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All-Optical Communications Command Missile Flight

May 2001
By Christian B. Sheehy

Surface-to-air projectile attains high accuracy through course adjustment in final meters to target.

Germany, France and Italy are experimenting with a new fiber optic guided missile system that will enable surface ships more precisely to track and destroy air and surface targets by using remote imaging sensor technology. With an onboard infrared camera and fiber communications system, the weapon can conduct long-range autonomous strikes, then relay critical information to the launch operator for the rapid processing of point of impact and kill assessment data.

A product of the Euromissile consortium comprising European Aerospace Defence and Space Company/ Lenflugkörper, Munich, Germany, and Consorzio Italmissile, Rome, the Polyphem missile system is being developed for surface- and submarine-mounted deployment as well as land-based coastal defense applications. Using a jam-secure, low-radar-detectable fiber optic control system, the weapon can eliminate a variety of line-of-sight and non-line-of-sight, mobile and immobile targets from air, surface and submerged platforms.

According to Polyphem project officials, fiber optic technology allows the weapon to transmit the target image it “sees” in flight to remote firing-station operators. Range is increased by fiber optic communications because the hardware is lighter than commonly used transmitting materials such as copper wire. Consequently, optical fibers carried by the missile can support strong signal transmissions over increased ranges. Because of its optical, rather than electrical nature, the fiber optic signaling architecture prevents interference from external jamming while the weapon is en route to a target.

The Polyphem firing station employs a global positioning system (GPS) Navstar navigation reference unit coupled with a command, control, communications and intelligence (C3I) interface for maintaining a precise track on a target’s location. Consisting of a missile guidance computer, operator control and tactical display framework, the station stores a digital map of the attack area, sending a streaming data signal on factors influencing target position back to the operator.

Prior to the engagement sequence, potential flight paths are displayed to the operator using image-processing algorithms that calculate the resulting directional vectors each path will produce. Factors such as wind velocity and altitude are considered in determining the optimal course to bring the missile autonomously into the designated target area. Once in the target area, the operator can adjust the trajectory using data relayed by an infrared imaging camera in the nose of the missile.

As the infrared imager scans the target area, the focal plane array (FPA) picks up images that enable the operator to discriminate the target from surrounding objects, consortium officials explain. In reconnaissance and surveillance activities, this information may be useful in determining weapons locations or performing damage assessments. Positioned on a gyro-stabilized, dual-axis platform, the imager moves and focuses automatically in response to changing environmental conditions and target distance. The operator can adjust the weapon’s final impact position to within 10 centimeters of the intended strike zone.

In the past, the problem with missile target location was the inability to get conclusive, timely data from the weapon system to the operator so that critical course changes could be made, officials remark. With advances in optical data transmission, the speed and clarity of the communications link between a launch command and weapon in flight has improved dramatically, they add. Data received by the operator gets increasingly accurate as the missile approaches its target, decreasing the target’s probability of evasion. In addition to gathering critical designation and location information, Polyphem’s imaging camera can collect surveillance data on other potential targets so that a larger picture of the attack zone can be developed.

Guided missile technology relies on clear data reception by launch command based on a weapon’s in-flight field of vision. As a missile approaches a target, the chance that a minor variation in a target dynamic will cause a major disturbance in missile precision increases proportionately to the weapon’s speed and target proximity. Once in the kill-imminent zone—usually the last 10 meters—the possibility that the missile will fail to hit the target peaks because even very fine movements have great significance at this point. It is at this critical juncture that sensors aboard the weapon need to collect data with the highest degree of precision to account for any unforeseen variables. The C3I capability enables greater accuracy through each phase of a missile’s flight, consortium officials explain. “With increased data reliability en route, fewer adjustments are necessary in the final meters to a target.”

Built around an all-optical communication system, Polyphem simultaneously transmits video data from missile to ground and command information from ground to missile at data rates greater than 200 megabytes per second. Digitally controlled spools, or bobbins, of fiber in the rear of the missile collect incoming signals that are transmitted to the navigation system. A GPS receiver and a laser inertial measurement unit process the information, redirecting the missile flight apparatus in accordance with data instructions.

