The Laser Target Recognition System (LTRS): A Lesson in Bleeding Edge Technology

The War in Southeast Asia  was many things to many people.  To some in the Defense Department under Secretary of Defense Robert S. McNamara, the war appeared to be a technological challenge that American know-how was meant to conquer.  To find an enemy who made the most of the cloak of darkness for movement and logistics was a significant challenge to the military services whose experience in using technology to turn night into day was limited to infrared projectors and optics and flares.  Given the short detection ranges that these limited technologies provided, the ability of airborne platforms to detect and attack ground targets at night were simply ineffective.

The technological limitations that the U.S. Armed Services faced in the Vietnam War grew increasingly frustrating as the conflict wore on, although it wasn’t until the initiation of Operation Rolling Thunder and the introduction of U.S. ground forces in 1965 that the scope of the problem became clearer.  The war in South Vietnam, which started as an insurgency, found that the enemy–in this case Viet Cong insurgents strengthened by North Vietnamese Army elements, used the lush jungle that covered most of the country and the night to mask the movement of units and logistics.  Lacking sensors that could penetrate either obstacle with the exception of the visual light from flares or searchlights, the Air Force struggled to find and attack targets on the move that would be vulnerable to air attack at night.

This frustration naturally led to a host of research and development projects whose purpose was to solve the problem of attacking ground targets at night and in poor weather.  Radar was an obvious sensor, and tactical aircraft designs originating in the late 1950’s incorporated radar as their primary attack sensor.  The two leading attack aircraft designs of the early 1960s carried radars designed to detect targets on the ground (the Air Force’s F-111A (AN-APQ-113 radar) and the Navy’s A-6A Intruder (AN-APQ-92 radar), but these sensors only provided an indication of a target–the Air Force weapons systems operators (WSOs) and Navy bombardier/navigators (B/Ns) saw only undefined representations of their targets on a radar screen on which to unload their ordnance.  In addition, clever enemies could spoof the radars with false targets which then led to wasted ordnance drops.

What was needed was a sensor that could provide the airborne operator with a real time image of the target, not just a blip on a radar screen, which brings us to a project uncovered in some recently declassified U.S. Air Force Research and Development case files.  The idea germinated at the Perkin-Elmer Corporation, an optical design firm that was founded in 1937, proposed that a continuous wave laser could provide imagery via a raster-scanned (horizontal scanning) real time display.  By February 1965, Perkin-Elmer’s R&D effort had progressed to the point that it could make a technical proposal to the Air Force for a nighttime real time viewing system incorporating a raster-scanned laser system.  Two years later, the Air Force authorized a sole-source procurement of what would become a Laser Target Recognition System (LTRS) under program 67-61698DF-6340680F for which $1 million dollars of Fiscal Year 1967 monies was applied.

The project involved Perkin-Elmer actually fabricating the sensor which was required to have an instantaneous field of view over a 5 to 1 range and be able to search 60 degrees in azimuth and 90 degrees in elevation.  The system should be able to operate at altitudes up to 10,000 feet and at ground speeds up to 400 knots.  The system should also be able to resolve a 2 1/2 truck-sized target to be recognized at a slant range of 1 mile.  The objective aircraft for the LTRS installation was the Martin B-57 tactical bomber, a versatile platform that was to serve in a number of specialized reconnaissance roles over the years.  LTRS utilized a 4 watt argon laser to generate its imagery.

The heart of the LTRS, a 4 watt argon laser.

A May 1967 revision of the contract statement of work specified that the LTRS was to be installed on a Douglas B-26B, Army serial 44-34538, civil register N6839D owned by Hughes Aircraft Company for flight testing, in this instance under contract number F33615-67-C-1901.  Unsurprisingly, testing soon revealed some problems.  LTRS was to support a video recording system to capture imagery from the laser scan.  It was determined that a 16mm film camera would resolve the imagery issue.  Secondly, the system could not track targets accurately to keep them in view.  The contractor recommended solution was to incorporate a doppler navigation radar into the LTRS pointing control subsystem.

