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3-2
BENTHOS
The AT-421 Directional Transducer and its beam pattern are shown in
Figure 1-7 on page 1-11. Whenever possible, this transducer should be
selected. Its transmit and receive beam patterns are that of a narrow cone;
hence it is a good choice when the depth and direction of the receiving
modem is known and is stable, and the transducer can be pointed at the
receiving modem. This transducer requires the least amount of power to
produce the equivalent source level of an omnidirectional transducer, and it
provides the best protection against the effects of multipath.
Battery Pack Selection
For the subsea modems that use battery packs, the capacity of the battery
packs determines both the amount of data a modem can transmit and the
modem’s useful deployment life. In many cases, the amount of data a modem
will transmit while it’s deployed depends primarily on the selection of the
transmitter/preamp battery pack, which powers the modem’s amplifier and
preamplifier. A second battery pack, the DSP/receiver battery pack will
provide power to the DSP and the receiver for over a year when the modem
is in the lowpower state. However, the actual useful deployment life of a
modem depends on both battery packs.
Battery Pack Currents
In the lowpower state, where the modem is switched on but is inactive, a very
small amount of current is drawn from the DSP/receiver battery pack,
typically from 0.7 mA to a maximum of 0.8 mA. Therefore, the DSP/receiver
battery pack can provide power in the lowpower state for over a year.
Similarly, a very small amount of current is drawn from the
transmitter/preamp battery pack. This is the transmitter/preamp standby
current, and is typically from 100 µA to a maximum of 150 µA. However, when
the DSP and the receiver are active, hence not in the lowpower state, a
relatively large amount of current is drawn while the modem either receives
or transmits data, typically from 170 mA to a maximum of 200 mA for an
ATM-870 Series and 85 mA to a maximum of 100 mA for an ATM-880 Series
Acoustic Telemetry Modem. And when the modem is transmitting, a large
amount of current is drawn from the transmitter/preamp battery pack,
typically 1 A. Therefore, unless a modem is to be deployed for well over a
year with few transmissions, it is the amount of data to be transmitted that
determines its useful deployment life.
User’s Manual
January 2001