Sonar Glossary Archives - Teledyne Reson Sonar Glossary Archives - Teledyne Reson

Sonar Glossary


03 /09 /10

Z-Kinking: The failure of cable conductors (characterized by the “z” shape of the damaged portion) resulting from apparent movement between the core and the jacket. At underwater cable terminations, this can occur with conductor extrusion under pressure (pistoning) from a flexible jacket into the dead end of a connector body.


03 /09 /10

Yaw: An instability characterized by the side to side movement of a ship or towed body about its vertical axis. Vessel and towfish yaw most often occur at low towspeeds and with quartering seas. Towfish yaw is distinctive in sonar data and has an effect in the sonar data similar to the effect of micro-turns.


03 /09 /10

Water Column: The vertical section of water from the surface to the bottom in which a sonar is towed; also the centre section of an uncorrected sonar record. The white strip in the centre of a non-range-corrected sonar record represents this water column above and below the towfish. This portion of the record can be informative to the survey when depicting the first bottom and surface returns as well as schools of fish and targets near the towfish.

Wavelength: The distance, measured in the direction of propagation, between two successive points in a wave that are characterized by the same phase of oscillation. Along with the transmit power of a sonar, the wavelength (directly related to sonar frequency) will determine the ultimate range of the system. Decreases in wavelength (increases in frequency) bring higher resolution with the trade off of reduced range.

Wharf: A structure serving as a berthing place for vessels.


03 /09 /10

Variation: Grid: Angular difference in direction between grid north and magnetic north. It is measured east or west from grid north. Also called grivation, or grid magnetic angle.

Variation: Magnetic: The angle between the magnetic and geographical meridians at any place, expressed in degrees east or west to indicate the direction of magnetic north from true north. Also called magnetic variation, or magnetic declination.

Velocimeter: See velocity profiler.

Velocity: A vector quantity equal to speed in a given direction.

Velocity of Sound: See sound velocity.

Velocity Profiler: An instrument used for the ‘in situ’ measurement of the sound velocity in the sea and other natural waters.

Vernal Equinox: See Equinox.

Vertical Beam Width : The angle of the transmitted (and/or received) side scan sonar pulse in the vertical dimension, typically between 40 and 70 degrees. The wide vertical beam in side scan sonar allows the acoustic pulse to propagate and insonify the seafloor over a long range. In a sonar with a narrow vertical beam, depending upon the downward tilt of the main lobe, some areas will be poorly uninsonified. If the narrow beam is tilted down significantly , range will be sacrificed, while near-range seabed will not be insonified if the beam is directed more to the horizontal.

Vessel Reference Unit (VRU): See motion reference unit.


03 /09 /10

Uplink: Provides all the sonar data from the sonar head to the processor.

UUV: Acronym for Unmanned Underwater Vehicle. There are two categories of UUVs: AUVs and ROVs.


03 /09 /10

Termination: The junction of either end of a towcable where it is fitted with a single or multi-pin connector. Lightweight towcables typically have connectors for the towfish at the “wet end” and at the control/display unit on the “dry end.” Armoured cable is usually terminated with a connector at the towbody end and wired to a block inside the winch drum at the other. This aids in troubleshooting the towcable if necessary. Quality terminations are important to a functioning system and are also a delicate component of the operational assembly. Termination “kits” are part of a spares inventory and include all the parts and tools needed to re-terminate a cable should the connectors or the cable become damaged.

Thermocline: A layer of water where the vertical temperature gradient is greater than that in the water above it or in the water below it. Thermoclines affect the ray path of acoustic signals underwater and can result in a range-limiting type of banding visible in side scan sonar data. Similar to the effects of a haline front, this banding is most evident at the outer ranges of sonar data where the beam’s angle of incidence to the thermocline is high. Changing the sonar geometry will minimize or eliminate the effects of thermoclines.

Tide: The periodic rise and fall of the surface of the Oceans, Bays, etc., due principally to the gravitational attraction of the Moon and Sun for the rotating Earth.

Time Synchronisation: The synchronisation, based on time, of the recorded data to produce maps or reports that require the merger of successive SeaBat profiles.

