Drones began to crash on the Ukrainian front, without much explanation.

For months, aerial vehicles supplied by Quantum Systems, a German technology company, had worked flawlessly for Ukraine’s military, swooping through the air to spot enemy tanks and troops in the country’s war against Russia. Then, late last year, the machines abruptly began falling from the sky as they returned from their missions.

“It was this mystery,” said Sven Kruck, a Quantum executive who received a stern letter from Ukraine’s Defense Ministry demanding a solution.

Quantum engineers soon realized the problem: the Russians were jamming the wireless signals connecting the drones to the satellites they relied on for navigation, causing the machines to lose their way and plummet to Earth. To adapt, Quantum developed AI-powered software to act as a kind of secondary pilot and added a manual option so the drones could land with an Xbox controller. The company also built a service center to monitor electronic attacks from Russia.

“All we could do is get information from the operators, try to figure out what wasn’t working, test it and try again,” Kruck said.

A battle is raging in Ukraine in the invisible realm of electromagnetic waves, where radio signals are used to override communication links with drones and troops, locate targets and deceive guided weapons. Known as electronic warfare, the tactic has become a game of cat and mouse between Russia and Ukraine, quietly driving momentum shifts in the 21-month conflict and forcing engineers to adapt.

“Electronic warfare has impacted the fighting in Ukraine as much as the weather and terrain,” said Bryan Clark, a senior fellow at the Hudson Institute, a think tank in Washington, adding that every operation in the conflict must now take into account the enemy movements. in the electromagnetic spectrum.

Electronic warfare has been a feature of warfare for more than 100 years. During World War II, the British imitated German radio signals to fool the targeting systems used by bombers, popularized by Winston Churchill as the “battle of the rays.” In the Cold War, the Soviet Union invested heavily in electronic weapons to gain an asymmetric advantage against U.S. missiles and aircraft.

In recent decades, the use of electronic attack and defense has been more unbalanced. In the Iraq War in the 2000s, the United States used devices called jammers to create so much radio noise that improvised explosive devices could not communicate with their remote detonators. More recently, Israel has mixed GPS signals in its airspace with electronic warfare systems to confuse possible drone or missile attacks.

The war in Ukraine is the first recent conflict between two large, relatively advanced militaries that widely deploy electronic warfare capabilities and evolve techniques in real time. Technologies once within the reach of trained experts have spread to frontline infantry troops. Ukrainian drone pilots said they were constantly refining their methods to stop invisible attacks. Some said that someday a new radio frequency might work. Next up, a different antenna.

The tactics have become so critical that electronic warfare received its own section in a recent essay by General Valery Zaluzhny, Ukraine’s top military commander. “The widespread use of information technology in military affairs” would be key to breaking what has become a stalemate in the conflict with Russia, he wrote.

The techniques have turned war into an indirect laboratory that the United States, Europe and China have closely monitored to determine what might influence a future conflict, experts said.

Gen. Charles Q. Brown Jr., chairman of the Joint Chiefs of Staff, raised the issue of electronic warfare this year in remarks prepared for a congressional hearing. NATO countries have expanded their programs to buy and develop electronic weapons, said Thomas Withington, an electronic warfare expert at the Royal United Services Institute, a British security think tank.

“The war in Ukraine has been the drug that improved NATO’s electromagnetic thinking performance,” he said. “It has been what concentrates minds.”

As Russian tanks advanced toward kyiv in February 2022, the Russian military initially made good on its reputation as one of the world’s best at electronic warfare. It used powerful jammers and decoy missiles to swamp Ukrainian air defenses, leaving Ukraine dependent on aircraft to fight off Russian aircraft.

Electronic weapons do not seem dangerous at first glance. These are typically satellite dishes or antennas that can be mounted on trucks or installed on fields or buildings. But then they emit electromagnetic waves to track, deceive and jam sensors and communication links that guide precision weapons and enable radio communications. Almost all communications technologies are based on electromagnetic signals, whether it’s soldiers with radios, drones connected to pilots, or missiles connected to satellites.

A basic but effective tool is a jammer, which disrupts communications by sending powerful signals on the same frequencies used by walkie-talkies or drones to cause so much disruption that transmitting a signal is impossible. Jamming is similar to blasting heavy metal in the middle of a university lecture.

