In--manyorfshore operations involving deep water or large horizontal ranges, long-baseline acoustic positioning systems are required to obtain the-needed accuracies. A long-baseline system, however, requires a transponder gridcalibration procedure that can take from several hours to over a day. In many operations, the time required for calibration will result in sufficient added operating cost to lead to the decision to use a short-baseline system and accept the lower accuracies. This paper describes how the time nece~sary to deploy and calibrate the transponders can be significantly reduced with a combined ultrashort-and long-baseline system. A procedure is presented which allows the transponders to be deployed and calibrated in the time previously required for deployment alone. This procedure eliminates the operating costs associated with calibration.Results of an analysis are presented comparing the calibration accurac~es obtainable via this new calibration procedure with those obtainable under the more traditional procedures. The results of this analysis indicate that this new calibration method will meet resolution requirements for many offshore applications.
Two aspects of the acoustoelectric aftercurrent in photoconducting CdS are investigated. First, the value of the aftercurrent immediately after the voltage is removed (the maximum aftercurrent) is considered. A linear model is shown to predict that the maximum aftercurrent is proportional to the difference between the ohmic current and the current just before the voltage is removed. The experimental results show good qualitative agreement with this prediction. Secondly, the time behavior of the aftercurrent is considered. The persistence time of the aftercurrent is found to be dependent on the maximum aftercurrent in agreement with the predictions of the model.
A special purpose synthetic aperture sonar (SAS) system has been developed and built by Hughes Aircraft Co. and Dynamics Technology, Inc. The main features of the system are a 3.2-m-long, 32-element linear array, and a data acquisition system capable of recording the elemental data in real time while preserving the phase and amplitude fidelity required by SAS processing. High-resolution, large (1002 m2) and small (32 m2) SAS images of various objects will be presented that demonstrate resolutions of 20 cm at 1000 m. The innovation of the work is that resolution achievable only with short-range, high-frequency sonar has been demonstrated at ranges obtainable only with a low-frequency sonar. [Work supported by DARPA.]
A dynamic positioning (DP) system was installed on Brown &: Root's derrick barge Atlas I and successfully used during the Lena guyed tower guying system installation. Modular design and onshore testing of the components resulted in rapid DP system installation and commissioning.Only minor DP equipment problems were experienced throughout the guying system installation.A long-baseline acoustic positioning system was used to measure the positions of the anchor piles, clump weights, and jacket during the Lena installation. The system provided relative position measurement accuracies on the order of +5 feet in 1000 feet of water.
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