Technology Explained – Cruise Missiles

But technology advanced and the Soviets did eventually acquire look-down/shoot-down capable fighters and this together with advances in radar capability meant that the geriatric B-52, SAC’s primary weapon, had little chance of successfully penetrating to heavily defended targets deep in the USSR, to launch its SAAM short-range nuclear missiles. The new B-1A bomber was to have assumed the B-52’s role, with a quoted 1/30 of the B-52’s radar cross-section, terrain following capability and very high dash speed. But the mid-seventies saw the rebirth of the cruise missile, as inertial guidance, computer and propulsion technologies reached a level of development which allowed a new generation of these weapons. The Carter administration, eager to gain political mileage, dumped the manned B-1A bomber in favour of rearming the B-52s with the new cruise missile.

Conceptually elegant, use an established platform to deliver a large load of missiles to the perimeter of the Soviet air defence system, launch these in numbers sufficient to saturate the defences, and fly home. The missiles would enter Soviet territory, hugging terrain to further reduce their very low detectability, eventually hitting their preprogrammed targets with 200 kT nuclear warheads. The 1,500nm range cruise missile would do away with the need to do the deep penetration missions and thus allow retention of the existing B-52, saving on a new fleet of expensive bombers. In retrospect, it is apparent that the Carter administration overestimated the true operational effectiveness of the weapon which though useful could not, just as any single weapon cannot, provide all the answers, so today we can see a revised B-1B in production to be armed with a mix of the traditional SAAM missile and the new cruise missile, flying a mission profile involving penetration and standoff missile launches. The cruise missile itself has entered its next generation in development, while still experiencing numerous teething troubles, the Soviets responded in a two-pronged exercise of upgrading their air defences with AEW and shoot-down capable air defence fighters and deploying their bomber-launched equivalent of the US’ cruise missile.

The Boeing AGM-86B Air Launched Cruise Missile (ALCM)

First of the cruise missile family to enter service and essentially the only dedicated strategic weapon in the family, the AGM-86B is a generic development of the early seventies Boeing SCAD program. That vehicle subsequently developed into the AGM-86A, which was supplanted by the longer-ranged AGM-86B which became the operational version of the missile. The Current AGM-86B is equipped with the basic Tercom (Terrain Contour Matching) inertial guidance system and this also reflects in the vehicle’s mission profile. A Tercom-equipped vehicle will employ an inertial navigation system (INS) to find its way to the target, however, the INS will accumulate a position error with time and this error must be eliminated or reduced if the weapon is to have any sort of useful accuracy. Tercom does exactly that.

The Tercom system uses a radar altimeter and barometric altimeter to measure the profile of the terrain beneath the aircraft. This information is then compared with stored terrain profile information within the memory of the Tercom computer to yield a position update. The update is then fed into the INS. The exercise of measuring and matching the terrain profile is carried out only several times during the weapon’s flight, this provides sufficient accuracy while limiting the size of the computer memory required to store the terrain maps. The missile is loaded with its position and a file of terrain maps for its flight prior to launch, it is then fully autonomous. It will hug terrain at several hundred to a thousand feet, following a very indirect flight path through hostile territory, this serves to confuse the defenders as to the exact nature of the target, alternately the flight planner may route the flightpath to avoid known early warning and air defence facilities.

The AGM-868 is a 20ft 10. 5 in (6. 3m} vehicle weighing in at 3,150lb (1,420kg) at launch. Most of the airframe is occupied by the four fuel tanks that feed the powerplant. That is a Williams International F-107 6001b thrust class low bypass ratio turbofan, weighing about 1501b (68kg) and occupying the last three feet of the airframe. The engine mounts above the set of actuators for the vehicle’s folding elevons, the tailpipe is shielded by a tail cone which conceivably blocks infrared emissions from the engine hot end. The use of a turbofan is advantageous in that it does provide better fuel consumption than a turbojet, with a reduced infrared signature. The penalty to be paid is in the thrust limitation, which will force the vehicle to follow terrain with larger clearance (see June 1984 issue, Terrain Following). This tradeoff of signature vs thrust was presumably made in the earlier phases of the weapon’s development when Soviet lookdown radar capability was far lesser. It has since been identified as a major limitation in this weapon’s penetration ability and is to be rectified in a $300 million engine upgrade program which will provide a new compressor, combustion system and ignition system. The result should be a 40% thrust increase (SL) to improve terrain following performance in the 100ft altitude region, a speed increase to 0. 65 Mach and several hundred miles of extra range.

