The Sea-Based X-Band Radar, a key link in the nation’s defense against North Korea, spent eight months at sea before returning to its home in Pearl Harbor last month.

But the powerful $2.2 billion discriminating radar can’t be used operationally anywhere near Hawaii, and its return here for “routine maintenance” highlights a major shortcoming of the system: its ship-based design. The need to return to port creates gaps in its ability to track North Korean warheads at a time when such a threat is becoming ever more real.

The Missile Defense Agency said in its fiscal 2018 budget overview that it requested funding “to extend on-station time from 120 days at sea to 330 days to expand contingency operations for defense of the homeland.”

The MDA also is racing to build replacement ground-based radars in Alaska and Hawaii. In the meantime it’s not clear when the SBX will head out again.

“For security reasons, we do not comment on the ship’s deployment schedule,” MDA Director Lt. Gen. Sam Greaves said in an email. The SBX “completed its previous mission and is currently in port conducting routine maintenance,” he said.

Riki Ellison, chairman of the nonprofit Missile Defense Advocacy Alliance, thinks the SBX will be back out on duty again before the end of the year.

“They will get it back out to sea as fast as they can and as soon as its upgrades and maintenance are done,” Ellison said. He added that the 280-foot-tall floating radar “has to be seaworthy and requires scheduled maintenance for its sustainment of operations — like all of our Navy ships that go on deployments for six months and come back to port for maintenance and upgrades.”

The SBX has been going out on longer deployments, he said. Most recently the radar was at sea from early January to mid-September. The longest deployment was in 2007-08 for 392 days, according to the Missile Defense Agency.

The SBX radar acquires and tracks ballistic missiles and discriminates warheads from rocket debris and decoys. The Union of Concerned Scientists said the SBX is the primary discrimination radar of the ground-based midcourse defense system, which would defend Hawaii and the mainland from North Korean missiles using interceptor rockets in Alaska and California.

Ellison said in a media release that additional interceptors are being emplaced at Fort Greely, Alaska, with a total of 44 expected to be in place in Alaska and California next month.

The Union of Concerned Scientists said the first notice of an offensive missile launch would come from early-warning satellites detecting the launch flame and from forward-based radars in Japan or on Navy ships with the Aegis SPY-1 radar.

The “threat cloud” would next be picked up by the Cobra Dane radar on Shemya Island at the far end of the Aleutian island chain and the SBX radar — if it has been moved into position, the science group said in a 2016 report.

News agency Reuters previously reported that the SBX left Hawaii in January to monitor potential North Korean missile launches from a destination about 2,000 miles northwest of the state.

The Union of Concerned Scientists said while the SBX “is the most capable discrimination radar” in the ground-based midcourse defense system, “it has a number of serious limitations,” including a very limited electronic field of view that affects its ability to track multiple launches and targets.

The scientific group noted that a Clinton-era plan called for a large ground-based radar on Shemya Island and eight or nine other ground radars for precision missile tracking — but the radars were never constructed.

Instead, the seagoing SBX was created, primarily for missile defense testing but also for operational purposes, with the MDA arguing the mobile radar could support a wider range of testing scenarios across the Pacific and was also cheaper to build, the group’s report said.

“Implicitly acknowledging the shortcomings of the SBX radar,” the MDA in 2014 announced plans for a long-range discrimination radar at Clear Air Force Station in central Alaska, according to the report.

Additionally, MDA now is pursuing a “Homeland Defense Radar – Hawaii,” a medium-range discrimination radar, for the enhanced protection of the state; it could cost $1 billion and see initial operating capability in 2023.

Ellison said the ground-based interceptor missiles “have adequate sensor discrimination capability to intercept without the SBX, but the SBX increases it, and therefore gives more reliability to the system on whole.”

In the future the SBX might go to the East Coast, or the military “will look at taking it off the platform and putting it on land, but it still remains a tremendous asset to have in the Pacific,” he said.