Bottom Snapshot From The Sonar Data example essay topic

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 battle space 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 mine hunting sonars in the 60's Navies have been interested in developing databases of the mine like bottom objects with the areas that they may operate - the battle space. These efforts have been plagued by a number of fundamental 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 reinvestigate d 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 600 m wide channel the investigation and discrimination of all these objects would involve a speed of advance for the dedicated mine hunting vessel of less than one knot! There had to be a more efficient way. In the early 80's the sides can sonar systems were being supplemented by the fabulously powerful 286 computers. This allowed the sonar signal to be digitized, displayed on a screen and recorded to magnetic medium then stored.

This opened the way for the sides can sonar to be used to define the battle space. 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 "mine like contacts" geographically and a consequent lack of care with storage of the original sonar data. The sides can 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 sides can 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 maximis e the effectiveness of battle space 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 battle space is defined by selecting areas most favourable to searching. Hard bottom, smallest number of mine like objects, poor burial conditions etc... Detailed survey to establish a complete knowledge of the battle space. This includes environmental data collection within the battle space. Repeated surveys at highest sonar resolution possible to develop an intimate knowledge of the battle space.

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 sides can sonar used for route survey?

In the first instance sides can sonars were analogue and transmission controlled from the surface. A standard commercial type sides can sonar uses a fixed line array, single beam transducer with no focussing 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 tow fish the beam width of a single beam, un focussed sonar is equal to the length of the array, typically 40 cm. When the towed body transmits acoustic pulses are transmitted out both sides of the unit. The beam 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 un focussed sonar set at 100 m range (200 m swath total) is about 4 knots or less when looking for mine sized targets.

Typically mines of WWII or Korean vintage were cylinders about 2 metre's long and about 50 cm in diameter. But over the last 30 years the improvement of mine delivery and fusing systems has allowed a wide range of mine sizes and shapes to be developed. The "classic" ground (or bottom) mine sides can sonar return is shown below. This means that the classification of "mine like objects" has become so difficult that the differentiation of mines from the environment on a single sonar pass is impossible. TECHNOLOGY ADVANCES Over the last 10 years there have been two major steps forward in the commercial sides can arena. The first was the change from a hybrid analogue tow fish and digital processor to a full digital single beam system.

These new digital systems, such as the Klein 2000, provide excellent results and good definition at short ranges but are still troubled by beam spreading effects at longer ranges. The second is the fully digital multi beam sides can. The Klein 5000 sonar is the first commercial sonar of this type. It comes in a number of configurations but typically, five beams are used per side. This sonar was used to collect the sides can data for the common data set.

The advancing technology has provided the solution to the speed limitation of the single beam sides can sonars. The technique is to use a sonar with additional sonar beams. These additional beams give a higher speed of advance at greater operating range whilst maintaining 100% bottom coverage. The Klein System 5000 multi scan (or multi beam) sonar is the only commercially produced sonar of this genre. In addition to the additional beams, each beam is now focussed electronically in transmit and receive to give additional resolution. Specifically some advantages of a multi beam sides can sonar are: Area coverage rate increased Resolution is improved Towing speed is higher Total operating costs are lower Coverage rate.

The use of five beams per side on the sonar, operating at a frequency of about 400 k Hz, permits a 100% bottom coverage, with a total swath width (Port & Starboard) of 300 m, at a speed of advance of 5 m / sec (10 knots). At a 200 m swath the speed can be increased to 12 knots without detriment to the 100% bottom coverage. The operational comparison of a single beam un focussed sonar (which at similar frequencies has a maximum effective range of 100 m), operating at the optimum speed of 4 Knots and the focussed 5 beam sonar operating at 12 knots gives an operational advantage of 3 times. This advantage remains at least this high even if the speed is reduced and swath increased to allow for the need for some operational overlap of successive sonar passes.

Resolution The resolution of the focussed sonar will be higher than the un focussed over the full speed and range of the instruments. The single beam sonar will suffer from beam spreading which will give a beam footprint of about 1 m along track size at 100 m range from the tow fish. The focussed beams of the multi beam sonar are effectively columnar at 20 cm width each at the same range. Thus an object with an aspect of about 1 m will be seen once by the single beam sonar and five times by the multi beam focussed sonar.

In mine countermeasures the consequence of this improved along track resolution is that the probability of detection of bottom objects is significantly enhanced. In addition the edge detection capabilities for manmade objects is improved which allows some rudimentary classification possibilities. Operating costs It is clear from the foregoing description of the speed and range benefits of the multi beam sonar over the single beam of about 3: 1 that the cost of achieving the same amount of bottom survey coverage is about one third less with the multi beam. In terms of vessel and crew costs per survey day (based on a 200 day survey year) this could amount to a saving of about $2,000,000 per annum for each multi beam sonar used is tead of single beam systems. Data collection summary The first generation systems have done a quite good job of achieving the aims when employed by efficient, well trained crews. In addition the storage of this early data was invariably based on the positioning of all the "mine like contacts" geographically and a consequent lack of care with storage of the original sonar data.

