THE SEARCH FROM THE SKY

Reproduced from Electronics Weekly (No. 314 - 7th September 1966) with kind permission of the publishers.

Ships and surface craft have become easy prey to airborne electronic reconnaissance, but stalking the elusive sub is another story. Told here by Cdr C. J. ELIOT (RN, Ret'd)

A major problem facing the Western maritime world today is the location of submarines. Russia is reputed to have over 400 and, with the advent of the nuclear submarine that can stay fully submerged for days on end, the problem of detection has become acute.

The primary vehicle used for searching out these elusive vessels is the maritime aircraft. Because of the amount of sophisticated electronic equipment that has to be carried, such aircraft are usually large shore-based types. There are six main types in use today.

Britain still relies on the Shackleton, but this faithful old aircraft has become too slow to cope with the modern submarine and is shortly to be replaced by a maritime version of the Comet 4C, the HS 801, which will be the first all-jet craft to be used for maritime reconnaissance.

Canada uses the Argus, a large four-engined, piston aircraft, somewhat similar to the Britannia. Its maximum speed is slightly greater than a Shackleton, but it is still too slow.

America for many years has used the Neptune but is now replacing it with the Orion. This latter is a fine modern aircraft made by Lockheeds with a turboprop engine. It is said to be capable of up to 400 knots and to have an endurance of nearly 13 hours on four engines or up to 17 on two.

Finally, there is the French Atlantic made by Brequet with subcontractors in Germany and Holland. It has twin turbo-prop engines made by Rolls-Royce and has an endurance of up to 18 hours at a slow speed.

The maritime reconnaissance aircraft probably has more electronic devices in it than any other. Lockheeds recently issued a list of the devices in the Orion and they number no less than 56 different items. Britain is rather more secretive, but it is safe to assume that the new HS 801 will have at least as many. They include various forms of sensors, communication equipment, ECM devices, sonobuoy recorders, navigational equipment and so on.

It would be impossible in the space of a short article to discuss all the possible devices that are fitted all that can be done is to take the most important.

Heading the list of sensors is, of course, the air to sea search radar. The three main types in use are the ASV21 (made by EMI Electronics), the AN/APS80 A (American) and the DR AA-2A (French).

All three radars are scanning types, designed to pick up snorts or periscopes. ASV21 has a PPI presentation and in addition to its primary use is capable of picking up ships and land at considerable distances. In addition, its scanner can be tilted so that it can be used for cloud or collision warning.

One of the snags of a radar that sweeps ahead of the aircraft is that its transmissions can be intercepted by the submarine's ECM equipment at about twice the radar detection range and this allows her plenty of time to dive before the aircraft arrives.

Various alternatives are being considered. One is a sideways looking radar similar to that which is being developed for the British Phantom. Another possibility is a line scan, which could be either optical or infrared. This is not a radar but a scanning device which searches the ground beneath the aircraft and records what it "sees" on film. Its range is limited, but it might be useful as an identification device. Sea trials have produced promising results.

Radar or scanning devices are, of course, dependent upon the submarine showing some portion of itself above the water. With modern submarines, the times when this is likely to occur are few and far between. The best scientific brains in the western world are therefore concentrating on devices that could be used by an aircraft to detect the fully submerged submarine.

The primary method of detection of a submerged submarine is by sonar, but an aircraft cannot tow a sonar set as does a ship. It is true that helicopters can be fitted with dunking (dipping) sonars, but the helicopter has to be more or less stationary. The maritime aircraft's only approximation, therefore, to the sonar set is the use of sonobuoys.

These buoys are thrown overboard from the aircraft and, when in the water, act as stationary sonar sets, transmitting any detections they make by radar to the monitoring aircraft. The buoys are normally of the passive type, that is to say, they are fitted with hydro-phones which listen for the minute noises made by a submarine. But there are also active types that transmit a sonar signal and receive the echo, as does the normal ship's sonar.

Sonobuoys can be surprisingly effective and it is possible for an aircraft to track a submarine by laying a large number of buoys to contain it. There is, however, a limit to the number of buoys that can be carried.

