September marks the 50th anniversary of the Global Positioning System, a U.S.-owned free utility that offers positioning, navigation, and timing services to users all over the world. It changed not only transportation but also agriculture, construction, emergency response, and precise timing for financial transactions and communications networks.

A testament to American ingenuity and technological progress, GPS is now crucial to how we navigate, communicate, and interact with others. But few in 1973 could have predicted this outcome.

On that Labor Day weekend, the Pentagon, with over 6 million square feet of office space and more than 25,000 employees, was nearly empty except for 12 people in a 5^th floor conference room who sketched out the design of the Global Positioning System. The air conditioning was turned off to save money. It was hot, humid and dark. For food they ate stale hamburgers out of the 4^th floor automat. The parking lot was nearly empty. On a regular day more than 8,000 cars would be there. It was downright eerie.

In 1973 there were no laptops or cell phones, and the single-chip microprocessor was granted a patent. George Foreman was a heavyweight boxing champion, rather than a grill salesman. Tony Orlando and Roberta Flack topped the music charts.

GPS was invented for the military, but it was extended to civilian use by President Reagan in 1983 after Russians brought down a Korean Air Boeing 747. The plane had strayed into Soviet airspace, and the crash killed 269 people on board, including Rep. Larry McDonald (D-Ga.).

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What began as a constellation of satellites designed for military navigation has evolved into an indispensable global utility that affects virtually every facet of modern life. A National Institute of Standards and Technology-sponsored study estimates that GPS has generated around $1.7 trillion in economic benefits for America since its launch half a century ago.

GPS, which serves both civilians and the military, relies on a system of 32 satellites that send signals for positioning and timing. The U.S. Space Force, which didn’t exist when GPS was launched, oversees satellite orbits, clock synchronization, and navigational updates. Users on the ground and flying in planes use GPS receivers to calculate users' positions and time based on satellite signals.

As sophisticated technology has become cheaper and more widely available, the system has become increasingly vulnerable. For example, in 2019 Iran interfered with the navigation system of the British tanker Stena Impero in the Strait of Hormuz. Although the ship’s crew were in Iranian waters, spoofing by Iran made it appear that they were in international waters. Iran captured the ship and its crew, and held them for 10 weeks.

Americans use over 900 million GPS receivers in cell phones, car navigation systems, emergency vehicles, commercial trucks and buses, and railroad operations. (That is nearly three receivers for every man, woman and child in the United States.) Over 100 million vehicles have car navigation systems. Ships, planes, and drones use GPS for navigation. Trucks use GPS both for location services and for driver safety.

Over 7,600 U.S. commercial aircraft use GPS, as well as 167,000 general aviation aircraft and 34,200 experimental aircraft, to navigate to and from airports, avoiding mountains and other dangerous terrain. When GPS doesn’t work, the Federal Aviation Administration grounds all planes.

Now firefighters and ambulances use GPS to get to your house in an emergency. Back in 1973 they used maps. In addition, automakers ranging from Jeep to Mercedes can track cars with GPS, notify them when maintenance is due, and slow or disable them if they are stolen.

Trains use GPS to maintain rail systems and for positive train control. It stops trains from crashing, exceeding the speed limit, and heading the wrong way down a one-way zone. Drones operated by GPS maintain tracks and railroad bridges.

Ships use GPS for navigation and to prevent crashes with other ships. Automatic GPS identification systems permit port authorities to track ships, providing data to measure port congestion.

Even surveying and construction now use GPS. Surveyors can take measures that are beyond line of sight by carrying GPS devices or by using survey stations. Construction companies can GPS for digital blueprints, which can help workers pinpoint locations of studs and nails, reducing mistakes. GPS use for tunnels makes it easier to ensure that a tunnel started on both sides of a mountain meets precisely in the middle.

For agriculture, John Deere now makes GPS-enabled tractors that enable farmers to put seeds in particular locations. Then, farmers can water and fertilize these seeds and add pesticides, reducing waste.

With the increased use of renewables such as wind and solar, electric power plants use GPS to synchronize generators and to distribute energy to the grid. Utility operators can use GPS timing services to prevent blackouts and synchronize generators with the grid.

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GPS can even measure climate change by measuring the relation between snowfall and ice mass. University of California at San Diego scientist Susheel Adusumill studies the ice mass in West Antarctica using GPS, one of many examples. Reflectometry, which uses GPS, can measure changes in snowfall; vegetation; ice coverage; and soil moisture.

Damage to GPS systems has in the past come from harm to satellites—and could easily do so in the future. Satellites could be damaged by electromagnetic storms or hostile military action, either of which could eliminate the signals. Further harm could come from hacking—interfering with GPS systems so they do not operate. Inexpensive equipment to do so is easily available online.

Another form of harm, as Iran demonstrated in 2019, is spoofing—meddling with GPS to make it appear that it is operating correctly, but in reality it is sending users flawed information to make them believe that they are in a different location, or that their transaction is occurring at a different time. The economic consequences of such misuse are so costly as to be immeasurable.

That is why three separate laws, most recently the Frank LoBiondo Coast Guard Authorization Act of 2018, tasked the Transportation Department with providing a backup to GPS. The LoBiondo Act required the Secretary to put in place a backup system for GPS by the end of 2020, subject to Congressional appropriations. But Congress has so far not appropriated the funds for backup.

This needs to change. After half a century of use, Americans depend on GPS. Outages could cause incalculable damage to the economy. Waiting for GPS to fail, or for hostile powers to spoof it, is waiting until it is too late. GPS needs to be more resilient for the modern era.