Scientists and engineers at NASA’s Langley, Va., Research Center are evaluating the integration of a number of separate systems and techniques which, when combined, could achieve significant safety improvements in the approach and landing phase. These include synthetic vision, infrared sensing, terrain warning, weather radar, radio altimetry, differential GPS, head-up and head-down displays, “highway in the sky” presentations, multilateration, voice commands and other technologies. The project–part of NASA’s wider-ranging Aviation Safety and Security Program (AvSSP)–aims under the agency’s 2000 strategic plan to reduce the fatal aircraft accident rate by 80 percent within 10 years and by 90 percent within 25 years.
All the systems are, of course, in varying degrees of use today, but several operate independently of the others, and it becomes the pilot’s task to mentally integrate their separate data and then make the right decisions. Under normal circumstances, this rarely poses a problem, but accident investigations have revealed cases in which overlooking just one key parameter has caused the loss of the aircraft. The researchers’ objective, therefore, is to develop ways to present the pilot with every piece of critical data relative to the current flight phase in an instantly assimilated form, while at the same time avoiding information overload. NASA synthetic vision system (SVS) project manager Dan Baize described the aim as being the creation of a “virtual VMC” environment under all weather conditions.
To accomplish this, NASA researchers opted for what might be thought of as an information “pyramid” of the various technologies, with the SVS as the foundation on which the pyramid is built. The SVS employs an onboard computer-stored terrain map that, when fed the aircraft’s position, altitude, attitude and heading, provides the pilot with a correctly oriented, correct-perspective view of the scene ahead. The resulting full-color picture–with the traditional blue sky and appropriate terrain colors–is presented on the primary flight display (PFD), where it is flanked by vertical altitude/vertical rate and airspeed bars, with heading prominently centered in the upper part of the screen. The SVS picture is also presented as a plan-view map on the multifunction navigation display (ND), and then overlaid with an EHSI. The PFD and ND SVS presentations are displayed from startup to shutdown, but all the other technology “building blocks” of the pyramid are automatically introduced at appropriate points in the flight profile, to minimize screen clutter, although they can also be selected at the pilot’s discretion.
NASA has been refining its SVS project since 2000, when AIN flew on early test flights from Dallas/Fort Worth in the agency’s highly instrumented Boeing 757. Since that time, NASA has made major steps forward, including the integration of several of the technologies mentioned above. Currently, project tests are being conducted in a Gulfstream V, wet leased through the Research Triangle Institute, while the 757 is undergoing a major systems refit.
NASA has made several significant modifications to the GV. The more visible ones are the installation of two Rockwell Collins eight-inch by eight-inch liquid crystal display screens on the captain’s instrument panel and a Rockwell Collins Flight Dynamics HGS-4000 head- up guidance system. Project officials are quick to point out that replacing the GV’s standard Honeywell units was not necessarily because of any superior performance characteristics of the Rockwell units, but rather because engineers had created software and other attributes around the Rockwell equipment over the project’s four-year development.
In NASA’s 757 tests in 2000, the SVS picture was presented as a gray/white monochrome rendition of the outside world, using a “photorealism” technique where exceptionally sharp satellite terrain photographs were used for the SVS database.
While the maps were extremely accurate, they were far more detailed than was operationally necessary, and the current system now provides a somewhat softer image in which Jeppesen has blended the satellite photography with a colored terrain database. The blending process also allowed the terrain data to be more precisely aligned with the more accurate satellite data. NASA researchers place strong emphasis on SVS presentation accuracy, since any lack of correlation between the ground features displayed by the SVS and their real equivalents when they became visible would greatly lessen pilot confidence.
Creating a ‘Virtual VMC’ Environment
To this end, the total system uses other independent sources to reinforce the correlation of the SVS presentation with the real world. The aircraft’s position shown by the SVS versus its true position is of primary importance, and NASA’s tests to date have obtained their “truth” data from an airport differential GPS installation, essentially identical to an FAA GPS local area augmentation system (LAAS).
(Depending on the future of LAAS, NASA will be able to use the somewhat less accurate WAAS for this role.) Also, the radio altimeter confirms that altitude inputs to the terrain database are correct, while terrain responses from the onboard TAWS/EGPWS units also overlay the SVS terrain depiction, with appropriately colored terrain warnings appearing on the EVS display at their correct geographical locations.
Additionally, the GV test aircraft is equipped with a Rockwell Collins WXR-2100 weather radar that incorporates a software modification to “sharpen” its scanning beam and reduce ground clutter. Normally, attempting to map the surface by tilting a conventional weather radar down is not rewarding, unless very prominent geological features, such as lakes, large rivers, coastlines and mountain ranges, are present, since otherwise the display is totally cluttered with ground returns.
