Airborne Information for Lateral Spacing (AILS)
Automatic Dependent Surveillance-Broadcast (ADS-B)
Most major US airports have closely spaced, parallel runways. AILS is a proposed system to allow paired aircraft to autonomously approach and land on these closely spaced, parallel runways. Instead of an air traffic controller individually talking each aircraft down, an automated system would be put in place that would bring the local (tower) controller's view of the airspace surrounding the airport into the cockpit. This view would automatically give the pilot the navigational information necessary to keep the side-by-side landings safe. Because AILS would be an autonomous system, additional technology needs to be developed. That technology is called Automatic Dependent Surveillance-Broadcast (ADS-B) technology. ADS-B requires on-board equipment that broadcasts data to transceivers on the ground. The data is comprised of an aircraft's airspeed, altitude and whether the aircraft is turning, climbing or descending. The data appears on controllers' computer screens and also allows pilots to track other aircraft in the area on a cockpit display. ADS-B is now operational at Bethel in southeast Alaska.
During good weather, aircraft routinely land on closely spaced, parallel runways. As long as each aircraft knows its exact position as directed by the controller and there is good visibility, there are few problems in this procedure. However, performing such a maneuver under Instrument Meteorological Conditions (IMC), "errors" or "blunders" could cause a dangerous incident or accident. An "error" is a deviation from the expected. A "blunder" is where a totally unexpected event occurs, such as an approach to the incorrect runway. Using AILS would necessitate a need for precise navigation. Such precision in navigation can be achieved with Differential GPS (DGPS). DGPS is a satellite-to-ground-based navigation system. It uses the same GPS technology that was developed by the Unites States Department of Defense, but adds a ground-based reference station (a high-performance GPS receiver). Bear in mind that while GPS is a highly accurate worldwide system for navigation, it is still only accurate to within 20 meters. Other sources of error (which we will not go into now) limits GPS accuracy from 100 meters 95% of the time up to 300 meters 5% of the time. Greater accuracy is needed 100% of the time when flying aircraft. The accuracy problem can be overcome by making differential corrections using a reference station of a known, exact location. This receiver's antenna picks up the signals from the various satellites (just like any other GPS unit) and because it knows its own exact location, it can determine errors in the signals. It determines the error by making a comparison. It compares the measured ranges to each satellite (as received via the signal) to the actual ranges (as calculated from its known position). The difference between the measured and calculated ranges will provide the total error that is mathematically applied to the equation used for computing the location of the GPS unit. This corrected location message is then transmitted to the GPS user. This increased level of accuracy occurs because of the reference station-to-DGPS correction receiver exchange. Sophisticated on-board GPS receivers can achieve accuracy of 1 meter or less. When flying "side-by-side" landings on close parallel runways, this type of accuracy is critical to safety.
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