Chapter 1530 Three-dimensional Search

A few hours later.

Inside the command cabin of the Soryu submarine.

Captain Kenji Sato leaned over and stared at the constantly refreshing data stream on the electronic nautical chart, tapping lightly on the edge of the console with his fingertips.

The sweat beads on the forehead of sonar operator Koichi Yamamoto shone slightly under the cool instrument lights. His fingers moved quickly on the control panel, adjusting the sensitivity of the ZQQ-7 integrated sonar system.

"Captain, we have arrived at survey point B7," Deputy Captain Onodera reported in a low voice. "Sonar shows that there are no signs of large ships within 25 nautical miles."

Sato nodded slightly and sank into the wide backrest of the command chair:

"Begin diving and prepare to carry out surveying and mapping operations, focusing on recording terrain features suitable for submarine concealment and the placement of bottom mines."

Compared to when he first entered this sea area, his voice was obviously much more confident:

"In addition, we shut down the Stirling engine and only use the battery to maintain the lowest speed... We are still using the nautical charts provided by the United States in the 70s, and the error may reach several hundred meters. We can't take risks at this time."

As the order was given, the Soryu began to cruise slowly at a speed of less than 2 knots, and the high-resolution side-scan sonar at the bow emitted high-frequency sound waves to the seabed on both sides.

Traditional submarine navigation equipment is actually not complicated. It only needs to emit high-frequency sound waves diagonally downward to measure the distance to obstacles and avoid collisions.

However, the Soryu was designed with more complex intelligence gathering missions in mind, and therefore a hydrological detection device commonly found on oceanographic survey ships was specially installed.

As a price, the submarine's broadside area was reduced by about 20% compared to its later commissioned sisters.

Yamamoto concentrated on adjusting the parameters of the multi-beam navigation sonar, transmitting the seabed topography data to the central computer in real time.

Almost immediately, terrain features that were completely different from those on the old nautical chart appeared on the integrated display screen.

"Captain," Yamamoto said in a slightly higher voice due to surprise. "The seabed topography on the starboard side has changed significantly. Three parallel trenches have appeared at the location originally marked as a flat seabed. They are about 15 meters wide and 8 meters deep, which perfectly meets the requirements for the deployment of bottom mines."

Sato walked to the sonar console and squinted his eyes to examine the jagged lines. "Record the coordinates and mark them as priority points." He pointed to one of the most hidden gullies. "Especially here, this kind of terrain can completely avoid detection by conventional sonar equipment."

Chief Engineer Nakamura came from the engine room, holding a report on the Stirling engine's operation over the past few hours. "Captain, the engine is running smoothly, but long periods of low-speed cruising will cause the battery charging efficiency to decrease."

Sato took the report and glanced at it, then returned his gaze to the screen: "Hold on for a while longer, this is the last scanning point in area B. After we complete the mapping of this point, we will speed up the transfer."

Yamamoto suddenly looked up and said, "Captain, the passive sonar detected an abnormal echo in the distance. The characteristics are like some kind of man-made structure, but it is not marked on our nautical chart."

Sato's eyes flashed with a hint of alarm. "What's the exact location?"

"Northwest direction, about 2 nautical miles away, 85 meters deep." Yamamoto adjusted the filter. "Preliminary judgment is that these are steel pipe piles prepared for land reclamation, but there are no signs of subsequent construction. It may be left over from a failed attempt."

The atmosphere in the command cabin became slightly solemn.

Sato and Onodera exchanged glances - intelligence showed that China was likely to launch a large-scale island-building project within the year, but now it seemed that similar attempts were by no means recent.

If these "islands" that originally existed only in the sense of international law were to become naval and air force bases with real strategic value, then China's actual control over the surrounding waters would undoubtedly be greatly enhanced.

"Record the coordinates, but don't get close." Sato ordered, "Continue with the original surveying mission."

Meanwhile, above the water.

An Air-to-Submarine 200 anti-submarine aircraft was flying by at a low speed at an altitude of 150 meters. Inside the cabin, weapons operator Wang Lei stared at the screen of the magnetic anomaly detector with a frown.

"Captain, the third round of scanning is complete. No obvious abnormalities have been found in area B." He reported into the headset. "The sensitivity of the proton precession magnetometer is only 0.1nT. If the target is nearly stationary or its heading is approximately parallel to ours, it will be almost impossible to detect it...Should we use acoustic detection?"

The calm voice of Captain Li Qiang came from the headset:

"Not yet. We cannot risk alerting the enemy before the Star City fleet is in place. Fleet Command is certain that the target is in this sea area. We will conduct another cross-search."

Wang Lei sighed and began to adjust the equipment parameters.

He knew that the captain's caution was justified. Once the active sonar buoy was deployed, it would be equivalent to telling the other party that it had been discovered. And under the premise that it could not actually drop anti-submarine torpedoes, fixed-wing anti-submarine aircraft actually had no way to restrict the activities of a submarine.

"Taishan II, please note, turn to heading 245, and search in parallel at intervals of 5 nautical miles." Li Qiang's voice sounded again, "The target has been sailing for a long time and has crossed multiple latitudes. The magnetic signal should be slightly stronger than the theoretical value."

