'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 contact?' 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 — the probability envelope of uncertainty.
'I have a second bipolar contact,' Sessions said, 'bearings three one nine and three two one! I merge and designate this Master 2!'
'There's two of them?' Jeffrey said.
'Affirmative!' Sessions shouted. 'Shoal echoes put their speed at twenty knots!' The odds just lengthened badly, Jeffrey told himself.
The enemy boats were at long range for him to make a good torpedo shot, and they were closer to Diego Garcia than Challenger was. They must be going all out now on batteries. Their endurance at that speed was short, clear indication they were up to something — something nasty — and it was coming very soon.
'Flood tubes one through eight,' Wilson said. 'Equalize the pressure and open all muzzle doors.'
They were still well away from the targets, Ilse knew. Was this so they wouldn't hear us?
'Sonar,' Jeffrey said, 'what's target depth?'
