"And why not, Fire Control?" Wilson said.
"Captain, it's the same old problem we always have with a diesel/AIP running off its silent storage batteries. We can't hear him except for flow noise when we're practically on top of the guy. And we can't go active without giving ourselves away."
"Now tell me something I don't know."
Jeffrey winced. "Well, sir, there's no way his sonar and signal processors can be as good as ours, 'cause his combat payload's small. We know we can run rings around the guy thanks to our nuclear propulsion. We know we can dive deeper."
"But we can't assume he doesn't know that, since they have boats that do that too." Jeffrey hesitated. "We know he has to snorkel eventually and run his noisy diesels, and if we can force him to sustain high speed, his quiet fuel-cell fuel won't last long either."
"So what do you think his mission is?"
"Could be one of several things. To lob a nuke at Diego Garcia. To lay mines in the approaches."
"And.:?"
"Could be an anti-surface-shipping patrol, against our sea lines of communications. Could be intel gathering or a commando op."
"In other words, Mr. Fuller, it could be anything."
"Yes, sir," Jeffrey said. He watched Ilse's fingers delicately tap her keys. He noticed she wore no jewelry, though her ears were pierced.
"How long till he's in missile firing range of the atoll?" Wilson said.
"Er, those boats don't have vertical launch capability, sir. The land-attack missiles they fire from the torpedo tubes have a range of about seventy-five nautical miles."
"Sir," Sessions offered, "the Israeli datum combined with Integrity's puts the enemy's mean speed of advance at sixteen knots."
"Right," Jeffrey said. "Since we're ninety miles from Diego now ourselves, that gives us about an hour."
Wilson snorted. "Sixty minutes till the base gets nuked?" Jeffrey nodded. "If that's their intent."
"You want to count on D.G.'s antiaircraft defenses splashing all those missiles?"
"Of course not, sir."
"Should we request more backup? Just plain admit that we can't find the guy?"
"Not yet, sir," Jeffrey said. "We're the best platform to prosecute, and we're in the best position. The air assets have plenty else to do."
"So what's the call?" Wilson said.
"That's what I've been thinking about, Captain."
"Well, break it down. How do you think he got this far?"
"I'm guessing he's a lurker, snuck in past Ranger west of here. After she was hit, I mean." Commodore Morse came over. "You're saying they planned it this way? Coordinated movements?"
Behind Morse's friendly manner there was a challenge to his tone. Morse had actually gone through the dreaded British Perisher, where they didn't just teach, they aggressively eliminated. The U.K. had fewer SSNs than the U.S.: ten compared to barely fifty — before war losses, that is. The Brits were nearly beaten by unrestricted submarine warfare twice in the last century, and they took the business seriously indeed. Jeffrey wished America had taken the business more seriously these past twenty years — when the U.S.S.R. broke up, the U.S. Navy had had twice as many SSNs as now, but far too many had been retired prematurely and not enough new ones built.
Morse was a foreign guest, Jeffrey told himself, but he was also the most senior man aboard. His interest here was not just academic. Everybody's life depended on it, and once again the U.K. was almost on the ropes. If this new Hot War went on much longer, the badly overworked attack-boat groups might not meet their commitments — the Allied fleets' initial six-month surge capacity was fast running out, the units badly needed maintenance and upgrades, and the Axis knew it.
Jeffrey met Morse's eyes. Morse's expression seemed to say, Go ahead, lad, impress me if you can.
"The problem, sirs," Jeffrey said, "is that the helos' air-dropped active sonobuoys don't have much power. If they ping while we listen for echoes, the detection probability would be low"
"That's true," Morse said.
"Their dipping sonar's better, but not by much. They just don't have the electrical capacity aboard. It would almost be better if we drew back, went deep so the dome wouldn't cavitate, and we pinged at max power, with the helos listening from closer in."
"Bipolar," Morse said, "but with the roles reversed."
"That's all fine in theory," Wilson said. "But … "
"But two things," Jeffrey said. "They can almost certainly hear the helos the same way we can, from the shock waves off their supersonic blade tips, and they don't know we're here, though they may suspect. Plus the helos don't have much lifting capacity either, so their lightweight torpedoes have a lower PK than ours. They just don't pack the punch." Jeffrey knew that the kill probability, PK, of Challenger's Advanced Capability Mark 48 torpedoes — called ADCAPs — was outstanding, but the Seahawk ASW helicopters simply couldn't lift the things and still have useful on-station time. Instead they each carried two air-droppable littoral-capable Mark 46 fish, whose warheads barely weighed a hundred pounds.
Wilson stood there, waiting.
"Do you mind if I look at the chart, sir?" Jeffrey said. "Go ahead, you're the fire control coordinator." Wilson gestured for Sessions to join them.
Jeffrey walked to the navigation plotting table, where Morse and Wilson had been caucusing before. The others followed. The navigator, Lieutenant Monaghan, a tall and gaunt man, respectfully stepped aside, then looked on.
Jeffrey wasn't interested in finding deep water now. Quite the contrary. "This is really an old idea, Captain, but we could have a helo drop a weapon set to go off right away, like a depth charge. Not close to the expected target, but here, toward these shoals in the distance." Jeffrey pointed to the digital nav display, to the East and Great Chagos banks, then continued.
"The explosive power might be enough to get an echo off the diesel, no matter how stealthy his hull coatings. Set the fish to blow at a hundred feet, and well astern of us to minimize our own signature. That would give two bearings, one from our bow sphere and another from the towed array."
"And the one," Morse said, "would solve that pesky ambiguity in the other."
"Exactly," Jeffrey said. "And their intersection would give the range." Unlike the bow sphere, Jeffrey knew, the towed array always showed a contact at two bearings on the wideband display.
"Captain," Jeffrey said, "doing it this way we'd also spare the helos the risk the target takes a shot at them, in case they get too close. Those Polyphem antiaircraft missiles they can fire from underwater have a nasty sting. A standard-sized torpedo tube holds four."
"Considerate of you to think of that," Wilson said.
"Then we can shift position this way," Jeffrey said, moving his finger on the chart again, "while waiting for the explosion's reverb to bounce back from the shoals."
"After a useful time delay," Morse said, "that reflected noise would hit the target from behind."
"He'll have moved," Jeffrey said, "and we'll now have him localized, so we get another set of bearings using our starboard wide-aperture array in hole-in-ocean mode. That'd be enough to give a complete firing solution, with target course and speed." Jeffrey knew hole-in-ocean passive sonar detected targets not by their radiated self-noise, but by their tendency to block the constant ocean sounds from farther off. The wide-aperture arrays were three widely spaced hydrophone complexes worn rigidly along the side of the hull like saddlebags, allowing electronic beamform scanning.
"Why not just go deep," Wilson said, "and use our ambient detection mode to watch him go by over us?" Ambient sonar was another form of covert active search — it used sea noise reflected from the target instead of own-ship pinging.
"Won't work, sir," Jeffrey said. "The water's deep enough to give a nice-sized look-up search cone, but the sea state locally's too mild to generate much surface noise and there're no nearby ships whose propulsion plants could act as sound projectors. Those diesel boats are small, barely half of our dimensions, so a quarter of our footprint."