“May as well shoot it now, before the helicopters are close enough to hear any banging.”
“I understand,” Henri said. “I will see to it. I need fifteen minutes to backhaul a countermeasure and insert a bathythermograph.”
“Very well, Henri. Launch it when ready.”
He terminated the communications with the Specter and shot a glance at his executive officer.
“Okay,” Walker said. “I give in. What’s a bathythermo-whatever? Some sort of temperature sensor?”
“Correct. You shoot it from a submarine, and it stays tethered to the launcher until its line breaks. It tells you what the temperature is all the way down.”
“What’s the point? We can’t dive anywhere near as deep as the bottom.”
“Sound will bounce off the bottom, or it might curve away from it. Sound will also bounce off the surface but usually curve back up to bounce off it again. Sound’s behavior is predictable based upon pressure, salinity, and temperature, but temperature is the overriding factor.”
“What’s it all mean?”
“It means, that if the sound velocity profile is favorable, you can find a good depth for listening for other submarines. Or you can find a good depth where you can hide with your sound being trapped in a layer of water.”
“Hide, as in hiding from helicopters?”
“Perhaps. Depth may provide us protection, and I want to know where that protection might begin.”
Cahill looked at the overhead view of icons on his screen. With the Chinese warships and helicopters outside acoustic detection range, their stale location data had decayed to a guess. A glance at his executive officer’s slumped shoulders showed him lamenting the same staleness.
“How close do the helicopters need to be to hear them?” Walker asked.
“That’s an answer of mixed parameters and mixed emotions. For a conventional submarine, hearing a helicopter only means that it’s close enough to be dangerous. However, on this beauty, hearing a helicopter may provide enough warning to surface and fight.”
“I see,” Walker said. “Now I understand how you’ve mitigated the risk. Unless one lowers its dipping sonar right on top of us, we’ll have a chance to fight back before we would find ourselves underneath a torpedo.”
“Right. Helicopter torpedoes have less range than heavyweight torpedoes, which means a helicopter needs to get closer. That gives us reaction time before one can launch.”
“But if a hostile helicopter achieves a position atop of us, we’re dead, right?” Walker asked.
“Yes. Quite dead, barring a miracle of this ship’s size or speed granting us a second chance.”
A green icon glimmered on Cahill’s screen, and he tapped it to accept a voice connection with the Specter.
“Cahill,” he said.
“This is Henri, sir. I’m ready to launch a bathythermograph.”
“That was fast.”
“Antoine had already thought of it and had a unit staged by the launcher.”
“Are you waiting for an order? I said you could launch when ready.”
“Not waiting. Just informing you. I didn’t want to surprise you in case this creates a metallic transient noise.”
“Very well. Launch it.”
Moments later, Cahill heard a subtle thud. As he questioned if the sound of the sensor’s launch were real or imagined, a loud clank confirmed that the unit had banged against a support arm on its journey to the depths.
“Thank God there’s nobody close enough to hear that,” Walker said.
“Agreed. But let’s trust and verify.”
“Verify? You mean to risk a radio transmission?”
“Perhaps,” Cahill said. “But first let’s see if Pierre’s already doing us the favor of transmitting useful information. As long as I’ve made noise, I may as well risk exposing our radio antenna for a download.”
A frown cast a shadow over Walker’s face.
“What’s wrong?” Cahill asked.
“I’m not sure that Pierre isn’t already doing us the favor. Look at his low-bandwidth transmissions.”
As Cahill called up a history of recent transmissions from Pierre Renard, he recalled the communications protocol.
Renard had borrowed the bandwidth of high-endurance P3 Orion aircraft that the South Korean Navy flew to communicate with its submarines. With the Goliath submerged, the water above its antennas absorbed the electromagnetic energy of all but the aircraft’s lowest transmission frequencies. Low frequencies meant low baud rates and lethargy of information flow.
Every minute, Renard sent a fifty-character message of gibberish so that the Goliath could verify its low-frequency reception. When the Frenchman needed to send a message of content, the characters changed, and an algorithm in the transport vessel’s radio system raised an alarm if it recognized English words.
As Cahill studied the history of symbols, he noticed numbers that appeared meaningless. But equipped with Walker’s queuing, he noticed the pattern, and then he noticed an error in the system.
“Good gracious, mate,” he said. “Nice catch, Liam.”
Walker nodded and smirked.
“I’ll make note to have the system updated to recognize the word ‘helo’. Apparently, such shorthand escaped the grasp of those who wrote the English detection algorithm.”
“Right,” Cahill said. “He’s sending us coordinates of helicopters. Helo one and then helo two, each with latitude and longitude to the sixth decimal.”
“Right.”
“Are you checking this on the chart?”
“Already ahead of you. Done.”
On Cahill’s screen, the icons of two helicopters jumped through hyperspace and appeared ahead of the Goliath’s track. Twenty miles separated the ship from the nearer aircraft.
“Damn,” Cahill said. “Those pilots have made good guesses on where to look for us.”
“When do you start to worry?”
“Worry? I don’t intend to worry. But I will shoot them down before they approach within five miles.”
“Should we maneuver?”
“No change to course or speed. That’s just a guessing game the way helicopters maneuver so quickly. But depth is still in play. Time to check on the sound velocity profile.”
He opened the channel to his cargo submarine, and he heard a new French accent.
“Specter, Antoine here.”
“Antoine? Where’s Henri?”
“He’s right here beside me in the control room, but he knew that you wanted to ask about the sound-velocity profile. So he had me answer. I just finished computing it. Do you want me to send you the graphical data?”
“No. Just let me know if there’s a layer I can dive below.”
“It’s close to your diving limits. There’s a layer at one hundred and eight meters. You’re only designed for one hundred meters. So it’s a coin toss if one hundred meters will mask any of your noise. The Specter will be exposed above the layer, unless you go to one hundred and fifteen meters.”
Cahill’s mind stalled as he faced the hardest decision in his short tenure as the Goliath’s commanding officer. Seeking the acoustic layer would risk overpressure damage but improve his chances of unfettered travel. Staying shallow would hasten surfacing if he needed to fend off his hunters.
“Terry?” Remy asked.
“Yes, yes, Antoine. We’ll stay shallow. Thank you for the information.”
“Okay. I will listen for helicopters, then.”
As Cahill secured the voice channel, he noticed an icon appear eight miles from his ship.
“You see the update from Renard?” Walker asked.