Time Varied Gain: (TVG) A process where amplifier gain is changed based on time and matched with the returning signals between outgoing pulses of a side scan sonar. Because of attenuation of a sonar beam, the receiver gain must be increased as the acoustic returns from greater and greater distances arrive at the transducer. Because these returns are received over a predictable and consistent time, the gains can be increased over a time curve. In many sonar systems, this curve can be adjusted by the operator.

Track: The actual path or route of a craft over the ground or sea bottom, or its graphic representation. In Air Navigation also called track made good.

Track: Great Circle: The track of a craft following a Great Circle, or a Great Circle which a craft intends to follow approximately.

Transducer: The electromechanical component of a sonar system that is mounted underwater and converts electrical energy to sound energy and vice versa. The transducer formation determines the beam shape and is the basis for image formation in side scan sonar. Its condition and stability help determine the final image quality. Transducers can be surrounded by various types of acoustically transparent urethanes or epoxies, or can be within an oil-filled assembly. For the purposes of side scan, transducers are almost always mounted on a towbody which also contains firing and amplifying circuitry. They are towed over a cable instead of hull mounted in order to maximize stability by de-coupling them from ship motion.

Transducer Face: The rounded rubber end of the sonar head.

Transmitter Sync: An option that provides the means to synchronize external devices with the SeaBat transmitter ping.

Transverse Resolution: The ability of the sonar to image, as separate and distinct, objects that lay in a line parallel with the towfish track. Transverse or along-track resolution is determined, in part, by the horizontal beam width of the sonar. A narrow pulse width will display two targets close together as separate and distinct anomalies. The same two targets, when insonified by a wider beam, can be simultaneously enveloped by the single outgoing pulse. This can result in the two objects appearing as one in the sonar display. Transverse resolution decreases with range from the towfish because of beam spreading.

Trigger Pulse: The signal provided to sonar transducer firing circuitry to initiate the outgoing pulse; also two parallel lines on the centre of a sonar record that represent the position of the fish in relation to the sonar image. Many sonar displays sense the trigger pulse in order to synchronize other subroutines with the trigger. Because the display of the trigger pulse in data is caused by transducer firing, it is useful in system troubleshooting


03 /09 /10

Salinity: A measure of the quantity of dissolved salts in sea water. It is normally defind as the total amount of dissolved solids in sea water in parts per thousand (0/00) by weight when all the carbonate has been converted to oxide, the bromide and iodide to chloride, and all organic matter is completely oxidized. These qualifications result from the chemical difficulty in drying the salts in sea water. In practice, salinity is not determined directly but is computed from chlorinity, electrical conductivity, refractive index, or some other property whose relationship to salinity is well established. Because of the Law of Constancy of Proportions, the amount of chlorinity in a sea water sample is used to establish the sample’s salinity. The relationship between chlorinity (Cl) and salinity (S) as set forth in Knudsen’s Tables is:

S=0.03 + 1.805 Cl

A joint committee of IAPSO, UNESCO, ICES, and SCOR proposed the universal adoption of the following equation for determining salinity from chlorinity:

S=1.80655 Cl

It was adopted by IAPSO in 1963 and ICES in 1964.

Scale Marks: Equidistant, regular marks on a sonar display used to assist in the mensuration of targets and anomalies and to provide information on the range displacement of targets from the towfish path. Most sonar systems allow the user to choose how and where to display scale lines or scale marks. Common settings in side scan are 10, 15, 25 or 50 metres.

Scattering: The diffusion of the sonar signal in many directions through refraction, diffraction and reflection, primarily due to the material properties of the insonified areas. Scattering is one of the causes of attenuation in sonar, resulting in signal weakening. See also Backscatter.

Sea: The great body of salt water in general, as opposed to Land. One of the smaller divisions of the Oceans. The state of the surface of the Ocean with regard to wave or swell, as a calm sea. See Cross Sea, Head Sea, Beam Sea, Following Sea, Quartering Sea, Sugar Loaf Sea.