The two anti-submarine aircraft adjusted their formation, like two sea eagles circling over their prey.

Wang Lei noticed that the numbers on the magnetic anomaly detector suddenly jumped - a tiny change slightly larger than 0.1nT, which lasted less than two seconds.

"A momentary anomaly was discovered!" he reported immediately, but there was not much expectation in his tone.

This is not the first time that the magnetic anomaly detector has detected abnormal signals, but previous times were all proved to be interference from seabed manganese nodules in shallow waters.

In order to use a magnetic detector to find a deliberately hidden submarine, in addition to performance, a certain amount of luck is also required.

"Let's do another confirmation flight." Li Qiang remained as passionate as ever. After all, for a pilot, flying a jet airliner at low altitude and low speed is exciting enough.

When the anti-submarine aircraft flew over the suspicious area for the second time, the magnetic anomaly detector captured the weak signal again, which lasted for 2.5 seconds and the intensity rose to 0.2nT.

This time, Wang Lei also became excited.

Manganese nodules on the seabed generally do not produce such obvious directional anomalies, and there is a high probability that something else is hidden underwater.

Although it is not certain that it is a submarine, it is enough for surface ships to conduct a more detailed inspection...

……

Soon, the Soryu completed the surveying mission of the current area. Sato Kenji looked at the submarine clock and the pointer on it pointed to 1030.

"Retrieve the towed sonar, ascend to a depth of 70 meters, heading 195, speed 5 knots." He ordered, "Prepare to go to the next survey point."

"Captain," Chief Engineer Nakamura said hesitantly, "A speed of 5 knots will drain the power of the Stirling engine. Should we consider reducing it to 3 knots and replenishing the batteries?"

Sato rolled his eyes, obviously thinking about the other party's suggestion.

But in the end, he shook his head and said, "Time waits for no one, Nakamura-kun."

His eyes swept over every officer in the command cabin:

"Intelligence shows that the Chinese have begun to deploy underwater monitoring systems in Zhongsha and Xisha. In five years at most, this area will be covered by their monitoring network. By then, the difficulty of carrying out similar missions will increase exponentially, so this time we must map as many key points as possible."

The submarine began to slowly rise, with a slight metallic stress sound coming from the hull.

Sato walked to the sonar console and watched Yamamoto skillfully operate the recovery procedure. "Stay alert," he reminded in a low voice, "The hydrological conditions in this area are complicated, and there is no guarantee that there will be no obstacles at a depth of 70 meters..."

Just as the Canglong gradually accelerated to a speed of 5 knots, Wang Lei on the ADS-200 anti-submarine aircraft suddenly sat up straight. "Captain! The magnetic anomaly detector has a continuous response!" His voice trembled slightly with excitement. On the screen, a blue curve was showing regular fluctuations.

Li Qiang immediately turned the nose of the plane: "Mark the coordinates, turn 90 degrees and confirm again!"

The plane drew a tight arc, and the magnetic anomaly detector under the wing swept across the suspicious area again.

This time, the signal was even more obvious. "Confirmed abnormality, strength 0.4nT, there must be a big guy nearby!" Wang Lei sent the precise coordinates to the approaching surface ship formation through the data link while calling over the radio, "Taishan calling Longgong, a highly suspicious area has been found..."

"..."

In the combat command center of the destroyer Xingcheng, the fleet commander, Rear Admiral Yang Yuelun, stared at the updated information on the big screen, took a deep breath with confidence, and then immediately turned to the electronic chart table and drew three routes on it with his fingers:

"Order Jiangcheng to rush to the target coordinates. Xingcheng and Huangshan to deploy to the northeast and northwest respectively, keeping a distance of 15 nautical miles..."

He did not ask all three ships to rush to the coordinate point discovered by Taishan No. 1, but instead drew out a rectangular search strip.

The reason for doing so is that, on the one hand, there are a large number of submarine manganese nodules distributed under the South China Sea, and the magnetic field signal itself is extremely complex, so the reliability of the magnetic anomaly detection results is not high. On the other hand, we must also consider the possibility that the opponent is accelerating to move in other directions, and try to ensure a larger effective search range.

"Commander," asked the captain of the Xingcheng, Zhao Gang. "The coordinates provided by the anti-submarine aircraft are less than 140 nautical miles away from us, which is completely within the anti-submarine combat radius of the Z-12. Should we launch a ship-borne helicopter?"

"You can get the helicopter ready, but don't take off yet..." Yang Yuelun shook his head and explained, "We don't have much experience operating fixed-wing anti-submarine aircraft and anti-submarine helicopters in the same sea area at the same time. Besides, the fact that the target can be found by the magnetic anomaly detector means that the depth is not very high. The low-frequency noise of the helicopter rotor decays too slowly underwater and can easily alert the enemy."

Then, he pointed to an area on the electronic chart and said, "Tell the anti-submarine aircraft to continue monitoring and wait until we get within 50 nautical miles before considering deploying buoys."