An integrated navigation system which combines data from the Transit satellite system, a network of underwater acoustic transponders and an assortment of secondary sensors is described. Particular emphasis is placed on the techniques developed for relative and geodetic calibration of the acoustic grid. The system is scheduled for deployment in Spring 1977 for survey operations in the North Sea.
Integrated navigation systems have evolved significantly during the recent past to keep pace with the needs of the offshore oil and exploration communities. As production and exploration activities move beyond the range of shore based navigation aids, there has been an increased incentive to produce accurate, portable navigation aids for use at sea. The use of underwater acoustic beacons has been one approach to this problem.
Acoustic navigation systems are typically comprised of the following components: a network of underwater transponders deployed on the ocean floor, a hull mounted or towed transducer connected to the vessel to be navigated and an interrogation or control unit. The transponders are designed to transmit a pulse of acoustic energy in response to an interrogate pulse issued by the interrogation unit. The time delay between transmission of the interrogate pulse and receipt of the reply pulse by the vessel's transducer provides a measure of the slant ranges to each of the underwater transponders. When properly calibrated, these systems can provide precise navigation over a limited area.
The system described in this paper is designed to use this type of acoustic structure in combination with the Transit Satellites and the various navigation aids discussed below.
Fig. 1 is a block diagram of the system configuration. The navigation sensors include a gyrocompass, speed log or sonar, satellite receiver, Decca main-chain or Loran-C receiver, and the acoustic subsystem. The keyboard/printer is the primary operator interface device. The CRT is used for display of real-time navigation parameters. The track plotter is used to provide a graphic record of the vessel's path.
The data logging unit may be a magnetic cassette device or a reel to reel tape drive. In addition to recording survey data it is also used to log satellite data necessary for the geodetic calibration of the acoustic array. At the heart of the system is a Hewlett Packard minicomputer with a 24K word memory.
The acoustic subsystem is interfaced via an eight channel multiplexer to the CPU. Each channel in the multiplexer contains its own range counter unit, which uses a stable 100 KHz output from the satellite receiver as its frequency reference. This arrangement obviates the need for a full BCD interface with the acoustic interrogation unit, since only start and stop pulses are required to obtain range measurements.