Sea Clutter: The images created in a sonar display by acoustic returns from a rough sea surface. When using side scan sonar, some energy is projected above the horizontal from the wide vertical beam. If the sea surface is rough and within the range setting of the system, formless patches may overlay the normal seabed returns. This display “clutter” may be more noticeable on one sonar channel depending on the direction of sea surface waves relative to the towpath.

Sea Level: The Height of the surface of the sea at any time. The surface of the sea used as a reference for elevation. The expression is often used as a short expression for Mean Sea Level.

Sea Level Datum: A determination of Mean Sea Level that has been adopted as a standard datum of Heights although it may differ from a later determination over a longer period of time.

Sea Water: The water of the seas, distinguished from fresh water by its appreciable salinity. The degree of the salinity greatly affects the water’s physical characteristics.

Seafloor: The bottom of the Ocean when there is a generally smooth gentle gradient. Also referred to as a sea bed, sea bottom.

Secular Change: An increase or decrease of intensity and/or change of direction of the total magnetic field over a period of many years.

Sediment: Particulate organic and inorganic matter which accumulates in a loose unconsolidated form. It may be chemically precipitated from solution, secreted by organisms, or transported by air, ice, wind, or water and deposited.

Sediment: Bottom: In general all sedimentary material regardless of origin found on or in the submarine bottom, including ballast or other material dumped into the sea by man. More specifically it is limited to unconsolidated mineral and organic material forming the sea bottom, not including coral reefs or bedrock.

Sedimentary: Formed by the deposition of sediment.

Shadow: A light area on a normal sonar record that is less insonified than the surrounding region; most often caused by signal blocking from an acoustically opaque object on or above the seafloor. Shadows in side scan data are an important aid to accurate record interpretation. Often, an acoustic shadow will divulge more about a reflector than the actual acoustic returns. Shadows are also used to calculate the height of an object standing proud of the seabed. The calculation uses an algebraic solution of similar triangles formed by the height of the towfish, the range to the target and the length of the shadow.

Side Scan: a category of sonar system used to map large areas of the sea floor for a wide variety of purposes. These include making nautical charts, detecting and identifying underwater objects and bathymetric features, conducting surveys for maritime archaeology. Side-scan sonar imagery is also used to detect debris and other obstructions on the seafloor that may be hazardous to shipping, rigs and platforms.

Slant Range: The straight-path time of arrival of a sonar signal along the hypotenuse of a triangle described by the towfish, the seafloor directly below it, and the seabed point of interest. In side scan sonar, because the imaging source point (transducer) is not on the seafloor but rather above it, slant range does not represent the true range between any two objects. Below the towfish, the data is compressed. Further away from the towfish, the data becomes less compressed with the least error at the outermost ranges. The near range compression can be corrected using algorithms within modern sonar systems.

Slant Range Correction: A computerized repositioning of sonar data on the display to counteract range data compression. See also Slant range.

Slip Ring: An electromechanical component, most often used on a winch, that allows full electrical continuity of a sonar cable during winch drum operation. When a winch is used for deployment, altitude control and recovery of a side scan towbody, it is important to keep the control display unit and towfish operating during these processes. A slip ring has an internal core usually attached to the turning drum, while the outside of the slip ring is attached to a non-turning part of the winch. The towcable conductors are attached to certain parts (rings) of the core while the deck cable conductors are attached to a part (brushes) of the non-turning portion. In this manner the brushes maintain electrical contact with the rings as they turn along with the drum. Poor quality or dirty slip rings will cause noise in sonar data during drum movement and a bad individual ring can cause a blank sonar channel. A quality, well maintained slip ring will have good continuity and be noise free.

Sonar Geometry: The spatial relationship between the sonar transducers and their environment, including the seafloor, targets and the sea surface. Because of acoustic paths in the ocean environment, sonograms may provide puzzling imagery at times. Accurate data interpretation sometimes requires an understanding of the sonar geometry. A good example of this is when sonar signals return to the transducer over several different paths. See also multipath.

