Sharov heard the winch start up at the same time he got Lukin’s report. The sonar winch was a big thing, filling half the helicopter’s cabin behind the pilots with one hundred fifty meters of stout cable and the one-hundred-and-eighty-kilogram sonar array on the end. An opening in the cabin floor allowed the array body to be lowered into the water, while the operator, Lukin, monitored the procedure.
The winch started and stopped automatically, the Lira system stepping through a standard procedure: Lower the sonar projector to a water depth of twenty-five meters, listen for thirty seconds, then lower it to fifty, listen again, then one hundred, and finally one hundred and fifty meters. The entire sequence took several minutes.
Sharov read the status indicators on the mission display while splitting his attention between the ice-covered horizon, the engine instruments, and Red 50’s position, loitering a little to the northeast. It was waiting for Red 81 to finish her search. The results would determine where she had to dip. It was very unlikely that the first dip would be right over a contact, but if Red 81’s sonar picked up anything, then Red 50 would do its best to dip closer to the contact. Working as a team, the helicopters would use leapfrog tactics to first detect, then localize, and finally attack any submarine in the area. Assuming there was anything to find.
“Passive search completed, no contacts,” Lukin reported. “Request permission to go active.” If their sonar didn’t hear anything, then they could send out active pulses to look for a very quiet contact.
“Yes, go to active search.”
The intercom report was expected, but it still startled everyone in UCC. “Conn, Sonar. Active sonar, bears one seven one degrees.” While Cavanaugh was still trying to figure out where that was in relation to everything else, LT Ben Ford immediately ordered, “Walter, José, all stop, and hug the bottom.” Behind Ford, the commodore gave a small nod of approval.
As Petty Officer Alvarez typed on José’s console, Petty Officer Frederick sang out, “Walter holds the active sonar at one eight six degrees.” Alvarez gave the bearing from José seconds later. Jerry pointed to the fire control technician at the geoplot, instructing him to plot the bearings and get a position on the helicopter that was pinging away.
Sonar came back on to announce, “Conn, Sonar. Active sonar is classified as a Lamb Tail. It’s unlikely that they detected José. No chance of seeing Walter, and own ship is well out of range.”
A moment after that Jerry reported over the intercom, “Control, UCC, we’ve got a three-point fix on the Lamb Tail. It’s two thousand one hundred yards to the south of José. Recommend sending José northwest before bringing him back. Own ship should also head west.”
Weiss’s voice came up on the circuit and responded “Concur, changing course to the west.”
Cavanaugh felt the deck shift slightly. Carter had started turning.
“Commodore, what kind of sonar is a Lamb Tail?” he asked, breaking his long silence.
“It’s a NATO code name for a dipping sonar. It’s a high-frequency set, and relatively short-ranged, but putting it on a helicopter makes it very mobile, and of course we usually can’t hear the helicopter until it puts the sonar in the water and starts pinging.”
“And it didn’t detect the UUV,” Cavanaugh stated hopefully.
“No, or his partner would already be dipping on top of José, and we would be in a very different situation,” Mitchell explained.
“His partner?”
“ASW helicopters operate in pairs, usually from ships, but these were probably flying from the island.”
“Why didn’t the active sonar detect the UUV?”
“Like Carter, the UUVs have anechoic coating that absorbs active sonar pings, reducing the amount of energy that is reflected. That can cut the detection range by roughly half. And since the UUV is very small, was moving slowly, and was close to the bottom, the acoustic processor would have a hard time telling José from a rock. Just to be sure, LT Ford had them put José on the bottom and stationary until the sonar stopped transmitting. Right now, the two helicopters are positioning for another dip. They’ll listen first, and then if they don’t hear anything, they’ll ping.”
“Then shouldn’t the UUV shut down its own sonar?”
“No, it doesn’t need to. José and Walter use really short-range, very high frequency sonars. They operate at several hundred kilohertz, but the Lamb Tail is transmitting and listening at twelve to fourteen kilohertz. It simply can’t hear the UUVs’ sonar, the same way we can’t hear a dog whistle.
“Same thing goes for the acoustic modems that allow us to communicate with the UUVs. They don’t operate anywhere near as high as the imaging sonar, but the modem frequency is still up there and is outside the frequency range of the dipping sonar. That and the transmissions use pseudo-random noise sequencing to hide the signal.”
“And the sensor can’t change the frequency it listens to?”
“Nope,” Jerry announced confidently. “They’d have to have a separate receiver. And it’s hard to passively search for sounds at such high frequency. Remember, Doctor, the higher the frequency, the greater the attenuation loss. The UUVs use it for close-range navigation and imaging, so range doesn’t matter as much. Its best range is just over a hundred yards. The Lamb Tail is a dedicated search sonar. It can see and listen to contacts several thousand yards away…”
“And Carter’s sonar is even lower-frequency, for greater range,” Cavanaugh concluded. The physics made sense, once he remembered to apply it properly.
“Lower frequency means larger size, too. The UUV’s sonar transducer is the size of a microwave oven. The Lamb Tail’s sonar is the size of a mini refrigerator. And Carter’s active bow array is like a big hot tub by comparison.”
During their discussion, José had been motoring west at a brisk four knots, but still just off the bottom.
The intercom chirped “UCC, Control. Report status of comms with Walter?”
Jerry relayed the request to team Walter, and Petty Officer Frederick checked the display. “We’re good, sir. Signal strength is still strong, but I’d recommend limiting any transmissions. At least for now.”
“Very well, have Walter head west at slow speed and secure communications.” Clicking his mike, Jerry replied, “Control, UCC. Comms are good, however, we are securing them for the time being. Walter has been ordered to head west.”
Sharov put in a search axis of three five zero, based on nothing more than a guess. Standard tactics was to dip on each side of the axis, with an interval just slightly less than twice the sonar’s detection range.
The sonar contact had been northwest of Center. While there was more open water straight north, the intruder might zig west instead of zagging north. The Lira combat system calculated the two next dip points, based on the sonar conditions and the contact’s estimated speed, which Sharov believed was slow. Both helicopters would go active simultaneously this time, but Sharov had biased the dip points so that the sonars’ detection ranges just barely overlapped.
The air was still rough, and he kept one eye on the engine instruments. He’d heard stories of turbulence shaking things loose, and considering how low they were to the water, there would be little time to correct, or even autorotate down if the engines failed. They wore immersion suits, which would give them a little time in the water before they died of hypothermia, but hopefully long enough for Red 50 to fish them out.