Then, after patrolling in a motorboat along the Heilong River, which forms part of the dividing line, they watched a live broadcast of the congress on a cellphone.

To get a good signal, they had to continually move their patrol boat up and down the river in a place where low temperatures are already around 0 C.

The island is located at the confluence of the Heilong and Wusuli rivers and serves as a natural border between China and Russia. Xu's station has three officers and three soldiers to cover 13 kilometers of river.

"I've been stationed on the border for 13 years, since my second year in the Army," said Xu, 36, an officer of the Heixiazi Island border police at the country's easternmost station. "I have seen the changes at different border stations in the past decade, and I'm honored to be a Chinese soldier."

Their main duty is preventing illegal fishing, investigating smuggling and monitoring suspect vessels and airplanes.

"We spend most of our time on the patrol boat," he said.

He said border officers once had motorboats whose engines were started by pulling a rope by hand. They were difficult to use and often broke down.

"Now, all the motorboats have electric engine starters, which is more convenient," Xu said.

About 20 km away, another border team was steering its ship along the Wusuli River. The vessel, with a displacement of 150 metric tons, has been in use since 2015 to patrol the 130-km waterway.

"Conditions have improved greatly," said Wang He, 45, a border soldier for 27 years. "Twelve border officers and soldiers live on the ship from April to November. We now have stable power and a heating supply on the ship, which we couldn't have imagined before."

The vessel's interior, about 300 square meters, includes four dormitories, a bathroom and a kitchen.

Wang said he wants to pass the things he has learned to others before his retirement.

Fuyuan county, about 40 km from Heixiazi Island, also has seen big changes. At the end of 2012, it was linked to China's passenger rail network after a line was extended from Qianjin.

The new railway meant residents could reach Harbin, the provincial capital, in about 13 hours. In May 2014, the completion of Fuyuan Dongji Airport meant that the journey could be shortened to one hour and 20 minutes, with Beijing just a four-hour flight away.

"Convenient transportation is important for border officers like me," said Li Hui, 31, of the county's Zhengyang border police station. "I have only 20 days of vacation each year to return to my hometown in Lianyungang, Jiangsu province. In the past, I had to spend about a week on the road. Now it takes four days."

Li said he is looking forward to his next trip. "I hope I can make it to celebrate my daughter's first birthday on Nov 22," he said. "I believe life will be even better when she grows up."