GPS Timeline Summary

The events involved in developing technologies needed for the original GPS, and in establishing the joint GPS program under the Air Force leadership, are described in Chapter 1 of this book, along with other references. This section summarizes early history as well as describing more recent events.

On-orbit testing of GPS began in 1974 and continued through 1985, as enough test satellites were launched to validate the underlying concepts and assure that performance expectations would be achieved or exceeded.

In 1983, shortly after the Soviet Union shot down Korean Air Flight 007 when the 747’s guidance system malfunctioned and it wandered into prohibited airspace, President Reagan recommitted the United States to provide free civil use of GPS for improved navigation and air safety. Later in the 1980s, deployment of the GPS constellation was slowed by funding challenges, combined with the 1986 Space Shuttle Challenger disaster - the plan had been to deploy operational GPS satellites using the space shuttle, but after the Challenger explosion, the decision to use Delta II rockets introduced a two-year delay.

In 1990, Selective Availability (SA) and anti-spoofing (AS) capabilities were introduced. SA degraded the accuracy of the signal available to civil users, while AS cryptographically protects the military signal.

Operation Desert Storm in 1991 involved the first significant use of GPS in military operations. Ground troops used GPS to maneuver with unprecedented accuracy through featureless desert terrain, in sandstorms, and in the dark. GPS receivers were also used in aircraft to enable more accurate air strikes, and to enhance combat search and rescue. Since there were not sufficient numbers of military receivers, thousands of civil receivers were purchased and employed. Consequently, SA was temporarily turned off to enhance the accuracy provided by these civil receivers.

The Desert Storm experience led the military to more fully recognize the benefits of GPS. GPS began to be widely integrated into systems, operations, and tactics, eventually influencing the US military force structure. In 1993, the 24th operational satellite was launched and brought into use, leading the Department of Defense (DoD) and Department of Transportation (DOT) to announce Initial Operational Capability (IOC). By 1995, 27 satellites were operating, leading to declaration of Full Operational Capability (FOC).

Meanwhile, the US Coast Guard began development of differential GPS (DGPS) service for coastal coverage of the continental United States, the Great Lakes, Puerto Rico, portions of Alaska and Hawaii, and portions of the Mississippi River Basin. Maritime DGPS uses fixed reference stations with radio beacons that broadcast corrections to receivers. The receivers use the corrections to remove errors from their measurements, enhancing accuracy. IOC for maritime DGPS was declared in 1996, and the system was expanded subsequently to provide nationwide coverage.

Almost simultaneously, in 1994, the DOT and FAA began to develop the Wide Area Augmentation System (WAAS), a nationwide differential correction service whose signals are broadcast by transponders on geostationary satellites, using signals like the GPS C/A-signal. The primary motivation for WAAS was to provide enhanced accuracy and integrity to aircraft equipped with satnav receivers. Aircraft equipped with certified GPS/WAAS receivers could perform precision approach and landing without the need for infrastructure at every airport. The WAAS signal was activated for general aviation, covering much of the continental United States as well as portions of Alaska.

WAAS became the first Satellite-Based Augmentation System (SBAS). As described in Chapter 13, other SBASs are deployed and under development. Although each system is regional, their combined service regions promise to cover most of the Northern Hemisphere’s landmass, with increasing coverage of the Southern Hemisphere as well. All SBASs conform to a set of standards maintained by the International Civil Aviation Organization (ICAO), allowing the interoperability of receivers with different SBASs.

Ironically, the corrections provided by DGPS and SBAS remove the effects of SA. Thus, in parallel with the development of SA by DoD, civil agencies were investing in technologies that counter it. In May 2000, at the direction of President Bill Clinton, the US government discontinued its use of SA to make GPS more responsive to civil and commercial users worldwide.

In September 2007, the US government announced its decision to procure the future generation of GPS satellites, known as GPS III, without SA. This statement makes the policy decision in 2000 permanent, and eliminates a source of uncertainty in GPS performance that had been of concern to civil GPS users worldwide.

While the original GPS was still being deployed and uses were still developing, the National Research Council performed a seminal, forward-looking study. Among the recommendations in the study’s report are the following: eliminating SA, adding a new civil signal to provide dual-frequency civil service, adding more monitor stations, and evaluating prospects for a new military signal. Similar recommendations followed from other groups. These recommended GPS enhancements led to what is now called GPS Modernization.

