Receiver/Dataloggers

ERDC TN-DOER-E18

February 2005

disadvantage of using a hard-wired monitoring system is the limitation on cable length due to

noise propagation, which typically limits deployment to within 100 m from shore.

Advancements in wired hydrophones (Lotek Wireless Model LHP-1) enable cable runs of up to

1000 m by incorporating an amplifier that generates a differential signal drive. The LHP-1 can

be configured for either omni-directional or directional operation with a bandwidth of 20 to 80

kHz, and can be used for both datalogging and manual tracking applications. Most

manufacturers market a number of models of "wired" hydrophones since it is necessary to

match tag frequency and receiver type.

Wireless hydrophones are a hybrid technology that incorporates the flexibility of radio telemetry

in acoustic applications, while eliminating the inherent limitations created by lengthy cables

between the transducer and the receiver. Wireless hydrophones are capable of detecting

underwater acoustic signals and relaying those signals through the air by way of a VHF

transmission to a radio telemetry receiver located on a shore-based monitoring or recording

station. Wireless hydrophones are typically tethered to a buoy and anchored at the monitoring

site. This deployment method significantly reduces deployment time over conventional acoustic

hydrophone or bottom-moored datalogging systems. A typical deployment scenario would

include one or more wireless hydrophone deployed across the cross-channel profile of a river

creating a monitoring gate through which all tagged fish migrating upstream would have to pass.

Data from each of the wireless hydrophones could be transmitted to a single shore-based

datalogging receiver. This automated tracking feature allows for real-time, around-the-clock

monitoring with minimum personnel requirements.

Wireless hydrophone models are typically less than 20 in. in length and can be deployed to

depths of 100 m. They are deployed using "D-cell" batteries as a power source, which last from

4 to 6 months and are easily changed in the field. Both single- and dual-acoustic channel

retransmission via VHF models are available. Single-acoustic models are typically used for

studies that examine movement patterns of large numbers of solitary animals, while dual-

acoustic models are suited for applications involving larger numbers of tagged animals, which

may congregate in the same study area.

Receiver/Dataloggers. Most manufacturers of telemetry equipment offer several models of

receiver/dataloggers that enable the researcher to conduct automated tracking studies, carry out

real-time field tracking, or collect detailed positioning data. Receivers can be fixed to boats or

aircraft for manual tracking, or land-based monitoring stations for automated tracking studies.

Manual tracking is often used in studies of gross fish movement, migratory patterns and the

location of spawning grounds. In the past, manual tracking involved a manual search for signals

on every assigned frequency. This labor-intensive process has been eliminated with the

development of scanning receivers that automatically listen for a small period of time at each

assigned frequency. When a signal is detected, the scanning cycle can be suspended to locate the

animal. Scanning receivers can be classified by their intended usage as either "general-purpose"

or "specific application." Two examples manufactured by VEMCO include the VR60; a general-

purpose ultrasonic receiver designed for manual tracking of aquatic fish species, typically from

small boats, and the VR25, which is used in fish and small animal tracking specifically around

hydroelectric utilities. Note that some receivers are designed for either manual or automatic

tracking, but don't have the capability to do both functions in a single unit. Examples of manual