Technology Explained – Man Portable SAMS

Beam Riders – Bofors RBS-70

Beam riding guidance is one of the most conceptually simple techniques available. A beam riding missile will typically be equipped with a set of aft facing antennas (or optical detectors) usually mounted on the tips of its cruciform wings. These antennas {detectors) will sense the missile’s orientation within a microwave {laser) beam which will track the target. The missile will continuously adjust its flightpath to maintain its position within the beam {ie ‘ride’ the beam) until it collides with the tracked target. So much for the basic idea. In practice beam riding is a somewhat more complex affair. The flight of a beam rider will be divided into two phases, gathering and guided flight. The gathering phase takes place immediately after launch when the missile’s position relative to the beam centreline is uncertain, this results from various tolerances in the hardware, variations in propellant performance, wind velocity and jitter/p ointing errors in the launcher.

Gathering involves the use of a radio or optical command link to steer the missile into the centre of the tracking beam, the position of the missile is usually sensed by an infrared device which tracks the missile exhaust plume. Once the missile has been gathered into the beam the beam riding guidance may be engaged and the missile can then corkscrew its way up the beam until collision with the target.

The strength of the beam rider lies in simplicity, complete or nearly so immunity to jamming and seduction and the ability to kill

targets from all aspects, particularly head-on. A generic weakness of beam riders is a poor kill rate against crossing targets {ie

beam aspect) which results from the very high sustained turn rates required of the missile in order to stay within the rapidly slewing beam. Under such circumstances, the missile’s control surfaces may stall resulting in loss of control and destruction of the weapon. Operator skill level is very important (this requirement itself may be considered a weakness), a clever operator can apply some lead bias in tracking the target during the missile’s flight thus pre-empting the above. The most widely deployed beam rider today is the Swedish laser beam riding AB Bofors RBS-70. This weapon first flew in 1971 entering production by the mid seventies. The weapon fire unit is comprised of an integral missile container/launch tube, a sighting/guidance unit, both attached to a tripod stand/operator seat. The sighting/guidance unit provides a gyro stabilised optical sight with target fine tracking by thumb lever, the optics are bore sighted with the laser. Target tracking is achieved by keeping crosshairs on target thus directing the laser beam at it. The missiles are fielded in the sealed container/launch tubes which are discarded after use. The high explosive warhead is proximity and impact fused.

Command to Line Of Sight Guidance – Shorts Blowpipe and Javelin

An alternative form of guidance with many similar characteristics is command link guidance, a specific type of which is Command to Line Of Sight {CLOS) guidance. Commonly used in land based and naval point defence SAMs, command link guidance involves fitting the missile with radio {usually VHF to microwave) band receivers via which it receives steering commands from its launcher/operator. The Blowpipe and Javelin are both optically tracked CLOS (. . . to target) weapons, where the operator directly {or indirectly) steers the missile into the line- of-sight (LOS) to the target and eventually collision. Like beam riders, CLOS missiles must first be gathered into the field of view of the operator and then steered to impact. As with beam riders the strength of CLOS guided weapons lies in implicit immunity t o seduction, high resistance to jamming and an all aspect capability. Their weaknesses are also alike in that operator skill is a prerequisite and performance against crossing targets can be poor, although in this respect a CLOS guided weap on is unlikely to fall out of control but rather fail to sustain the required turn rate and consequently miss the target. The most comm only used CLOS man portable SAM is the Shorts

Blowpipe. This weapon was introduced over a decade ago and now equips twelve users, with a number of weapons used by the Mujahedeen in Afghanistan. The Blowpipe missile is fielded as two assemblies, a reusable Aiming Unit and a factory sealed expendable launching canister. The missile itself is a metal tube containing a two-stage solid propellant rocket, a warhead and at its tapered nose, a nose cone fitted with cruciform delta control surfaces. The nose cone is attached via a low friction bearing, the missile is steered by moving the whole nose. The missile has tail mounted cruciform wings which are attached to a sliding collar. Stowed in the launcher the collar and folded wings occupy together with the nose cone/controls the large forward part of the launcher.

