Ask Captain Lim ~ All about Aviation

GPS in Air Navigation and Air Safety

Flying has torn apart the relationship of space and time: it uses our old clock but with new yardsticks.

— Charles A. Lindbergh

How pilots navigate in the air?

The trick is using the GPS or Global Positioning System, our navigator, so to speak. For centuries, navigators and explorers have searched the heavens for a system that would enable them to locate their position on the globe with the accuracy necessary to avoid tragedy and to reach their intended destinations.

The GPS was developed by the USA military and operated by the Department of Defense. Besides aviation, the GPS is also used by other agencies for navigational purposes. They include the shipping, automobile and other industries where exact positioning on the globe is essential.

In aviation, the GPS receiver can be used to complement visual navigation or just general navigation in airways when it becomes impossible to locate one’s position visually due to weather or at night. However, GPS can also be used to guide airplanes to land in place of other less reliable aids like the NDBs (non-directional beacons).

The information one gets from the GPS to determine the one’s exact location is derived from 24 satellites orbiting 11,000 nautical miles above the earth. These satellites are placed at 6 orbital planes with 4 on each plane (6x4=24). The GPS constellation are designed by engineers so that the GPS receivers can observe at least 5 satellites from anywhere on the earth.

For normal navigation, a minimum of four satellites must be available and received to provide the position of the airplane. However, for landing approaches, where accuracy is of utmost importance, five satellites are needed.

The satellites broadcast signals that contain timing data, status data and satellite position. The airborne GPS receivers process the above signals for the exact location of the satellite and calculate how long it takes the signal to reach the receivers. The signals are processed through triangulation where the resultant airplane position is then determined. However, there is an error of between 100 to 200 feet, good enough for navigation at 35,000 feet but not good enough for precision landing onto airports.

The Department of Defense provides 2 levels of services. General aviation and ground GPS units use the first level. It has position accuracy of 100 meters or less with a probability of accuracy of 95 %. As stated, this is not suitable for GPS precision approach for guiding airplanes to land onto airports. The Military and some civilians however, use the other more precise level of services known as the Precision Positioning Services. This incredible new technology was made possible by a combination of scientific and engineering advances, particularly the atomic clocks that are precise to within a billionth of a second.

This precision services are extensively used in the US-Iraqi War. This precision level is intentionally downgraded to the other less accurate one by introducing errors to the system. This is to prevent terrorists and enemy nations from using this technology against friendly target. (Now you know how the US won the Iraqi War!)

The aircraft’s computers use the latitude and the longitude information from the GPS to navigate from one point to another. Once a particular route, say from New York to Dubai has been entered into the computer, it will direct the aircraft, through the autopilot, to fly straight or turn according to the route. This is executed automatically. There is no need for the pilot to look out and fly visually like the good old days when one must have exceptional eyesight to fly an aircraft!

How pilots prevent collision between airplanes in the air?

There are standard Rules applicable for all airplanes that operate in airways or outside them. Generally, the vertical separation between aircrafts is 1000 feet. Thus, aircraft flying East maintain an odd level whereas those flying West will fly an even level below a certain height.

However, as the airspaces become more congested, to accommodate more flights, certain airspaces have reduced the vertical separation at higher levels. Only aircrew that are certified are allowed to operate in these higher reduced separation levels. Further, only approved aircraft, of which the Boeing 777 is one of them, are allowed into such airspaces.

In areas where ground radar is available, the ground air traffic controllers control aircraft horizontal separation. Where no radar services are available, time restrictions are imposed, usually 10 minutes separation or about 70 to 85 miles apart, depending on the speed of the aircraft.

Traffic alert and Collision Avoiding System

In very congested airspaces where safe separation are insufficient, most modern aircraft, the Boeing 777 included, are installed with an equipment known as TCAS or Traffic alert and Collision Avoiding System that alert the pilots of any traffic that is on a collision course.

Basically, the TCAS is an airborne equipment that sends a signal to another aircraft that is also equipped with similar equipment. From the replies, the system analyze the intruding aircraft's altitude, closure rate, projected flight path to predict the penetration of the airspace above the threatened aircraft.

The system then displays the intruding aircraft visually in the cockpit instrument panel and aurally alerts the pilot, (such as, "Climb, Climb, Climb!' or 'Descent, Descent, Descent!) of the potential threat.

When the threat situation has passed, the aural alert 'Clear of Conflict!' will notify the pilot that the dangers have been averted.