Sonograph: A hard copy display of sonar data generated either in real time or from recorded data. Also known as sonograms, hard copy “records” of sonar data are generated either by a sonar printer, specialized graphics printer or with modern computerized sonar displays, by any drafting printer in colour or black and white. Early sonar printers used a wet paper technology creating dark and light zones on the paper through the migration of ions from a consumable print head or “blade.” These records were not dimensionally stable and shrank upon drying. Dry paper recorders were developed in the 1970s ultimately resulting in the use of thermal paper recorders in the 1990s. Unfortunately, many thermal recorders, although clean and easy to use, do not have the dynamic range of other dry paper machines and thermal paper is not an archive-able media. As a result sonar manufacturers and users must depend upon digital processing and mass storage of sonar data for archiving.

SOS: See speed of sound.

Sound Velocity: The rate of motion at which sound energy moves through a medium. The velocity of sound in sea water is a function of temperature, salinity, and the changes in pressure associated with changes in depth. An increase in any of these factors tends to increase the velocity. Also called speed (or velocity) of sound (SOS).

Specular Reflector: An object, to which incident sonar beams are largely normal to its surface, making it a strong reflector from a variety of angles. Objects in this category include cylindrical objects such as pipes and pilings and spherical objects such as subsurface floats. Specular reflectors may provide very strong sonar returns and will result in hyperbolic shaped lines in side scan data. The hyperbola is formed when a target is reflective enough to return even the low level energy in the side lobes of the sonar’s horizontal beam.

Speed: Rate of motion. The terms speed and velocity are often used interchangeably, but speed is a scalar, having magnitude only, while velocity is a vector quantity, having both magnitude and direction.

Speed Correction: The proportional matching of sonar chart length with the over-the-ground speed of the survey vessel. When towing side scan sonar at a constant speed over the bottom, if the image generation on the recorder or display unit is too slow, objects in the data will appear to be compressed in the transverse dimension. If the image generation is too fast, they will appear to be stretched. See Correction and Compression.

Speed of Sound (SOS): The speed at which acoustic pulse travels through water. See Sound velocity.

Spherical Coordinates: See Coordinates.

Spheroid: An ellipsoid; a figure resembling a sphere. Also called ellipsoid or ellipsoid of revolution, from the fact that it can be formed by revolving an ellipse about one of its axes. In geodesy this term is frequently used to mean reference spheroid. See also Spheroid: oblate and Spheroid: prolate.

Spheroid: Oblate: An ellipsoid of revolution, the minor axis of which is the axis of revolution. The Earth is approximately and oblate spheroid.

Spheroid: Prolate: An ellipsoid of revolution, the major axis of which is the axis of revolution.

Spheroid of Reference: See spheroid.

Stop Bit: In asynchronous transmission, the last bit, used to indicate the end of a character which serves to return the data line to its original state.

Sun: The luminous celestial body at the centre of the Solar System, around which the planets, planetoids, and comets revolve. It is an average star.

Sun: Apparent: The actual Sun as it appears in the sky. Also called true sun.

Sun: Mean: A fictitious Sun perceived to move eastward along the Celestial Equator at a rate that provides a uniform measure of Time equal to the average Apparent Time. It is used as a reference for reckoning Mean Time, Zone Time, and so on.

Survey: The orderly process of determining data relating to the physical or chemical characteristics of the Earth. The act or operation of making measurements for determining the relative position of points on, above or beneath the Earth surface. The result of such operations.

Survey: Aerial: A survey using aerial photographs as part of the surveying operation; also the taking of aerial photographs for surveying purposes.

Survey: Cadastral: A survey relating to land boundaries and subdivisions, made to create units suitable for transfer or to define the limitations of title. Also called land survey.

Survey: Coastal: A Hydrographic Survey of coastal area including coast-lining.

Survey: Exploratory: A survey executed for the purpose of obtaining general information concerning areas about which such information was not, previously, a matter of record.

Survey: Geodetic: A survey in which the figure and size of the Earth is considered. It is applicable for large areas and long lines and is used for the precise location of basic points suitable for controlling other surveys.

Survey: Geologic(al): A survey of investigation of the character and structure of the Earth, of the physical changes which the Earth’s Crust has undergone or is undergoing, and of the causes producing those changes.

