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.

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