When military pilots prepare for a mission, they want to know the look and location of every house and ridge line in the target zone. Alexandria company Vector Data Systems Inc. is overseeing a computer simulation system that helps U.S. pilots in the Balkans take off with that depth of knowledge.

It's a computer with a joystick that looks much like what you'd use in a computer game. What's different is the processing power and the software driving the system. Drawing on satellite images, maps and other data, it creates a believable 3-D rendition of specific places, which pilots then "fly" through to get familiarized.

Vector Data engineer Jim Cummins, fresh back from Albania, ran the "Topscene" system through its paces in his Alexandria office recently, gripping the joystick in front of a computerized image of hills and valleys not unlike those facing military pilots flying in Kosovo and Albania.

With simple turns and twists of his wrist, he surveyed the image before his imaginary cockpit, examining obstacles as he navigated a flight path and made mental notes of landmarks. Stopping and hovering over the scene, he impressed upon his mind how the land rose and fell as he made his way to the target marked by a small white circle.

"I'm seeing a 3-D image here, instead of a flat map," he said of the plain and the rising and sometimes cragged hills on his screen. "You will remember it better if you see it in 3-D."

For years, the military has used computer simulators to train rookie pilots and hone the skills of experienced ones. The systems let pilots "fire" weapons and pitch or sway to mimic the sensations of real flights. Topscene is different. It's intended not so much to train a soldier's reflexes for combat as to permit a methodical planning of entering into harm's way in a particular place.

Cummins has been at Albania's Rinas airfield, where he helped crews of Apache attack helicopters perform the same tasks. As the pilots awaited orders (which never came) to fly into Kosovo to smash the tanks of Serbia's war machine, they practiced runs on Topscene that simulated real places across the border.

Robert Mace, one of Cummins's colleagues at Vector Data, was in Italy recently, where he helped U.S. pilots preview missions they were to fly into Kosovo.

He said that in one case pilots looking at a two-dimensional navigation chart picked out geographic features as landmarks, hoping the landmarks would help them zero in on their target. But when they previewed the mission with Topscene, those features were not readily visible from the point of view of the cockpit.

The system enabled them to substitute new landmarks that they would be able to see as they flew in to drop their bombs.

Although a ceasefire has come to Kosovo, the same technology will now support the international peacekeeping force going into Kosovo, said Mark Darder of Vector Data.

Topscene, which stands for Tactical Operation Preview Scene, employs Silicon Graphics R10000 processors and runs segments of multi-gigabyte geographical data through them to produce a moving, real-time image of any part of the world that has been surveyed digitally.

The core of the information is a database that holds measurements of the elevation above sea level at specific points in a region. Extra details, like the varying textures of forests or deserts, can be added. In cityscapes, buildings can be made to appear to scale.

It's not only pilots that use the system. Vector Data has constructed the interior of buildings for Army Special Forces exercises.

"They started building the Topscene database for the Balkans in Bosnia," Cummins said, "and as the crisis grew, they added Kosovo and Serbia." It now extends from the Adriatic Sea through Bosnia and to the Hungarian border and takes up 18 gigabytes of computer storage space, Cummins said.

Improved depictions of that mountainous terrain have been added. Cummins said power lines from hydroelectric dams are prevalent throughout Albania, and a special overlay of imagery was programmed into Topscene to alert helicopter pilots to the locations of these hazards to flying.

Topscene was developed by Loral Corp. in the 1980s. Through a series of mergers, Lockheed Martin acquired the contract to produce the system, and now makes it through its Vought Systems division. Vector Data has a contract to support use of the system. It has assigned seven people from its South Washington Street headquarters in Alexandria to install, maintain and troubleshoot the equipment and software for the Army. Vector Data operators say it takes only about 15 minutes to learn to use the system.

Operationally deployed in 1990, Topscene made its wartime debut during the Persian Gulf War when it was dispatched to the gulf aboard the aircraft carrier USS Theodore Roosevelt. Now the system, which comes as a package for about a half-million dollars, is standard gear in the ready rooms of the Navy's carrier fleet.

A toned-down, $100,000-plus version has worked its way into units of other services, like the Apache helicopters stationed in Albania.

Topscene is similar to PowerScene, a scene visualization package developed by Cambridge Research Associates in McLean. At Dayton, Ohio, in 1995, Gen. Wesley K. Clark used PowerScene to show Serbian leader Slobodan Milosevic routes that Bosnian Serbs could use after a proposed peace settlement. The session was credited with a diplomatic breakthrough ending the Bosnian civil war.

CAPTION: Vector Data engineer Jim Cummins, right, demonstrates the Topscene system for Dave Muollo.

CAPTION: Detail of simulated terrain from U.S. landscapes similar to ones used for pilot training in Kosovo.