Compensation: How It Works; Figure A-4: Seawater Signal - Teledyne 440 System Manual

A – Operating Theory

Figure A-4: Seawater signal

The seawater signal, to the 440's search coils, looks similar to a target. This means that as the vehicle
is lifted off the seabed, a voltage over and above the target signal is seen. This voltage could easily be
interpreted as a change in the target's position. To correctly interpret the coil signals, it is assumed that
this extra seawater signal affects all three coils equally. By comparing voltages between the centre and
starboard, and centre and port pairs of coils, the effect of the seawater can be minimised. The main
drawback of this approach is that it limits range.
The Teledyne TSS 440 system uses an active compensation method to remove the seawater signal
from the coil voltages. This allows a more flexible means of determining range to be used, which
provides both increased accuracy at short ranges and up to a 40% improvement in ultimate range.
Also, effects due to the three coils receiving different seawater signals are minimised.
A.2.4 Compensation: How it Works
The Teledyne TSS 440 uses two separate methods of compensation: background and seawater
(active). Background compensation removes contributions to the signals which do not vary with
altitude, for example the ROV and the metal in the coil connectors. These effects are accounted for by
a compensation procedure with the vehicle on the sea bed. The coil voltages reported at the SDC also
provide a useful check as to whether the system is operating correctly.
Technically, background compensation is the recording of the coil voltages with the vehicle stationary.
These voltages are then subtracted from subsequent readings.
The system measures the coil voltage at three points. This yields two voltages, termed "early" and
"standard". The "early" voltage will be more sensitive to both target and seawater than the "standard"
voltage. The "early" voltage will be approximately 20 times more sensitive to seawater than the
"standard" voltage. However, it will be only three times more sensitive target signals. It is this different
sensitivity is exploited to remove the seawater signals.
DPN 402196 Issue 4.1 © Teledyne TSS A- 5