Overview:
When bottom mounted wave and velocity measurements are required, but the coastline bathymetry or heavy boating activity do not allow a long power & data cable, acoustic underwater modems (UWM) may be used to transmit data to a shore station or offshore structure. In this situation, the bottom mounted equipment is powered by external battery packs. Data are transmitted from an UWM on the bottom mount to an UWM at a shore station pier or other offshore structure. If the structure is not cabled to shore, data may be sent via RF/GSM/Iridium telemetry to shore (see Example 4). The AWAC is a common choice for bottom mounted directional wave and current profiling equipment.
Advantages:
- Can be located in areas of heavy boating and fishing activity
- Can be located in areas where coastline does not allow cable deployment
- Efficient pre-processing of wave and velocity data for UWM transmissions
Limitations:
- Relatively low bandwidth communications
- Requires periodic replacement of batteries
Technical considerations:
Because UWM’s have comparatively low bandwidth, the copious amount of data required for wave measurements must be pre-processed for efficient UWM telemetry. To accomplish this, Nortek has developed the Nortek Internal Processor (NIP). The NIP is a small processor (about the size of a credit card) running on the Windows CE platform. The NIP fits into the standard pressure housing of the AWAC. It is user programmable to output a variety of wave statistics. The wave statistics represent a more manageable amount of data for the UWM to transmit compared to the raw wave data. UWM performance and transmit range depend on acoustic frequency, water conditions (acoustic noise, density gradients, etc) and available power. UWMs may have directional or omni-directional transducers. Directional transducers are more efficient, but must be aligned accurately and are not applicable on moving platforms (i.e. surface buoy or boat). Directional transducers would be used when a bottom mounted AWAC (fixed in location) is communicating with a pier or offshore structure (also fixed in location).
Typically, UWM’s would be used to transmit data horizontally over tens to hundreds of meters. Under ideal circumstances, UWM’s may have a range of up to 1000 m.
Battery power is typically the limiting factor for this kind of deployment. The AWAC, NIP and UWM all require power. Thus, choosing the best data sampling interval, wave processing strategy and acoustic telemetry are all important considerations. Typical deployments can last 3-6 months before requiring a new battery.
An ActiveX toolbox with current and wave processing commands is supplied for real-time data conversion. These processed data are then ready for display on the users web site in real-time.
Examples:
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