In 1996, recognizing the importance of GPS to civilian users as well as military users, US President Bill Clinton issued a policy directive. This directive reaffirmed that GPS is a dual-use system. It also established a governance structure, the Interagency GPS Executive Board (IGEB), led by representatives of the DoD, and the Transportation Department, to manage GPS as a national asset.

The timeline for GPS Modernization thus begins with turning SA off in 2000. Subsequent events, described in more detail in subsequent sections, involve adding new civil signals, new military signals, and new monitor stations. Related upgrades and replacements to GPS satellites, the GPS ground segment, and military GPS receivers are ongoing. It is increasingly apparent that GPS Modernization is not an event or even a phase in the history of GPS, but an ongoing process that will continue indefinitely.

Yet another event in the 1990s turned out to have great influence on the 21st century, and on satnav more generally. In recognition of the increasing use of mobile devices, the Federal Communications Commission introduced requirements that mobile phones become “location-capable” when making emergency E911 calls - reporting position to first responders. While the requirements were technology-agnostic, there was a widespread selection of GPS as the most desirable way to meet this mandate.

The result was two important contributions to the utility and use of GPS:
- The development of Assisted-GPS (A-GPS), which makes consumer device use of GPS more responsive and more robust,

- Significant investment in small, inexpensive, and low-power GPS chipsets that drastically increased the utility and utilization first of GPS and then other satnav systems.

GPS history since 2000 has had two primary focuses: implementing GPS Modernization and adapting to the internationalization of satnav. The details of GPS Modernization are described in Sections 3.1.2 through 3.1.5. Internationalization, however, is described below.

While GPS has always provided service to an international set of users, GPS’s international interactions expanded in the 21st century to include a leadership role with other satnav system operators. Although GPS and GLONASS had coexisted in the 1990s, there was little interaction between the two service providers. Not even spectrum coordination was needed since GLONASS and GPS signals use distinct frequencies. As Europe’s development of Galileo began after the turn of the century, however, two realizations occurred in the United States:

- It became clear that GPS and GLONASS would be joined by other satnav systems, with their signals sharing the limited frequency spectrum allocated by the International Telecommunications Union (ITU) for Radionavigation Satellite Service (RNSS) use,

- It was obvious that receivers could benefit by using signals from multiple satnav systems.

These realizations led to a US decision to catalyze international efforts to coordinate compatibility and interoperability among satnav systems. Negotiations between the United States and European Community, which started in 2002, led to an agreement on GPS-Galileo cooperation.

The work during these negotiations introduced the formal concepts of compatibility (multiple signals and systems can be used separately or together without unacceptably degrading or otherwise affecting use of an individual service or signal) and interoperability (receivers and applications can use multiple signals and multiple systems to obtain better capability than could be obtained using only one signal or system). Methods for assessing radio frequency compatibility developed by GPS were subsequently formalized for international use.

Spreading modulations originally developed by GPS and subsequent variants proved valuable in designing signals that are compatible and interoperable, while offering superior performance. The design effort for the newest GPS signal, L1C, included consultation with experts representing other satnav systems leading to widespread adoption of similar signal designs to enhance interoperability.

These contributions from GPS have formed the basis for bilateral coordination and technical working groups between GPS and other satnav systems, as well as between other pairs of systems. In addition, the International Committee on Global Navigation Satellite Systems (ICG), founded in 2005, has provided a forum for multilateral discussions not only among satnav providers, but also with users across the world.

Internationalization also continued to affect US policy. In 2004, President Bush updated relevant US policy. Even the title is notable, referring to PNT rather than GPS in the earlier policy. This new policy recognized foreign satnav systems and the need for compatibility and interoperability, extensively addressing interactions and relationships with foreign systems. The policy also replaced the IGEB with a National Executive Committee for Space-Based Positioning, Navigation, and Timing, defining higher level co-chairs than IGEB leadership.

President Obama’s 2010 National Space Policy took further steps toward internationalization, permitting the use of foreign PNT services to augment and strengthen use of GPS. The 2011 National Security Space Strategy went even further, seeking collaboration with partners to augment US capabilities, sharing risk and cost.