To their left, on a large circular TV screen, the tactical navigator was constructing his plot of the water below. The line of sonobuoys was marked by eight small green squares, each identified with a radio channel.
‘Fifty and twenty-seven are top buoys!’
The voice on the intercom indicated the Jezebels giving the strongest signals, the ones closest to the target.
Looking over the shoulder of the tactical navigator, the AEO saw that the top buoys were at each end of the chevron.
‘Spot on! He’s coming straight for us,’ he shouted with satisfaction.
Suddenly one processor operator jabbed a finger at the top of his sonar display, the green ‘waterfall’ sound pattern detected by buoy ‘36’ at the apex of the chevron. He was detecting something more than ripples in the pattern created by the distant Victor.
‘Hey, I’ve got something!’ he snapped into his boom-microphone.
He spun a roller-ball to move the cursor to the low-frequency noise that had caught his eye, a frequency too low to be audible to the human ear.
His fingers flicked switches to focus the narrow-band analyser onto it.
‘I’m getting doppler effect on thirty-six,’ he snapped again.
‘Same on forty-two,’ the second operator reported.
A minute reduction in the frequency detected told them something other than the Victor had just passed between two hydrophones and was heading away from the line.
The tactical navigator moved a cursor across his video map, to the position of the new target. He pressed a key to fix the co-ordinates in the navigation computer. The aircraft turned on its new heading.
‘Prepare DIFARS seven-five and zero-nine,’ the TacNav ordered.
In the rear of the plane aircrew selected directional buoys from a storage rack, set the radio channels, and loaded them into the ejection tubes.
A button on his control panel launched the first of the buoys. ‘Seven-five, gone. Turn now,’ sang out the TacNav. The plane banked sharply to reach the launch position for the second.
The AEO clutched the edge of the processor housing to steady himself. The ‘G’ force in the sharp turn threatened to buckle his knees.
‘Zero-nine, away.’
He crouched in front of the processor screens. The DIFAR buoys, directional and highly sensitive, would give the speed and bearing of the target.
Ten buoys in the water was no problem for the AQS.901. Sixteen could be monitored simultaneously on the four displays.
‘DIFAR seven-five gives bearing one-seven-zero, and decreasing.’
‘Zero-nine gives two-five-seven, increasing.’
‘Any classification yet?’ the AEO asked.
The operators studied the pattern emerging on their screens. Listening didn’t help; nothing but squeaks and crackles from shrimps and other marine life. It was down to the computer to analyse the low-frequency vibration of the target.
‘Looks like a bloody Trafalgar! That’s the noise signature!’
The second operator nodded in agreement.
So that was it. That was why the Victor had headed north.
‘Bet he’s never been that lucky before! A Victor tracking a Trafalgar? Impossible, according to the bloody Navy!’
‘They’re both doing nearly 30 knots!’
The bearings from the DIFARs changed rapidly as the target passed between them. The Russian boat was coming up fast through the Jezebel line.
‘They’ll both be deaf, going that fast,’ the AEO remarked.
‘Hang on!’ called the TacNav. ‘Our chap’s slowing down.’
‘She’s sprinting and drifting. This is where he finds out he’s picked up a tail. Could get interesting!’
While they waited to see what the Victor would do, the AEO grabbed the signaller’s clip-board of intelligence signals. Very odd. Not a word about a RN boat being in the area.
The portly, middle-aged AEO chortled inwardly at the chance of embarrassing the Navy. He drafted a brief, sarcastic signal to the joint Maritime headquarters at Northwood, reporting their contact, and asking if they knew where all their own submarines were. The radio operator hunched over his keyboard, encrypting the message from a code card.
Tim Pike was controlling the boat from the ‘bandstand’, a circular railing in the centre of the control room.
‘Depth fifty metres,’ yelled the helmsman.
‘Control Room, Sound Room!’ the communications box crackled.
Pike clicked the switch and acknowledged.
‘Contact’s gone active, sir! The sod’s pinging us!’
Cordell threw himself at the AIS Console. Sonar data were transferred automatically from the sound room to the AIS.
The intercept sensor projecting like a stubby finger from the upper casing of Truculent had detected the faint ‘ping’ from below the thermocline. The computer gave them a bearing and range.
Cordell saw from the amber lines snaking across the screen that the ‘ping’ had been too weak to detect them. The sound-absorbing tiles coating their hull would have prevented an echo.
‘Out of range,’ he called over his shoulder. ‘Contact bearing two-six-zero. Range five-thousand-three-hundred yards. Depth three-hundred metres.’
‘Closer than we thought,’ Pike breathed, leaning over the TAS officer’s shoulder. ‘Odd! The Soviets don’t usually go active — don’t want to give away their frequencies.’
The use of active sonar was a last resort for submariners; the signal inevitably revealed the position of the transmitting boat.
‘He’s just pinged again. Different angle. He’s searching for us.’
‘Time to show him our tail,’ warned Pike. ‘Steer zero-eight-zero. Revolutions for thirty knots. Clear the datum.’
‘He’s dead keen to keep tabs on us,’ Cordell mused. ‘Perhaps there’s a promotion in it for him!’
The submarine banked to starboard; the men in the control room gripped fittings and grab-rails.
Truculent would make more noise going fast, and her sonar would be deaf, but they needed the distance. Just a few minutes’ sprint, then they’d slow down to listen.
‘Give us room! Give us room!’ Pike muttered to himself, his body spring-tight with tension. ‘When he realizes we’re not deep any more, he’ll come up here looking for us.’
Silence fell in the control room, anxious eyes fixed on dials and screens.
‘Still pinging?’ Pike checked.
‘No, he’s stopped,’ replied Cordell.
‘Are we out of range if he pings again directly at us?’
‘Probably not. Stern on, we’re a small target, but he might get an echo off the propulsor.’
Truculent’s propulsion system was like an aircraft’s turbojet, a double row of compressor fans encased in a tube. Only from directly astern could the blades be detected on sonar.
‘Another course change,’ Pike ordered, swinging round to the planesman. ‘Port five. Steer zero-six-zero, and be ready to go deep again.’
‘How long now at this speed?’ Pike asked.
‘Six minutes, sir!’ answered the navigator.
Three miles they’d covered; three miles further from the Victor, he hoped. Time to listen again.
‘Reduce speed. Revs for fifteen knots,’ he called.
The instruction was relayed aft to the manoeuvring room. The response from the propulsion plant was almost immediate.