Sharpening the radar beam, however, significantly reduces ground returns. In addition, on an approach the modified radar can detect the edges of the runway ahead, plus terrain and other features that create measurable radar “shadows,” all
of which can be used for SVS correlation. NASA personnel expect that with further software development, the radar should be able, from three miles’ range, to detect individual objects on the airport surface having a frontal area of as little as one square meter.
The infrared image from the enhanced vision system (EVS)–in this case, the GV’s standard Kollsman All-Weather Window–also provides a comprehensive “ground truth” overlay of the SVS presentation, covering the terrain ahead and the airport approach lights, runways, taxiways, buildings and other structures. The EVS data is displayed on the PFD as a square, smaller than full screen, image centered on the approach path, thereby providing the pilot with another SVS correlation check.
But the main role of EVS is to provide, via the head-up display (HUD), situational awareness during the approach and after landing, with special emphasis on detecting aircraft and vehicles moving on the airport surface. The SVS cannot display these potentially hazardous targets because its database stores only pre-recorded fixed objects. Also, because of the current limitations of HUDs to project a range of colors, a comprehensive SVS presentation on the HUD identical to that on the PFD is not yet practical. As an interim step, Rockwell Collins has developed a simple grid overlay that flows over the terrain and provides a useful surface depiction on the HUD, but only of fixed features.
After touchdown, the EVS provides low-visibility guidance on the HUD and the PFD by marking the runway centerline with a series of arrows, accompanied by a line of traffic cone-like symbols along both edges of the runway. The HUD displays the distance to go to the next turnoff, and the pilot initially sees the turnoff in correct perspective further down the runway. The display perspective changes as the airplane approaches the turnoff, with similar centerline arrows and edge cones continuing through the turnoff and onto the follow-on taxi route, with appropriate stop lines displayed, to the ramp.
The turnoff exit swings to the center of the HUD when the airplane rolls off the runway into the turn, and as the aircraft heading alters, so does the display’s view ahead, in exactly the same way as the pilot’s perspective would in good visibility. The taxiway comes into view on the display when the turn is complete, and similar centerline arrows and edge markers guide the aircraft along its cleared taxi route to the ramp.
Remaining in the ‘Tunnel’
Most pilots will be familiar with the “highway in the sky,” or HITS, presentations, where the future flight plan track is shown as a series of picture frame-like boxes that diminish in size with their distance ahead. Flying the aircraft through the center of each box keeps one on track.
But while the presentation is instinctively intuitive, pilots have occasionally experienced difficulty in re-entering the “tunnel” after inadvertently leaving it. Also, since the required future track is usually a straight line for many miles, the consecutively smaller and more distant boxes tend to add clutter to the centerline and can, following a relatively minor heading change, cause the tunnel to move slightly in the opposite direction in its attempt to bring the aircraft back on track, which can be disconcerting to new pilots.
NASA’s solution to this characteristic is to remove the boxes once the aircraft is established on the centerline, and show only their four corners. This achieves two things. First, the display is effectively de-cluttered, allowing the pilot to keep track more precisely. Second, it then allows a more positive warning of any divergence from the centerline by displaying just the one side of the box toward which the aircraft is drifting, thereby allowing the pilot to make a correction instinctively.
Should the drift off track be uncorrected and the aircraft leave the tunnel, the single side disappears and is replaced by the three other sides of the box, clearly indicating the direction required for track recovery. For example, if the aircraft inadvertently climbs, just the top side of the box will appear first as a caution. But continuing the climb will cause the top side to disappear and be replaced by the other three sides, which form a squared-off U shape below the aircraft symbol on the display.
Correcting the climb and descending back to the tunnel will cause the U to be replaced by the single top-side symbol, denoting that the airplane has re-entered the tunnel, and this symbol will disappear as the descent continues to regain the centerline.
Multilateration is also part of NASA’s total avionics suite. This technique, already entering service, employs several small ground stations arranged around an airport to achieve line-of-sight reception coverage of the airport surface and the local airspace and to listen continuously for transmissions from aircraft transponders replying to ATC radar or TCAS interrogations, plus ADS-B signal bursts and military IFF signals. Each ground station passes its received data to a central computer that uses a sophisticated triangulation program to determine the position of each aircraft, and these positions, along with their related idents and altitudes, are then displayed in the airport tower and at other locations. The system, which is more accurate and has a much faster update rate than radar, promises to be a valuable tool in airport surface traffic management. It also alerts controllers to potential runway incursion and collision situations, thereby allowing controllers to warn the pilots or vehicle operators involved.
But the controller warnings to pilots can sometimes leave little time for avoidance, and warning times of less than 11 seconds have been reported. The ideal solution would be to provide pilots with the same information as the tower controllers, and NASA is testing a runway incursion prevention system developed by Rannoch. Here, aircraft or vehicles near the active runway are clearly portrayed on the SVS airport depiction and will trigger flashing conflict alerts on the HUD and the PFD, plus an aural alert, during the aircraft’s takeoff and also during its approach, landing and rollout.