Deep Sound Channel

by Joe Buff

  "I suppose I'll find out soon."

  Jeffrey glanced away for a moment, then looked back. "You speak American English very well."

  "I spent four years in San Diego. I got my Ph.D. from Scripps."

  "Impressive."

  "So I've been on lots of research ships. Diving is a hobby. And I dated a few navy guys, out of Coronado. . . . No, they didn't blab about your precious submarine. I've been briefed, on the way out here. Only up to a point, apparently, but by COMSUBPAC

  himself."

  "Really?" Jeffrey sounded almost jealous.

  "He said this boat's assigned to DEVRON TWELVE, as if that explained everything."

  "It would, to a submariner. Development Squadron Twelve . "

  "And if you're wondering why they picked a woman, I wasn't their first choice. The other three they tried were men, but none of them had the guts for it." Ilse wriggled her bottom to get more comfortable in the seat. She put the first CD-RW

  into the drive, worked the console's keyboard, and massaged the trackball with her palm. She called up a menu she knew by heart, then leaned over to talk to the sonar officer, a slightly plump lieutenant sitting to her right. Step one, she presumed, would be to enhance his ocean models, used to compute sonar detection and target counterdetection zones. That should keep her busy for a while.

  "Anyway," Jeffrey said, "I'm going forward to talk to Webs." Ilse hardly noticed. The smell of coffee began to fill the air as the messenger put up a fresh pot in a nearby pantry alcove. Then he came around. Ilse and the sonar officer both took their coffee black.

  Sonar said his name was Robert Sessions. He looked in his mid-twenties. If war made people older, Ilse told herself, he must be very young. At almost thirty she felt ancient.

  "Mmmm," Sessions said, drinking deep. "Caffeine. drug of choice of the Silent Service." Ilse couldn't help smiling. She had some coffee herself, very strong and hot. That's better. She'd slept badly on the plane.

  She leaned over to examine Sessions' waterfall displays, which she barely understood. " Whose do you think it is?"

  "You mean the other sub?"

  Ilse nodded.

  Sessions shrugged. "No whiffs at all yet. Doesn't matter much. They're the same designs, right? Same crew training, far as we know, Germany and South Africa." Like many of the other crewmen, Sessions wore a baseball cap, with the ship's name and number and an emblem. On the emblem was a dragonfish, a black deep-sea creature with a distorted fish's body and a hideous face with long sharp teeth and a dangling luminous barb. The dragonfish was grinning, clutching in its stunted fins a torpedo and a missile. Challenger? Of course! Ilse realized the sub wasn't named for the blown-up space shuttle after all. HMS Challenger was the first dedicated oceanographic research vessel, a British sailing ship in 1872.

  Ilse liked the hat. She wanted one.

  4 HOURS LATER

  "Captain," Jeffrey said, "the boat's closed up at battle stations antisubmarine. We are rigged for ultraquiet. Our course is one eight zero, speed six knots; depth is fifteen hundred feet."

  "Very well, Fire Control," Wilson said. At general quarters Jeffrey as XO was fire control coordinator, overseeing Sonar, Weapons, and the TMA—target-motion analysis—team. The captain had the deck and conn as approach coordinator, grand strategist in the upcoming duel.

  Jeffrey paced the three steps back to Sonar. "Still nothing, Commander," Sessions said. " We continue tracking Sierra 6, that convoy." Jeffrey leaned between him and Ilse, looking over Sessions' shoulder as he pointed to the wideband and narrowband displays.

  "Those are the fast auxiliaries and escorts," Jeffrey said, "heading west to replenish the Reagan carrier battle group."

  "Correct, sir," Sessions said. "These thick lines here're the battleship Wisconsin." The display was labeled across the top like an unraveled helix, showing each bearing several times but from different incoming depression/elevation angles. Jeffrey hit a switch on Sessions' console, then read the screen.

  "Blade rate on Wisconsin's four shafts says she's doing thirty-one knots."

  "She's so noisy we hold contact in the surface duct at a hundred nautical miles."

  "Survivable as hell, though," Jeffrey said, "in this sort of war."

  "Agreed, sir," Sessions said. "The main thing for big surface units is keep moving fast. . .

  . These two contacts are our ASW helos." He tapped a pair of thin lines slanting across the broadband waterfall. "They just relieved the first ones that were running low on fuel.

  "

  "They doing ladder searches?" Jeffrey said.

  "Yes, sir," Sessions said.

  "Dropping sonobuoys or dipping sonar?" "Intermittent plopping sounds. No pinging yet, so the hostile can't steal echoes off our hull."

  Jeffrey nodded. "No sign the helos have a passive contact?"

  "Negative, Sir, neither one is circling."

  "That would be our cue," Jeffrey said. "Let's hope they hold their fire, on the off chance they pick us up. Them's the rules of engagement—no one wants a blueon-blue." Jeffrey glanced at Ilse, who looked up at him. She was busy rewriting some computer code. Jeffrey thought her lips seemed very sensuous in the subdued control room lighting. Her pupils were nicely dilated. Then he caught himself.