Other than the powerplant, the tail region of the vehicle also houses a thermal battery and the small #4 fuel tank. The engine inlet is on the top of the fuselage, usefully shielded from GCI radars by the airframe. The fuselage of the vehicle, from the rear propulsion section to the forward payload/control section, is roughly equally split into three fuel tanks, #3, #2, #1 from tail to nose. Prior to launch the vehicle’s wings, elevons and vertical stabiliser are stowed, they deploy upon release from the launch aircraft. The spine of the fuselage carries the control instrumentation cables from the nose to the propulsion section. The instrumentation bay houses then the INE (Inertial Navigation Element), Flight Controller and Radar Altimeter. Aft and below the instrumentation bay is the payload bay, housing the W-80 200 KT nuclear device and its arming mechanism. The vehicle’s nosecone underwent modification in the early production phase, transitioning from the flattish shark nose to the rounded “beluga” nose, apparently to reduce radar cross-section, In spite of the fact that the ALCM airframe was not optimised for minimum radar signature, either in shape or materials, it has a small cross section which will apparently be reduced by the use of stealth technologies in future upgrades. The USAF is also considering the use of chaff and flare dispensers to further improve Survivability.

The ALCM Guidance System

The core of the ALCM’s guidance system is the Litton LN-35 inertial navigation unit, built around a P-1000 inertial platform, The P-1000 uses two dry-tuned rotor gyros and three accelerometers (see June 1984, AJQ-20 BNS, F-111C description for INS operation), the set has a combined drift error rate of 0. 6mm/hr (the drift error develops through the combined inaccuracies in the gyro and accelerometer pickoffs, mechanical tolerances friction and presumably also A/D quantisation errors, and as such is intrinsic to all conventional inertial platforms). The inertial set is complemented by a Tercom computer and Honeywell radar altimeter. The radar altimeter, designed with power management, minimised sidelobes and variable PRF (pulse rate) for low detectability, is used to generate a continuous stream of altitude readings. These are used throughout the flight for terrain following. furthermore, while the missile is over a known’ mapped area, they are subtracted from the barometric altimeter reading to generate terrain elevation readings. The mapped areas are stored as MxN matrices of point elevation readings (further matrices) in the computer memory. The terrain contour matching is then carried out by the computer, which takes the series of height readings and uses a mathematical algorithm to find the best ‘fit’ of the flightpath/position in the matrix (map) and thus determines the position of the vehicle with some known (subject to the quality of the altimeters and maps) error. This position information is then used to reduce the inertial set’s error.

To get the best possible update, the computer uses a technique termed Kalman filtering. The Kalman filter is a mathematical algorithm that, accounting for the INS errors and Tercom matching errors with a mathematical model, allows optimum mixing of the inertial set’s reading and the Tercom 70 system’s reading. Thus the Kalman filter will take readings from both systems to enable the calculation of a ‘better’ position estimate, which is then loaded into the inertial set. One of the nicer aspects of this technique is the fact that the algorithm apparently calibrates the gyros, as with each update it gets a better and better idea of what the particular gyros drift rates are. Furthermore, the matrices become finer and finer in resolution as the missile nears its target. Thus the Tercom guided missile becomes more accurate the closer it gets to the target (see Gelb A. , Sutherland A. A. ; ‘Software Advances in Aided INS, Navigation, Vol. 17, 70/71). As is intuitively obvious, the Tercom system performs best over rugged terrain and is useless over ocean. In operation the ALCM would fly on inertial guidance between individual matrices, where it updates its position and changes heading if required, therefore it is only really necessary to have a series of well-known rugged/unique areas that can be used as matrices. Critics of the cruise missile have focussed strongly on the Tercom system. The first target was the radar altimeter, allegedly easy to detect and jam. The second target was the software/system error. Northern Siberia is very flat and covered with annually variable ice therefore critics believe that uniqueness cannot be guaranteed.

