Today, the civil positioning, navigation, and timing (PNT) community needs a clear and concise statement of PNT needs. This statement should incorporate all aspects of PNT services and all applications and modes of PNT use. With such a statement, providers of PNT technology and services will be better able to plan for, develop, and implement their PNT services. While I offer my thoughts on this need for the United States, the arguments I present are useful for consideration by other nations as well.

The tremendous success in GPS has brought to the fore a common positioning and timing capability, as well as universal reliance on these services. Along with this increased reliance on GPS has come more stressing demands in PNT services, such as landing aircraft in all weather conditions, implementation of positioning accuracy with integrity for intelligent transportation applications, and PNT services in urban environments.

The United States and other nations have been looking at ways to consolidate these needs to reduce unique solutions and promote a more general approach, in which common services are used for many, though not all, positioning and timing needs. In order to determine which common services must be applied, a unified statement of needs is required. This can be provided in a PNT requirements document.

The challenge of civil requirements

For years, the US military has had a formal process for collecting and documenting requirements. In this process each military branch is asked to identify its mission shortfalls, that is, those capabilities not met by current equipment or services. These are called operational requirements. These operational requirements are rolled into a common document, called the capability development document, which is used as the basis for procuring systems, equipment and services to meet operational needs. The US military recognized early on that despite the diversity in missions of its many service branches, there is much commonality in what these service branches do, and thus military leaders encouraged the development of common solutions.

US civil agencies have typically not followed this model, however. Historically, each agency is separate from the others, having differing missions and objectives. US government departments and agencies were created at different times and for different purposes. Some departments began quite early, such as the Departments of Agriculture (1862) and Commerce (1903). Others came later as needs developed, such as Transportation (1966) and Homeland Security (2002). Each has by and large operated independently of the others, and in ways that fit its own purpose and scope. With the common PNT service provided by GPS, however, came the common purpose of civil agencies to balance their needs against those of the military. In the mid-1990’s after GPS became operational, the US Government took steps to preserve through executive order and law the dual use nature of GPS to serve both military and civil interests. The White House under President Bill Clinton formed the Interagency GPS Executive Board to oversee the policy matters of GPS, and assigned to the US Department of Transportation the role of coordinating the needs of the civil departments and agencies. This was strengthened by US law in 1998 when Congress enacted US Code Title 10 Section 2281 directing the US Government to sustain and operate GPS for military and civilian purposes. The White House under President George W. Bush extended the role of the Department of Transportation to cover all space-based PNT services in 2004 with the establishment of the Space-Based Positioning, Navigation, and Timing Executive Committee to replace the Interagency GPS Executive Board.

The US Department of Transportation was placed then into a somewhat artificial leadership role in the area of space-based PNT services. By use of the word “artificial”, I do not mean its authority was not real, but rather that its role was “unusual”, not following the normal protocol between US government agencies. The US Department of Transportation’s leadership role in PNT is somewhat that of “herding cats”, for despite the authority granted the US Department of Transportation, each department and agency still tends to pursue its own goals and interests. In fact, in areas other than PNT, seldom do civil departments and agencies have to unify in purpose, motive, and implementation. Thus despite this effort for unity, the civil agencies continue to operate separately and uniquely in support of their own mission needs.

So even though GPS began as a uniform PNT service, each department overseeing its own areas of interest (e.g., aviation, maritime, space) has developed unique augmentations to GPS to support its own particular mission. The FAA developed the Wide Area Augmentation System, or WAAS, to support aviation users in the US national airspace. The US Coast Guard developed the Maritime DGPS service to support maritime users in US coastal waterways and inland river valleys. The Federal Railroad Administration advocated for the expansion of the US Coast Guard system into a Nationwide DGPS service to support railway applications. The US Department of Commerce through its National Oceanic and Atmospheric Administration’s National Geodetic Survey implemented the Continuously Operating Reference Stations (CORS) network to support surveying applications. And the National Aeronautics and Space Administration has implemented a global differential GPS service to meet the needs of its department’s space and terrestrial research. The pattern is clear. Each agency has adopted GPS, yet each has uniquely adapted it to meet the specific needs of its constituent user base.

