Test, Evaluation, and Implementation of Current Measurement Systems on Aids-To-Navigation.

Abstract

The Physical Oceanographic Real-Time System (PORTS®) is a program of the National Ocean Service’s Center for Operational Oceanographic Products and Services (NOS/CO-OPS). PORTS® provides real-time marine environmental information to support safe and efficient maritime commerce and informed coastal resource management. In response to user requests for water current information near navigation channels, CO-OPS entered into a partnership with the U.S. Coast Guard (USCG) to place current measurement systems on existing USCG aids-to-navigation (ATON) buoys. This document describes measurement and communication technologies employed, the design considerations which ensure that the PORTS® payload does not interfere with the primary mission of the buoys, the system test and evaluation undertaken, the field procedures for installation, repair and maintenance of the systems, the data flow configuration, data quality control procedures, and finally, the products generated and disseminated to the public from these current measurement systems. The system, developed from commercial off-the-shelf (COTS) technology, consists of a “clamparatus” (produced by the Oceanscience Group) that secures a current profiler (manufactured by Nortek) and electronics box to the buoy. The entire package weighs ~200 pounds and is easily deployed using a small boat and a block and tackle. Data are sent to shore via spread spectrum radios. Deployments of up to seven months have been achieved thanks to the low power consumption of both the profiler and radios. This system satisfies the maritime community’s requirements for current information within navigation channels, while removing the restraints of previous current meter placements, which were limited by cable length and the need to stay out of the channel. Underwater cable to shore has long been a PORTS® operations weak link because cables are often snagged and broken. The ATON-mounted system complies with all USCG requirements. The most important concern is ensuring that the system does not interfere with ATON utility or maintenance. Other USCG requirements include: a completely battery powered system; the ability to deploy and recover with the buoy on station; the ability to remove the entire package prior to ATON servicing; the use of allsimilar metals and coloration; and minimal alteration of buoy profile and characteristics. NOS/CO-OPS uses the David Taylor Model Basin to evaluate all acoustic Doppler current profilers (ADCPs). Current profilers are mounted to a carriage that travels through the tank at precise speeds; the current speed recorded by the profiler is then compared to the carriage speed. CO-OPS policy requires that all Nortek current profilers used in ATON current measurement system be tow-tested at David Taylor before operational use. The David Taylor tests showed a mean speed difference of between one centimeter per second (cm/s) and five cm/s, depending on the carriage speed and Nortek bin sampled. These results are completely in line with current profilers of other manufacturers used by CO-OPS. CO-OPS performed a field intercomparison in the Potomac River in about 60 feet of water. A bottom-mounted RD Instruments 600 kilohertz (KHz) Workhorse ADCP configured with one-meter bins was located approximately 250 meters north of the Nortek one-megahertz (MHz) Aquadopp current profiler mounted on Buoy B at Piney Point. The Nortek was also configured with one-meter bins. Performance was evaluated by comparing the differences in current speed and direction. The speeds compare very well, within the anticipated error of individual instruments. There was no obvious bias in the ATON, with the mean difference being ~ 3 cm/s equals 0.06 knots. The results of comparisons with current directions from bottom-mounted current profilers show agreement to within +/-21° (standard deviation of 26°) when all data are included. The difference improves to 12° when speeds less than one-quarter knot are omitted. These results are encouraging, considering three general categories of source of differences between the two direction measurements: 1) the error in bottom mount direction; 2) the error in ATON direction; and 3) the real environmental differences between the respective volumes of water. CO-OPS performed another intercomparison at the mouth of the Freeport River, Texas, in about 11 meters of water. Data from a bottom-mounted ADCP located about 70 meters from an ATON-mounted profiler were analyzed. As in the Piney Point comparison, an RD Instruments 600 KHz Workhorse ADCP configured with one-meter bins was deployed in a bottom mount to evaluate the Nortek oneMHz Aquadopp profiler mounted on Buoy 6, which was also configured to sample one-meter bins. The mean speed difference was ~3 cm/s with no bias observed. The Freeport River empties into the Gulf of Mexico and, although the flow in the region is not tidally dominated, the rapid reversals in the alongshore current are evident in both records of direction. In spite of large directional differences at times of current reversal, the mean direction difference was ~15° and ~12° when only times with speeds greater than one-quarter knot are considered.The general navigation requirements are for reported current speeds to be accurate to within +/- 0.10 knot and direction accurate to within +/- 15° in order to aid in the maneuvering of large vessels. The ATON-mounted current measurement system, as designed and deployed with the recommended procedures, meets these speed and direction requirements. The authors recommend that CO-OPS senior management approve the ATON current measurement system for use in PORTS® and begin the operational dissemination of the data via the web, phone and text pages (CO-OPS homepage). Additional system testing (outlined in Section 8.0) is recommended. As presented in this report, the ATON current measurement system provides the navigation community with important data in those areas where it is impracticable to use either the traditional bottom-mounted or sidelooking current profilers. CO-OPS management personnel have reviewed this document and concur that the evaluated sensor/system, when deployed and implemented as described, meets the defined requirements and is suitable for operational use. While additional testing may lead to superior performance or more economical operation, the existing sensor/system configuration is sufficient as described.