Introduction - Understanding GPS: Principles and Applications - page 12

gain significant advantage with integrated GPS, communications, moving maps,
and database technology for more efficient tracking and dispatch operations. One
concept employed is called geofencing , where a vehicle's GPS is programmed with a
fixed geographical area and alerts the fleet operator whenever the vehicle violates
the prescribed “fence.”
Since the writing of the first edition of this topic, recreational usage has
increased tremendously. A variety of low-cost GPS receivers are available from
many sporting goods stores or through various Internet sources. Some have a digi-
tal map database and make an excellent navigation tool; however, the prudent user
will still carry a traditional “paper” map and magnetic compass in the event of bat-
tery failure or receiver malfunction. Some recreational users participate in an
adventure game known as geocaching [18]. Individuals or organizations set up
caches throughout the world and post the cache locations on the Internet. Geocache
players then use their GPS receivers to find the locations of the caches. Upon finding
the cache, one usually signs the cache logbook indicating the date and time when
one found the cache. Also, one may leave an item in the cache and then take an item
in exchange.
Many of the world's military ground forces are GPS-equipped. Depending on
the country and relationship to the United States, the receiver may be either SPS or
PPS. Numerous countries have signed memoranda of understanding with the U.S.
DOD and have access to the GPS military signals.
1.9.2 Aviation
The aviation community has propelled the use of GNSS and various augmentations
to provide guidance for the en route through precision approach phases of flight.
The continuous global coverage capability of GNSS permits aircraft to fly directly
from one location to another, provided factors such as obstacle clearance and
required procedures are adhered to. Incorporation of a data link with a GNSS
receiver enables the transmission of aircraft location to other aircraft and to air traf-
fic control (ATC). This function, called automatic dependent surveillance (ADS), is
in use in various classes of airspace. In oceanic airspace, ADS is implemented using a
point-to-point link from aircraft to oceanic ATC via satellite communications
(SATCOM) or high-frequency datalink. Key benefits are ATC monitoring for colli-
sion avoidance and optimized routing to reduce travel time and, consequently, fuel
consumption. ADS techniques are also being applied to airport surface surveillance
of both aircraft and ground support vehicles.
A variant of ADS is automatic dependent surveillance-broadcast (ADS-B). This
service employs a digital data link that broadcasts an aircraft's position, airspeed,
heading, altitude and other information to multiple receivers on the ground as well
as to other aircraft. (The ADS-B datalink can be thought of as a point-to-many link.)
Thus, other aircraft equipped with ADS-B as well as ground controllers obtain a
“picture” of the area air traffic situation. At the time of this writing, the U.S. Federal
Aviation Administration (FAA) had implemented ADS-B and related data link tech-
nologies in a collaborative government/industry program called Safe Flight 21. The
Safe Flight 21 initiative focuses on developing the required avionics, pilot proce-
dures, and a compatible ground-based ADS system for air traffic control facilities.