NASA has launched the final stage of a four-year effort to develop a national traffic management system for drones, testing them in cities for the first time beyond the operator's line of sight.

Multiple drones took to the air at the same time above downtown Reno this week in a series of simulations testing emerging technology that someday will be used to manage hundreds of thousands of small unmanned commercial aircraft.

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In this Tuesday, May 21, 2019 photo, a drone takes off from the roof of the Cal-Neva casino parking garage in downtown Reno, Nev. as part of a NASA simulation to test emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages. It marked the first time such tests have been conducted in an urban setting. (AP PhotoScott Sonner)

NASA has launched the final stage of a four-year effort to develop a national traffic management system for drones, testing them in cities for the first time beyond the operator's line of sight.

In this May 21, 2019 photo, a drone flies over downtown Reno, Nev., before landing on the Cal-Neva casino parking garage, as part of a NASA simulation to test emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages. It marked the first time such tests have been conducted in an urban setting. (AP PhotoScott Sonner)

Similar tests have been conducted in remote and rural areas. The Federal Aviation Administration has authorized individual test flights in cities before but never for multiple drones or outside the sight of the operator.

In this Tuesday, May 21, 2019 photo, a drone flies over downtown Reno, Nev., before landing on the Cal-Neva casino parking garage, as part of a NASA simulation to test emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages. It marked the first time such tests have been conducted in an urban setting. (AP PhotoScott Sonner)

In this Tuesday, May 21, 2019 photo, a drone flies over downtown Reno, Nev., before landing on the Cal-Neva casino parking garage, as part of a NASA simulation to test emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages. It marked the first time such tests have been conducted in an urban setting. (AP PhotoScott Sonner)

In this Tuesday, May 21, 2019 photo,  a researcher positions a drone for take off on the roof of a casino parking garage in downtown Reno, Nev., where NASA conducted the first drone tests of their kind in an urban setting. A series of simulations are testing the emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages. (AP PhotoScott Sonner)

In this Tuesday, May 21, 2019 photo, a researcher positions a drone for take off on the roof of a casino parking garage in downtown Reno, Nev., where NASA conducted the first drone tests of their kind in an urban setting. A series of simulations are testing the emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages. (AP PhotoScott Sonner)

In this Tuesday, May 21, 2019 photo, Chris Walach, executive director of the Nevada Institute of Autonomous Systems, points to the the launch pad on a casino parking garage in downtown Reno, Nev., where NASA conducted the first drone tests of their kind in an urban setting. A series of simulations are testing the emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages.  (AP PhotoScott Sonner)

In this Tuesday, May 21, 2019 photo, Chris Walach, executive director of the Nevada Institute of Autonomous Systems, points to the the launch pad on a casino parking garage in downtown Reno, Nev., where NASA conducted the first drone tests of their kind in an urban setting. A series of simulations are testing the emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages. (AP PhotoScott Sonner)

In this Tuesday, May 21, 2019 photo, two drones fly above Lake Street in downtown Reno, Nev., on, as part of a NASA simulation to test emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages. It marked the first time such tests have been conducted in an urban setting. (AP PhotoScott Sonner)

In this Tuesday, May 21, 2019 photo, two drones fly above Lake Street in downtown Reno, Nev., on, as part of a NASA simulation to test emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages. It marked the first time such tests have been conducted in an urban setting. (AP PhotoScott Sonner)

In this Tuesday, May 21, 2019 photo, Ron Johnson, NASA's project manager for Unmanned Aircraft Systems Traffic Management, points to an image detailing airspace elevations in downtown Reno. Nev., where NASA conducted the first drone tests of their kind in an urban setting. A series of simulations are testing the emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages.  (AP PhotoScott Sonner)

In this Tuesday, May 21, 2019 photo, Ron Johnson, NASA's project manager for Unmanned Aircraft Systems Traffic Management, points to an image detailing airspace elevations in downtown Reno. Nev., where NASA conducted the first drone tests of their kind in an urban setting. A series of simulations are testing the emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages. (AP PhotoScott Sonner)

NASA officials say it's the most complicated technical challenge they've faced since the simulations began in 2015.