Survey: Gravimetric: A survey made to determine the acceleration of gravity at various places on the Earth’s surface.

Survey: Ground: A survey made by ground methods, as distinguished from an aerial survey. A ground survey may or may not include the use of photographs.

Survey: Hydrographic: A survey having for its principal purpose the determination of data relating to bodies of water. A hydrographic survey may consist of the determination of one or several of the following classes of data; Depth of water; configuration and nature of the bottom; directions and force of currents; Heights and Times of Tides and water stages; and location of fixed objects for survey and navigation purposes.

Survey: Land: See Survey: Cadastral.

Survey: Large Scale: A hydrographic survey at a large scale. Large scale surveys are usually intended to furnish detailed information for dredging, or other types of harbour improvement.

Survey: Magnetic: A survey conducted to measure the strength and direction of the Earth’s magnetic field at specific points on or near the surface of the Earth.

Survey: Magnetometer: A survey wherein the Earth’s magnetic field is mapped by the use of a magnetometer.

Survey: Oceanographic: A study or examination of any physical, chemical, biological, geological of geophysical condition in the Ocean, or any part of it. An expedition to gather data, samples or information to conduct such studies or examination.

Survey: Photogrammetric: A survey using either terrestrial or aerial Photographs.

Survey: Preliminary: See Survey: Reconnaissance.

Survey: Reconnaissance: A hasty preliminary survey of a region made to provide some advance information regarding the area which may be useful, pending the execution of more complete surveys. Also called preliminary survey.

Survey: Running: A hydrographic survey in which the greater part of the work is done from the ship steaming along the coast. fixing chiefly by dead reckoning and astronomical observations, and at the same time observing angles and bearings to selected and prominent coastal features. A line of sounding is run on the course made good and any additional information of a hydrographic nature which can be obtained whilst under way, is collected.

Survey: Wire-Drag: A hydrographic survey made utilizing a wire drag. Mainly used prior to the availability of Multibeam Echo Sounder Systems, and used in areas of rocky bottom or where submerged obstacles such as wrecks are present, a wire-drag survey represents the most practical way of making sure that all obstructions or dangers have been found and least depths over them obtained. Also called wire drag sweep.

Swath Width: The lateral coverage of side scan sonar on the seabed. Because side scan sonar projects a beam out to the side of the towpath it creates a wide region of insonified seafloor. Both right and left sonar channels make up the swath. Swath width changes with range settings and is a factor in determining coverage and lane spacing.


03 /09 /10

Range: A sonar setting which represents a distance, usually measured in metres, that is the maximum distance from the towfish that the sonar will display (the range setting on the sonar also determines the time between outgoing sonar pulses); also synonymous with the cross-track dimension. Once the range setting on the system is set, when returning echoes arrive from that distance away from the transducer, the ping cycle starts again and a new acoustic pulse is transmitted in to the water.

Range Data Compression: Sonar image compression resulting from the geometry of slant range side scan sonar displays. The time differences of returning pulses from the seabed in the near ranges below the towfish is very small. As a result, the separation of different portions of seabed is inaccurately displayed in conventional side scan data. As the pulse travels away from the area below the towfish, the time difference between one section of bottom and another begins to accurately represent their separation. Because of this, slant range compression is most severe nearest the centreline of two channel side scan data.

Range Overlap: The area of seabed, lateral to the towfish track, re-insonified on successive tracks during a survey; equal to the range per side less the lane spacing, usually expressed in metres. Overlap can be a crucial component of a side scan survey. See overlap.

Range Resolution: The ability of the sonar to image, separately and distinctly, objects that lay in a line 90 degrees to the towfish heading. The range resolution is determined, in part, by the pulse width of the sonar. A narrow pulse width will display two targets close together as separate and distinct anomalies. The same two targets, when insonified by a wider pulse width, can be simultaneously enveloped by the pulse. This results in the two objects appearing as one in the sonar display.

Ray Bending: Changes in the speed and direction of a sonar beam in the water. Ray bending is a major cause of data distortions from thermoclines, inverted thermoclines and haline fronts. Side scan, like many acoustic imaging instruments, rely on the fact that most ray paths are relatively straight. When they are not, as a result of by ray bending, it is important for the sonar operator to recognise it.

