By James Webster, Capt. Anil Hariharan and Capt. Chris Mendoza, Global Positioning Systems Wing
/ Published March 02, 2010
LOS ANGELES AIR FORCE BASE, Calif. -- The first thought for most people when they hear the term "GPS" is a device that that gets them from place A to B. GPS stands for the Global Positioning System and started out as a US military system for space-based radio navigation. Over the past decade, the use of GPS has exploded beyond simple positioning to become a global phenomenon used in everything from financial transactions to cell phone calls. Despite how integral GPS has become to our everyday lives, few people understand what it is or how it works. Over the next few pages, we will give you a quick overview of who manages GPS and how it operates. Additionally, we will present a brief history of GPS and how it relates with other positioning systems planned or deployed by foreign countries. Finally, we will touch on how GPS is used by the Civil Engineering community and end with a look into the future plans of GPS.
What is the Global Positioning Systems Wing?
The Global Positioning Systems Wing (GPSW) is a joint service effort directed by the United States Air Force (USAF) and managed at the Space and Missile Systems Center (SMC), Air Force Space Command, Los Angeles Air Force Base in El Segundo, Calf.. This joint service includes many civil organizations such as the Federal Aviation Authority (FAA). The GPS Wing mission is to "deliver sustained, reliable GPS capabilities to America's warfighters, our allies, and civil users around the world. This is done by maintaining GPS performance today, fielding new capabilities, and developing more robust, modernized capabilities for the future."
What is GPS?
Broadly described, GPS is a space-based radio-positioning system comprised of at least 24 satellites. These satellites, along with the associated hardware, software, systems engineering, operations and sustainment are operated by the USAF and provide both the military and civilian communities Positioning, Navigation, and Timing (PNT) services free of charge.
There are a total of six GPS orbital planes, each with at least four satellites. GPS satellites are approximately 20,200 kilometers from the surface of the Earth in a Medium-Earth Orbit (MEO), which is nearly circular, and inclined to 55 degrees. GPS is based on the fundamental concepts of time, distance and the speed of radio waves (approximately that of the speed of light). The GPS signal is a radio wave that is emitted from each satellite and is basically the satellite's position and time. A GPS receiver on the ground "receives" these signals and based on the time the signals are transmitted and received, can calculate the distance from itself to the satellites and use that to formulate a positioning solution. It takes a minimum of four GPS satellites to determine a position on Earth. This position combined with geographical map or grids in the receiver, allows the user to discern their location on the earth.
The above methodology applies to all receivers, civilian and military. The GPS system is comprised of three major segments: Space, Ground, and User. The Space Segment consists of the GPS satellites that orbit the Earth. The Ground, or Control Segment, consists of the Ground Antennas (GA) and the Master Control Station (MCS) located at Schriever Air Force Base, CO. The User Segment consists of the antennas, processors, and receivers that process the GPS signals.
The GPS service consists of four authorized signals and four open signals operating over three frequency bands. Authorized signals are for federal use and are encrypted. Open signals are signals available to anyone to use free of charge. The first set of open signals are in the L1 band and are centered at 1575.42 MHz. The most widely used signal is the L1 C/A signal, currently used by almost all commercial receivers in the world today. The L1C signal will also be available on this frequency with the launch of GPS III, the latest generation of satellites, and will be available for public use around 2020. This signal will offer much improved performance from the current L1 C/A signal. The next open signal is the L2C signal, centered at 1227.6 MHz. This signal is currently being broadcast from eight satellites and will be available for use in approximately 2014. The final open signal is L5, which is centered at 1176.45 MHz. This signal is currently broadcasting in an experimental mode and will be available for general use in approximately 2016.
History of GPS
The concept of using satellites to obtain information originated during the earliest days of the Space Age. As an interesting note, in June 1959, Walt Disney studios released the short film "Eyes in Outer Space," which introduced mass audiences to the idea of a system of satellites utilized for weather forecasting and control. Although not specifically mentioned, the sophisticated (and as yet unrealized) weather control techniques depicted in the film imply an ability to coordinate communications between satellites and ground stations to precisely determine locations on the Earth's surface. Thus, at a time when public attention was already focusing on the manned flight "Space Race," the idea of improving the quality of life on Earth through satellites was already in the air.
