150 TECHNOLOGY BRIEF 5: GLOBAL POSITIONING SYSTEM
Technology Brief 5: Global Positioning System
The Global Positioning System (GPS), initially developed in the 1980s by the U.S. Department of Defense as a
navigation tool for military use, has evolved into a system with numerous civilian applications, including vehicle tracking,
aircraft navigation, map displays in automobiles and hand-held cell phones (Fig. TF5-1), and topographic mapping.
The overall GPS comprises three segments. The space segment consists of 24 satellites (Fig. TF5-2), each circling
Earth every 12 hours at an orbital altitude of about 12,000 miles and transmitting continuous coded time signals. All
satellite transmitters broadcast coded messages at two specific frequencies: 1.57542 GHz and 1.22760 GHz. The
user segment consists of hand-held or vehicle-mounted receivers that determine their own locations by receiving and
processing multiple satellite signals. The third segment is a network of five ground stations, distributed around the
world, that monitor the satellites and provide them with updates on their precise orbital information.
GPS provides a location inaccuracy of about 30 m, both horizontally and vertically, but it can be improved to
within1mbydifferential GPS. (See final section.)
Principle of Operation
The triangulation technique allows the determination of the location (x0,y
0,z
0)of any object in 3-D space from
knowledge of the distances d1,d2, and d3between that object and three other independent points in space of known
locations (x1,y
1,z
1)to (x3,y
3,z
3). In GPS, the distances are established by measuring the times it takes the signals to
travel from the satellites to the GPS receivers, and then multiplying them by the speed of light c=3×108m/s. Time
synchronization is achieved by using atomic clocks. The satellites use very precise clocks, accurate to 3 nanoseconds
(3×10−9s), but receivers use less accurate, inexpensive, ordinary quartz clocks. Consequently, the receiver clock may
have an unknown time offset error t0relative to the satellite clocks. To correct for the time error of a GPS receiver, a
signal from a fourth satellite is needed.
The GPS receiver of the automobile in Fig.TF5-3 is at distances d1to d4from the GPS satellites. Each satellite sends
a message identifying its orbital coordinates (x1,y
1,z
1)for satellite 1, and so on for the other satellites, together with a
binary-coded sequence common to all satellites. The GPS receiver generates the same binary sequence (Fig. TF5-3),
Figure TF5-1 iPhone map feature.
Figure TF5-2 GPS nominal satellite constellation. Four
satellites in each plane, 20,200 km altitudes, 55◦inclination.