The Tercom system does have built in measures to deal with fix problems, specifically, it will take three Tercom fixes and check if at least two of the three agree in position before it commits itself to an update of the inertial set. Only if no agreement can be found will the missile degrade down to pure inertial guidance, disarm itself and function as a decoy, effectively.

One option under review as an alternative to Tercom is the use of the Navstar Global Positioning System (GPS) L-band satellite navigation system. GPS offers 15m positioning accuracy virtually worldwide and would eliminate the need for a costly global mapping exercise. On the other hand it can be jammed and many SAC commanders are apprehensive about the ability of the 18 satellite constellation to survive the initial phases of a nuclear exchange, not to mention the GPS receivers coping with atmospheric phenomena and EMP effects. Two Tomahawk cruise missiles have been fitted as testbeds for GPS, though little has been published on test results to date. Another option being, developed at this stage is a Laser Terrain Following Radar. Laser-based radars have been tested in the past, using eg CO 2 lasers and offer both precise narrow beam pointing and precise range gating. The USAF-funded development program envisages the use of a high-range resolution laser radar for cruise missile terrain following, navigation and terminal targeting. Hughes Aircraft are currently involved in the 42-month advanced development phase for the system, General Dynamics will be providing a Tomahawk flight test vehicle.

Earlier demonstrations of laser radar have shown the ability to easily resolve objects such as wires, power lines, telephone poles and trees. With sufficiently powerful signal processing this information can be exploited both for very low-level terrain follow- ing. Tercom and target recognition from its geometry. Limitations may however exist due to atmospheric absorption under some (eg water vapour-laden) conditions.

ALMC/Launch Vehicle Interface

The AGM-868 is currently carried by the Boeing B-52G bomber and will also equip the Rockwell B-1B bomber. The SAC B-52s carry twelve ALMCs, six per underwing pylon and will eventually carry a bomb bay-mounted internal rotary launcher with another eight rounds. This launcher termed the CSRL (Common Strategic Rotary Launcher), is compatible with ALCM, the current SRAM defence suppression missile and the future ACM (stealth cruise missile) and AASM (Advanced Air-Surface Missile, SRAM replacement). The B-1B will carry ALCM internally on its CSRL and also externally, twelve rounds carried tangentially attached beneath the fuselage, flanking the aft bomb bay. forward bomb bay and flight deck access well. The Northrop ATB “Strealth Bomber’ will also be fitted with the CSRL.

The ALCM/Launch vehicle interface is typified by the installation being developed for the B-1B. The Boeing-designed OAS (Offensive Avionics System) is built around an IBM AP-101F 16- bit computer, which ties into two Sundstrand data transfer units and the ALCMs. The OAS will load the terrain matrices (up to twenty) into the ALCM prior to launch, the IBM computer having the ability to communicate with each missiles LN-35 INS and computer to provide the OAS operator with exact status information on each round. Though current limitations in the number of available matrices apparently restrict targeting to a set of prebriefed targets, SAC’s eventual objective seems to be the ability to select targets at will if necessary at the discretion of the launch aircraft crew. Once the matrices and flightpath/target data have been loaded into the missile, it is ready for launch. Once the launch aircraft reaches its release point, the LN-35 is updated with the final position supplied by the bomber’s nav/attack and the missile can be launched.

Mission Profile

In the event of a SAC strike against the USSR, the ALCM armed B-52Gs (later also B-52Hs) would scramble from continental US airbases and head for the Russian coastline. Typical approach paths would be over the North Pole as the far North of Russia is sparsely populated and thus difficult to effectively monitor. The aircraft would cruise at high altitude until several hundred miles from the Russian coastline, where the large GCI radars come into play. To elude these, the B-52s would descend to a low altitude and activate their ESM/ECM systems. In theory, this would allow them to approach to within 200nm of the coastline, where they would launch the ALCMs.