What has actually occurred, however, is that these augmentation systems have expanded to support users outside their original sphere. WAAS is used in agriculture. The Nationwide DGPS service supports agriculture, emergency service, and transit needs. The CORS network is used by the National Weather Service to determine precipitable water vapor. And the NASA global differential GPS service is employed by land-based scientists worldwide in research and scientific investigations benefiting our planet. The systems and services overlap each other like threads of a cloth, tightly woven together and covering the world with space-based PNT services.

 The Federal Radionavigation Plan

In 1980, the US Government began the process of unifying the radionavigation-based PNT services used by the disparate military and civil government agencies through the publication of the Federal Radionavigation Plan. Issued biennially, this document provides a unified statement of the needs and capabilities for radionavigation services for US military and civil agencies, – whether for aviation, maritime, highway, rail, timing, or static and dynamic precise positioning applications.

Perhaps as an indicator of the difficulty in coordinating among all these departments and agencies, the release dates of the Federal Radionavigation Plan are telling. Although congressional direction has been to issue these documents “not less often than every two years”, two of the last three issues have been late, with the 2005 edition arriving four years after the 2001 edition, followed by the 2008 edition three years later. Clearly this is a difficult job.

Despite these difficulties, there is much benefit to coordinating and harmonizing agency needs. When government departments and agencies work together, they can establish and attain common objectives more easily and with greater cost savings and efficiencies.

 The US National PNT Architecture

The most recent effort to coordinate between the various departments and agencies has been the National PNT Architecture [[]] development, intended to unify the military and civil interests and needs for PNT services. This effort has been in progress since 2006. The architecture development resulted in a set of recommendations to guide US policy in growing its PNT capability over the next 20 years. It was developed with the cooperation of 31 US government departments and agencies having a stake in PNT services. The National PNT Architecture team identified an overarching strategy of “Greater Common Denominator”, meaning the US Government would support the development of common PNT services and capabilities while accommodating specialized solutions which otherwise would be either inefficient or inappropriate to provide using a common solution. This strategy is a head nod to the practical effect the common GPS service has had to users worldwide for over 20 years. Prior to GPS, precision navigation systems were high cost and often limited in range and capability. GPS was a game-changer, in which PNT users were given access to precise, low-cost PNT services anywhere. With the advent of GPS came its rapid incorporation into numerous applications, — surveying, timing, maritime operations, aviation, land navigation, and agriculture, to name a few. GPS thus became the first instantiation of a “greater common denominator” approach in PNT, in which everyone had access to a core service with a common interface.

 A Civil PNT Requirements Document

While the National PNT Architecture has taken steps to unify US Government strategy and vision, practical steps are still needed to move PNT into the future. While the military moves forward with their capability development document, there is yet no single document that consolidates all PNT needs of civil departments and agencies. Is such a document needed? If there is to be any progress towards unifying the development and deployment of PNT services, the answer must be Yes.

I do not think the issue is so much about harmonizing all the applications between the various civil missions. After all, what the aviation community needs in operating aircraft is radically different from the needs of the surveying community. The issue rather is how to move forward with new common PNT services, most notably those provided in areas where space-based radionavigation services today are not available. These are predominantly in urban areas and inside buildings. As it had done with GPS, the US Government may be able to develop and operate a common PNT service for urban areas or inside buildings. Or perhaps it can foster the development of this capability by commercial interests, in much the same way it has supported the development and use of the airwaves for cellular voice and data services.

One significant way the US Government can know what is needed by its populace is to collect in one place the current and future PNT needs of civil users. This can be done by the development of a civil PNT requirements document. Civil agencies should overcome their predilection to “go it alone”, and seek to cooperate in a way to provide a uniform and consistent statement of PNT needs for all users.

