Over the past decade, the NAVSTAR Global Positioning System (GPS), managed by the U.S. Air Force (USAF) Space Command for the Department of Defense (DoD), has become the most exploited space-based asset that the U.S. government has ever developed. As GPS provides space-based radio navigation for anyone with a GPS receiver, both civilian and military uses have increased exponentially.

In fact, everything geospatially oriented today is reliant on GPS. Most military and intelligence operations depend on knowing precisely where something is located. National Geospatial Intelligence Agency’s (NGA) mission is to provide the accurate, timely, and actionable geospatial intelligence (GEOINT) to our mission partners, when and where they need it most. From the warfighter on the front line to the local search-and-rescue team seeking flood victims, NGA provides GEOINT to support operational and decision-making needs.

For example, we provide the geospatial products that enable our warfighters — soldiers, sailors, and airmen — to accurately locate and hit targets. Our analysts will build a picture for the warfighter by layering natural features, such as rivers, hills, and waterways, with man-made features, such as roads, power lines, and buildings, to develop a 2D or 3D picture for common use. Knowing that a target of interest exists is important; knowing where exactly that target of interest is located ensures accurate targeting and minimizes the risk of collateral damage. To target an object, the warfighter needs accurate geographic coordinates. Subsequently, each data layer used in the development of a GEOINT product is referenced to a standard coordinate system.

Ensuring Accuracy + Reliability
U.S. national security, transportation and navigation safety, economic interests, and scientific uses all rely on GPS. This increasing dependence demands that the coordinate information and reference system be both accurate and accessible. NGA plays an essential role in maintaining and improving the accuracy and reliability of GPS by providing the DoD with precise GPS orbits, satellite and station clock corrections, and Earth orientation information. NGA is a daily consumer of GPS as well as a robust contributor, as well.

NGA and its predecessor organizations partnered with the DOD to develop the World Geodetic System 1984 (WGS 84) as the standard geodetic frame of reference. The WGS 84 global reference frame provides a mathematical representation of the Earth’s shape, a 3D coordinate system, and a gravity model that is essential for computing satellite orbits and precise locations on, above, or below the Earth’s surface. WGS 84 provides a common, standardized reference frame for interrelating and integrating all geospatial data, including GPS-derived position location information. This global reference information is what allows users to determine their locations on Earth based on the precise positions of GPS satellites in space.

Prior to the 1950s, coordinate systems were developed regionally. Once satellites became available in the 1950s and 1960s, we were able to establish an Earth-centered, global coordinate system. Today, the WGS 84 coordinate system used by GPS is defined by the 3D coordinates established by the combination of the U.S. Air Force and NGA satellite tracking stations distributed around the world. The more accurately we know the positions of these tracking stations, the more accurately we can determine the GPS satellite positions. Currently, we estimate the accuracy of these station coordinates within a few centimeters or less. As a by-product of this data processing, we can also detect small variations in the Earth’s orientation in space and its rotation rate. This information is crucial for the accurate and precise orientation and geopositioning of satellite imagery.

Precise timing is the key to GPS’s accuracy. Every DoD GPS tracking station and GPS satellite is equipped with an atomic clock, each of which runs at slightly different rates. NGA collocated a tracking station with our nation’s master timekeepers at the U.S. Naval Observatory (USNO) in Washington, D.C. This allows us to take advantage of the stability, precision, and accuracy of the USNO time by defining it as our GPS “master clock,” and then to adjust all the other satellite and station clocks to the master.

Reliability of service is essential to GPS effectiveness. Beginning in the 1980s, NGA provided personnel support at the GPS Joint Program Office (now the GPS Wing) at Los Angeles Air Force Base (LAFB) and the Operational Control Station (OCS) at Schriever AFB in Colorado Springs. Additionally, we invested in building and operating a global network of unmanned GPS tracking stations to augment the Air Force’s permanent GPS tracking stations. The result has been substantial benefits to the entire GPS user community. As part of a major accuracy improvement initiative, NGA stations now feed real-time data to the GPS through the OCS at Schriever AFB. These data are incorporated into the real-time estimation process for GPS orbit determination, resulting in increased accuracy and integrity of GPS navigation signals for GPS users.

Looking Forward
Future improvements and maintenance of GPS, augmentations, and backup capabilities are necessary to meet growing national security, economic, commercial, and scientific requirements and opportunities. For example, new foreign-based Global Navigation Satellite Systems (GNSS), such as the Russian GLONASS and the European Galileo System, provide additional options for current and future GPS users. These foreign systems are not yet as robust as our GPS system, but may be in future years. With ongoing efforts to ensure interoperability among all the systems, every system may be vulnerable to the same intentional or unintentional interference. The sheer number of combined GNSS satellites, upwards of 60–100 in the future, may help to mitigate these effects.

The defense community is exploring new mitigation strategies to counter electromagnetic radiation interference caused by solar flares or geomagnetic storms, as well as intentional and unintentional radio jamming caused by man-made techniques. DoD’s development and implementation of a military-only code and other new satellite features are designed to protect and preserve U.S. strategic access to GPS, even in hostile environments.

As we look toward the future and the next evolution of GPS, we must ensure interoperability and compatibility in the context of geospatial information. NGA will continue our strong collaboration with the USAF to ensure future satellite procurement and technological decisions to consider GEOINT needs and capabilities. Additionally, NGA’s continued participation in the International GNSS Service, the international organization that produces state-of-the-art GNSS data and products for the scientific community, will also help ensure that NGA stays up-to-date on the latest GNSS science and technology.

As both a consumer of, and a contributor to, GPS, NGA is committed to integrating and working collaboratively with our mission partners as we make the best decisions to ensure our national security, safety, and stability.

About the author
Vice Admiral Robert B. Murrett is the director of the National Geospatial-Intelligence Agency (NGA).

Editor’s Note: This article previously appeared in the November/December 2008 issue of NGA’s Pathfinder magazine.