VEMCO – VR4-UWM Manual
7
1.3.1
Effect of Local Conditions on Modem Performance
Several aspects of local water conditions can have a significant impact on modem performance. These
conditions include:
•
Noise, from either natural or man-made sources. In particular, noise in the 22-27 kHz band where the
modem operates will directly interfere with communications. Noise that is not in this range can also
have an adverse impact if it is quite loud. Common noise sources include engines, depth sounders,
acoustic profiling instruments, shipping, industrial activity, wind, waves, and noisy organisms such as
snapping shrimp.
•
Attenuation. This can be worsened by suspended sediment or biological matter in the water.
•
Echoes. Sound waves will reflect from the ocean floor, the water’s surface, the shoreline, etc. Hard
surfaces, such as rock or concrete, will reflect more. The echoes will interfere with acoustic
communication, increasing the error rate.
•
Thermocline and halocline. These conditions create a barrier layer that reflects and refracts sound
waves, making it difficult to communicate acoustically across the cline.
•
Fouling. A build-up of sediment of biological growth on the receiver can attenuate the signal and
reduce performance.
•
Obstacles. The modem requires a clear line-of-site path from the VR4-UWM to the Deck Box
transducer. Large obstacles, such as an underwater ridge, will block the signal.
•
Wind and current. These can cause the VR4-UWM and/or the Deck Box transducer to move around
making it difficult to control their position and orientation.
When planning a VR4-UWM study, consider the local conditions and choose locations where the adverse
effects are manageable.
1.3.2
Modem Address
Each VR4-UWM is assigned a modem address, which is a number from 1 to 249 that is used to distinguish
different VR4-UWM receivers if more than one is located within range of the Deck Box. You must know the
modem address of a VR4-UWM in order to communicate with it using the acoustic modem. If the modem
address is not known, the Scan feature in VUE (see section 2.5.3) can be used to identify all VR4-UWM
receivers in the area. The Scan feature is time consuming and is best used when the desired modem address
is not known.
Due to the limited address range, it is not possible to
assign unique addresses to every VR4-UWM. At the
factory, VR4-UWM receivers are assigned a modem
address that can be found in the VR4-UWM
specification sheet provided with your order.
When deploying VR4-UWM receivers, plan their placement so that units with the same modem address, if
there are any, are not located near one another. They should be at least 5 km apart to avoid any possibility of
an address conflict. Keep a record of the serial number and modem address of each deployed unit and its
location. Refer to these records when visiting the receivers to offload data.
Do not deploy VR4-UWM receivers
with the
same modem address
within
5 km
of each other. Doing so could
disable modem communications with
the surface acoustic modem.