Recognition: The acknowledgment by the sonar operator of the existence of a target or anomaly as displayed in the sonar data. Lack of anomaly recognition can be problematic during sonar operations. Catastrophic towfish altitude loss, severe fish instability and search targets all should be recognized by the operator any time they occur. Recognition is distinguished from detection as being operator dependent. See detectability.

Reconnaissance Survey: See survey.

Rectangular Coordinates: See coordinates.

Reference Direction: A direction used as a basis for comparison of other direction.

Reference Line: Any line which can serve as a reference or base for the measurement of other quantities. Also called datum line.

Reference Spheroid (or Ellipsoid): A theoretical figure whose dimensions closely approach the dimensions of the Geoid. The exact dimensions are determined by various considerations of the section of the Earth’s surface considered. The Spheroids of Bessel, Clarke, Delambre, Everest, Hayford, Helmett and others have been adopted as reference spheroids in geodetic work by different countries. Also called spheroid of reference, or ellipsoid of reference. See also Spheroid: Oblate

Refraction: The change of direction of a sound beam when passing obliquely from one medium into another, where its wave velocity is different. Refraction is a type of ray bending that will affect acoustic returns for proper sonar imaging. This occurs when sonar pulses encounter thermal and haline discontinuities. In a normal summer thermocline environment, a side scan beam can be refracted sharply to the seafloor severely limiting range. See also Ray bending.

Reverberation: The echoing of a sonar signal from a target or targets. Echo and reverberation are often used interchangeably, although targets are more often described as returning an echo whereas large insonified areas such as the seafloor are described as reverberating under the influence of sonar.

Rhumb Line: A line on the surface of the Earth making the same oblique angle with all Meridians; a Loxodrome spiralling toward the poles in a constant true direction. Parallels and Meridians, which also maintain constant true directions, may be considered special cases of the rhumb line. A rhumb line is a straight line on a Mercator Projection. Sometimes shortened to rhumb.

Ringing: In a transducer, this is the reception of the transducer output pulse at the time of transmission. In active sonar systems, the projector and the hydrophone are one and the same, so the hydrophone receives its own outgoing pulse. In a target, this is a well documented phenomenon resulting from multiple echoes from certain types of targets due to the acoustical physics of sound pulse wrap-around and reflections internal to the target.

Rip-Rap: See Groin.

Rise: A long, broad elevation that rises gently and generally smoothly from the sea floor.

The increase in the height or value of something, as the rise of the Sea due to tidal action, or a rise of temperature. The opposite is fall.

Roll: The rhythmic movement of a ship or towbody about its longitudinal axis. When working from a well designed support vessel, roll will not contribute to towfish instability to the same extent as heave in any given sea state. Roll can affect the ability of crews to perform at sea, as well as destabilize unsecured equipment. However, to preserve data quality, many surveys will use a different vessel heading allowing the vessel to roll instead of pitch or heave.

Rotation: Turing of a body about an axes within the body, as the daily rotation of the Earth.

ROV: Acronym for Remotely Operated Vehicle (submersible). This is an underwater vehicle that is connected to an operator via power and/or communication tether. See also AUV.

Rub-Test: The process of manually creating friction on a transducer face in order to test system electrical continuity. Before a sonar towbody is put in the water, it is a common practice to test the system on deck. Because air is a high impedance medium for sonar, the best method of testing system function is to tap or rub the transducer face. In a dual channel side scan sonar, one transducer is rubbed, then the opposite then the first one again. This is to clarify that the transducers are not wired to the wrong display channels.


03 /09 /10

Quay: A wharf approximately parallel to the shoreline and accommodating ships on one side only, the other side being attached to the shore. It is usually of solid construction, as contrasted with the open pile construction usually users for piers.

Quench: The loss of a sonar signal, most often due to water borne discontinuities and resulting in blank sonar display areas. It should be noted that quenching will affect both the outgoing sonar pulse and the returning echoes. Also, water borne acoustic obstructions might absorb or scatter part of an outgoing pulse, but the returning echoes are of a much lower intensity and their blockage by the same obstruction is more often the result of blanking than the loss of the outgoing pulse.