At launch a thermal battery is fired up, the launcher cap blown off by gas pressure, the missile gyro fired and the first stage engine ejects the missile from the canister, extracting and unfolding the wings during exit. The pistol grip aiming unit contains the command link radio transmitter, an optical sight, an autogather- ing device and optionally an IFF interrogator. Controls comprise a trigger, thumb control joystick, fuse, autogather and command frequency selection switches.

An engagement involves clipping the Aiming Unit to the launcher, acquiring the target in the graticuled optical sight and squeezing the trigger. After the first stage burns out at a safe distance the second stage brings the missile up to supersonic speed. Autogathering steers the missile into the centre of the optics field of view (FOV) after which the operator steers the weapon with the joystick to impact judging missile flightpath by the exhaust flare.

The Blowpipe has not been as successful as the heat-seeking Stinger in Afghanistan, primarily due to the need for a skilled operator who can judge the missile and target trajectories and apply appropriate lead. As the operator must account for gravity drop and crosswind drift, accuracy can suffer. This can be difficult and the newer Javelin, evolved from Blowpipe, solves much of this with the use of Semi-Automatic CLOS (SACLOS) guidance. This technique involves the tracking of the missile’s flightpath and the automatic transmission of steering commands which keep the missile on the LOS between the aiming reticle and the target.

The Javelin Aiming Unit is more complex with additional electronics and optics. An engagement will proceed much like with Blowpipe but with the Aiming Unit projecting a stabilised illuminated aiming mark into the operator’s field of view. The target is initially tracked to gain lead, the missile launched and steered to target by the operator who keeps the aiming mark on The Javelin is a SACLOS derivative of the established Blowpipe. This missile is virtually immune to jamming and highly effective against clos- ing threats. Note the fins unfolding at launch.

target with the joystick. In addition to Blowpipe switches and controls the Javelin is fitted with an automatic crosswind cancellation switch. Both Blowpipe and Javelin are proximity and impact fused.

Infra-Red Homing FIM-92A/B/C Stinger Guidance – General Dynamics The Stinger

family of missiles evolved from the FIM-43A Redeye, itself conceptualised by General Dynamics and US Army MICOM in the 1950s, developed in the early sixties and deployed in 1966. The Redeye was designed to shoot down hostile Close Air Support (CAS) aircraft operating against US Army land forces and was the first such weapon ever fielded. The design of such a missile was no mean feat as the state-of- the-art in heat seeking missiles, the AIM-9 Bravo Sidewinder was a cumbersome 70kg/2. 8m weapon with an uncooled lead sulphide (PbS) detector and two channel rotating reticle seeker (refer TE March 1982, Heat Seeking Missile Guidance) capable only of tail chase engagements. A new approach was required and GD pioneered several new design features to create the Redeye.

Redeye was the first Rolling Airframe Missile (RAM). Unlike conventional roll stabilised missiles which are steered in two axes, pitch and yaw, by two (pitch, yaw) control channels a RAM uses a single control channel which is ‘phased to introduce pitch and yaw commands subject to the missile’s instantaneous orientation (roll angle) in roll. In this fashion a single pair of control surfaces can do the work of two pairs saving weight and volume with some penalty in manoeuvre performance. GD applied further new technology to Redeye designing all of the guidance and control electronics with solid-state transistor and integrated circuit technology, a first in tactical missiles.

Another major weight saving measure was the use of electrical control actuators displacing bulkier conventional hydraulics. Internal wiring harnesses in the missile were replaced with lighter flexible flat printed wiring harnesses. Finally, the seeker itself employed conical scanning never previously used in a heat seek- ing missile.

The Redeye warhead was also an unconventional titanium design, built to actually burn through the skin of the target. The Redeye’s short wavelength seeker however limited it to tail aspect shots and it was found to be susceptible to flares, which seduce a heat seeker by presenting a greater infra-red signature than a real target. This was recognised by the US Army who, together with the US Marine Corps, sought an all aspect Redeye II, subsequently redesignated Stinger.

