GENERAL DESCRIPTIONS
The Ultra-Short-Baseline (USBL) acoustic positioning method, also referred to as Super Short Baseline (SSBL), involves measuring the range and bearing from a vessel based transceiver to a single, subsea transponder.
The seabed transponder which may be mounted on a mobile target such as an ROV or on a fixed target such as a seismic cable ground station, is initially positioned using data from the ship’s GPS. The USBL system then provides a range and bearing estimate of the transponder relative to the ship’s position.
Range is calculated by measuring the time taken from sending a transponder interrogation signal to receiving its reply. Bearing is derived by comparing the small differences in the time of arrival of the reply signal at each receiver element within the transceiver.
Such small delays (microseconds) are analysed as ‘time-phase’ differences and this data processed to provide a best fit solution. Because there are more than three receivers in the transceiver head, the redundant information can be used to get a measure of consistency and therefore quality of the position fix.
DYNAMIC POSITIONING
One of the main applications for USBL is as a position reference input for vessels fitted with a dynamic positioning (DP) system.. By deploying a transponder on the seabed and using the USBL system to calculate its relative position, a ship’s DP computer can use the information to hold the vessel in the same position, on the same heading whatever the disturbing forces from wind, current and wave action.
Operationally, the USBL method is very efficient as acoustic positioning can commence as soon as a transponder has been deployed in the water. At the core of the Sonardyne’s USBL system is the under hull acoustic transceiver and Navigation Processor.
USBL TRANSCEIVER
The innovative design of the USBL transceiver means that unlike other more complex systems, there is no need for an expensive mechanical tracking head. By providing near hemispherical acoustic coverage below its fixed deployment position, the transceiver is able to interrogate reference transponders and mobile beacons whatever their direction from the vessel.
A key feature of the transceiver is the use of separate transmit and receive elements. This both reduces the level of receiver noise and increases the sensitivity achievable. It also allows the receiving elements and their amplifiers to be optimised. Other contemporary systems use the same elements for receiving and transmitting.
NAVIGATION PROCESSOR
The Navigation Processor commands the transmission of acoustic signals from the transceiver to the transponders, decodes the data received back and formulates the appropriate telegram to pass to the DP desk. The unit also processes signals from VRUs, Gyro Compass’ and (D)GPS.The standard operator interface consists of a 1 7″ rack mountable colour monitor with integrated keypad. This facilitates system initialisation, whereby job specific parameters can be changed and positioning tasks monitored. Alternatively, the USBL display can be integrated within a customer’s DP console.
TRANSPONDERS
Sonardyne USBL is compatible with the vast majority of Medium Frequency (MF) transponders that are currently in use world-wide including Sonardyne’s own ‘COMPATT’ transponders and new Super Sub-Mini which can be used to position ROVs and towfish. Options available with Sonardyne transponders include high power transducers, acoustic release mechanisms, depth rating to 6,000 metres and a choice of battery packs depending on length of deployment and anticipated interrogation intervals.
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