Another device for underwater detection, in use by the Americans, is M.A.D. or magnetic anomaly detector. This consists of a magnetometer installed in the aircraft at the end of a long boom on the tail to keep it clear of the magnetic parts of the aircraft. The principle is that a metal body, such as a submarine, can use the distortion of the earth's magnetic field which, although small, can be measured. The aircraft has to fly very low to get any readings at all and he devises, although useful, cannot be regarded as a primary means of detection.

There is one further device in use that senses diesel exhaust gases. All that it can tell the aircraft's crew is that diesel exhaust gases are present, which might mean that a submarine had been present on the surface shortly before. It could also be the trail left by a surface ship using diesel.

A more useful form of the detection device is DF and ECM analysis equipment. Here, of course, detection depends on the submarine's emitting some form of transmission, either wireless or radar. Submarines are very chary of using either, but radar is probably necessary when carrying out an attack and wireless may occasionally have to be used to report casualties, position, etc. With modern techniques this can probably be done by some form of condensed transmission lasting only a second or so, thus the ECM devices have to be very sophisticated. Probably both an ECM direction finder and a signal analyser would be fitted. No report of an enemy submarine is of any value unless the position of the reported enemy is accurate. Navigation of aircraft on patrol for hours on end is not easy, thus it is essential that the best possible navigational systems should be fitted.

A modern maritime aircraft would be fitted with both Inertial and Doppler navigation. The former system, as is well known, is an automatic dead reckoning device that is up-dated by any other means of fixing available. These might consist of Loran. Decca, VOR, and in the future navigational satellites. The navigational equipment alone might amount to as many as 19 different pieces of hardware.

A maritime aircraft has two main communication roles-to communicate with its base which may be hundreds or even thousands of miles away and to communicate with co-operating ships and aircraft.

In spite of trials of other systems, HF has been found to be the most satisfactory for the former, but it is still far from perfect and, in Canada, trials have been carried out, in an endeavor to improve it, using ionospheric sounders. An ionosonde receiver is fitted in the aircraft and sounding transmissions made from the base. The maximum usable frequency is thus indicated to the aircraft operator, who then has to select a frequency nearest to that indicated. The system is still in its infancy but might effect considerable improvements in this difficult form of communication.

Ordinary morse transmissions have been in use, but are being replaced by radioteletype. In the future, when communication satellites are a really viable concern, it should be possible to use them for communication over long distances.

Communication with co-operating ships and aircraft is normally by voice using UHF. In addition, the aircraft has to be fitted with VHF voice for ordinary traffic control communication.

Communication equipment takes up a considerable amount of space in the aircraft, particularly as some of it has to be duplicated for safety reasons. The American Orion has two HF transceivers, one VTF transmitter two VHF receivers and two UHF transceivers.

With a number of sources of information, fast I moving targets, not to mention own speed, the human brain would find it difficult to correlate and plot all the details obtained, so, in a modern maritime aircraft, this is done by a computer.

The system used is not unlike the Navy's Action Data System now fitted in Eagle and other ships, but, of course, much smaller.

All the information is fed into the computer where it is stored and can be called up when required. In addition, a tactical plot is provided on which target positions can be displayed to captain and navigator.

Considerable efforts are being made to find a really efficient way of detecting a submerged submarine from the air, but, as yet, there is no sign of a real breakthrough.

One possibility is infrared. A submarine raises the temperature of the water around it and a sensitive IR device could detect the change in temperature and record it on film. The Americans have done trials and claim to have recorded heat in the water several hours after a submarine had passed.

Another, somewhat doubtful, the idea is that a submarine submerged but close to the surface causes a "bump" in the ocean and it is thought this might be detectable.

For the detection of surfaced or partially surfaced submarines an image intensifier television is being considered.

The device is for use at night and, provided there is any ambient light at all (and except on the very darkest overcast night there usually is), objects in the sea will show up quite clearly.

Its advantage is that it emits no form of transmission and thus cannot be detected by the submarine. But it cannot measure range.

With all the improvements that have been made in electronics since the last war surface craft in the future will find it difficult to escape the watch from the sky, the same cannot be said of submarines. Provided the latter can remain fully submerged the chances of detecting them from the air are remote indeed.

What is needed is an underwater detection device capable of being operated by an aircraft at speeds of 200 knots or more. In the present state of the art, such a device does not appear feasible.