McM Sonar Technology

ABSTRACT

Searching for mines is a time consuming and relatively hazardous operation that is heavily weighted in the favour of the miner if the defending force has not prepared the battlespace beforehand. In simple terms, if the environment in which the enemy is likely to launch a mine attack is known, and the defending forces are familiar with the bottom conditions then the enemy mines are more easily localised and subsequently eliminated. The method of achieving this familiarity with the environment is known by a number of terms but most commonly used is Q-Route Survey or just Route Survey.

From the introduction of minehunting sonars in the 60's Navies have been interested in developing databases of the minelike bottom objects with theareas that they may operate - the battlespace. These efforts have been plagued by a number of fundemental equipment and philosophical problems. Apart from the difficulties faced with precisely positioning the mine-like objects on the bottom the general navigation and plotting accuracies of the vessels was very poor. This contributed to so great a lack of confidence by succeeding vessel commanders about the validity of the database of bottom objects that the databases invariably failed. The failure of a MCM database is catastrophic for the defenders, after an attack, as it means that all bottom objects would need to be reinvestigated to prove they were not mines.

Even in moderately cluttered bottom conditions such as in harbours or approaches where there may be 300-400 objects per kilometer of 600m wide channel the investigation and discrimination of all these objects would involve a speed of advance for the dedicated minehunting vessel of less than one knot!

There had to be a more efficient way. In the early 80's the sidescan sonar systems were being supplemented by the fabulously powerful 286 computers. This allowed the sonar signal to be digitised, displayed on a screen and recorded to magnetic medium then stored.

This opened the way for the sidescan sonar to be used to define the battlespace. The first generation systems have done a quite good job of achieving the aims when employed by efficient, well trained crews. However, this has been the exception rather than the rule, and the quality of the data so far collected probably less than optimum. In addition the storage of this early data was invariably based on the basis of positioning all the "minelike contacts" geographically and a consequent lack of care with storage of the original sonar data.

The sidescan sonar tools used by navies for the task of route survey are typically about 10 years old and comprise a hybrid Analogue sonar with digital recording and display of the data. A few Navies have developed first generation MCM GIS systems and sidescan sonar digital mosaic production facilities.

Over the past few years the Analogue sonars have become obsolescent with the introduction of fully digital sonars offering the advantage of better performance over long cable lengths and better control over data quality. Also, during the same period there has been a plethora of GIS packages come into the marketplace at very small investment costs for the underlying software.

These changes offer the opportunity to maximise the effectiveness of battlespace preparations at a very low investment cost. The key to success in this type of operation is the use of effective and proven techniques for the data collection and management with good quality control. The storage and review, including processing and reporting, of the collected data centrally using a Geographic Information System (GIS) linked to an Image Processing System (IPS) is the final step in the process.

Route survey is normally divided into a number of discrete phases. These are:

 Route or area selection, where the battlespace is defined by selecting areas most favourable to searching. Hard bottom, smallest number of minelike objects, poor burial conditions etc..

 Detailed survey to establish a complete knowledge of the battlespace. This includes environmental data collection within the battlespace.

 Repeated surveys at highest sonar resolution possible to develop an intimate knowledge of the battlespace.

In peacetime these procedures are capable of being performed by commercial vessels (fishing boats etc) or COOP (Craft Of Opportunity) as the basic, not unreasonable, assumption that is made that mines do not presently exist in a time of peace. In time of rising tension however, these commercial vessels will probably be replaced with autonomous or remotely controlled vessels, so that the hazard to the vessel crews from mine explosion is eliminated. This will especially be the case if mining has been detected.

How is a sidescan sonar used for route survey?

In the first instance sidescan sonars were analogue and transmission controlled from the surface. A standard commercial type sidescan sonar uses a fixed line array, single beam transducer with no focusing

or steering. The transducer design results in a beam pattern which is fairly narrow in the horizontal plane and broad in the vertical plane. When operating close to the towfish the beam width of a single beam, unfocused

sonar is equal to the length of the array, typically 40cm.

When the towed body transmits acoustic pulses are transmitted out both sides of the unit. The beam insonifies a swath on the seafloor with the total swath on each side being controlled by selecting the operating range. Due to the speed of sound the pulse repetition rate is determined by the range set. The physics of the environment cause this as sufficient time must be allowed for the acoustic pulse (or sound) to travel out to the maximum range and return to the sonar before the next pulse is transmitted.

Consequently, the sonar must be towed at a speed at which the soar only travels a distance equal to the beam width covered in a single ping (pulse), if 100% of the seafloor is to be covered. Speeds higher than this will introduce gaps into the data coverage which will reduce the probability of detection for small bottom objects.

The optimum operating speed for a single beam unfocused

sonar set at 100m range (200m swath total) is about 4 knots or less when looking for mine sized targets.

Typically mines of WW2 or Korean vintage were cylinders