Although the system is fully automated, operators can intervene in a mission scenario. Data received from the weapon can be used to aid in decreasing the time interval required to process final approach target data. Maintaining a man-in-the-loop, open architecture facilitates the movement of information through the system so that more data can enhance overall performance. At an average speed of 160 meters per second, controlling information the instant it becomes available requires processing at both the launch and missile ends of a strike.

Polyphem image resolution uses an FPA composed of platinum silicide. The FPA enables operators to positively identify targets at distances up to 8 kilometers. Rotating on a dual-axis base, the imager lens opens to accommodate a 90-degree peripheral field of view in the search and detection phase of a strike. Once the target destruction phase has begun, the FPA aperture angle decreases, maintaining an exclusive focus on the target. Surveillance data collected in the search and detection phase can be used to track other targets for simultaneous engagement or identification.

The period from launch until the weapon attains normal cruising altitude is controlled by a predetermined set of coordinates, consortium officials state. Directional changes can be made during this period to accommodate target movements that are inconsistent with the missile’s trajectory. This, however, aborts the original mission, requiring increased operator intervention to guide the weapon to the target. The search phase commences once the weapon has reached a steady cruising speed and altitude. As the missile comes to within 8 kilometers of the target, it adjusts its course to attain the optimal inbound position angle. An adjustable depression angle and variable attack altitude give the operator a range of target approach possibilities. “This flexibility in range is particularly useful for non-line-of-sight operations,” consortium officials explain. “Because the best route around an island is not always over it, Polyphem seeks the route of least resistance.” Variables such as target speed and direction are considered in determining the optimal flight path. Environmental conditions such as wind and weather are computed continually throughout the mission and coupled with the target information as it is collected.

Tracker lock-on occurs after the missile has detected the target and the operator has confirmed positive recognition and identification. At this point, the imager focuses completely on the target area, collecting visual data that is used to adjust the missile’s trajectory. Once locked on, the weapon is committed to the target with about 30 degrees of error in which to readjust course if needed. Inside the last 50 meters or so to the target, the missile will begin to weave back and forth as it positions itself for final impact. In non-line-of-sight situations, the operator must rely on signals from the weapon to guide it above the level of precision offered by the onboard navigation systems,”consortium officials explain.

During ground-to-missile communications, the tail bobbin that carries 60 kilometers of optical fiber unwinds, which sends the infrared video through a fiber optic datalink to the launch command station. Missile steering commands are then sent back to the weapon via the same datalink. The operator can observe the missile’s flight in real time as the infrared images are being taken. This online interaction between the operator and missile enables the prosecution of timely course changes to precisely target a location.

In instances when a target is obscured by electromagnetic interference or a natural obstacle like an island, the fiber optic connection is maintained because of the very low power levels used to transmit the signals. The pulses are continuous because they stay beneath higher-level disturbance, giving off little infrared radiation across a low-frequency radar band. Problems such as electromagnetic jamming do not prevent optical fiber transmissions because signals are passed optically, as opposed to electrically. Bending light pathways allow data to be received around obstacles and through electrical jamming patterns.

A key Polyphem feature is its ability to hit a single target with multiple missiles. Called ripple fire, as many as four missiles can be launched from one platform, each simultaneously hitting the same target from different directions. If the operator sees that one or more of the missiles will not strike the designated location, the weapons can be called off individually and repositioned for a second attempt. In non-strike operations such as reconnaissance or surveillance missions, Polyphem can be used to make several passes over an area to collect more visual data than could be gathered in one fly-over.

In surface operations, Polyphem offers easy integration with shipboard fire control support systems and combat information centers, developers say. The C3I interface enables pre-engagement mission planning using 20-inch, high-resolution screen displays of target areas. Infrared magnification of potential flight paths aids in determining no-fly zones and other restricted airspace quadrants to 33 nautical miles. Aside from low radiation output, the low signature format of both firing stations and missile designs also help the system avoid radar detection by the intended target.

Polyphem completed the first phase of operational evaluation testing in September 2000. The second phase of experimental testing involving demonstrations at the system level is ongoing, and potential exists for incorporating the product into the German navy’s new K-130 Corvette class of surface ships. Experimentation with helicopter and submarine variants, the latter to be used in conjunction with class U-212 German submarines, is also underway. A fully operational surface-to-air format is scheduled to enter service in 2004.