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egm502 seafloor mapping

lecture 8 navigation and positioning

Marine Positioning Systems – Surface and Underwater Positioning

•  All observations at sea need to be related to a geographical position. •  To precisely locate the position of features imaged on the seabed, we need to know (a) where the survey vessel is located (on the sea-surface), (b) where the sonar platform is located (below the sea-surface) and (c) which area of the seafloor is insonified.

•  This precise positional data is given by a combination of position fixing (latitude, longitude and altitude above a reference datum), heading, speed and attitude (heave, roll, pitch, yaw) information.

•  The remainder of this lecture will detail the techniques used for measuring the position of the survey vessel at the sea-surface, the position of the sonar platform (if not attached to the vessel) and their relative variations in attitude.

(a) Surface (Ship) Positioning

•  Since the GPS revolution in the early 1990s, ship positioning techniques have changed in ease and accuracy.

•  Majority of offshore navigation systems superseded by the GPS system – commercially released in 1993 (see MAR101 lecture notes for details).

•  Global Positioning System comprises a constellation of 24 satellites in synchronous orbits, associated ground stations and user equipment.

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•  The resolution of GPS can be enhanced with Differential GPS (DGPS). •  In DGPS, a high-precision GPS reference station receiver is surveyed-in with centimetric accuracy and its position is programmed to memory.

reference station

satellite constellation

(fixed receiver, known position)

remote station

(roving receiver)

error corrections

•  The reference station calculates its position using the same satellites as the ship.

•  As the reference station knows its exact position (pre-programmed), it can identify errors in the satellite signal time measurement, compute a correction and transmit the correction to the roving receiver (survey vessel).

•  Corrections update regularly (typically every second) and broadcast to the survey vessel.

•  This technique works because the satellite signal time distortion is almost identical for any 2 points on the Earth which are less than 500km apart.

•  Differential corrections can be obtained from several sources: (a) your own base station, (b) the worldwide network of DGPS radio-beacons, or (c) a satellite differential service provider.

•  The network of DGPS radio-beacons throughout the world is rapidly expanding and the signals that most radio-beacons transmit are free.

•  There are also a number of satellite differential service provider options including subscription services you pay for, and now the Wide Area Augmentation System (WAAS) or EGNOS satellite systems for the US and Europe respectively that will improve integrity, accuracy and availability of GPS for users.

•  For very high precision surveys, sub-metre DGPS accuracy is not enough.

•  That's where centimetre-level solutions using Real-Time Kinematic (RTK) can provide even greater accuracy.

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(b) Sub-surface (Towfish) Positioning

•  In many applications, the sonar platform (towfish) is towed by the ship, sometimes only a few metres above the seafloor.

•  The towfish may be positioned two ways: (i) by manually calculating how far it lies behind and below the ship, or (ii) by using acoustic techniques.

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(i) Manual Positioning

•  The Towfish Layback is the distance the towfish is being towed behind the vessel.

•  It is important to remember that the sonar image data is not being generated at the tow vessel but at the towfish. Also remember that the GPS fix being generated is where the GPS antenna is located on the tow vessel not where the towfish is. The lay back correction can be calculated fairly accurately by using the Pythagorean theorem.

•  The information you need is as follows: (1) The amount of cable that has been deployed (the hypotenuse).

(2) Establishing the depth of the towfish (one leg of the right triangle). This can be obtained by subtracting the height of the towfish off the bottom (a measurement made on the sonar record) from the total depth of the water (ships depth sounder).

Layback = Cable Deployed 2 – Towfish Depth 2

(ii) Acoustic Positioning

•  Acoustic positioning systems are base don two types of configuration: short baseline (SBL) and long baseline (LBL):

LBL (Long BaseLine)

An LBL system has two parts or segments:

•  The first segment comprises a number of acoustic Transponder Beacons moored in fixed locations on the seabed. The positions of the Beacons are described in a co-ordinate frame fixed to the seabed. The distances between them form the "baselines" used by the system.

•  The second segment comprises an acoustic transducer on a Transceiver which is normally temporarily installed on the vessel - or on a Tow Fish.

Sonardyne, 2006

USBL (Ultra Short BaseLine)

•  USBL principles are very similar to SBL principles (in which an array of acoustic transducers is deployed on the surface vessel) except that the transducers are all built into a single transceiver assembly - or the array of transducers is replaced by an array of transducer elements in a single transceiver assembly.

•  The distances or ranges are measured as they are in an SBL system but the time differences are now much less. Systems using sinusoidal signals measure the "time-phase" of the signal in each element with respect to a reference in the receiver. The "time-phase differences" between transducer elements are computed by subtraction and then the system is equivalent to an SBL system.

Sonardyne, 2006

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Sonardyne, 2006

Sonardyne, 2006

Case Study Fredrik Søreide and Marek E. Jasinski - Ormen Lange Deep-Water Shipwreck Excavation (IJNA, in press)

•  Ormen Lange field located in the Norwegian Sea, 100 km north-west of the coast of Mid-Norway.

•  Historic wrecks in 170m water depth.

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(c) Attitude Information

•  The final issue of which portion of the seafloor is being insonified is addressed by measuring the attitude of the ship or sonar platform.

•  Four types of movement are recognised: heave, roll, pitch and yaw.

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Inertial Measurement Unit (IMU) or Motion Referencing Unit (MRU)

•  An Inertial Measurement Unit is a closed system that is used to detect altitude, location, and motion.

•  Typically installed on survey vessels, it normally uses a combination of accelerometers and angular rate sensors (gyroscopes) to track how the vessel is moving.

•  Typically, an IMU detects the current acceleration and rate of change in attitude (i.e. pitch, roll and yaw rates) and then sums them to find the total change from the initial position. These corrections are then applied to the geophysical data to correct for the effects of pitch, roll and yaw.

•  An IMU stands in contrast to the GPS system, which uses external satellites to detect the current position.

EXAMPLE SYSTEM

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WITHOUT COMPENSATION WITH COMPENSATION