(Bloomberg) -- It wasn’t the eruption of rocket fire from Gaza that rattled soldiers at Israel’s southern frontier on Oct. 7. It was the unusual hum overhead that they hadn't heard before. 

A fleet of drones that are available online for as little as $6,500 filled the skies above Israel’s $1 billion border fence. They were rigged to carry explosives and knock out cameras, communications systems and remote-controlled guns, setting the stage for the unprecedented massacre. 

Militaries have been using drones in conflicts for more than two decades. Israel itself boasts one of the largest armies of unmanned aerial vehicles in the Middle East. Today, a new generation of cheap, commercially available systems — like the ones Hamas used in the Oct. 7 attack — is emerging, challenging some of the world’s most technologically advanced forces. 

The war with Hamas is a wakeup call for top-tier militaries about their deadly potential, according to Heven Drones Chief Executive Officer Bentzion Levinson, whose company supplies the Israeli army with heavy lifter and hydrogen-powered drones.

“We have these huge drones, these UAVs, we have planes, our technology is much more advanced,” Levinson said. “What this war did is that we realized that this is happening in our backyard, both on the defense and the offensive side.”

Hamas’s use of modified commercial drones to stage attacks — a strategy also used by Ukraine in the early days of Russia’s invasion — exposed a significant vulnerability in Israel’s vaunted air and ground defenses. The tactics overwhelmed a far more advanced opponent, all on a shoestring budget.

With the high-tech surveillance systems compromised, thousands of Hamas militants overwhelmed the border in trucks and paragliders. The attack on southern Israel was the deadliest day in the country’s history, with about 1,200 people killed and some 200 taken hostage. It took days for the army to fully regain control of the territory. 

Israel launched a ground invasion of the Gaza Strip on Oct. 27. Over 19,000 Palestinians have been killed in Gaza since the war began, according to the Hamas-controlled health ministry in the territory.

A spokesperson for the Israeli army declined to comment on how it was countering drones or the failure of its early warning systems. “Questions of this kind will be looked into in a later stage,” after the war, the spokesperson said. 

The Israel Defense Forces use its UAV fleet for surveillance and bombing targets. They are also increasingly turning to drones in urban warfare in Gaza to scout out buildings and defuse explosives before sending in troops, according to Aviv Shapira, the chief executive officer of Xtend, which provides UAV operating systems to the US and Israeli militaries. 

Israel has already upgraded its Iron Dome system — which uses interceptors to protect against incoming short range missiles — to detect large UAVs, but many Hamas drones are still able to slip through. The army is testing a laser-based system designed to intercept smaller ones and short-range rockets, although it won’t be ready for at least another year.

Some Israeli startups and tech volunteers have already drawn up new defenses, as the army’s troops involved in the ongoing invasion of Gaza come under frequent DIY kamikaze drone attacks. Videos posted by Hamas’s military wing since the start of the war, which could not be independently verified, show drones dropping grenades on Israeli troops and damaging armored vehicles.

A team of volunteers working out of a WeWork space in Tel Aviv — just across the street from Israeli military headquarters — has already gotten the army’s attention. The Israel Tech Guard initiative grew out of a Discord server that was formed on Oct. 8 by dozens of Israeli tech workers, including from Google and defense contractor Rafael, according to Mor Ram-On, a co-founder of the group.

One of their systems, developed in four days and now undergoing field testing on army bases, is an app that links two mobile phone cameras and audio systems to scan the skies for drones. It uses a 3D-printed case that can be mounted to vehicles and the group hopes to roll out the cheap alert system quickly.

Hamas drone attacks remain a potent threat, according to Liran Antebi, a research fellow at the Israel-based Institute for National Security Studies.  

“It gives you the ability to use precise or guided munition, which is something that until several years ago, only very advanced countries could do,” Antebi said. “With a criminal mind and small equipment, you can do terrible things like the first attack of Hamas.”

Hamas developed the tactics with its ally Iran and Mohamed Zaouari, a Tunisian engineer who led the group’s effort to develop UAVs. He was assassinated in 2016 in a killing the militants blame on Israeli intelligence. A model of attack drones is named after him and 35 of them were used in opening salvo.

Photos released by Hamas and the Israeli army appear to show off-the-shelf drones, including models similar to one made by Chinese producer DJI intended for aerial photography and industrial applications.

Three DJI drones were being studied at Sentrycs, a Tel Aviv startup that designs systems to counter UAVs. They are one type of aircraft that were rigged with explosives by Hamas, which is designated a terrorist organization by the US and European Union.

The effectiveness of the Hamas drone program also exacerbates growing concerns that non-state actors could develop deadly weapons with dual-use tech whose sales can’t be tracked. Even as militaries spend record amounts on sophisticated technology, simple equipment can allow marginal players to coordinate devastating attacks. 

Many soldiers in Gaza have been resorting to shooting the slow-moving drones out of the sky. The Israeli army said in a November blog post it has assigned a handheld SmartShooter precision targeting system to a soldier in every infantry unit for the first time. 

The system can be mounted on assault rifles and improves accuracy for moving targets like drones or enemy combatants. 

Israel had at least one system on the Gaza border on Oct. 7 specifically designed to counter drones, but it was not yet operational. The final stages of testing were scheduled a few days after the surprise attack, according to Sentrycs, which developed it. The system can detect and take control of drones from several kilometers away, rerouting them away from their targets. It's now deployed on Israeli military vehicles, along the border and near strategic assets, said Sentrycs Vice President Rotem Epelbaum. 

“We were off by a week,” Epelbaum said. “It could have been a game-changer.”

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