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Vertical Motion Simulator
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The out-the-window (OTW) graphics, the computer-generated images that simulate the outside world for the pilot, offer both flexibility and accuracy. Numerous rural and urban locations, such as airports in the United States and abroad, have been simulated, and all can be modified as needed for specific simulation experiments. When required by researchers, new locations, whether real or imaginary, can also be generated.

Each OTW scene depends on a database, the software that contains the relevant characteristics of a geographic location. Numerous three-dimensional moving and stationary models are created independently, then included in the database. Aircraft, ground vehicles, and buildings are common three-dimensional models. Ships, including several aircraft carriers for experiments in landing and takeoff, have also been simulated.

The VMS maintains two image generators, one with five channels and one with six. Each channel corresponds to the image displayed in a single window. The image generators are capable of independent eyepoints; in other words, they can display the scene from different positions simultaneously. This enables the pilot and copilot to view the scene accurately from their slightly different positions. It also allows the exterior of the simulated aircraft to be viewed by the researcher from any location in the database.

Objects at a distance are seen with less detail than those nearby, and the image generators save time by omitting unseen detail. To accomplish this, databases are designed with up to six levels of detail. The image generators present the transitions between levels smoothly so the simulation pilot is unaware of them.

These graphics of a carrier demonstrate three levels of detail. The carrier is shown the same size in each graphic to make the difference easy to see. Normally, however, the carrier with the least detail would be the smallest, and the carrier with the most detail would be the largest.

Computer generated image of aircraft carrier at low resolution Low Resolution Computer generated image of aircraft carrier at medium resolution Medium Resolution Computer generated image of aircraft carrier at high resolution High Resolution

The largest object simulated at the VMS is the earth, used in simulating the landing of the Space Shuttle Orbiter. Because any part of the earth may be seen from higher elevations, all areas have been developed at the lowest level of detail. As the shuttle gets closer to the earth, visible areas are displayed with increasing detail. However, to avoid taxing the image generators, only those areas used for low-level flight are developed at the highest level of detail. Areas as large as 10,000 square kilometers have been simulated at this level.

The process of image generation begins when the host computer samples the state of the aircraft, including its position, orientation, and speed, along with any input from the pilot via the flight controls. The host computer then extrapolates the aircraft’s state for the instant the pilot will view the image; in this way, the pilot will not see a graphic that's a split-second old. This information is sent to the image generator.

The image generator also performs an extrapolation, this time to account for the fact that it operates at a different rate than the host. The image generator then computes which scenery the pilot would see and prepares it with the appropriate level of detail. The image generator next converts the three-dimensional information for display on a two-dimensional surface. Finally, it prepares the image to be displayed electronically and sends it to the cab.

In the cab, the pilot does not directly view the monitor. To make the image appear distant, it is first reflected by the back of the window onto a section of spherical mirror. Because of the mirror's shape, the reflected light rays are parallel, resulting in an image that appears distant. The image then passes through the window to the pilot’s eye. Because of this dual function, the window is called a beam splitter.

Because the image generator computed the scene for a split second into the future, the image arrives at the pilot’s eye at just the correct moment. At any one time, three images are at one stage of processing or another. The OTW system displays a new image 60 times a second, delivering smooth and accurate visual cues to the pilot.

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Last Updated: April 5, 2017

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