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THE GLOBAL POSITIONING SYSTEM (GPS)

December 1998 IBEW Journal

What a marvel to navigate a course on land, sea or air using the global positioning system (GPS). GPS is a radionavigation system consisting of three basic parts: a satellite component consisting of 24 Navstar satellites orbiting the Earth 11,000 nautical miles away; a ground-based monitoring component consisting of five monitoring stations, three ground antennas and a Master Control Station (see map); and a user component consisting of antennas and receiver/processors that report position, velocity and precise timing to the user. In summary, working together these GPS components can precisely locate the user’s position anywhere on the globe.

The preciseness of the GPS measurements varies, depending on the level of service that the user is permitted to access. The Precise Positioning Service (PPS) is available to military and other authorized users and is the most accurate. The Standard Positioning Service (SPS) is available to civilian and commercial users. In addition, a technology developed by the U.S. Coast Guard can augment the precision of SPS. This error-correcting technology is called the differential global positioning system (DGPS).

History of GPS

The U.S. Department of Defense (DOD) began construction of a sophisticated satellite positioning system in the mid 1970s to permit military ships, aircraft and ground-based vehicles to determine their location anywhere in the world. DOD began launching Navstar GPS satellites in the late 1980s. In April 1995 the U.S. Air Force Space Command (AFSC) announced that the constellation of 24 Navstar GPS satellites was fully operational.

For purposes of U.S. national security, DOD developed an encryption technology to guard the satellites’ signals from being corrupted. This technology called selective availability encrypts the signals by introducing slight error in the satellites’ atomic clocks, a timekeeping component. With PPS, the military and other authorized users can decode the encrypted signals to get precise measurements and time. Position accuracy for the military is within 15 meters. With SPS, civilian and commercial users do not have access to decode the encrypted signals, so the receiver’s time, velocity and position calculations are not as precise. Position accuracy for SPS users is within 100 meters.

Components of GPS Satellites Each satellite weighs approximately 2,000 pounds, provides 24 hours of daily coverage, and has a lifespan of 7.5 years. Each satellite is inclined at an angle of 55 to the equatorial plane and orbits the Earth in a circular path every 12 hours. There are four satellites in each of six different orbital planes, equally spaced 60 apart.

These satellites are tracked by five ground stations, three antennas, and a Master Control Station (MCS) at Schriever Air Force Base in Colorado. The MCS collects data from the five ground stations and calculates each satellites’ orbital data (ephemeris), frequency drift, frequency error, and clock time error. The ground antennas upload this information to the satellites for the satellites to broadcast accurate position and time.

Each GPS satellite has a main body and two wing-type solar arrays.

The main body is made up of about 65,000 parts, including 12 antennas, four atomic clocks and a transmitter. The transmitter broadcasts the timed electromagnetic signals (or radio signals) at high frequency reaching to all areas of the Earth. SPS transmits on the L1 frequency (1575.42 MHZ), and PPS uses the L1 and L2 frequencies. The L2 frequency is 1227.6 MHZ.The signals are encrypted with time and navigation data. Antennas are positioned toward the Earth and pick up orbit-correction data computed from the Master Control Station and uploaded by the ground antennas. The atomic clocks used in the GPS system are referenced to the United States Naval Observatory’s (USNO) Master Clock, which follows Coordinated Universal Time (UTC). UTC replaces Greenwich Mean Time (GMT) as the world’s official time. Each satellite has four clocks in case one stops or malfunctions.

On either side of the satellite’s main body are wing-type solar arrays. These are positioned toward the Sun and convert the solar energy to approximately 710 watts of electricity, which activates the satellites’ transmitters to broadcast timed navigation pulses to GPS receivers.

 Components of GPS Receivers

There are two types of GPS receivers, roving and stationary. Each GPS receiver has an antenna and a timekeeping component such as a quartz-crystal clock. The antenna receives the satellites’ transmitted radio signals. The timekeeping component aids in a receiver’s rapid internal calculations of the signals’ departure times from any four Navstar satellites and their arrival times at the receiver’s antenna. These complex calculations result in accurate position, velocity, and time — latitude, longitude, altitude, course and speed.

1) A receiver’s position is determined by calculating the distance between a receiver and any four satellites. The distance is equal to the signals’ travel time multiplied by the speed of light. Travel time is the difference between the time a signal departs from a satellite and the time it arrives at the GPS receiver. The distance measurements from the four satellites are then compared and adjusted to coincide with the satellites’ orbit-correction data.

(2) A receiver’s velocity is determined by calculating the changes in the receiver’s position over time and/or by the satellite’s Doppler frequencies.

(3) A receiver’s time is set to GPS satellite atomic clock time.

 Innovative GPS Applications

IBEW members have been very involved in GPS technology for several years. Working at the Rockwell Collins Government Systems’ plant in Coralville, Iowa, members of IBEW Local Union 1634 manufacture military standard handheld and vehicular, aircraft and missile GPS receivers. IBEW Local Union 1362 also manufactures GPS receivers at the Rockwell Collins plant in Cedar Rapids.

Both individuals and industries are utilizing the global positioning system in numerous ways. GPS can guide hikers over unknown terrain, pleasure boats into foggy harbors and passenger cars along unfamiliar routes. The system assists in commercial aircraft navigation, monitors the movement of cargo containers on ships in port, and assists the railroad industry in mapping the location of railroad tracks, their structures and features. Currently, the railroad industry is involved in testing different GPS applications. Geologists use GPS surveying equipment to measure small movements in the Earth’s crust and to locate areas prone to earthquakes. Farmers rely on GPS data for precision farming, which is a technique for applying agricultural products to specific sites on a field through the aid of a digitized map showing the varying soil characteristics of that field.

The twenty-first century holds great promise for continued exploration of ways to utilize the information provided by that constellation of 24 Navstar satellites orbiting the Earth 11,000 nautical miles away.