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Mobile phone networks are being used to track the whereabouts of military units on both sides of the Russia-Ukraine war.
Cell-site simulators, which mimic phone masts, are being hidden inside drones flown over the battlefield to pick up mobile phone signals.
The simulators then measure the strength and direction of these signals, allowing combatants to analyse the likelihood of troops being in the vicinity before launching artillery strikes.
Russians are known to be using the Leer-3 electronic warfare system to this effect. It comprises two drones and a command vehicle.
The Ukrainian army is using a similar system, and it was reported by The New York Times that a Russian general was killed in mid-March after a phone call he made was intercepted.
The purpose of tracking mobile phone networks is not always to kill. Sometimes they are used to access vital intelligence.
"The consequences of this aren't always immediately apparent, which can lead to groups developing security practices that don't take into account just how much they are monitored," John Scott Railton, a senior researcher at the University of Toronto's Citizen Lab, told Sky News.
"Maybe they're not valuable enough targets to immediately send a missile at, but they might be valuable enough to track and glean intelligence from."
The tracking of troops in this fashion has been called the digital equivalent of a careless cigarette lighting.
To counter the threat, the Ukrainian army previously issued a directive to its troops fighting Russian separatists in the Donbas region on how to use phones on the battlefield.
Ukraine army's nine-point mobile phone directive
1. Leave your own SIM card at home.
2. The best place to obtain a SIM card is in the zone of conflict itself.
3. If you plan to make a phone call, do it at least 400-500 metres from squad positions.
4. Don’t walk away alone, take an armed friend with you for cover.
5. The best place to make a phone call is in locations with a lot of civilians, preferably in recently liberated towns.
6. Always keep your phone off. Your life depends on it. Grad missiles will hit your whole squad.
7. Do not accept refill codes or cards from the locals. The young woman that brings you a refill card from the neighbouring village may be working for the enemy. Right now FSB and SBU [Ukrainian security services] have to process enormous amounts of data to identify the mobile phones of our own people and of the enemy. Do not make their job easier.
8. Watch over your comrades – a friend calls his girlfriend and an hour or so later your position gets shelled or attacked.
9. Remember, the enemy could be listening to your conversations regardless of which SIM card or which telecom operator you are using.
The cost-benefit analysis of disabling networks
Given the hijacking of mobile phone networks both for destructive and surveillance purposes, surprise has been expressed that Ukraine's mobile network infrastructure remains intact.
The calculation on both sides is that the benefits it provides outweigh the cons – and disabling a network is hard to achieve in any case.
"Destroying a cell phone network is hard, there are lots of cells everywhere with good coverage," Sam Cranny-Evans from the defence think tank Rusi told Sky News.
"In Mariupol and Iziyum, for instance, they are only recently beginning to lose all access to their cell phone networks after a few weeks of fighting, and even then they can gain access in some instances."
To add an extra layer of complexity, even when a phone is switched off its underlying operating system can still be accessed.
Removing the battery is one option but often easier said than done with today's smartphones.
Another factor to consider is boredom.
Despite knowing the dangers of using phones, troops sitting for hours on end with nothing to do sometimes decide they would rather take the risk of using their phones.
"Sitting out there in the dugouts, trenches and bunkers for days and even weeks with nothing to do, people start going out of their heads. You need something to take your mind off of things," a soldier in Donbas told a researcher in 2017.
