Next year, Bombardier's Global 5000 and 6000, fitted with Global Vision cockpits powered by Rockwell Collins Pro Line Fusion avionics, will be the first production business aircraft to enter service with head-up displays with synthetic vision system (SVS) background imagery.

Their HGS-6000 HUDs will use the same digital terrain elevation database (DTED) as the head-down primary flight displays, but the 3-D imagery will conform to the view of the outside world through the HUD on a clear day and the SVS picture will be monochromatic green to provide contrast with the rose-tinted combiner glass. Global Vision's SVS imagery also will be displayed on the head-down PFDs in full color and with a much wider angle field-of-view.

We had the opportunity to fly the Global Vision HUD with SVS in late September. It was installed on the right side of the cockpit of the Rockwell Collins Challenger 601-3A flying test bed, along with a full set of Global Vision flight instruments including a 15-in. PFD. Synthetic vision rapidly is emerging as a de facto standard for head-down displays in business aircraft, so a logical next step is to move it up to HUDs, according to Rockwell Collins engineers.

HUDs with SVS have been in use aboard military aircraft for more than a decade, especially tactical aircraft that fly high-speed, nap-of-the-earth sorties. We've witnessed the capabilities of just such a system while flying a low-level demo hop in a Dassault Rafale in central France.

Now, Rockwell Collins is pioneering the use of SVS for HUDs in civil aircraft. The goal isn't to give civil pilots the ability to use SVS to fly down canyons and skim over ridgelines in IFR conditions, but to enable them to fly down to lower minimums on published instrument approaches in accordance with FAR Part 91.175, much the same they can in some aircraft equipped with HUDs having approved enhanced vision systems (EVS).

We've flown several aircraft with HUDs having EVS. These systems use external sensors, typically short-wave infrared cameras, to peer through clouds and darkness to detect terrain contours, electric lights and runway surfaces. What they detect then is projected as background imagery on head-up or head-down displays to aid pilots in “seeing” the runway environment better than they can with unaided vision.

One of EVS's key assets is its ability to detect obstacles in the landing zone in real time. If, for instance, a vehicle, aircraft or animal moves onto the runway, an IR EVS may be able to detect the threat beyond the range of unaided vision. The HUD then displays the intruder's image so that the pilot can take evasive action. Certification authorities have extensive experience with IR EVS equipment and many countries allow such systems to be used to fly to lower weather minimums than are possible using unaided vision.

But prevailing weather conditions can limit the ability of IR cameras to detect terrain, pavement and intruders. While the IR equipment can “see” through fog at roughly twice the distance of human eyes, the system can be blinded by thick fog or precipitation. And the performance can be impaired by thermal inversion layers and other temperature gradient anomalies.

SVS, in contrast, uses a DTED stored aboard the aircraft, along with precise GPS position, attitude, heading, track and altitude data, to generate terrain contour, and obstacle and runway imagery on a display. DTEDs are available in several levels of resolution. Level 0 with 1-km resolution, Level 1 having 100-meter resolution and Level 2 offering 30-meter resolution are the most common DTEDs available for use in civil aircraft.

SVS is an autonomous system that is immune to the weather impairments that can plague IR cameras. It doesn't attempt to “see” through clouds. It just generates synthetic imagery by relying upon onboard computers.

SVS's greatest compromise also is its reliance on a published terrain database. It can only show terrain, obstacles and runways, among other features, that have been previously surveyed and recorded in the DTED. It can't detect what's actually going on in the runway environment in real time. Historically, the FAA's position has been that SVS is fine for enhancing situational awareness, but it could not be used for operational credit for lower visibility minimums on approach.

However, Tim Etherington and Sarah Barber, two of Rockwell Collins' lead avionics engineers, believe that ultimately, SVS should be granted such credit. Garmin, Honeywell and Universal Avionics Systems engineers, among others, also are fully involved in such research efforts.