on impact, it is not economical to incorporate complex, expensive
components and systems in general-purpose bombs and smaller missiles. A popular
approach for these types of ordnance is laser guidance. In this technique, a laser
beam marks the surface of the target with an intense spot of radiation. An Nd:YAG
at 1.06 μm typically does the marking. This wavelength offers the advantage
of stealth (it is invisible) while being easily detectable with silicon PIN photodiodes.
In operation, a laser beam is directed
at the target either from the launch platform or by a third party, including covert
ground operations personnel. Light reflected from the target enters the front of
the missile through the so-called dome. In most cases, the dome optics are molded
polycarbonate plastic, to minimize overall cost and weight. These optics also include
a bandpass filter to block light outside the laser wavelength. In this way, a high-contrast
image of the laser spot is focused onto a photodetector assembly, mounted immediately
behind the dome.
Typically, this photodetector is a
quad cell — a photodiode whose active area is partitioned into four discrete
quadrants. The outputs from these four quadrants are independently amplified by
four high-gain transimpedance amplifiers. These are often hybridized (integrated
into the photodiode assembly) to maximize signal-to-noise and, hence, range. The
ratio of the signals from the four amplifiers enables easy determination of the
position of the image spot on the quad cell. Of course, the molded optics may introduce
aberrations and may produce a distorted spot on the photodiode array, but these
effects are easily calibrated out during missile assembly and test.
The launch platform is equipped with
a similar system to allow it to track and observe the target before and after destruction.
Here, the photodiode quadrant is usually mounted on a pair of orthogonal gimbal
mounts, which provide ΘX and ΘY adjustment/motion, allowing the platform
to “look” at targets at wider angles. Information from this detector
allows the bomb or missile to be automatically launched in the right direction.
This type of system is referred to
as “fire and forget”; after launch, signals from the onboard quad detector
in the missile are used to adjust directional vanes on the projectile during its
flight. No further direct communication occurs between the launch platform and the
missile. Today the military uses these systems to guide both passive projectiles
and actively propelled missiles.
In a typical battlefield situation,
several targets may be simultaneously designated and marked with separate lasers.
To avoid confusion and crosstalk, each laser beam emits a coded sequence of pulses.
Each missile is programmed to respond to only one of the code sequences.
Another type of ordnance that relies on photodiodes
is the wire-guided missile — e.g., the tube-launched, optically tracked weapon
(Figure 1). The missile remains in direct communication with the launch platform
via a long wire that uncoils during flight. This is not a “fire and forget”
system; the launch platform — which may be a Bradley vehicle, a helicopter,
a Hummer or even a simple portable tripod — tracks the projectile and supplies
corrective trajectory information. The idea is to incorporate only the less expensive
guidance components in the missile itself, with most of the “smarts”
in the launch platform, thus allowing repeated use and lower event cost. Unlike
laser guidance, there is no third-party target designator.
Figure 1. In wire-guided missiles, such as the tube-launched optically
tracked weapon, most of the “smarts” are incorporated in the launch
platform to minimize cost and weight. Courtesy of US Department of Defense.
A bomb-site/telescope arrangement
acquires it; the operator sights the target (usually a tank) in the crosshairs of
a telescope. The system computer registers the direction of the target from angular
encoders on the telescope gimbals. The missile is launched through a tube, which
can be at a large (45°) angle from the telescope, depending on the target’s
location. The computer immediately corrects the trajectory to direct the missile
toward the target.
The system tracks the entire flight
by following a xenon lamp in the tail of the missile. The tracking system contains
two subsystems to provide wide-field (lower resolution) and narrow-field (high resolution)
tracking and correction. These subsystems are similar except for f number
and detector size. In each subsystem, the xenon lamp is imaged through a rotating
prism onto a two-element photodiode assembly with a characteristic chevron shape
(Figure 2). The optics form a separate image on each of the photodiode “legs.”
As the prism rotates, this image moves across the detector, producing a modulated
signal. This arrangement is used instead of a quad cell because the optics are designed
so that the two legs of the photodetector produce orthogonal information on the
trajectory angle, which can be directly converted into pitch and yaw corrective
commands for the missile.
Figure 2. In the tube-launched, optically
wire-guided missile, a chevron-shaped photodiode assembly provides corrective pitch
and yaw data.