 Features to consider

As the US Government looks to document these needs, I believe there are several key areas to consider.

  • Support for the larger mission. There will be a tendency for civil agencies to focus only on their user needs, and to secure solutions that are very narrow in focus. This attention to their user base is well placed, but if left to such a narrow focus, it threatens to scuttle the opportunity to serve the larger mission of PNT services. The lessons of GPS, the ultimate “greater common denominator” PNT service, should not be lost. If the US Department of Defense had not taken the radical step to offer a dual-use service that could be used by civil as well as military users, then the renaissance of GPS would not have occurred, and this readily accessible, ubiquitous service would not exist today. But provide it they did, and the world is better for it. Failure to learn this lesson could result in another “dark age” period in which the development and deployment of PNT capabilities becomes unnecessarily arduous and limited in capability.
  • Standardized definitions. Need for standardized definitions of PNT services, notably positioning and timing accuracy, availability, continuity of service, integrity (including alarm limit and time to alert), and coverage area. There is a tendency for individual applications to use their own lingo when describing their needs. The value to standardization of definitions is that it provides a common set of metrics against which PNT services can be developed and evaluated against. Although individual agencies may not think in these terms, they should support the transformation of application unique formats and services into this normalized set of definitions.
  • Time phased needs. In defining needs, consider capabilities that meet not only current PNT needs, but future needs. For example, the GPS L5 signal came out of the desire to support aviation needs into the new century. Likewise, future highway needs require accurate positioning with integrity and improved map databases, to alert drivers of dangerous curves, approaching traffic stops, and congested areas. Eventually with further refinement of PNT services there could even be autonomous vehicle operation. Some needs are immediate, some will be required within the next ten to twenty years, and some may not occur for forty years or more.
  • Implementation independence. When defining needs, the scope of the need must be independent of a solution. It is not appropriate to state that a need is for “better WAAS” or “better Nationwide DGPS”, or even improved GPS. The need should rather be stated in a form that describes the accuracy required, the availability of the accuracy, the area of operation and so on. How the need can be implemented would be determined later once the individual PNT needs have been identified.

 Final thoughts

To support the expanding demands for PNT services into the future, civil PNT requirements need to be identified. Civil agencies and departments can do this by supporting a standardized set of terms for PNT services, and state their needs without regard to how they will be implemented. This method worked well for the US military in developing and fielding GPS, and it will work well for civil departments and agencies in fostering the next generation of PNT services in years to come. The efforts being made by the United States to define PNT requirements can be adopted by other countries as well to support a universal statement of civil PNT requirements.

Copyright © 2010 John W. Lavrakas

A copy of this article may also be found in the March issue of Coordinates magazine.

2 Responses to “Defining Civil Positioning, Navigation, and Timing Requirements”

  1. on 25 Mar 2010 at 9:25 amgeoff tanner

    Dear Mr. Lavrakas:

    I just read a message regarding Oregon wave energy and your report. I note that you are a GPS expert with many years in the industry. Please, if you know the names, who was / is Walter Melton and Dr. James Spilker? Thank you.

  2. on 25 Mar 2010 at 2:07 pmJohn Lavrakas

    Dear Geoff,
    I did not know either Walt Melton or Jim Spilker personally, but over my career I heard of Jim Spilker’s role in defining the GPS signal structure. You can see evidence of this in his article “Signal Structure and Performance Characteristics” published in the GPS Redbook Volume 1 (available from the Institute of Navigation, This article lays out everything you to know about the basic signal structure for GPS. I have likewise heard Walt’s name mentioned over the years. As I understand it he was at Stanford Telecommunications, one of the earlier manufactures of GPS receivers, and the one who supplied the receivers used by the US Air Force in implementing the ground control network for GPS.

    How is that you know and/or ask about these two individuals, if I may ask?
    John Lavrakas

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