Development of the Stinger proceeded from 1972 to 1977 concurrently with a post 1974 Aeronutronic Ford Alternate Stin-ger semiactive laser homing weapon which was later abandoned. The Stinger design was a much improved Redeye, 1. 52m rather than 1. 2m long, weighing 15kg, a 16% increase in weight. The missile fuselage is divided into functional blocks. The tail of the Propulsion Section mounts a launch rocket engine with canted nozzles to impart spin (roll) during launch, it burns out and separates within the disposable wound Kevlar launch tube. The missile fuselage boat tail mounts the tail assembly with its folding canted cruciform tail surfaces, these lock after launch and sustain the fuselage roll during flight. The dual burn Atlantic Research engine high energy propellant is claimed by GD to be the state-of-the-art in production propellants, it will accelerate the missile to cca twice the supersonic speed of Redeye. Forward of the Propulsion Section is the Warhead Section also containing the Motorola proximity fuse. The missile is designed to hit the target and inflict as much damage as possible kinetically, the 3kg Picatinny Arsenal fragmentation warhead will enhance this damage.

The nose of the missile contains the Guidance Section, the aft part of which contains the missile battery, controls and umbilical interfaces. One pair of unfolding cruciform canard surfaces is fixed and the other controlled by the seeker. The battery powers both electronics and controls.

The Alpha model seeker uses gyro stabilised optics to focus infrared energy received through the nose window onto a gas cooled detector. The missile will fly a proportional navigation trajectory homing in on the target’s exhaust plume until its terminal phase, where a Target Adaptive Guidance (TAG) algorithm steers the missile into the target’s aft fuselage to damage structure and powerplant hot end. GD claim it has the ability to acquire, track and hit targets from all aspects.

The cooled single colour seeker was a vast improvement over Redeye but didn’t provide the flare rejection sought by users. In 1986 it was supplanted by the FIM-92 Bravo Stinger-POST (Passive Optical Seeker Technique) seeker which introduced a new rosette scanning dual band infra-red (IR) and ultraviolet (UV) detector/optics assembly. The POST seeker exploits the low UV reflectance of aircraft compared to a sky background and initially acquires and guides the missile onto the UV ‘hole’ in the sky represented by the target. The concurrent use of UV and IR allows unambiguous rejection of flares which are bright in both the UV and IR bands.

The expectation that the threat will use IR jammers led to the 1989 phase in of the FIM-92 Charlie Stinger-AMP (Reprogrammable MicroProcessor) version which is field reprogrammable with new guidance software if required. A memory module in the gripstock can be swapped, it would contain executable software which is downloaded to the memory of the microprocessor chip in the missile via the umbilical interface.

The missile is supplied as a complete round with the launch tube sealed and pressurised with nitrogen to keep out moisture, the seeker sees out through a fragile IR and UV transparent membrane. The gripstock contains the launcher control electronics and mounts a pistol grip with trigger and a Battery/ Coolant Unit (BCU). The BCU supplies electrical power to the gripstock electronics and missile prior to launch and Argon gas coolant to cool down the detector. If a launch doesn’t occur a fresh BCU is fitted. A lightweight IFF interrogator is also fitted with electronics and a battery in a belt pack.

A typical engagement will involve the visual acquisition of a target by the observer in the two man fire team, using field glasses. The gunner will then clip a fresh missile to the gripstock while the observer tracks the target. A safety switch on the gripstock is then used to apply coolant and power to the missile spinning up the seeker gyro and cooling down the detector. The seeker is initially caged to the missile centreline, the gunner must track the target in his graticuled optical sight for seeker acquisition. Once the seeker has acquired the target, an acquisition tone is produced and the seeker may be uncaged to track the target, this is done with a gripstock switch. To provide proper lead against a crossing target and elevation to compensate gravity drop during missile launch, the gunner must track the target with one of three markers in the optical graticule, one for each aspect. Depressing the trigger then fires the missile battery which retracts the umbilical connector, this in turn fires the launch engine after which the missile exits the tube.

The use of optical homing with proportional nav means that the missile will collide with an approaching target or pursue a crossing or receding target. As Stinger is a true fire and forget missile the fire team may quickly run for cover since the missile exhaust plume has betrayed their location.

Next issue: Technology Explained examines the Soviet range of man portable SAMs and also looks at the various defensive options available to defeating this significant threat.