Shortly after dawn on a southern Taiwanese beach, Robin Hsu's iPhone pings with the first radio message of the day from Taiwan's air force as it warns away Chinese aircraft.
"Attention!" a voice says on the radio, speaking in Mandarin to a Chinese military plane flying at an altitude of 3,500 metres. "You have entered our southwestern air defence identification zone and are jeopardising aviation safety. Turn around and leave immediately."
Taiwan, which China claims as its own territory, has complained for years of repeated Chinese air force missions into its air defence identification zone (ADIZ), which is not territorial airspace but a broader area it monitors for threats.
Although Taiwan's Defence Ministry details these almost-daily incursions on its website, including maps outlining the activity, a band of Taiwanese radio enthusiasts like Hsu has been tuning in to related radio traffic and publishing the recordings online.
"The Chinese Communist planes are like flies on your dining table. If you kill them on your plate then your meal is ruined," said Hsu, 50, a tour guide and a military enthusiast. "All you can do is to wave them away."
The action ebbs and flows. On one day in May, when Reuters accompanied Hsu, nine other warnings were broadcast to Chinese warplanes after the one at dawn.
The Chinese aircraft have not fired a shot and have not come near Taiwan's shores, according to the island's military.
But for Taiwan, such incursions amount to a low-key war of attrition, as the island frequently scrambles aircraft to intercept Chinese planes. Taiwan's defence ministry has termed the flights "grey zone" tactics, designed to exhaust its air defences physically and financially.
Halfway through lunch, Hsu's iPhone - which is linked to a separate radio antenna - tracked another broadcast, this time in English.
"Chinese Air Force, I am a United States aircraft operating in international airspace and exercising these rights as guaranteed by international law," the transmission said. "I am operating with due regard to the rights and duties of all states."
On a flight tracker app, a US military refuelling plane was flying east off Taiwan's southwest into the Bashi Channel that separates the island from the Philippines.
A spokesperson for US Indo-Pacific Command said that there are routine flights conducted in international airspace in accordance with international law, and that the language in the broadcast is consistent with US military aviation units operating in the Indo-Pacific.
The same day, Taiwan scrambled jets to warn away six Chinese aircraft, including two H-6 bombers and one Y-8 anti-submarine aircraft, according to the island's defence ministry.
China's defence ministry and Taiwan Affairs Office did not respond to requests for comment.
'On our doorstep'
Hsu and his team have set up a dozen reception points on hills across Taiwan. With the help of those stations and flight-tracking apps, Hsu has counted 317 Taiwan warnings to Chinese warplanes from the start of the year to early May, 3 per cent more than in the same period a year earlier.
"I want people to know the Chinese Communist planes are very close to us," said Hsu, who releases his recordings on a Facebook page that has attracted nearly 16,000 followers and is widely tracked by Taiwanese media.
He said making such information public could boost support for Taiwan's armed forces, which are dwarfed by China's. Taiwan's defence ministry said it respects any opinion that helps boost defence, but had no further comment.
On rare occasions, Chinese pilots have responded to Taiwan's radio communications.
"This is China's People's Liberation Army. I'm conducting routine exercises. Please do not interrupt my activities," one message in Mandarin said in late 2020, minutes after Taiwan warned away a Chinese warplane.
Hsu, a former navy radar operator, described a pattern of activities off Taiwan's southwest in which US military planes, often refuelling and surveillance aircraft, are shadowed by Chinese planes, who are warned away by Taiwan's air force.
Military experts say the strategic waters there, where the largely shallow Taiwan Strait descends in depth, provide submarines a location for an ambush, making it a hot spot for anti-submarine and surveillance aircraft.
Su Tzu-yun, a research fellow at Taiwan's top military think tank, the Institute for National Defence and Security Research, said Hsu's efforts help the public understand the threat from China is "very real and immediate".
He said the transparency could also help both militaries avoid an accidental conflict.
At one of his reception points, a hilltop cafe overlooking the Taiwan Strait, Hsu said he planned to expand his network to include communications in Taiwan's northern skies, where China, Japan and Taiwan air forces regularly cross paths.
"On the surface, we have peace. But the reality is Chinese Communist planes are flying on our doorstep every day," he said. "People need to be aware of the crisis."
