Perspectives: Terrain Avoidance: When COTS Won’t Do

By Paula A. Jackson, Dennis Bostich and John Padukiewicz | June 1, 2005
Send Feedback

In the late 1980s the U.S. Navy decided to develop its own ground proximity warning system to prevent flight into terrain. Eventually, the service designed its own terrain awareness warning system, too. Why didn't it use commercial off-the-shelf (COTS) equipment?

In 2003 the U.S. secretary of Defense and the secretary of the Navy challenged the services to reduce their aircraft mishap rates by 50 percent. "Safety programs are not discretionary," they said. "Fully funding them should be a priority." There was certainly an urgent need to do so. Navy Safety Center data from 1990 through 1999 showed that 131 lives and 51 aircraft had been lost in controlled flight into terrain (CFIT). The cost in aircraft exceeded $898 million; the human cost was immeasurable. And the impact on readiness and capability was significant.

But commercial off-the-shelf (COTS) products couldn't meet the necessary safety requirements, given the very small market that tactical aircraft represent. Commercial terrain avoidance systems work because air transport pilots are not flying low-altitude, high-speed maneuvers. These systems are intended to correct for momentary losses of situational awareness on safe, predictable air routes--controlled in every phase of flight--and give pilots enough warning to recover their aircraft with minimum impact on passengers.

For many of the Navy's transport aircraft, modified COTS ground proximity warning systems (GPWS) provide adequate protection. The Navy has purchased and installed COTS systems in large transports such as the KC/C-130, VP/UP-3, C-2 and aboard the CH-46E and H-53 helicopters. COTS systems, however, would be insufficient, to say the least, for F/A-18s doing 400 to 500 knots a few hundred feet above the ground.

Combat pilots intentionally operate their aircraft close to their physical limits. They fly into a hostile environment at low-level, high-speed, high-G and high angle of bank, with little reaction time. A simple look at the terrain directly below provides insufficient information to avoid impact in such a dynamic environment. A COTS box would provide a nuisance warning every time the pilot pushed over into a bombing run.

First Step: GPWS

When the Pentagon edict appeared in 2003, work at the Naval Air Systems Command was already under way. The chief of naval operations in 1987 had directed the command to develop a system to reduce CFIT.

The first systems went through developmental and operational testing in the command's manned flight simulator (MFS) and mobile avionics test station (MATS), which provided laboratory flight test simulation. The command has used MFS since the beginning of the program; MFS continues to be an integral part of the testing process. A baseline version of the generic GPWS algorithm evolved through this process to satisfy basic requirements.

In 1987 the Air Combat Electronics Program Office (PMA209) formed an integrated product team to develop a GPWS. The team defined the threat as the loss of air crew, the loss of aircraft, and the decline of readiness. It set out to design, test and implement a terrain avoidance system for tactical aircraft.

The Navy developed its own look-down system, combining altitude data from the radar altimeter, vertical speed from the air data computer, attitude from the inertial navigation system (INS), and own-aircraft dynamics data to project the trajectory, predict the time of impact, and issue voice and visual recovery commands. The system has five voice warnings: Roll Right, Roll Left, Pull Up, Power and Check Gear.

Last-Second Warning

The Navy GPWS provides a 3- to 7-second warning before impact and includes 1.2 seconds for pilot response. The GPWS also calculates trajectory, using only 80 percent of the aircraft's maximum available G for recovery in order to provide a margin of error if the pilot does not pull to the maximum available G or delays initiating the recovery. The system also incorporates individualized aircraft performance data. An F/A-18 at 450 knots, for example, has 6 to 7 Gs available to maneuver. The Hornet's GPWS knows the ability of the F/A-18's engines to generate power at a certain speed and point in the trajectory.

Why so little warning time? The intent is to avoid false and nuisance warnings. A false warning means there is no impending CFIT condition. A nuisance warning means that actual terrain is setting the system off, but the pilot consciously ignores it because he is deliberately operating the aircraft in this environment and has full situational awareness of the surroundings. Either type of warning is undesirable. Both cause pilots to shut the system down and reinforce negative training. With the military GPWS system, Navy pilots are trained to take immediate action when they hear the warning.

One obvious limitation is that the radar altimeter illuminates the ground in only a 30-degree cone. If the pilot banks, dives or climbs at angles greater than 30 degrees, he loses radar altitude. At such moments the system goes into a 2-minute "coast mode," using the last, best altitude and aircraft dynamics data to predict aircraft position relative to the terrain.

The GPWS provides voice alerts and visual cues on the head-up display (HUD) to get the pilot away from the ground. Voice alerts, for example, call "pull up! pull UP!" or "roll left! pull up!" And large left-, right- or upward-pointing arrows indicate roll left, roll right, or pull up. Voice is the primary alert mode, but graphics reinforce what the voice says.

The government-owned GPWS algorithms can be hosted in any onboard computer that can receive the required sensor data, complete the dynamic calculations, and output information to the appropriate annunciator or display.

In the F/A-18s and AV-8Bs, the GPWS algorithms are loaded into the mission computer's operational flight program. In the EA-6B, the software will be part of the control display unit, the interface to the flight management system. The Navy expects to conduct development testing of the EA-6B application this year, as part of a block upgrade. The entire fleet of F/A-18s and AV-8Bs now uses the software. More than 1,500 of an authorized 2,000 systems have been installed. Future development of the GPWS software will focus on the Navy's helicopter fleet, including UH-1Y, AH-1Z and MH-60R/S.