In the fast-moving arena of aerospace in the 1960s, efforts to put these ideas into practical effect were not long in coming. The United States Navy Navigation Satellite System, known as Transit, became operational in 1964. At the same time, the other military services and NASA also became interested in developing satellite navigation systems. The first serious study of the possibility of a comprehensive satellite navigation system appeared in 1967, entitled "Study of a Navigation and Traffic Control Technique Employing Satellites," published by TRW Systems Group (now part of Northrop Grumman) of Redondo Beach, Calif.
The TRW study's purpose was to develop a concept for a navigation and satellite control system to initially provide coverage for the Atlantic region, with the option of expanding such coverage into a worldwide network. The report also included what may be the first recorded use of the term "NAVSTAR," which was an early synonym for the GPS system. The study proposed a network of only four satellites for Atlantic coverage, with the stated objective that "[i]f a user can view three satellites and knows his own altitude, he can calculate latitude and longitude." The basic framework presented in the study is mirrored in the present-day GPS system: Space, Control and User Segments.
The earliest uses and strongest interest in future development of GPS-type systems came from the military. In 1969, the Department of Defense moved to "consolidate the independent development efforts of each military service to form a single joint-use system." The responsibility for developing and deploying the "NAVSTAR GPS" system was assigned to the Air Force in 1973, and the GPS program was formally initiated in 1975 with the establishment of the GPS Joint Program Office (JPO), the predecessor to the GPSW (Joint Service Charter for the Management and Administration of the NAVSTAR Global Positioning System Acquisition Program, 26 February 1975). The term "joint" in the designation JPO reflected that the initial military uses of the system would be used by all branches of the U.S. Armed Forces, with eventual applications made available to the civilian user community. The first GPS satellite, named NAVSTAR 1, was launched from Vandenberg Air Force Base on Feb. 22, 1978. Three further launches followed by end of that year, and on Jan. 8, 1979 the original TRW vision of a four satellite constellation was realized when NAVSTAR satellites 1 through 4 were "set healthy" to successfully transmit navigation signals.
The 1980s and early 1990s were a time of continued development of the GPS constellation, resulting in a 24-satellite constellation in 1993 and Full Operational Capability of the system in 1995. A significant development in making GPS available to the general public was the discontinuance of "Selective Availability", which was the deliberate degradation of GPS signals for security reasons. This discontinuance occurred on May 1, 2000 at the direction of President Clinton, in order to "make GPS more responsive to civil and commercial users worldwide." (Statement by the President Regarding the United States' Decision to Stop Degrading Global Positioning System Accuracy, May 1, 2000). This resulted in high accuracy signals being available to civilians for an ever-widening field of private and commercial applications. Today, the GPSW supports a worldwide PNT service to the military and global civil communities.
Effect of Foreign Satellites
GPS is a type of Radio Navigation Satellite Service (RNSS). RNSS is a generic term for the science of using satellites to determine a position on the earth. GPS is the United States' contribution to the RNSS world and by no means is it alone. Other countries have or are developing their own RNSS technology. Currently there are four other global RNSS systems: GLObal NAvigation Satellite System (GLONASS) - Russia, Galileo - (European Union), COMPASS - (China), and Global Indian Navigation System (GINS) - India. Additionally, there are regional RNSS systems such as the Japanese Quasi-Zenith Satellite System (QZSS) and Indian Regional Navigation Satellite System (IRNSS) that provide PNT for specific region.
Of these systems, the only other operational RNSS system is GLONASS. GLONASS has been operational since 1991 and nominally consists of 24 satellites in three orbital planes. The other three RNSS systems are still in development. The Galileo and COMPASS systems currently have experimental satellites in orbit and plan to be operational by 2013 and 2015 respectively.
