Story highlights Patriot anti-missile batteries appeared overnight this week in Japanese capital

They are deployed in case North Korea carries out its threat to test-fire a missile

Would take between five and 10 minutes for missile to reach skies over Tokyo

Japan has deployed these measures before during previous launches

Two PAC 3 Patriot anti-missile batteries currently occupy what is normally a baseball pitch in the leafy grounds of Japan's Ministry of Defense. They point northwest -- in the general direction of North Korea.

They were moved here to the heart of the Japanese capital in the early hours of Tuesday morning, alongside further batteries in two Tokyo suburbs.

And if the intelligence is right, and North Korea has moved one, maybe two mid-range missiles to its eastern coast, any test firing it might choose to conduct could conceivably have its trajectory over Japan.

Pyongyang is just 800 miles (1,287 kilometers) from Tokyo. It would take between five and 10 minutes for a missile to reach the skies over Tokyo. The concern here is not that North Korea has Japan or indeed any particular target in its sights, whatever its grandiose claims, it is that something might go wrong with a missile test.

"The North Koreans are not trying to attack Japan but to scare the Americans," says Narushige Michishita, director of security studies at the National Graduate Institute for Policy Studies (GRIPS) in Tokyo. ''The problem is the missile might malfunction and part of the debris might land on Japanese territory."

Residents whose homes overlook the Ministry of Defense are getting used to these military apparitions outside their bedroom windows. The Patriots have been deployed here three times since 2009, each time North Korea has announced it will launch what it claims is a satellite, but what most other countries call a missile.

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"The situation is acute," Japan's defense minister told the unit commander on a visit to the site laid on for journalists. "Be ready to take prompt action whenever the order is made."

It is arguable that this deployment in the heart of the capital is partly for show -- to guarantee photographers an easy front page and thus reassure nervous Tokyoites that their safety is secured.

But the missile defense shield the Japanese have developed with the U.S. to safeguard this region is one of the world's most sophisticated. These latest deployments are about fine-tuning. If the previous three "satellite" launches are anything to go by, the Japanese government will have positioned Aegis Destroyers equipped with SM-3 missile interception systems in the Sea of Japan, also known as the East Sea.

It has been widely reported that has happened -- though the government won't confirm it, unwilling to "show its cards," a spokesman said, to the North Koreans.

These deployments should suffice, says Michishita. "Generally a sea-based system is more useful because it has a wider area it can defend. PAC 3 is more contained but together they are effective.''

But Kim Jong Un's belligerence has some Tokyo residents clearly worried.

Junro Kato came out in search of TV crews like us armed with a map showing how far North Korea's missiles can reach. "North Korea can hit Japan in five minutes," he said.

"If there are miscalculations between U.S. and North Korea, military conflict will happen."

For others though, the overriding emotion is astonishment -- astonishment at their puffed up neighbor's endless posturing.