The first-generation GPWS system uses altitude and velocity vectors to calculate an aircraft's state and recoverability. The vectors also are used to warn the air crew when they are dangerously close to impact and to provide an escape maneuver. While this is good for over-water flight paths, it has limitations. Since it is a look-down only system, it can't see forward in the direction of flight and warn of rising terrain. And the system provides only limited protection outside the radar altimeter envelope.

Nevertheless, the Navy feels that the benefits of GPWS, even with its limitations, are far better than nothing, as the equipment provides some immediate protection to air crews. And the system already has recorded two saves. But more capable, 3D, look-ahead performance is desired for high-speed aircraft.

Enter TAWS

In parallel with the GPWS development, the Navy began planning a TAWS system to generate the more dynamic 3D warning and recovery solutions needed in combat. What evolved was an application using data from the National Geospatial Intelligence Agency's worldwide digital terrain elevation database (DTED).

DTED currently is hosted in the moving map system, known as the tactical aircraft moving map capability (TAMMAC), that is installed in F/A-18 aircraft. The service has been fielding TAWS software in F/A-18 TAMMACs since November 2004.

The radar altimeter remains the primary sensor in the Navy TAWS system, but it is supplemented by GPS outside of the altimeter's 30-degree illumination area. The system constantly calculates trajectories in all three dimensions.

TAWS adds terrain elevation and GPS latitude and longitude information to the radar altimeter, inertial navigation system, air data computer and aircraft dynamic performance data. GPS locates the aircraft within the database, and the database provides information on what's ahead of and around the aircraft. (There is no active, look-ahead sensor.) TAWS is always on and requires no input from the pilot. The key difference from commercial systems is that the warning is held off till the last possible second to reduce nuisance alerts.

The TAWS algorithm--using real-time aircraft position, attitude and altitude--continuously calculates aircraft altitude along its trajectory and compares the altitude to elevation data in the embedded 3D virtual terrain model. The system doesn't warn of approaching terrain unless its parameters define the situation as unintentional ground impact.

The TAWS algorithm computes two recovery options, one based on an increase in load factor (G-loading) at the current angle of bank (AOB) and one based on a roll-to-wings-level and pull-up. The increase in G-loading at current AOB is designed to reduce nuisance cues and false alarms when flying over mountainous terrain.

The system uses aircraft sensor information to constantly update aircraft trajectory. When the predicted trajectory intersects the ground, the system displays and annunciates warnings to the pilot to roll and/or pull away from the danger.

As the algorithm continues to be developed, so does the prospect of such enhancements as autorecovery and obstacle avoidance. If the current system was coupled to the flight control system, for example, TAWS could automatically initiate a recovery maneuver instead of, or in addition to, giving the pilot a warning. The Navy is considering funding the development of such autorecovery technology for FY2008. Obstacle avoidance would require additional sensors or a sufficiently accurate database to include obstacles and generate TAWS warnings.

The integrated product team has studied the use of a complementary forward-looking infrared (FLIR), and other active and passive sensors, to verify and augment database information, as well as to provide the direction of and range to possible obstacles.

The Navy TAWS system is being tailored for the F/A-18C/D/E/F and T-45. A digital video recorder will host the software in the T-45. Other potential platforms include the V-22, Joint Strike Fighter, heavy lift requirement helicopter, unmanned air vehicles, and the Joint Primary Aircraft Training System (JPATS). Foreign military sales to Australia, Canada, Finland and Switzerland are under way.

GPWS Saves

Initially, there was some heartburn about the U.S. Navy's custom-built ground proximity warning system (GPWS). Because tactical aircraft operate in a low-level environment to avoid detection by enemy sensors, flight crews were concerned about the possible false and nuisance warnings they could get while flying just above the terrain.

But any doubts have now been put to rest. By April 2004 the GPWS had recorded two saves. In the first incident, an instructor pilot was acting as a safety-of-flight observer to a replacement pilot during training in low-altitude tactics. This was the replacement pilot's first flight alone in a jet at 200 feet, and the instructor was monitoring the aircraft's flight path to keep the pilot away from mountain ranges on either side of the valley. While concentrating on the replacement pilot, the instructor pilot flew into rising terrain and received a GPWS warning. He initiated the proper recovery maneuver and bottomed out at 70 feet, doing 420 knots.

In the second incident, a pilot was conducting a close air support mission when he went head-down to verify target coordinates. He received a GPWS warning and recovered the aircraft at 120 feet, doing 431 knots.

At A Glance

High-speed, high-G, tactical aircraft have more stringent terrain avoidance requirements than can be addressed by commercial software. So the U.S. Navy developed the following:

  • A look-down, radar altimeter-based, ground proximity warning system (GPWS), using dynamic aircraft performance data--installed in all F/A-18s and AV-8Bs, with future installation in E-2D, EA-6B, MH-60R/S, AH-1Z, and UH-1Y, and

  • A terrain awareness warning system (TAWS)--with a 3D terrain elevation database, altimeter and GPS inputs, using dynamic aircraft performance data--which began fielding with F/A-18E/Fs in November 2004 and is planned for the F/A-18C/Ds and T-45s.

About the Authors:

Paula A. Jackson is the conflict avoidance systems integrated program team leader for PMA-209, Air Combat Electronics, Naval Air Systems Command. Dennis Bostich, with support contractor DCS Corp., chairs the GPWS/TAWS team Software Engineering Process Group; John Padukiewicz is an engineer with DCS. The command's GPWS/TAWS team, led by Jackson, supports both fixed and rotary wing aircraft. It manages the development, integration, installation and life-cycle support of GPWS/TAWS--both COTS- and government-developed. The team has achieved Level 3 Software Engineering Institute maturity. It holds a U.S. Patent for its TAWS algorithm. For more information, contact

Receive the latest avionics news right to your inbox