Why would China use a spy balloon when it has satellites?

News of an alleged Chinese spy balloon floating over the US has left many wondering why Beijing would want to use a relatively unsophisticated tool for its surveillance of the US mainland.

China has said the balloon, spotted over the state of Montana, is merely a "civilian airship" which deviated from its planned route, but the US suspects it is a "high-altitude surveillance" device.

Whatever the capabilities of this particular balloon, the US has taken the threat seriously enough to postpone Secretary of State Antony Blinken's visit to China, which was due to take place on 5 and 6 February.

Balloons are one of the oldest forms of surveillance technology. The Japanese military used them to launch incendiary bombs in the US during World War Two. They were also widely used by the US and the Soviet Union during the Cold War.

More recently, the US has reportedly been considering adding high-altitude inflatables into the Pentagon's surveillance network. Modern balloons typically hover between 24km-37km above the earth's surface (80,000ft-120,000ft).

"Beijing is probably trying to signal to Washington: 'While we want to improve ties, we are also ever ready for sustained competition, using any means necessary,' without severely inflaming tensions," independent air-power analyst He Yuan Ming told the BBC.

"And what better tool for this than a seemingly innocuous balloon?"

The balloon's anticipated flight path near certain missile bases suggests it is unlikely it has drifted off course, He Yuan Ming said.

The US Department of Defence on Thursday said the balloon is "significantly above where civilian air traffic is active".

But China expert Benjamin Ho said Beijing had more sophisticated surveillance technology at its disposal.

"They have other means to spy out American infrastructure, or whatever information they wanted to obtain. The balloon was to send a signal to the Americans, and also to see how the Americans would react," explained Dr Ho - coordinator of the China programme at Singapore's S Rajaratnam School of International Studies.

It may even be the case that China wanted the US to detect the balloon.

"It's possible that being spotted was the whole point. China might be using the balloon to demonstrate that it has a sophisticated technological capability to penetrate US airspace without risking a serious escalation. In this regard, a balloon is a pretty ideal choice," said Arthur Holland Michel from the Carnegie Council for Ethics in International Affairs.

Nevertheless, the experts point out that balloons can be fitted with modern technology like spy cameras and radar sensors, and there are some advantages to using balloons for surveillance - chief of which is that it is less expensive and easier to deploy than drones or satellites.

The balloon's slower speed also allows it to loiter over and monitor the target area for longer periods. A satellite's movement, on the other hand, is restricted to its orbital pass.