PEARL HARBOR — The outside of the 280-foot-tall Sea-Based X-Band Radar, a platform large enough to accommodate 18 basketball courts on its six pillar legs, is impressive enough.

But inside the domed top, the nine-story phased-array radar — an octagonal flat panel studded with 45,000 radiating elements delineated in Aztec-like geometry — really gets the "wow" factor.

The radar is so powerful it can detect a baseball-sized object thousands of miles away. Part of the United States' developing missile defense system, it will be used to track targets and can tell nuclear warheads from decoys.

The $900 million-plus radar returned to Hawai'i last month from the waters of the Aleutian Islands for $27 million in repairs and upgrades — up to $12 million of which will go to BAE Systems Hawai'i Shipyards.

Boeing is in charge of development of the system for the Missile Defense Agency.

The towering radar will be a fixture on the Pearl Harbor skyline until early August, when it will head out to sea for systems and ballistic missile defense testing, and then return in October or November for several more months before returning to its home base in Adak, Alaska.

There, it will be moored to eight anchor chains for the first time, or moved around the ocean for missile tracking tests. In March, the radar and two Aegis destroyers tracked a long-range missile fired from California.

Army Col. John Fellows, project manager for the sea-based radar, yesterday said the plan was to move the one-of-a-kind platform to Adak and use it as a test asset.

"As we learn more and more about the system, we want to make sure that we have the right capability, so we have adjusted the plan," Fellows said.

The radar has come and gone multiple times from Pearl Harbor, sometimes as a result of sea testing and sometimes to fix problems, since it first arrived in January 2006 from Corpus Christi, Texas, perched on a heavy-lift vessel.

Valve damage to ballast piping was discovered, then an independent examination in June 2006 found a host of other problems with the former oil-drilling platform.

Among the improvements being made at Pearl Harbor are the addition of a second crane, improvements to its helicopter landing platform, rescue boat changes, improvement of the galley and addition of satellite TV for the crew of up to 100.

The self-propelled radar platform can take on ballast to ride 45 feet lower in heavy seas. It endured 100-knot gusts and 50- to 60-foot waves on the winter shakedown trip to the Bering Sea.

"When you are ballasted down, you don't feel the movement of the ship. Generally, it's a lot better than any cruise I've been on," Fellows said.

"It's different for sure," said Edward Pillert, a Kane'ohe resident and captain of the Sea-Based X-Band Radar, or SBX.

In April, Lt. Gen. Henry "Trey" Obering, the director of the Missile Defense Agency, told a Senate panel that "ballistic missile threats are real and growing," with the pace of foreign missile testing twice that of last year.

Since June 2004, Obering said, the U.S. had placed 17 long-range interceptors in Alaska and California, modified 16 Aegis ships for missile tracking, upgraded three land-based early warning radars and delivered transportable radars, including the Sea-Based X-Band radar.

Reach William Cole at wcole@honoluluadvertiser.com.