Navigating to within three inches of your destination is made possible by algorithms and software developed by NASA. These power a NASA system that augments the raw navigation signals provided by the U.S. Air Force's GPS satellites to support airplane navigation around the world, direct emergency responders and, soon, guide self-driving cars.

The Air Force began launching global positioning satellites in 1978, and it continues to operate and maintain the satellite network to this day. But over the decades, NASA has played a critical role in improving the system we rely on in our daily lives.

GPS satellites orbit thousands of miles above Earth, which means there are delays and distortions as the signals they send pass through the atmosphere. The signals are also subject to errors in satellite positions and noise and drift in the satellites' atomic clocks.

As a result, positions based on raw data from GPS can be off by up to 30 feet. In contrast, corrected results from NASA's Global Differential GPS (GDGPS) can be accurate to within three inches.

At the time the first GPS satellites were launching, NASA's Jet Propulsion Laboratory in Pasadena, California, had long experience working with radio signals from faraway sources. For example, in the 1960s, NASA used a network of radio telescopes and a technique called very large baseline interferometry to track radio signals from distant quasars, some of the brightest objects in the universe.

By measuring how long it took for the same radio signals from those quasars to hit different telescopes around the world, JPL scientists were able to get a highly accurate picture of Earth's size and shape (a field called geodesy).

In the early 1980s, JPL started building out a network to track the GPS satellites, putting the first ground stations where the very large baseline interferometry tracking sites already were. These ground stations acted as known fixed points that helped scientists calibrate the location information provided by GPS. Today, that network includes more than 80 receivers worldwide.

The ground stations were only the start. Another crucial piece was software: JPL created algorithms to model delays, correct for errors and perform GPS orbit determination and receiver positioning. The program, called GIPSY-OASIS (GPS-Inferred Positioning System and Orbit Analysis Simulation Software), quickly became one of NASA's most widely licensed software programs.

However, it took hours and days to collect the tracking data from the nework via telephone modems, and the software required days' worth of data to perform its orbit determination.

In the mid-1990s, with some funding from NASA's Deep Space Network and an assist from the internet, JPL developed the Real-Time GIPSY software. This program could receive a stream of measurements from the remote sites over the internet and make the orbit and clock corrections every second.

Another key investment in 2000 from NASA's 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," said JPL's Yoaz Bar-Sever, who manages GDGPS. "That led a lot of the industry to us."

While NASA remains a key user of GDGPS, its ongoing development has been funded almost entirely by other government users and commercial companies that pay for the service through reimbursable Space Act Agreements.

Safety, Smartphones and Social Media
JPL recognized "that real-time GPS processing on a global scale could be revolutionary," said Bar-Sever. "But we were still surprised by the scope of the impact of the new capabilities we were building."

One early use was geolocation of cell phones, especially during emergency calls.

In the early 2000s, the Federal Communications Commission required that all cell phone service providers include the ability to immediately locate 911 callers. Comtech Telecommunications Corporation turned to NASA's GDGPS.

On an emergency call, an initial, rough location can be determined from a network of mobile towers and GPS satellites, explains Tsega Emmanuel, Comtech's product manager for advanced location services. Then the company's positioning engine uses JPL data to refine the location. Since the caller might not stay in one place, the positioning engine provides responders with periodic updates.

Besides increasing safety and security, the assisted GPS capability is the reason smartphone navigation applications have a short "time to first fix" - locating themselves almost immediately, unlike car GPS units that can take time to acquire a satellite connection.

Today Comtech provides emergency locator information for about half of cell phone owners in the United States, as well as millions of others around the world.

With the help of GDGPS, Comtech has also become one of the top providers of location-based services for navigation, social media and even pet tracking. For these applications, an equipment manufacturer or app writer creates and markets the product under its own name but relies on Comtech's access to JPL data for positioning. The company computes more than 10 billion locations per month around the world.

JPL's system "is part of the value chain," Emanuel said. "If we don't have it, we don't have accurate location. We don't have a product."

Accuracy Opens New Possibilities
But precision GPS is used far beyond the world of cell phone locations. Among other uses, it provided the basis for the Federal Aviation Administration's Wide Area Augmentation System (WAAS), used by tens of thousands of planes for navigation and guided landing in North America. WAAS and its implementations in Japan and India are based on the Real Time GIPSY software JPL enhanced with support from Raytheon and the FAA.

GDGPS has also enabled modern automated precision agriculture: while John Deere eventually developed its own navigation technology for self-driving farm equipment, an earlier partnership with JPL helped kick off the technology and bring it to prominence. Today precision agriculture saves farmers resources, increases yields and cuts down on pollution worldwide.

Since 2002, the Air Force has sponsored a dedicated JPL service to provide a wealth of real-time performance-monitoring information about the satellites to the military's GPS operators. "In supporting operational GPS, the GDGPS system contributes to the reliability of the entire GPS enterprise," said Bar-Sever.

And the JPL team continues to work on upgrades in partnership with the Air Force and other sponsors. In 2014, the next-generation software, RTGx, replaced Real-Time GIPSY as the operational engine of GDGPS. RTGx is slated to become the navigation software for Air Force GPS operations.

These upgrades will help power future innovations. The JPL team is working, for example, to achieve unprecedented accuracy to guide self-driving cars. "The accuracy and reliability that industry demands is very difficult to achieve," Bar-Sever said. "We're working on that with the support of the aggregate of our customers."

He noted that the many GDGPS customers share both the load and benefits of continually updating the system's capabilities, a necessity in the rapidly evolving domain of satellite navigation.

"What's emerged is a self-supporting system, providing its customers and NASA many returns on their investment," Bar-Server said. "This is the poster child for the tremendous value of NASA to people everywhere."

Related Links
Global Differential GPS
GPS Applications, Technology and Suppliers

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Northrop Grumman awarded $1.39B for new Air Force navigation system
Washington (UPI) Sep 30, 2019
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