NASA Technology

In the 1960s, NASA used a network of radio telescopes and a technique called very large baseline interferometry (VLBI) to capture images of quasars in distant galaxies. In the following decade, scientists reversed the process to determine the precise locations of the telescopes, painting a picture of Earth’s shape and orientation in space. Today, an evolution of that technology supports another location-based system that has arguably become the world’s most important communication infrastructure: the Global Positioning System (GPS).

When the first GPS satellite was launched in 1978, NASA’s Jet Propulsion Laboratory (JPL) already had experience in tracking radio signals from faraway sources and extracting valuable information from them. Using VLBI, for example, JPL scientists measured the time difference between when the same radio signals from quasars—some of the brightest objects in the universe—hit different telescopes around the world to conduct geodesy, assessing Earth’s size and shape.

These capabilities served as the springboard that let JPL recognize early on the potential that GPS held and compelled the center to invest in the technology and infrastructure to support it.

In the early 1980s, as the U.S. Air Force continued launching GPS satellites, JPL started building out a tracking network, collocating the first ground stations with VLBI tracking sites. These offered the necessary communications infrastructure and precise location information to tie the geodetic reference frame to that of GPS. Today the network includes more than 80 receivers, making it the world’s largest geodetic-quality, centrally managed GPS tracking network.

Raw GPS data can produce positioning errors of 30 feet or more if not calibrated for signal delays caused by electrons and gases in Earth’s atmosphere, errors in GPS satellite positions, and noise and drift in the satellites’ atomic clocks. The software JPL developed to model delays, correct for errors, and perform GPS orbit determination and receiver positioning was called GIPSY-OASIS (GPS-Inferred Positioning System and Orbit Analysis Simulation Software). It quickly became one of NASA’s most widely licensed software programs, with hundreds of licenses to academia and industry.

But no one was able to make these corrections in real time on a global scale until the Internet became widespread in the mid-1990s. Shortly after the GPS satellite fleet became fully operational in 1994, JPL received a small amount of funding from NASA’s Deep Space Network program to figure out how to use GPS data in real time to enable faster turnaround time for deep-space navigation. That seeded the development of JPL’s Real-Time GIPSY (RTG) software, which could make all these corrections and update them every second.

A small jet flies into storm clouds
The Federal Aviation Administration invested in Global Differential GPS (GDGPS) early on for the development of its Wide Area Augmentation System, which now lets tens of thousands of smaller airplanes land at most North American airports in low visibility, using GPS guidance.

The corrections also were globally and uniformly valid, in contrast to the prevailing approaches at the time, which corrected signals based on local information, leading to corrections that were only valid regionally and suffered from degradation when the satellites rose and set on the target region.

Once JPL had demonstrated precision global GPS in real time, a host of new possibilities opened up.

“We took an innovative approach to the opportunities and challenges the free Internet offered, and we also had the vision to recognize that real-time GPS processing on a global scale could be revolutionary,” says Yoaz Bar-Sever, who was one of the developers of GIPSY and RTG and is now the manager of the Global Differential GPS (GDGPS) system at JPL. “But we were still surprised by the scope of the impact of the new capabilities we were building.”

Technology Transfer

In the mid-1990s, both industry and government began forays into the use of GPS corrections to improve positioning, navigation, and timing. In 1995, Satloc, a small company vying for the U.S. market of industrial GPS corrections, licensed RTG and became the first provider of RTG-corrected GPS service across most of North America.

Two tractors work a field
The first major commercial customer for GDGPS was John Deere, which worked closely with JPL scientists to make the system practical for self-driving farm equipment, a technology that would eventually make efficient precision agriculture commonplace around the world.

The following year, the Federal Aviation Administration (FAA) selected the software as the prototype for its Wide Area Augmentation System (WAAS), with the goal of providing accurate GPS navigation for pilots, and offered funding for JPL to further mature the technology.

“We had the track record, and we had the only proven software to do what they wanted,” says Bar-Sever.

Another key investment in 2000 from the NASA Earth Science Technology Office helped JPL put in place the operational technology and infrastructure needed to launch a reliable, global, real-time service, and the GDGPS system was born. “As the Internet became more prevalent, we were able to get orbital determination in seconds globally,” Bar-Sever says. “That led a lot of industry to us.”

While NASA remained a key user of GDGPS, its ongoing development ever since has been funded almost entirely by other government users and commercial companies.

As word spread of the accuracy and global coverage of GDGPS, in 2002, the Air Force sponsored the development of a dedicated service to provide a wealth of information to its GPS operators, a service that has been enhanced over the years to adapt to the evolving GPS constellation and continues to this day. “In supporting operational GPS, the GDGPS system contributes to the reliability of the entire GPS enterprise,” says Bar-Sever.

Also in the early 2000s, GDGPS found its first major commercial customer in the farm equipment company John Deere, which invested heavily to help JPL engineers make the system practical and reliable for guiding self-driving tractors (Spinoff 2017). The technology made precision farming commonplace around the world.

By this time, the system enabled positioning accuracy to within less than three inches.

Another early adopter was Comtech Telecommunications Corporation, which became a customer in 2002 and remains a major provider of location-based services, among other technologies. As cell phones became more prevalent, the Federal Communications Commission mandated that all providers include the ability to immediately locate 911 emergency callers. This capability was initially Comtech’s main use of GDGPS, and it still provides the service for about half of U.S. cell phone owners, as well as millions of others around the world.

Comtech, headquartered on Long Island, sells its software and applications to mobile phone operators and original equipment manufacturers, explains Tsega Emmanuel, the company’s product manager for advanced location services. On an emergency call, an initial, rough location can be determined from a network of mobile towers and GPS satellites in the vicinity, and then company’s positioning engine uses JPL data to refine the location based on satellites available to the mobile device. Since the caller might not stay in one place, the positioning engine provides responders with periodic updates.

“GDGPS connectivity has to be reliable. To assure device location accuracy, everything has to be reliable,” Emmanuel says.


Two firefighters check a smartphone
Comtech, one of the earliest commercial customers for GDGPS, uses the data to provide first responders with locations for emergency cell phone calls for about half of U.S. cell phone users and millions of others around the world.

Between the mid-1990s and the mid-2010s, GDGPS technology and data were at the heart of three technological revolutions with broad societal benefits, Bar-Sever says. “The first was the creation and deployment of WAAS, which revolutionized the safety and economy of commercial aviation.” This was followed by the broad industrial application of corrected GPS, especially for precision agriculture. “And finally came its widespread usage to enhance the safety and utility of geolocation for mobile wireless users.”

Comtech has long been at the forefront of that latest revolution. In addition to increasing safety and security with accurate emergency call locations, the assisted GPS capability provided by the company and others is the reason smartphone navigation applications have a short “time to first fix”—locating themselves almost immediately, unlike vehicle GPS units that can take time to acquire satellite connections.