WAAS Up?

The current worldwide Air Traffic Control (ATC) system had its origins in the mid 1940s and was launched in conjunction with the formation of the International Civil Aviation Organization (ICAO). It only took about 40 years to realize that growth in the aviation industry was beginning to stretch the limits of the system.

It was 1983 when ICAO commissioned a committee charged with developing the operational concepts of future air navigation systems (FANS). The plan was to improve communications, navigation, and surveillance (CNS) while at the same time providing new technology air traffic management (ATM) equipment.

Required navigational performance (RNP) is a means of classifying aircraft navigational capability. The FAA is moving toward a performance-based national airspace system. The ability to fly in certain areas may be governed by the aircraft and flight crew’s ability to achieve precise navigational performance within specific tolerances. RNP-0.3 will be used for approaches; it refers to 0.3 nautical mile accuracy. This accuracy may be achieved through various means but the aircraft will be certified to a particular RNP.

Navigation improvements
The main thrust in the area of navigation improvements has revolved around a transition from inertial methods of navigation to those employing the use of space-based satellites, otherwise known as the global positioning system (GPS).

The NAVSTAR constellation achieved full operational capability in 1995. It originally functioned with 24 satellites circling the earth twice a day at an altitude of around 12,000 miles and a velocity of 7,000 mph. Very precise orbits were calculated and attained while the satellites transmitted a continuous encoded signal. The planned orbits would routinely allow a minimum of six satellites to be in view from most points on the earth at any time.

The GPS receiver installed in an airborne or ground-based vehicle will acquire as many satellites as are in its reach. As long as a minimum of three are detected triangulation can occur, allowing calculations factoring in time to take place, resulting in an accurate location to the GPS user. A GPS receiver must be locked on to the signal of at least three satellites to calculate a two-dimensional position (latitude and longitude) and track movement.

With four or more satellites in view, the receiver can determine the user’s three-dimensional position (latitude, longitude, and altitude). Once the user’s position has been determined, the GPS unit can calculate other information such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time, and more.

Although typically very accurate, there are several factors that can cause degradation in GPS accuracy and they include:

  • Ionosphere and troposphere delays — The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to correct partially for this type of error.
  • Signal multipath — This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors.
  • Receiver clock errors — A receiver’s built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.
  • Orbital errors — Also known as ephemeris errors, these are inaccuracies of the satellite’s reported location.
  • Number of satellites visible — The more satellites a GPS receiver can “see,” the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors,
    underwater, or underground.
  • Satellite geometry/shading — This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping.
  • Intentional degradation of the satellite signal — Selective availability (SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defense. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.

Wide Area Augmentation System (WAAS)
Wide Area Augmentation System (WAAS) is a product of the FAA to enhance the global positioning system and is accomplished by improving accuracy, integrity, and availability. The intent is to enable aircraft to use GPS for all phases of flight, including precision approaches to landing.

As of March 2008, there are 31 actively broadcasting satellites with those above the original 24 intended to improve the precision of GPS receiver calculations by providing redundant measurements. The increased number of satellites changed the constellation to a non-uniform arrangement. This arrangement was shown to improve reliability and availability of the system, relative to a uniform system, when multiple satellites fail.

The system takes advantage of ground-based reference stations in North America and Hawaii to measure the satellites’ signals and determine position error. Information from the reference stations are routed to master stations, which queue the received deviation correction (DC) and send the correction messages to geostationary WAAS satellites in a timely manner (at least every five seconds or better). Those satellites broadcast the correction messages back to Earth, where WAAS-enabled GPS receivers use the corrections while computing their positions to improve accuracy.

ICAO calls this type of system a satellite-based augmentation system (SBAS). Europe and Asia are developing their own SBASs — the Indian GPS Aided Geo Augmented Navigation (GAGAN), the European Geostationary Navigation Overlay Service (EGNOS), and the Japanese multi-functional satellite augmentation system (MSAS), respectively. Commercial systems include StarFire and OmniStar.

The WAAS specification requires it to provide a position accuracy of 7.6 meters or better (for both lateral and vertical measurements), at least 95 percent of the time. Actual performance measurements of the system at specific locations show it typically provides better than 1.0 meters laterally and 1.5 meters vertically throughout most of North America. With these results, WAAS is capable of achieving the required Category I precision approach accuracy of 16 meters laterally and 4 meters vertically.

Upgrades needed
Having a stand-alone GPS receiver or even a GPS-equipped flight management system (FMS) does not automatically mean the system is WAAS capable. Most satellite navigation systems manufactured prior to 2007 will require upgrades to utilize maximum system capabilities. These changes can be as simple as a software upgrade or strapping change but may involve installation of a new GPS receiver and possibly a new antenna.

Although the physical alterations or modifications may not sound involved, the certification to satisfy Airworthiness Authorities may be more complicated. In many cases airframe manufacturers will issue service data that can be used as a basis for airworthiness approvals, but trying to upgrade an out of production model may become more of a challenge. Some satellite navigation receivers are self-contained within the navigation display unit and an upgrade may involve returning the device to the manufacturer or simply contacting an authorized representative to conduct the upgrade procedure.

Frequently in larger aircraft, the GPS receiver is an internal component of a FMS. Most changes in operating system software require an aircraft flight manual change or issuance of a supplement. Changes not approved by the original equipment manufacturer may influence the certification basis and knock the modified aircraft out of a “Group Certification Basis” which may potentially affect the aircraft resale value. Supporting a “one off product” can be an additional challenge, especially if the agency issuing the certification happens to go out of business.

WAAS up with that?

Jim Sparks has been in aviation for 30 years and is a licensed A&P. Currently when not writing for AMT, he is the manager of aviation maintenance for a private company with a fleet including light single engine aircraft, helicopters, and several types of business jets.

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