Page 156 - Planning and Design of Airports

124 Airp o r t Pl anning

Airport Surface Detection Equipment
In use in many of the busier airports within the United States and
elsewhere, specially designed radar called airport surface detection
equipment (ASDE), often referred to as ground radar, has been
developed to aid the controller in regulating traffic on the airport.
The system gives the air traffic controller in the control tower a picto-
rial display of the runways, taxiways, and terminal area with radar
indicating the position of aircraft and other vehicles moving on the
surface of the airport.
ASDE technologies are most commonly available in two forms.
The airport movement area safety system (AMASS) integrates third
generation (ASDE-3) ground-based radar systems with audio and
visual warning systems to prevent runway incursions on the airfield.
ASDE-X, a system that integrates ASDE technology with transponder
systems to identify the aircraft operating on the aircraft, has been
employed at less busy commercial service airports throughout the
United States. These systems are monitored by air traffic manage-
ment personnel in the airport’s ATCT.

Satellite-Based Systems: Global Positioning System
Perhaps the greatest impact on air traffic management since the
beginning of the twenty-first century has been the development,
acceptance, and proliferation of navigational procedures which rely
on the global positioning system.
The global positioning system (GPS) is a satellite-based radio
positioning and navigation system. The system is designed to pro-
vide highly accurate position and velocity information on a continu-
ous global basis to an unlimited number of properly equipped users.
The system is unaffected by weather and provides a common world-
wide grid reference system. The GPS concept is predicated upon
accurate and continuous knowledge of the spatial position of each
satellite in the system with respect to time and distance from a trans-
mitting satellite to the user. The GPS system consists of 24 satellites in
near-circular orbit about the earth. GPS receivers onboard aircraft
automatically select the appropriate signals from typically four satel-
lites which are in view of the receiver and translate these signals into
a three-dimensional position, and when the receiver is in motion,
velocity.
GPS was developed by the United States military to aid in recon-
naissance and strategic operations. Under military operations, sig-
nals transmitted from the orbiting GPS satellites were encrypted to
reduce the accuracy of positioning for nonauthorized users. This pol-
icy became known as “selective availability.” However, on May 1,
2000, President Bill Clinton ordered the removal of selective avail-
ability, allowing the civilian world to take advantage of GPS position-
ing accuracy on the order of 1 to 3 m. This level of accuracy has been