Headlights illuminate a group of soldiers smoking and drinking steaming cups of coffee on the side of a road in northeastern Ukraine.

A fine sleet on another freezing night falls on the silhouetted barrel of a Soviet-era anti-aircraft gun mounted on an ancient, battered lorry.

These are the men of a mobile air defence unit, preparing for another night of tracking and attempting to shoot out of the skies the Russian drones heading for cities and power plants across Ukraine.

We joined the units over 48 hours in the southern sector of the city of Sumy, near the border with Russia.

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Captain Serhii, of the 117th Brigade, is a commander in charge of 160 of these men in eight mobile units protecting this area of northeastern Ukraine.

We met up with him in an empty car park next to a high-rise apartment block in Sumy.

Sitting in his 4×4 he squinted in concentration as his scanner showed a handful of drones on a flight path heading towards us from Russia.

Air raid sirens blared in the background.

He signalled for us to follow, and he sped off into the night weaving through checkpoints along empty, darkened country roads.

He had said the drone was near, but we were confused when he suddenly pulled off the road and jumped out of his cab.

Within moments the skies were lit up by mobile anti-aircraft batteries firing into the sky.

Tracers from the bullets flew over our heads and above us we could hear the monotonous sound of the drone as it passed overhead.

Searchlights reflected off a dank, dense fog that had enveloped the countryside criss-crossed in the sky as the units tried to spot the drone.

They didn’t spot or hit the drone, and slowly the sound of its motors grew dimmer and dimmer until there was silence.

“You can see the current weather conditions, it’s foggy, so we’re basically working off sound alone, as thermal imaging and other devices can’t pick them up,” Captain Serhii told me.

“A spotlight is completely pointless, the speed of these drones isn’t particularly high, so technically, we could hit them, but the weather makes it impossible,” he added.

Every night across Ukraine the tarpaulin covers are pulled off these Soviet-era anti-aircraft guns and prepared for action.

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On battered trucks they rumble into the night to take up defensive positions.

These old guns are the country’s main defence against an increasing number of drone attacks from Russia.

The units here say they mostly encounter two types of drones which are usually deployed at the same time: Gerberas and Shaheds.

Gerberas do not carry an explosive payload but instead are designed to confuse Ukraine’s air defence, while the Iranian-supplied Shahed drones carry explosives with pre-programmed targets.

One of the soldiers, Volodymyr, tracks the drones on a handheld tablet. All the men wait, watch the screen, and listen.

“They generally fly between 200-300m up to 3km,” Volodymyr tells me.

“If they’re flying lower than 300m above sea level, the radar won’t pick them up.”

The men start to gather around their anti-air battery unit as a drone flies closer towards our location.

The gunner then jumps onto the truck and searches for the target on a radar attached to his weapon.

He will fire when it’s within a 10km range of his position.

The night-time silence in the middle of the field is suddenly shattered by the deafening sound of firing as the gunner goes for the drone.

Then in the fields all around us other units join the attack – it’s so dark we had no idea they were even there.

It’s absolutely relentless.

Captain Serhii tells me their critical job would be made easier with more sophisticated weapons like the American-made Patriot system.

“We need a stronger, more automated air defence system that operates independently of human involvement,” he said.

“Winter is approaching, and our guys who are on duty 24/7 will face incredibly tough conditions, they aren’t made of steel, so having an automated system would be ideal.”

He believes any political negotiations with Russia will just end badly for Ukraine.

“A frozen conflict is typically just a temporary pause, and you can’t trust [Russian President Vladimir] Putin, because any agreement made will likely work in Russia’s favour,” he told me.

“They’ll stockpile resources, more Shaheds, rockets, soldiers, even North Koreans, and then they’ll return, and they’ll come back stronger than they did in 2022.”

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We join another mobile unit, this time by the side of an empty road as they track another Iranian Shahed drone.

It’s flying in our direction and they’re waiting to see if it will come into range.

It does, and they too attack – firing powerful rounds at their target.

The red, orange, and yellow flashes of colour as the weapon fires light up an otherwise pitch-black sky.

In the 48-hour period we spent with the anti-aircraft units in this part of the Sumy region, Russia deployed a record number of drones into Ukraine.

Ukraine, for its part, deployed a record number of drones against Russia too, but the number is dwarfed by the Russian assault.

Night after freezing night these Ukrainian defenders, hundreds of them around this city, will man these guns.

The last line of defence – not good, but better than nothing.