User Manual for the Type 8300 Compatt 6
(and Variants)
UM-8300-Compatt 6
Issue B3
Appendix A – Basic Principles of Transponder Interrogation
and Reply
The interrogation signal transmitted to a 6G® Wideband®2 Transponder is a phase-encoded acoustic
signal. This signal is around 8 milliseconds in length. The acoustic transducer converts the acoustic
signal into a low level alternating voltage. All ambient noise is converted into an electrical noise voltage
covering a wide frequency range. The pre-amplifier in the transponder amplifies the combined signal
and noise voltages, and filters to weaken frequencies outside the band used by the transponder
channels. The receiver uses Digital Signal Processing (DSP) to detect the interrogation signal, even in
the presence of other noise and signals.
After a specific time-interval known as the ‘turn-around time’ logic circuits in the transponder and
transmitter generate a powerful acoustic reply. The reply is on a different channel to the interrogation
signal, but is the same length.
The equipment sending the interrogation signal will detect the reply signal from the transponder. The
delay in receiving the reply will allow the equipment to calculate the range (distance) to the
transponder.
The equipment calculates the range as follow:
l
Measures the total time delay = A
l
Subtracts the turn-around delay = B
l
This gives the true ‘through water’ time delay = C
l
Multiplies by the speed of sound in water = D
l
This equals the two way range = E
A – B = C
C x D = E
Sound is reflected by the sea surface, seabed and other reflecting surfaces. This causes a received
signal to appear much longer in duration than the other transmission. This extended signal has
random phases and random decreasing amplitude. This sound reflection is known as reverberation.
The transponder has a built in ‘blocking period’ which allows the reverberation to face away before a
transmission starts. The ‘blocking period’ prevents multiple replies from being transmitted in response
to the reverberation of the initial interrogation signal.
The 6G® Wideband®2 transponder is set to always receive its Individual Interrogation Signal (IIS)
which is set by its acoustic address. The IIS can be followed by an acoustic command. If no command
is present the transponder will reply on the Common Reply Signal (CRS). This interrogation mode is
used for baseline measurements between transponders, where only one transponder at a time will be
replying. The turn-around time in this fixed mode is 320 milliseconds.
The transponders can also reply to a Common Interrogation Signal (CIS), this reply can be enabled or
disabled by acoustic command. If enabled, it will reply to the interrogation signal using its Individual
Reply Signal (IRS), which is set by the acoustic address after the turn-around time. There are different
CIS signals (CIS0 to CIS14) these allow independent operation of several Long Baseline (LBL)
navigation systems in the same area. The CIS used and the value of the turn-around time can be set
by serial port commands or acoustic telemetry commands.
To maximise battery life, the transponder goes into a sleep mode. To wake the transponder from this
mode a wake up signal is sent before interrogation. The transceiver which generates the interrogation
can be instructed to transmit a wake-up signal immediately before sending the interrogation signal.
Appendix A – Basic Principles of Transponder Interrogation and Reply
125