Quenching: The great reduction in underwater sound transmission or reception resulting from absorption and scattering of sound energy by air bubbles entrapped around the sonar dome. See aeration and attenuation.

Q-Route: A route of safe passage through a mined waterway. Side scan sonar is often used not only in mine hunting, but also as a mapping tool to gather baseline data for safe harbour routes in the event of conflict. Once baseline data is gathered for a particular section


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PAL: Format of the video signal transmitted from the SeaBat (operator selected). This format is used throughout Europe and other countries, excluding the United States and Japan.

Parity BIT: A data bit, set at “0” or “1”, is a character advising you that the total number of bits in the data field is even or odd.

Parallel: A circle (or approximation of a circle) on the surface of the Earth, parallel to the Equator and connecting points of equal Latitude.

Pass: A single procession by a seabed anomaly during a sonar survey. Multiple passes by a target are important for the process of target classification. Also, interpretation of phenomena can require multiple passes. For example, thermal discontinuities rarely look the same on any two passes. A target that may be a school of fish can be properly identified by several passes because fish schools rarely stay in one place over any length of time.

Passive Sonar: A sonar system having only a hydrophone and capable of receiving signals but not transmitting them. An example of a passive sonar is a streamer array towed from a seismic vessel. Another is an array on a submarine that detects other vessel’s engine and propeller noise.

Path: A line of movement; course taken, as the path of a Meteor. A line connecting a series of points in space and constituting a proposed or travelled route.

Path-tracking: The ability of a towed body to accurately follow the path along which it is towed by a surface vessel. In the absence of high velocity currents and using short cables, most sonar towfish will track along the ship’s path well. However, if the sonar is towed at 90 degrees to a strong current and over long cable, it will tend to be displaced from the vessel’s path and heading. See lateral offset.

Phase Measurement: The method by which the phase is measured.

Phase(s) of the Moon: The various appearances of the Moon during different parts of the Synodical Month. The Cycle begins with new moon or change of the Moon at Conjunction. The visible part of the waxing moon increases in size during the first half of the Cycle until full moon appears at Opposition, after which the visible part of the waning moon decreases for the remainder of the Cycle. First quarter occurs when the waxing moon is at east Quadrature, last quarter when the waning moon is at west Quadrature. From last quarter to new and from new to first quarter the Moon is crescent; from first quarter to full and from full to last quarter it is gibbous.

Phase Velocity: Velocity, measured over a short time period, at which a particular wave crest is propagated through water or rock media.

Ping: A single output pulse of a sonar system; also the returns from a single output resulting in the lateral display of one individual line of side scan data. A side scan sonar transmits many pings into the underwater environment. Each of these outputs has resultant echoes coming from the seafloor. Any individual ping’s returning echoes does not provide imagery alone, but together many pings, juxtaposed in a display, provide the imagery. However, in modern computerized sonar processors and displays, individual ping and return signal strength “values” can provide information to the operator about system operation.

Pitch: An instability in the towfish expressed by the alternate rise and fall of the nose and tail about a horizontal axis. Most often, towfish design inhibits pitch during normal operations. When the towfish is lowered rapidly though, it will temporarily pitch in a nose-down attitude.

Plan View: A to-scale side scan sonar display constructed to represent the top view of a section of seabed. (See mosaic and mapping).

Polar Coordinates: See coordinates.

Pole: Either of the two points of intersection of the surface of a sphere or spheroid and its axis. The origin of system of Polar Coordinates. Either of the two magnetic poles of a magnet.

Position: Data which define the location of a point with respect to a reference system. The coordinates which define such a location. The place occupied by a point on the surface of the Earth, or in space.

Position: Dead Reckoning: The position of a craft determined by Dead Reckoning, or by advancing a previous position for courses and distances.

Position: Estimated: The most probable position of a craft determined by incomplete Data or Data of questionable accuracy. Such a position might be determined by applying a correction to the Dead Reckoning Position.

Position: Geographical: The position of a point on the surface of the Earth expressed in terms of Latitude and Longitude, either Astronomical or Geodetic.

