Chinese astronauts successfully performed the country's first tandem spacewalk on Sunday, working for seven hours on the outside of the new Tiangong station in orbit around Earth.

Tiangong's construction is a major step in China's ambitious space programme, which has seen the nation land a rover on Mars and send probes to the Moon.

Three astronauts blasted off last month to become the station's first crew, where they are to remain for three months in China's longest crewed mission to date.

On Sunday morning, two of them exited the station for around seven hours of work in the first spacewalk at Tiangong, the China Manned Space Agency said.

Researchers from the National Time Service Center (NTSC) of the Chinese Academy of Sciences have proposed a new method to realize absolute calibration of multi-mode satellite navigation receiver delay.

The new method can be applied to fields like navigation positioning, satellite timing, and time transfer, increasing user positioning and timing accuracy.

The calibration of global navigation satellite system (GNSS) receiver delay poses a technology difficulty in the field of satellite navigation. The widely used relative calibration method can only obtain the delay difference between a tested receiver and a reference receiver.

However, fields like GNSS time offset monitoring, satellite timing technology and satellite-ground time synchronization need absolute calibration.

NTSC researchers used satellite simulator hardware and high-speed oscilloscope to completely calibrate all kinds of GNSS receiver delay, with the calibration accuracy better than 1.5ns(1σ).

An atomic clock provided reference frequency for all equipment. Testing Time-to-Code (TtC) by the oscilloscope was used to calibrate the simulator delay, and the channel delay could be calculated by the pseudorange, 1and PPS output delay measured by a time interval counter.