Through The Canopy Glass

By Eric Hehs Posted 22 June 2011

“Pilots never see the outside world through a canopy. They see an image of it.”

Mike Jones, a Lockheed Martin Technical Fellow in optics, electro-optics, and directed energy, uses these lines to introduce visitors to the canopy mapping laboratory at the Lockheed Martin facility in Fort Worth, Texas. The laboratory, near the F-35 production line, is a required stop for every canopy before it is installed on a Lightning II.

“Every manufactured canopy optically distorts the view of the outside world in a unique way,” added Jones, who has been influential through the years in devising instruments to quantify these distortion differences for the F-16 and F-22. He is the inventor of the advanced canopy mapping system used for the F-35 canopy.

Mapping the optical properties of a canopy is akin to reverse engineering the optics of a pair of prescription glasses. However, these glasses don’t rest on noses—they surround the heads of fighter pilots. The mapping process is necessary because aerial and ground targets viewed through head-up and helmet-mounted displays are distorted by canopy thickness, curves, and material. Simply put, the canopy can have a direct effect on weapon accuracy.

Optical deviations vary widely for azimuth, which is defined by left and right head movement, and elevation, defined by up and down movement. Deviations also vary with eye position in the cockpit. That position changes with head movement and seat height.

The output of a canopy mapping system generates data that allows compensation for these optical deviations in the form of sets of numbers called look-up tables. These tables, which are associated with a specific canopy, interact with the avionics software to correct pilot displays. If a canopy is changed on a particular airframe, the look-up table must be reloaded as well.

The defined viewing area depends on the aircraft and the targeting system. For example, a more detailed map is required for an F-16 equipped with a joint helmet-mounted cueing system than for an F-16 without that system.

“The accuracy requirements and larger range of viewing angles for the helmet-mounted display system in the F-35 forced us to explore new technologies and approaches for compensating for canopy optics,” noted Jones. “Besides, the mapping systems we have for the F-16 and the F-22 could not physically accommodate the shape and structure of an F-35 canopy.”

Jones says the idea for the F-35 canopy mapper came to him from his background in directed energy, wavefront sensors, adaptive optics to correct for time-varying aero-optical and atmospheric turbulence, and even from his background in astronomy. The subject is as complicated as that last sentence reads.

Jones elaborated: “The key idea that occurred to me is that the F-35 canopy optically distorts and deviates the passage of light through it in a manner similar to aerodynamic flow fields and free-stream atmospheric turbulence, except that the optical deviation does not vary with time. Instead, optical deviation varies with where a pilot looks through the canopy. Once the canopy is manufactured, it does not change the way it deviates the outside world. So it acts as a frozen turbulence field. Deviation, which is not a trivial matter, is a function of where and from what direction an object is viewed through it.”

Previous canopy mappers use only one or two probe beams. The beam source is placed at specific locations inside the canopy and then projected through the canopy to a sensor. Deviation is measured as a difference in beam angle with and without the canopy. This process made measurement of canopy optical deviation over the total field of regard and for all head positions time consuming and tedious.

The older type canopy mappers are highly serial in nature. The measurements are taken sequentially for each azimuth and elevation angle one at a time at one eye position. Then the canopy has to be moved to the next eye position then the complete set of azimuth and elevation angle measurements must be repeated for each of several possible eye positions.

The F-35 advanced canopy mapper, or ACM, simultaneously measures optical deviations for the total region for a pilot’s head and eyes by using a large collimator and a laser to produce a large pattern of test beams into the wavefront sensors. This approach significantly reduces the time required to measure a canopy by eliminating the need to take measurements at different eye positions. “We still have to measure at different azimuth and elevation angles,” explains Jones, “but changes in optical deviation with head position are captured simultaneously in a single camera frame.”

Changes in azimuth angles are handled by rotating the canopy on a platform about the central test beam projector. Changes in elevation are handled by placing the wavefront sensors on a moveable arm. Canopy rotation motors and wavefront sensor cameras, as well as real-time data acquisition and storage, are fully under computer control. Canopies are lowered onto and removed from the fixture by the same overhead crane system that moves major assemblies on the F-35 production line.

The F-35 ACM has already demonstrated a significant savings in time and labor costs of at least forty times better than previous mapping technologies and equipment. Previous mappers would require more than one week of measurement time to fully measure the F35 canopy to the same resolution and range of angles. The new mapper finishes its complete set of measurements in only about one hour after the canopy is loaded—a significant benefit over the thousands of canopies that will have to be measured during F-35 production.

The prototype ACM has been operational since 2008. It is the product of a creative and talented joint engineering team between the F-35 program, the Lockheed Martin ADP (Advanced Development Projects, the Skunk Works) Mission Systems and Avionics group, and LM Aero Electronic Laboratories and Ranges personnel. Two more advanced canopy mappers are in the works to handle the increased F-35 production rates.

Eric Hehs is the editor of Code One.
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