  "I've traded e-mails with your onboard systems administrator more than once," Ilse stated. "This change will make better use of real-time surface temperature inputs and static-height anomaly data."

  "More accurate water column density gradients?" Jeffrey said.

  "Exactly."

  "Um, great."

  "Sir," Sessions said, "we also have Sierras 7 and 8, Klakring and McClusky, the ASW frigates on the outer picket line. They're intermittent in the second convergence zone, some sixty nautical miles, as they zigzag in their sectors. But nothing submerged."

  "We know the enemy's out there somewhere," Jeffrey said. "Integrity's passive towed array heard him running on air-independent propulsion. The PROBSUB's been upgraded to a CERTSUB."

  "After we picked up their data dump on EHF," Sessions said, "when we went back to '

  scope depth, I looked at it myself. Faint but clear low-frequency tonals from the fuel-cell reactant circulators, looked like a German Klasse 212."

  "They probably switched to batteries, at least for now."

  Jeffrey knew Integrity was a SWATH—small water-plane twin-hull—a steel catamaran displacing 5,000 tons and "armed" with a twin-line passive towed array. Fully 6,000 feet long, that gear had 180 acoustic channels and was optimized for hunting diesel subs by their near-infrasonic tonals.

  As Jeffrey and Sessions huddled, Wilson walked over. "Talk to me," he said.

  "Sir," Jeffrey said, "allowing for possible variations in enemy course and speed, our target should be somewhere on an arc to the west of us, within striking distance of our weapons, but we have no datum for a shot."

  "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 com

  bat 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 o
r 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."

  "That's one of their strengths," Morse interjected.

  "Exactly, sir," Jeffrey said. "Right here right now he wouldn't be acoustically illuminated well enough for us to see."

  "And you're not worried," Wilson said, "with your helo torpedo bipolar thing, that this enemy boat could use active out-of-phase sound emissions to break up the echo, and active feed-through noise to plug the hole?"

  "Captain," Jeffrey said, "first of all, those subs don't have that capability like we do, at least not to the degree they'd fool our signal processing algorithms. At a fifth of our displacement they're too small to carry the computers and the power supply"

  "Okay," Wilson said.

  "Secondly, what makes it even harder for them is that the explosion would be gray noise, not a coherent active ping, with very complex reverb coming back from off those shoals.

  . . . And thirdly, for hole-plugging emissions to do much good they need to know our relative bearing."

  "What if we go through this rigmarole and there's no conta
ct?" Wilson said.

  "We should shift position and try again," Jeffrey said. "I'd head closer to the atoll, since in this case the greater risk exposure is underestimating target speed." Wilson gave Sessions a piercing look. "And what do you think, Sonar?"

  "Urn, I concur with Commander Fuller, sir. We can also use the wide-aperture arrays to do synthetic instant range gating. The data would be soft, but it would help us validate the firing solution."

  "Good," Wilson said. "That's what the ARCI's for." "And I think the helos should stay here for a while,"

  Jeffrey said, "keep working this same area. Have them

  drop a few more 46s now and then."

  "Give the bad guys a false sense of security?" Wilson said.

  "Affirmative, sir," Jeffrey said, smiling. He glanced again at Ilse Reebeck, who seemed to be taking all this in while pretending not to. He made eye contact and she looked away.

  Jeffrey wondered if Morse and Wilson had intended this front/back bipolar trick themselves, all along. Maybe Wilson just wanted to see if Jeffrey could come up with it.

  "Sonar," Wilson said, "how's wave action topside?" "Long gentle swells now, Captain."

  "Coming way up from Antarctica," Jeffrey said, "as you'd expect this time of year."

  "Maybe three feet crest to trough," Sessions said. "Period?" Wilson said.

  "Four per minute, sir," Sessions said.

  "Any contacts overhead?"

  "Negative, Captain."

  "Very well, Sonar," Wilson said. "Fire Control, we'll use your plan." Wilson turned to Meltzer at the helm. "Bring the boat to periscope depth. Let's talk to those helos."

  "Chief of the Watch," Jeffrey said to COB, "stand by to raise the UHF antenna." Then Jeffrey grabbed a mike and punched in circuit 22. "Radio, Fire Control, prepare to send on short-range airborne tactical."

  "Sir," Sessions said to Jeffrey, "the helo's parachute-retarded torpedo has hit the water. . .

  . Unit has detonated, acoustic power spectrum confirms a Mark 46 conventional warhead."

  "Very well," Jeffrey said. Since the explosion was dead aft and relatively small, he didn't hear it through the hull.

  "Sir," Sessions said, "I have a submerged bipolar contact, bearings three two five and three two three on towed array and bow sphere! I designate the contact Master 1." Jeffrey watched the bearing lines appear and intersect on his target-motion analyzer window, all relayed to him by the combat control technicians sitting to his right. The range was over 30,000 yards. A 3-D representation of the tactical picture came up on the CACC's wide-screen situation display, with a multicolored halo around the contact—