In this Tuesday, May 21, 2019 photo, a drone takes off from the roof of the Cal-Neva casino parking garage in downtown Reno, Nev. as part of a NASA simulation to test emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages. It marked the first time such tests have been conducted in an urban setting. (AP PhotoScott Sonner)

In this Tuesday, May 21, 2019 photo, a drone takes off from the roof of the Cal-Neva casino parking garage in downtown Reno, Nev. as part of a NASA simulation to test emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages. It marked the first time such tests have been conducted in an urban setting. (AP PhotoScott Sonner)

Similar tests have been conducted in remote and rural areas. The Federal Aviation Administration has authorized individual test flights in cities before but never for multiple drones or outside the sight of the operator.

In this May 21, 2019 photo, a drone flies over downtown Reno, Nev., before landing on the Cal-Neva casino parking garage, as part of a NASA simulation to test emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages. It marked the first time such tests have been conducted in an urban setting. (AP PhotoScott Sonner)

In this May 21, 2019 photo, a drone flies over downtown Reno, Nev., before landing on the Cal-Neva casino parking garage, as part of a NASA simulation to test emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages. It marked the first time such tests have been conducted in an urban setting. (AP PhotoScott Sonner)

In this Tuesday, May 21, 2019 photo, a drone flies over downtown Reno, Nev., before landing on the Cal-Neva casino parking garage, as part of a NASA simulation to test emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages. It marked the first time such tests have been conducted in an urban setting. (AP PhotoScott Sonner)

In this Tuesday, May 21, 2019 photo, a drone flies over downtown Reno, Nev., before landing on the Cal-Neva casino parking garage, as part of a NASA simulation to test emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages. It marked the first time such tests have been conducted in an urban setting. (AP PhotoScott Sonner)

In this Tuesday, May 21, 2019 photo,  a researcher positions a drone for take off on the roof of a casino parking garage in downtown Reno, Nev., where NASA conducted the first drone tests of their kind in an urban setting. A series of simulations are testing the emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages. (AP PhotoScott Sonner)

In this Tuesday, May 21, 2019 photo, a researcher positions a drone for take off on the roof of a casino parking garage in downtown Reno, Nev., where NASA conducted the first drone tests of their kind in an urban setting. A series of simulations are testing the emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages. (AP PhotoScott Sonner)

In this Tuesday, May 21, 2019 photo, Chris Walach, executive director of the Nevada Institute of Autonomous Systems, points to the the launch pad on a casino parking garage in downtown Reno, Nev., where NASA conducted the first drone tests of their kind in an urban setting. A series of simulations are testing the emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages.  (AP PhotoScott Sonner)

In this Tuesday, May 21, 2019 photo, Chris Walach, executive director of the Nevada Institute of Autonomous Systems, points to the the launch pad on a casino parking garage in downtown Reno, Nev., where NASA conducted the first drone tests of their kind in an urban setting. A series of simulations are testing the emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages. (AP PhotoScott Sonner)

In this Tuesday, May 21, 2019 photo, two drones fly above Lake Street in downtown Reno, Nev., on, as part of a NASA simulation to test emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages. It marked the first time such tests have been conducted in an urban setting. (AP PhotoScott Sonner)

In this Tuesday, May 21, 2019 photo, two drones fly above Lake Street in downtown Reno, Nev., on, as part of a NASA simulation to test emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages. It marked the first time such tests have been conducted in an urban setting. (AP PhotoScott Sonner)

In this Tuesday, May 21, 2019 photo, Ron Johnson, NASA's project manager for Unmanned Aircraft Systems Traffic Management, points to an image detailing airspace elevations in downtown Reno. Nev., where NASA conducted the first drone tests of their kind in an urban setting. A series of simulations are testing the emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages.  (AP PhotoScott Sonner)

In this Tuesday, May 21, 2019 photo, Ron Johnson, NASA's project manager for Unmanned Aircraft Systems Traffic Management, points to an image detailing airspace elevations in downtown Reno. Nev., where NASA conducted the first drone tests of their kind in an urban setting. A series of simulations are testing the emerging technology that someday will be used to manage travel of hundreds of thousands of commercial, unmanned aerial vehicles (UAVs) delivering packages. (AP PhotoScott Sonner)