Post-processing: Sonar data processing after real time data generation and storage. Modern sonar systems allow the user to concentrate on the task at hand of gathering data by recording “raw” data which can be manipulated or “processed” later. Because ship time is expensive and operational crews must be fully attentive to proper data gathering, post processing is most effectively performed in the less time-budgeted environment of the on-shore laboratory. Post processing can include mosaic construction, slant range and speed correction, false colorization and hard copy print generation.

Pre-plots: Specific points, including track lines and navigation fix points, the positions of which are determined prior to the commencement of a survey. Planning is a crucial component to an effective side scan sonar deployment. Where the survey is to take place, the location of lanes and what “highway markers” or geodetic positions will be logged and checked are often determined before hand as pre-plots.

Precision: The degree of refinement of a value not to be confused with accuracy, which is the degree of conformance with the correct value.

Primary Great Circle: A Great Circle used as the origin of measurement of a Coordinate; particularly such a circle 90 degrees from the poles of a system of spherical coordinates, as the Equator. Also called primary circle, fundamental circle.

Profiler: An instrument that records a vertical section, or simple outline, of the seafloor along a survey line. Profilers come in many types and frequencies but generally the term refers to sub bottom profilers that work in the 0.5-5.0kHz region. These frequencies will penetrate some types of bottom sediments and provide an image that represents a cross section of the seabed. Other types of acoustic profilers are higher frequency and show only the outline of the seabed surface topography.

Projection: The presentation of a figure on a surface, either plane or curved, according to a definite plan. In a perspective projection this is done by means of projecting lines emanating from a single point, which may be infinity.

In Cartography, any systematic arrangement of Meridians and Parallels portraying the curved surface of the sphere or spheroid upon a plane. Also called map projection or chart projection.

Projection: Mercator: A Conformal Projection of the cylindrical type. The Equator is represented by a straight line true to scale; the Geographic Meridians are represented by parallel straight lines perpendicular to the line representing the Equator; they are spaced according to their distance apart at the Equator. The Geographic Parallels are represented by a second system of straight lines perpendicular to the family of lines representing the Meridians, and therefore parallel with the Equator. Conformality is achieved by mathematical analysis, the spacing of the parallels being increased with the increasing distance from the Equator to conform with the expanding scale along the parallels resulting from the Meridians being represented by parallel lines. Since Rhumb Lines appear as straight lines and directions can be measured directly, this projection is widely used in navigation.

Projector: A sonar transducer that translates an electrical signal into pressure waves (sound signals) and transmits them through the water. A projector almost always requires some sort of hydrophone to receive the transmitted signals or returning echoes. The hydrophone may be very close to the projector or, as in the case of Rafos and Sofar floats, may be thousands of kilometres away. With a few exceptions, conventional side scan sonar most often uses the same transducer crystals as the projector and hydrophone.

Propagate: The movement of sound waves through the water; also transmit. The manner in which sound propagates underwater is the basis for quality image formation using side scan sonar. Because it is known how sound behaves in the underwater environment, sonar systems are designed and operated based upon these physics. Understanding the propagation of sound also allows rapid interpretation of sonar imagery.

Protocol: A formal set of conventions governing the formatting and timing of message exchange between the SeaBat processor and an external device. Protocol consists of: baud rate, parity, data bits and stop bits.

Pulse: A short burst of sonar, typically measured as a function of time, distance or power. Each pulse of sonar is also known as a ping. However, the pulse is a more formal term and we use it to describe the length in time and width in metres of a sonar ping. (See pulse length).

Pulse Length: The length of time that an active sonar is transmitting one pulse, typically expressed in milliseconds. Longer pulse lengths allow more power to be put into the water at the expense of cross track resolution. This has the effect of gaining range for large area surveying.

Pulse Width: The distance of the insonified water, in the range dimension, at a given point in time, expressed in metres and determined by multiplying the pulse length by the speed of sound through the water. Pulse widths are measured to determine the maximum theoretical resolution of an imaging sonar. Some side scan sonar systems have variable pulse widths although most do.