"How new is this?"
"They were testing one on USS Memphis back in the late nineties at DEVRON TWELVE, our squadron. That's what we do. We're operational SSNs and we also test technology and tactics, working with the Naval Underwater Systems Center in Rhode Island, and the various contractors."
Ilse stared at the terrain estimation display. "You can see right through things!"
"Sure," Sessions said. "Matter's transparent to gravity, right? One seamount shows up past another. Good thing too, else we couldn't go this fast so close to the bottom."
"You must use some kind of stored data as backup, don't you?"
"We do," Sessions said, "and to speed these calculations also. We have decent bottom charts, for gross verification and course plotting."
"You don't just feel your way?"
"Not usually. Hitting a basalt cliff head-on would be embarrassing."
"Yeah," Ilse said.
"The helm guys need to stay real sharp," Sessions said. "At twenty-six knots we move one boat length every eight seconds. Watch this." On-screen Ilse could see that the canyon they'd been following took a sharp turn to the left. As if on cue to her thoughts the boat banked to port. She watched as Challenger followed the canyon leftward, still hugging the deep ravine's right wall, several hundred feet up. The boat leveled off as it came out of the turn.
"You can see why we don't stream a towed array," Sessions said.
"Neat," Ilse said. "But how come we don't stay right on the bottom?"
"Stealth. It's too obvious. If we follow one wall partway up, we still get all the benefits of terrain masking."
"And I guess that makes it tougher for the enemy to lie in ambush or plant a mine."
"You got it, Ilse. It also gives us more lateral clearance, room for turning sideways just in case. Right down on the canyon floor we'd be boxed in."
"Can you use this under ice?" Ilse said. "For ice avoidance?"
"Maybe someday," Sessions said. "It's great to fix your posit under the ice cap, update your inertial nav, since you can't just pop up for a GPS fix then, even in peacetime. Instead we orienteer from the gravimeter, based on distinctive bottom features and our charts. For ice avoidance we have to use our sail-mounted high-frequency sonar, which radiates and has short range and can't see past a bummock or berg. The problem with the gravimeter is the density gradient's not strong enough — rock or sediment versus water's one thing, but ice versus water's something else."
"Then what about detecting other subs? Or surface ships?"
"Smart question," Sessions said. "No good, unfortunately, is the answer. Floating surface units and submarines displace a mass of water equal to their weight, right? So unless you're really close, there's not enough change in the gravity field."
"What if you are close?" Ilse said.
"There's another problem if they're moving," Sessions said. "It adds a centripetal gradient that's unknown to the algorithms, not like own-ship velocity. And if you do know target range and course and speed, who needs the gravimeter?"
"What about a nuclear sub, though, one that's motionless or on the bottom? The reactor compartment must be extra heavy. Wouldn't that show up, compared to spaces full of air, as a mass field discontinuity?"
"Now you're getting into classified stuff," Sessions said. "But it's no secret gravimetry can help you avoid something man-made that's real heavy, like an oil drilling platform, since it doesn't move."
Ilse went back to her screens. Watching Challenger's swift progress through the benthic topography was fascinating. The boat followed an S-curve between underwater peaks, hard right and then hard left again. Ilse could see it coming, on the bird's-eye view, and she watched the other picture as they took the turns, like looking out the windshield of a car.
"I have another question," Ilse said, "if this isn't secret."
"Try me," Sessions said.
"I need to understand this to help you navigate. How come Jeffrey isn't giving orders?"
"You mean no helm commands?"
"Yeah. When we fought those diesel boats, Captain Wilson kept saying make your course this, make your depth that, hard right rudder … "
"Nap-of-seafloor's different. That's one reason we have senior people at the helm. Commander Fuller can overrule them, even take control himself if need be from his console, but this is different from maneuvers near the surface. Here we follow the terrain."
"So our detailed course is a given," Ilse said. "Mother Nature calls the shots."
"Pretty much."
"Could you fight a battle this far down?"
"I suppose," Sessions said. "The enemy would have to find us first. Down here you get lost in the sonar grass, just like with radar, and even look-down acoustic Doppler, which tracks suspended particles, gets confused by multiple currents at different depths."
"So more depth means more protection?" Ilse said.
"Yeah," Sessions said. "It's not just that you get more thermal and salinity layers to hide behind—"
"Plus the deep scattering layer too," Ilse said, "as schools of biologics migrate up and down the water column every day."
"That's right," Sessions said. "Anyway, the point is, if the enemy is sort of overhead, the spherical attenuation model holds. The intensity of our self-noise as received by the other guy goes down with the square of range, so ten times deeper means just one percent the signal strength."
"I should bone up on this stuff," Ilse said.
"You need to," Jeffrey interrupted. "There's a series of tutorials you can run on the computer, with homework problems and everything."
Ilse realized he'd been listening in. There really is no privacy on a submarine, she told herself. "Yes, Commander," she responded, slightly irked. "I'll look at the tutorials once I'm done with the gravimeter." Jeffrey turned back to his console. Ilse watched her screens again. She saw large boulders show up now and then. Probably from underwater landslides, from seismic activity she knew never really ceased. Sometimes she saw talus slopes, rubble built up over eons at the base of undersea escarpments.
"You can fiddle with the picture if you like," Sessions said. "Use your trackmarble to look ahead, or rotate the presentation and see things from a different angle." He showed her how.
"This is fun," Ilse said. "But I have another question. With nap-of-seafloor, we're not too exposed, you know, to enemy sensors planted on the bottom?"
"Yes and no," Jeffrey butted in again. "That's why we use broken terrain, where it's hard to place a trip-wire grid and seafloor current eddies make for lots of false alarms. We'd avoid a smooth, flat open-ocean basin at all costs, for just that reason."
"Oh, okay," Ilse said.
"Another thing," Sessions said, "with the exotic nonacoustic ASW methods, like surface-wake anomalies, is they need minutes or hours of supercomputer time to make sense of the raw data, by which point you're tens of miles away. That's why we always zigzag these days, even out in blue water, deny the enemy our base course and speed."
"Right," Ilse said. Boy, do these guys love their shop talk.
"And now," Sessions said, "please excuse me." He got up and started walking the line of sonar consoles, talking to his people. Ilse went back to her screens. Challenger plunged onward.
Up ahead, on the display, she noticed an interesting formation of big rocks on the bottom, where the canyon opened out. The rocks lay in a perfect triangle.
"What's that, Helm?" she heard Jeffrey say.
"More boulders, sir," Meltzer said. "We'll pass well clear."
"Very well, Helm," Jeffrey said.
As they got closer to the boulders, Jeffrey watched the gravimeter's resolution sharpen. " That middle one. COB, if you will, gimme a close-up."
The image changed as Challenger seemed to leap ahead.
Jeffrey stared. "All stop!" he shouted. "Hover on manual!"