DATA BASING OF SONAR INFORMATION When the first systems were introduced the methods for defining the location of bottom objects duplicated the traditional methods. This meant that the position of each bottom object was calculated and entered into the database with its attributes and subsequent surveys were meant to confirm these positions. Unfortunately, even with excellent navigational positioning, the best resolution that it was possible to obtain was a CEP of 12-15 metre's. This is obviously inadequate if the mine prosecuting vessel itself has a navigation inaccuracy of about 5 metre's. Pattern matching of the bottom object positions was then attempted with some success, although requiring significant computing power.

The solution to the positioning dilemma was bred from a MCM exercise sponsored by the Canadian DREP and conducted in Halifax Nova Scotia in 1986. At this exercise an area was side scanned to achieve a 400% bottom coverage from two directions. The number of bottom objects in this area was into the many thousands per Kilometer. After the initial survey, six mines or shapes were laid and the area re-surveyed to at least 200%. All records were compiled on wet paper recorder.

After the before and after records were compared all six mines or shapes were discovered. These were then dived by row and human divers and recovered. This interesting anecdote shows that the technique that needed to be employed was NOT positioning objects on the bottom but by comparing the image data (in this first case printed on paper) and using the human eye to detect changes. In Australia, the wheel went the cycle, starting with the traditional positioning then comparing images of sections of the bottom. This method was trial led over a couple of years and it was determined that the technique of pattern matching the image data is the only effective technique for route survey using sides can sonars. This is because the variation in mine sizes, types and burial conditions means that the "classic" mine return in the picture is very rarely seen except for older style mines on a clear hard packed sand bottom.

Pattern matching uses the basic premise described earlier that in peacetime, without a prior conflict in the area, the battle space is benign. The "picture" of the bottom is equally as important as the geographical positioning of objects on it. As described during the DREP trial in Halifax, after a number of passes through the area it is relatively easy to detect a foreign object amongst the known items in the battle space. To achieve the collection of data a system such as the Triton-E lics "ISIS" sonar imaging software is used. This system not only databases the position of contacts marked by the operator in real time, but also stores a size selectable, surrounding bottom snapshot from the sonar data. This makes subsequent geographical comparison of contacts much quicker.

The role of the operator on the route survey vessel is to collect the highest quality data possible and to mark objects seen as the sonar views the bottom (as a first line filter). This data is then transferred to a centralised data centre for review and certification. Control of the collected sonar and contact data must be managed centrally so that the routes are not compromised by independent authorities changing them. The quality of the data is kept standard and the database of contacts is not corrupted. This centralised system (for convenience sake) is named the Route Survey Database System (RSDS). The RSDS normally should comprise two major subsystems: Firstly, a geographic Information System (GIS), which should normally comprise five primary elements: i. provision of an automated central repository for route survey geographic data collected by the COOP units, ii. maintenance of a master file of all MCM routes, . provision of sophisticated data analysis and processing functions to aid the RSDS operator in the final assessment and correction / quality control of field data and management of route maps, iv. production capability for a tactical publication describing the battle space, and vs. provision of an automated system for generating operational packages for the field units including data disks of navigation and environmental data, pre-plots, set up parameters, survey instructions and survey priorities.

Secondly, the system must have a sonar Image Processing System (IPS), which should comprise four primary elements: i. provision of a review capability of sonar data recorded on the data collection vessels, ii. maintenance of an image based master file of all sonar contacts, targets and bottom classifications.. provision of sophisticated tools for correction of sub-sea navigational information, and iv. production of composite bottom mosaic printouts of sides can sonar data for Q-Route selection purposes. The sonar image processing facility is comprised of hardware and software that fall into two categories, image replay (and contact data review) and bottom mosaic generation. The IPS must be interfaced to the RSDS GIS across a data link that allows 2 way communication for control of navigational positional data to the sonar record. Techniques and procedures. Q-Route Operational Scenario description During survey operations the vessel is guided along pre-selected track lines via a helmsman's display. The surface position of the vessel is determined from a Differential GPS positioning system.

Often, the sides can tow fish is tracked relative to the vessel. During survey operations contacts are selected by the sonar operator via a cursor on the sonar screen display and contact position co-ordinates are computed using the associated navigation information (navigation information is recorded with each sonar 'ping' on the optical disc). The sonar display console stores the target co-ordinates, dimensions, optical disc index and operator annotations. (This is also stored on the optical disc). The contact position is shown on the tactical display in the correct navigational position. Post Mission Analysis Although not directly related to the survey vessel operation it is necessary that personnel understand that the optical disk recorded on the vessel is not the end product of an operation, but in many ways it is the commencement of another more involved one.

Furthermore, the ultimate quality and usefulness of the product acquired for delivery to the RSDS is dependant on an understanding of the ramifications of poor data collection quality. Subsequent route surveys will be carried out periodically to update the regional database. The new survey data is reviewed at the RSDS and compared to the current data to determine changes in the seafloor and submerged objects. Detected changes are then accepted into the database Conclusion The most important factor when considering the use of sides can sonars for route survey is the need for centralised control of operations and all quality and operational functions.

It is axiomatic that if quality data is collected by the survey vessels then future enhanced processing techniques and procedures will be able to be applied to it. The raw sonar data is critical to comparison analysis of the battle space and must be treated with the utmost care. Procedures must be introduced that enhance the confidence level in the database quality for the battle space being considered so that expensive ship time can be minimise d but the same level of clearance (or safety) confidence maintained..