Another key weapon sends a signal that pretends to be something it is not, like a satellite link. The false signal, called spoofing, can convince a drone or missile that it is way off course by giving it false coordinates. In other cases, counterfeiters imitate signals emitted by missiles or aircraft to trick air defense systems into detecting attacks that are not occurring.

Other tools listen for rays of radiation and seek to locate their source. These devices are often used to find and attack drone pilots.

After initial success using these tools, the Russian military stumbled, analysts said. But as the war has dragged on, Russia has innovated by making smaller mobile electronic weapons, such as anti-drone weapons and small jammers that form a bubble of radio waves around trenches.

“The Russians have been more agile in responding than we would have expected from their behavior on the ground,” said James A. Lewis, a former U.S. official who writes on technology and security for the Center for Strategic and International Studies in Washington. . “That should be worrying for NATO.”

The Kremlin did not respond to a request for comment.

To combat Russia’s century of Soviet know-how in electronic attacks and defense, Ukraine has turned to a start-up approach associated with Silicon Valley. The idea is to help the country’s tech workers quickly produce electronic warfare products, test them, and then ship them to the battlefield.

This summer, Ukraine’s government hosted a hackathon for companies to work on ways to jam Iranian Shahed drones, which are long-range unmanned aerial vehicles that have been used to attack cities in the country’s interior, said Mykhailo Fedorov, digital minister of Ukraine.

At testing grounds on the outskirts of kyiv, drone makers pit their ships against electronic attack weapons. In August, in a field in central Ukraine, Yurii Momot, 53, a former commander of the Soviet Union’s special forces and founder of the electronic warfare firm Piranha, showed off a new anti-drone weapon built for the conflict. .

The weapons had a checkered performance in the war, but Mr. Momot’s version worked. He pointed it at a DJI Mavic, a cheap and common reconnaissance drone, and pulled the trigger. The drone remained motionless. His navigation system had been overwhelmed by a burst of radio signals from the weapon.

“The whole system is more structured in Russia,” Momot said of Russia’s electronic warfare program, which he knows from his time with the Soviet military. “We’re catching up, but it’s going to take a while.”

Other Ukrainian companies, such as Kvertus and Himera, are building small $100 jammers or walkie-talkies that can resist Russian jamming.

At Infozahyst, one of Ukraine’s largest electronic warfare contractors, engineers recently worked on a project to track and identify Russian air defense systems. Iaroslav Kalinin, the company’s chief executive, said Russian anti-aircraft radars were not as easy to replace as tanks. But if enough were eliminated, it could be a turning point in the war.

“Once we control the sky, Russia will fail miserably,” he said.

This summer, Oleksandr Berezhny, a Quantum executive, traveled with one of Ukraine’s top drone pilots to share with NATO what they knew about electronic warfare. At a large roundtable held at a base in Germany, they explained the problems they faced to a rapt audience of commanders.

“We told them that probably 90 percent of the American and European systems coming into Ukraine were not prepared to meet the challenge of electronic warfare,” Berezhny said. “There was a complete understanding that something needed to change.”

As Ukraine offers a glimpse of how future electronic battles may be fought, potential combatants in those fights have taken note. The United States and Europe have paid close attention to how those weapons have performed against Russian systems, and some fear they are not responding quickly enough. Chinese experts have also comprehensively chronicled which Russian electronic attacks were most effective against NATO systems and, in turn, where Russia failed.

In a November 2022 report, a Chinese defense think tank detailed how a Russian electronic attack had fooled NATO detection equipment, prompting Ukraine to reveal the location of its own electronic defenses.

“The anti-drone combat capabilities of the Russian military are superior to those of the US military,” the report says.

As Ukraine develops its anti-jamming techniques, some of those tactics are flowing to the United States and its allies, said the Hudson Institute’s Clark.

“Now you’re starting to see countries, including the United States, using these smaller systems, just as you’re seeing people in Ukraine improvising them,” he said.

For many on the Ukrainian front, improvements cannot come quickly enough.

“Even if you make your drone invisible, your controller and antenna emit a signal,” said a Ukrainian drone pilot, who gave only his first name, Vladislav. The Russians can detect a window of about 200 square meters where a drone pilot could be, he added, noting that artillery once came within “15 to 20 meters” of hitting him.

“You can’t hide completely,” he said.

Olha Kotiuzhanska contributed reporting from kyiv, Dnipro and Odessa, Ukraine. Arijeta Lajka contributed to the production of the video from New York.