The missile is also directed to fly
a smart trajectory. A standard, “time of flight” pulsed laser rangefinder
is mounted inside the launch platform to automatically detect the range. (This also
uses a photodiode — in this case, a high-gain avalanche photodiode.) As the
missile nears the target (usually a tank), it makes an “up and over”
detour so that it can strike from above, effecting maximum damage.
Smart weapons also protect the launch platform
by countering incoming missiles. A large, slow-moving ship presents a particularly
inviting target. Until recently, missiles such as the Exocet could strike a ship
with devastating effect, as evidenced in the Falklands/Malvinas war between the
UK and Argentina in 1982. These missiles initially proved hard to combat because
of their low trajectory and high speed (up to Mach 2). The task is to accurately
direct a defensive missile at the incoming missile and to detonate it in proximity.
A successful solution has proved to
be the rolling airframe missile, which, for security reasons, cannot be discussed
in anything but general terms here (Figure 3). In this system, photodiodes are used
to sense the proximity of the incoming missile, rather than for guidance.
Figure 3. The new ship-based rolling airframe
missile system utilizes photodiodes to determine proximity to incoming enemy missiles.
Courtesy of Raytheon Co.
This missile is a “fire and forget”
system; once launched, it relies on internal tracking systems. Initially, it tracks
the incoming missile via the radio frequency signal it uses to lock onto the ship.
At closer range, it tracks the infrared from the plume of the incoming missile.
As its name suggests, this missile
rolls (spins) during flight at a high revolution rate. Laser diodes are arranged
to emit from its side. When the rolling airframe missile is passing alongside the
incoming missile, the laser radiation is reflected and collected through optics
and a bandpass filter mounted in its side, and then directed onto elaborate arrays
of photodiodes. The reflection signal is used as a proximity fuse to detonate the
warhead. The success of this missile is due to its ability to discriminate among
reflections from the target missile, sunlight, starlight or the ocean surface.
Figure 4. In the latest arc fault detector for the US Navy, a photodiode detects high-voltage
arcs by their UV emission. Integrated LEDs allow complete system test.
Laser diode sources and monochromatic
(bandpass filter) detection filter out ambient light. The shape of the photodiode
array and the characteristic profile of the pulsed signal that results from the
high-speed rotation of the missile protect against ocean reflections.
Because most of the smarts are incorporated
within this sophisticated weapon, the rolling airframe missile represents a higher
cost per launch. But in this case, cost is really not an issue, given the weapon’s
importance and the fact that the manufacturer, Raytheon in Tucson, Ariz., reports
a kill rate higher than 99.99 percent.
Photonics technology, and the photodiode
in particular, has been key in enabling smart weapons that can attack selected targets
with great accuracy, avoiding blanket destruction. Now, more than ever, this type
of weaponry is critical to the military and political success of warfare.
Meet the author
Brock Koren is president and CEO of Advanced Photonix
Inc. in Camarillo, Calif.
Arc Fault Detection Systems
Photodiodes play a role
in protecting the launch platform from internal failure problems. For example, ships
use high-voltage systems that can arc and create fires. If a fire is not promptly
detected and extinguished, it represents a serious risk to personnel — either
directly or by detonating stored ordnance. Because of this risk, the US Navy recently
developed an automated arc fault detection system that relies on silicon photodiodes
from Advanced Photonix Inc.
An electrical arc breakdown generates highly ionized
air molecules that emit UV and visible radiation. The arc fault detector gathers
this light through an input lens and passes it through a UV bandpass filter. A single-element
photodiode that is hybridized with a transimpedance amplifier detects the UV. As
soon as an electrical arc occurs, it triggers this device, and the signal is used
to automatically shut down the local electrical system before a fire can start.
The UV filter eliminates false alarms caused by changes in ambient light.
Because of its importance to the ship’s
safety, the system is designed to self-test. Several LEDs are incorporated within
the photodiode hybrid assembly and are directed at the front window of the detector
(Figure 4). During test, light from the LEDs bounces off the inner surface of this
window onto the photodiode, which allows a full system test, including automatic
electrical shutdown. Visible LEDs can be used because they are inside the photodiode
assembly; their light never passes through the UV bandpass filter.
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