Global concerns over North Korea's latest nuclear test and long-range rocket launch have shone a spotlight on the perennial, high-tech game of hide-and-seek played around Pyongyang's advanced weapons programmes.

On the seeking side are analysts using high-resolution images from a circling constellation of commercial satellites to pick up whatever data they can on the North's fast-developing nuclear and ballistic missile capabilities.

It's a challenging task that more often than not is reduced to educated guesswork, as the number of clues they have to work with is diminished by an increasingly effective North Korean concealment programme.

Satellite imaging has been around for decades, but for most of that time was almost exclusively for government and military use. It was only at the start of the 21st century that high-resolution commercial images became publicly available.

- A new window -

For civilian North Korea watchers, the aerial snapshots provided by organisations such as DigitalGlobe and Airbus DS opened an entirely new window onto one of the most closed regions in the world.

Two of the main monitoring targets are on opposite sides of the country: The Punggye-ri nuclear test site in the northeast and the Sohae satellite launch complex in the northwest.

"From early on the demand for informed analysis on North Korea was higher than for any other country, with the possible exception of Iran," said Allison Puccioni, an imagery analyst with expertise in North Korea.

"The fervour over the Punggye-ri test site was unlike anything else, especially in terms of media interest."

Effective analysis requires high-level expertise in areas like nuclear fuel cycles and advanced weapons systems.

But the key raw materials are images and, in recent years, the North Koreans have become far more adept at hiding their activities from the satellite lenses.

"They have certainly adopted additional concealment measures," said Puccioni.

"They are clearly well aware of the orbital patterns of the satellites and are operating around those patterns to avoid detection," she added.

Another tactic is to work at night or on cloudy, rainy days.

- Under wraps -

In a recent post on the 38 North website run by the US-Korea Institute at Johns Hopkins University, imagery analyst Jack Liu noted how North Korea's fourth nuclear test on January 6 had taken the international community by surprise.

"Unlike the 2013 test, when heavy activity was seen in the weeks prior to that test, very little preparation activity was visible leading up to the 2016 test," Liu wrote.

"The North Koreans have continued to improve their operational security and concealment procedures, limiting what is visible to commercial imaging satellites," he added.

38 North specialises in satellite imagery analysis of North Korea, which is also provided by Jane's IHS and the Washington-based Institute for Science and International Security.

Managing editor of 38 North, Jenny Town, said North Korea had quickly become wise to the satellite monitoring process.

"They've learned what we are looking for, and so they've learned when not to do things," Town told AFP.

They have also covered, screened or moved underground structures at sensitive sites to shield them from observation.

This is particularly true at the Sohae satellite launch site where a rocket can now be brought in, assembled and erected on the launch pad without being directly caught on camera.

"We can see movement, but we still don't know what, if anything, is actually being moved," said Town.

During preparations for the North's first successful space rocket launch in December 2012, satellite images showed the three-stage carrier sitting on the launch pad.

By contrast, the rocket that blasted off on Sunday was never seen, although an image collected the day before the launch showed a tractor-trailer that was "probably" bringing in the third stage, Town said.

"Again, that's down to the concealment procedures. They just didn't reveal what was there during the satellite hours," she added.

- Sharper eyes -

North Korean efforts to avoid detection have been partly mitigated by technical developments on the monitoring side.

In June 2014, US federal law permitted American companies to sell satellite imagery with a resolution of 25 centimetres (10 inches) per pixel -- a dramatic 400 percent increase in resolution from the sharpest images previously available.

An even stronger weapon is the development of radar and infrared imaging.

"With radar, you still get returns on objects under concealment as the radar hits right through the camouflage," Puccioni said.

At the same time, there is an increasing number of commercial companies providing images.

That will result in the launch of around 70 new satellites over the next five years, adding to the 14 already in orbit, Puccioni said.

"So obviously, that's a much larger constellation."

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IHS Global Insight

DIGITALGLOBE