Each system has its own benefits, but in an effort to reduce the "congestion" in the available spectrum and to improve the user's performance, the United States spearheaded an effort to define signals that would be common among all the RNSS systems. This effort led to the development of the L1C and L5 signals. These "interoperable" signals can be broadcasted from every RNSS satellite and theoretically provide users up to 120 satellites to receive services from, thereby significantly improving availability and accuracy.
GPS Applications to Civil Engineering
GPS is used extensively in the Civil Engineering community for construction, surveying, mapping, site exploration and a myriad of others due to its ability to provide high accuracy positioning. For example GPS surveying instruments offer measurements in 3D (X, Y, and Z planes) that enable a surveyor to take measurements accurate up to a millimeter from every point on the area. Another example of how of GPS is used in Civil Engineering com is the use GPS to monitor the structural integrity of highways, bridges, and dams. For instance, three GPS receivers were used to monitor the shifting and deformation of the Pacoima Dam. This allowed the builders to use new types of risk mitigation measures to further ensure the safety of the inhabitants downstream from the dam.
High precision measurements are required in order to conduct these activities with the accuracy they require. GPS solutions from stand-alone receivers are accurate to within a few meters. Better accuracies, on the order of a meter or so, can be obtained through the use of differential GPS (DGPS) techniques that employ pseudo-range measurement corrections supplied by external reference systems at operating at surveyed locations. GPS solution accuracies of a centimeter or better are commonly achieved using kinematic carrier phase tracking (KCPT) techniques which -- although somewhat similar to DGPS with information being supplied by external reference systems - are made with high-end GPS receivers which take very precise measurements of the phase of the incoming L-band carrier signals in addition to the standard pseudo-range measurements.
Receivers can often take 20 to 30 minutes to produce an accurate KCPT position solution using the C/A-code signal on L1. A popular strategy is to just accept the delay. This is frequently the best choice, especially when the KCPT solution can be done after-the-fact. However, there are significant benefits to be had in many applications (road construction, agriculture, mining, etc.) to obtain positioning to a centimeter or better in real time. This can be done by obtaining additional KCPT measurements on the military's Precise/Encrypted (P(Y)) code signal broadcast on the L2 carrier. These "codeless" and "semi-codeless" techniques for kinematically tracking the broadcast L2 carrier phase despite the presence of the P(Y)-code signal on the L2 carrier allows for real time, high precision receivers.
GPSW's Plans for the Future
The GPSW is in the process of modernizing GPS. The GPS Block IIF satellite, due for launch in 2010, will allow enough satellites to bring the first modernized signal, L2C, into operational use. The Block IIF will also introduce the second new modernized signal, L5 that was specifically designed for the aviation community to provide safety of life navigation. The future of GPS satellites that is currently in development, GPS III, will introduce a third modernized signal, the L1C. This signal is designed to allow for faster acquisition and better multipath performance than the current L1C/A signal. In order to efficiently provide these modernized signals to the user, the GPSW is updating the ground control segment, known as OCX. These new capabilities will ensure increased signal availability, accuracy and integrity of GPS.
GPS is a very fascinating concept and an incredibly useful technology that it supports both civilians and the military, and is a free service available to all users, which is provided by the United States Government. GPS has saved countless lives, solved crimes, located missing persons, and is used in countless applications around the world included civil engineering. GPS has become the mainstay of transportation systems worldwide, providing navigation for aviation, ground and maritime operations. GPS has provided positioning services as can be seen in the emergence of "telematics" by locating automobile accidents, unlocking doors, or slowing down vehicles for police when vehicles are stolen. GPS has also provided timing services as can be seen in banks, ATMs, and cell towers.
The U.S Air Force and Air Force Space Command have been the diligent stewards of GPS since its conception in the 1970s and continue its commitment to this critical component of our National Infrastructure. The current GPS constellation has the most satellites and the greatest capability ever. The Air Force is committed to maintaining the current level of service, while striving to improve service and capability through on-going modernization efforts.