Ground Radar Doesn’t Tell Whole Story
Ground radar is a relatively unsophisticated piece of equipment that will tell an airport controller that something is moving on the ground, but it will not tell the controller whether the moving object is an aircraft or a truck. And it will not automatically warn the controller if a collision on the ground is likely to occur.
Designed to help flight controllers keep track of ground traffic at busy airports when visibility is reduced by such things as fog or rain, ground radar “provides an opportunity to know where an aircraft is (on the ground,) versus where a pilot might think he is,” said Fred O’Donnell, spokesman for the regional office of the Federal Aviation Administration.
Los Angeles International Airport is one of 13 airports in this country with ground radar systems, but its system was not working when a USAir jetliner crashed into a commuter aircraft on a runway Friday night.
Essentially, all radar works on the same principle, whether its purpose is to help harried flight controllers sort out approaching aircraft or guide a missile to its destination.
Radar, an acronym for radio detection and ranging, bounces high-frequency radio waves off a target and then receives an echo. Aircraft and ships use radar to keep track of weather systems--which also reflect the signal--as well as other traffic in their area. The length of time it takes for the signal to bounce back tells the system how far away the object is.
The primary tool used by air traffic controllers is a sophisticated radar system that sweeps the skies and continuously feeds them critical data. Since its inception in the early 1930s, radar has grown increasingly complex, and today’s radar systems can tell much about approaching aircraft.
“Radar gives us information in terms of altitude, speed of aircraft, direction of flight, plus a collision alert capability that recognizes the potential for conflict,” O’Donnell said.
Ground radar differs from air radar in that its antenna sweeps the ground instead of the skies, telling controllers what is going on along the many taxi strips and runways at a busy airport such as Los Angeles International. But it is more limited than its sky-watching partner in that it will only tell the controller that something is there, not what it is.
“Basically, it’s a surveillance radar,” O’Donnell said. The ground radar screen has an outline of the airport superimposed on the screen. As the radar antenna sweeps around the airport it detects planes or other vehicles on the ground and allows the controller to know precisely where they are.
It does not provide the kind of information available from radars that sweep the skies. It is not designed to automatically alert controllers that a crash is likely.
HOW RADAR WORKS
A radar system transmits radio waves which echo back from a distant target. The time it takes the signal to bounce back reveals the distance to the object.
AIR - The more sophisticated aerial radar sweeps the skies and tells flight controllers the distance, speed and course of aircraft in the area. The transmitter (1) sends a signal to the antenna (2) which transmits it to the aircraft (3). The signal bounces back from the aircraft and the echo is received by the antenna and sent to a computer (4) which displays the data on the controller’s screen (5).
GROUND-The ground radar system at Los Angeles International Airport uses an antenna on top of the tower to scan the airport. It tells controllers the location, but not the type, of vehicles on the airport grounds. It was not working at the time of Friday’s crash.