This tactic would certainly apply to attacks on targets within the Leningrad-Moscow-Kuybyshev-Sverdlovsk arc, with weapon release over the Barents sea. Aircraft targeted for the Sverdlovsk-Novoskibirsk-Irkutsk zones, including the Abalokovo battle management radar and ICBM fields, would presumably cross the Russian coastline between the Taimyr Peninsula and Bering Straits, hitting PVO and Ciristallations on the way in, using SRAM missiles (200kT). They would release their ALCMS over the Siberian landmass. One would presume this attack profile would also apply to strikes on Far Eastern targets, with a convenient approach over land mass rather than ocean.

The ALCMs would after launch, be it over ocean or land, head for their initial matrix and update. The first matrix is very coarse and is quoted at 26nm, successive matrices are smaller and smaller, the terminal matrices being described by some sources as a mere 1nm wide. The weapon would then zig-zag along from matrix to matrix, penetrating deeper and deeper. Presumably, it would avoid PVO defences and populated areas, approaching its target from a bearing least convenient to the defender and probably with little or no warning. Only 85% of prospective targets can be hit from offshore launch zones and it is certain that many aircraft would be expected to penetrate. It was that limita- tion in the weapon’s range that prompted SAC to reinitiate the B-1B, which would upon introduction take over the penetration role, leaving the B-52 with only the standoff launch role.

SAC intends to arm 168 B-52Gs with the ALCM and all B-1Bs, though it is not clear at this stage how many of the B-52Hs may be armed. As one can see, a full-scale strike would present the Russians with a sizeable headache, to the tune of several thousand terrain-following missiles.

The Russians have responded strongly, they have deployed the new SA-10 SAM, phased array air defence radars, the 11-76 SUAWACS airborne early warning platform, the new MIG-31 Foxhound and Su-27 Flanker fighters, both with lookdown radar and shootdown missiles. The vigour with which they have pursued the development of their AWACS illustrates the importance they place upon the cruise missile. One could presume the ultimate goal would involve setting up an AEW barrier along the Northern coast of European Russia, to hit launch platforms and incoming missiles over the ocean, together with AEW cover for key Siberian and European targets.

Another move involves the deployment of Russian ALCMs, carried by the massive Blackjack bomber and the trusty Tu-20 Bear, this is currently being countered by increased US deploy. ment of AWACS and F-15 aircraft from continental US bases. Over-the-horizon (OTH) radar and orbital infrared surveillance platforms are both currently under review as options for early detection of incoming cruise missile carriers, prior to handoff to the AWACS/interceptors. Though the ALCM has had numerous teething troubles, both in software (resulting in the recent flight tests over Northern Canada serving to acquire calibration data on flat polar terrain performance and Coriolis effect compensation) and in powerplant performance, it is an effective system. Not every target will have the optimum set of defences or the appropriately non-unique terrain profiles along its approach paths, The current development of the General Dynamics ACM (Advanced Cruise Missile) is bound to create further problems for the Russians. This composite/polymer skinned/structured missile, equipped with a low heat signature Williams International F-112 engine will offer a 40% range increase over the ALCM, at a fraction of the ALCM’s radar and IR signature. Whether it will use Tercom or GPS is not clear at this stage, it is though expected to carry extensive ECM and ECCM systems. The ALCM achieved IOC in December 1982, production was cut in April 1983, bringing the total to 1,700 rounds at $4. 3 billion. SAC plans to further acquire 1,300 ACMs, with a projected IOC of 1989.

The deployment of the ALCM has had a considerable effect in Increasing the US’ ability to hit the Soviet hinterland-deployment of even partial defensive measures has cost the USSR considerably and if continued is certain to further divert resources from offensive weapons programs.

The overall significance of the ALCM is however far lessened when one considers also the effects of submarine and ground-launched theatre nuclear cruise missiles, weapons of comparable capabilities with very flexible modes of deployment. These together with conventional derivatives of the cruise missile family will be the subject of Part II.