This angular contraption is the Laser Target Recognition System (LTRS) installed on B-26B 44-34538, civil registration N6839D, 8 January 1969.  The stenciled warnings are a testament to the argon laser’s light path vulnerabilities

A series of monthly status reports by Hughes Aircraft illustrated that progress in manufacturing the system was slow, and there were problems meeting project timelines.  A Research and Technology Resume (DD Form 1498) dated 29 January 1968 stated that the fabrication of the sensor was 60% complete.  Once the sensor had been completed, it would be installed on a rooftop for stationary testing.  After testers completed the rooftop phase of the project, the system would be installed on the Hughes B-26 for airborne tests.  Rooftop testing was expected to take place in March 1968 with airborne testing taking place shortly afterward.  As it turned out, rooftop testing did not actually take place until September 1968.

An LTRS image of a GMC M135 2.5 ton truck on the Culver City range.  The LTRS was mounted on the Hughes Aircraft Company facility rooftop, and the truck is roughly 200 yards away from the sensor, November 1968.


The Hughes Aircraft Company Monthly Status Reports were filled with the details of the efforts to overcome the technological hurdles the LTRS project uncovered:

The third major problem involved temperature rise of the scanner motors which could ultimately degrade the bearings and introduce scan wheel imbalance (15 July 1968).

The Laser Heat Exchanger output has been checked and found to be approximately 3 gallon (sic) rather than the specified 5 gpm (15 August 1968).

Currently the system may be operated for 45 minutes continuously and then 15 minutes cooling time is required (15 September 1968).

Warping of optical elements exposed to atmosphere on one side and to the vacuum of the scanner housing on the other lens side (20 September 1968).

The entry and exit windows to the scanner assembly housing have been replaced (entry window anti-reflection coating imperfection was the source of 32% loss in laser energy) (15 October 1968).

Checkout flight tests of the LTRS installed on the Hughes B-26 finally commenced in November 1968.  A flight test summary appended to the 15 April 1969 status report revealed the following: out of a total of 13 test flights listed (LTRS-0 through 12), five aborted because of technical problems with the LTRS.  The eight flights on which LTRS actually functioned operated in the vicinity of Hughes Aircraft Company Culver City facility, Edwards Air Force Base, Marine Corps Base Camp Pendleton, Los Angeles harbor and vicinity, and the coastal waters adjacent to the Los Angeles area.  In addition to sensing targets in the Culver City area such as runways, housing, office buildings, and vehicles on roads, and armored vehicles at Camp Pendleton, the LTRS also scanned Navy Underwater Demolition Team (UDT) swimmers and beach obstacles, various ships, and even a Navy submarine (USS Bashaw (AGSS-241)).

The LTRS operator’s station in the Hughes Aircraft Company B-26B, 8 January 1969.

The outcome of this limited testing was disappointing to those who had hoped LTRS would prove to be the answer to targeting vehicles and other point targets from the air at night.  The very best test results (accomplished on the last test flight) detected targets at a range of less than 3,000 yards and could identify those targets at a range just under 2,000 yards.  Now that performance is about what the Air Force requested when it began the LTRS project back in 1967.  However, in reality, those distances were simply too close to allow a safe attack on the identified targets by low speed propeller-driven aircraft, and much less so for a turbojet-powered attack aircraft such as the B-57.  The reason for the marginal performance of the system was simple–LTRS’s argon laser suffered from significant light loss because of its complicated light path.  While the laser had an output of 4-6 watts in the laboratory, once installed in LTRS in an aircraft that output had been reduced to 1.2 to .6 watts. The end came swiftly for the LTRS project.  The close out report is dated 20 October 1969.  In terms of cost, the LTRS project cost the Air Force some $2.4 million dollars in 1969, $16.6 million in today’s budget climate.

In addition to the laser output problem, LTRS suffered from a more fundamental problem.  The system’s argon laser projected light that was reflected from the target back towards the sensor where a scanner then assembled the various reflections into an image of the target.  In order to illuminate the target with enough laser light, the laser must be of a continuous wave (CW) type–in essence the laser beam is projected continuously through the aircraft’s sensor pass over the target.  The technology of the day could not support reliable CW laser technology, which eventually appeared in such developments as CD-ROM readers, DVD players, and bar code scanners.  Sensors that use lasers for imaging had to await new laser and computer developments.

Eventually imaging lasers appeared in LIDARs (light detection and ranging) where the distances measured by individual laser light pulses could be assembled into clear images. LIDARS are at the heart of geographic information systems today.  They serve other purposes as well, most notably as sensors for unmanned ground vehicles whose artificial intelligent guidance systems requires precise knowledge of the distances to objects around these vehicles.  The thing to remember though is that LTRS tried it first more than five decades earlier.




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