The Global Positioning System (GPS) is a U.S. military space system operated by the U.S. Air Force. The system is used for position location, navigation, and precision timing. It accomplishes this using three segments: (1) satellites, (2) ground control centers, and (3) receivers. The most common consumer application is giving real-time directions to drivers; there are thousands of popular government applications, along with some special public uses such as tracking.
Satellites are the first and most widely known segment of GPS. These 24 orbiting satellites are stationed 11,000 miles above the Earth. These satellites send one-way, time-tagged transmissions that radiate down over the entire Earth. This timing is produced by four atomic clocks located on each satellite. The Department of Defense officially named the satellites NAVSTAR, which stands for Navigation Satellite Timing and Ranging. These satellites were launched in phases, with the first phase of four satellites going up in 1978 and the final phase completing full global coverage 17 years later in 1995.
The second segment of GPS is the control segment, consisting of five tracking stations located across the world. The master control center is located at Schriever Air Force Base, in Colorado. This is home to the Air Force’s 50th Space Wing, the unit that provides command and control for defense warnings, along with navigational and communicational satellite tracking. The other four control centers are unmanned and controlled from the master control center. These four control centers are located in Hawaii and Kwajalein in the Pacific Ocean, Diego Garcia in the Indian Ocean, and Ascension in the Atlantic.
The location portion runs across the GPS, but all tracking services utilize some other technology to transmit the GPS coordinates. It is important to note that GPS by itself is not a tracking system. GPS receivers use the passive, one-way transmissions of signals from the orbiting satellites to determine a position fix. Nothing in the GPS infrastructure enables system operators to know who is using the signal or where they are. Conversely, a GPS unit cannot use the system to transmit its own location. For GPS to become part of a tracking system, it must be coupled with a communications device. Network-based positioning systems that use a telecommunications infrastructure can determine a user’s location. This location information is either available on the device or needs another technology like cellular to transmit. A GPS-equipped digital phone can also be used to track its location and movement. These phones provide users with greater control over their own privacy because such units typically allow users to block the transmission of the phone’s location information.
Looking at GPS security, one can start with its public usage debut. During President Reagan’s term, GPS was made freely available worldwide for commercial use. When the system was released, it was set up to beam down two signals: (1) the Precise Positioning Service (PPS) or Precision code (PC), and (2) the Standard Positioning Service (SPS) or the Coarse Acquisition code (CA). The dual signals arose from a national security concern of allowing near-perfect positioning to enemies. This PC code is the one the military encrypts with confidential cryptographic equipment, keys, and specialized receivers. The PC code also increases its accuracy by using a second carrier wave that allows receivers to measure the small delay caused by the signals having to move through the atmosphere. The other code (the CA code) was the one released to the public. It was less accurate and easier to jam. It is easier to acquire the CA code than the PC from the Earth; it is so easy that the military first homes in on the CA code. Once it has found and tracked the CA code, it authenticates using their cryptographic key, allowing them to transfer to the PC code. This CA code is defused by a technique called selective availability (SA). This technique allows the U.S. Government to modify the GPS signal accuracy in any place on Earth at any time. This selective availability could be used by the U.S. Government to change the course of an enemy military relying on GPS for navigation.
Although the Department of Defense tried to limit the accuracy of commercially available GPS, the GPS receiver companies soon found legal ways around selective availability. This technique, which had major involvement from the U.S. Coast Guard, is called Differential GPS. The technique worked by using the exact position of the base stations and comparing it with the base station’s location through the GPS CA code. Once the level of error was determined, it could be applied to the location of the receiver, making it almost dead accurate.
Well after a number of techniques similar to the one described above were created to get around the selective availability, a major event took place. This event changed all of these concepts and techniques. On May 1, 2000, President Clinton signed an order ending selective availability and making civilian GPS readers a lot more accurate. This led to the commercial end of techniques to get around selective availability, and also created the massive proliferation of commercial GPS devices.
There have been many U.S. Government documents, reports, and appointed teams tasked with researching vulnerabilities in the GPS. Most of this work leads to the notation that the main risks in GPS are signal jamming and spoofing. Looking at what kind of jammer one would need depends on the amount of space in which one wants to prevent the GPS signals. To have any real effect on the system, one needs a jammer capable of jamming up to 10,000 watts. This would mean one would need a large truck or plane. Luckily, for the military, the bigger the jammer, the easier it is to track down and destroy. The real threat to GPS comes from a large number of small, man-portable, 500- to 1000-watt jammers. With these jammers running simultaneously and spread out in a large area, an enemy could easily create a large GPS jamming capability. For this reason, the use of small wideband jammers is of great concern to the U.S. military.
