Purpose
RACK-CS is designed to register specific to underwater communication signals, which are then classified by an operator who can make the decision to jam them actively for a certain period of time by emitting powerful hydroacoustic impulses in an appropriate frequency range.

The underwater part of the RACK-CS is positioned on the seabed at a depth of up to 30 m and registers the exchange of information between ultrasound underwater communication devices in the 10 – 60 kHz frequency range. Effective suppression of the underwater communication is achieved by emission of pulsed interference signals with a frequency close to that of the communication signals and an amplitude sufficient to achieve a level above the threshold of the receiver.

RACK-CS is designed to meet the following requirements:

• to register the availability of communication signals in a maritime area;
• to receive and analyze hydro acoustic signals emitted in the full frequency range of the systems for underwater communication;
• to be activated remotely via a cable, when information for possible underwater targets is received from a radar, a sonar, by visual means, etc.;
• to emit a series of pulses with tonal interference;
• to provide non-isotropic emitting of signals – omnidirectional radiation in the horizontal plane and directed radiation in the vertical plane;

Monitoring, recording and classification of the signals for underwater communication is carried out by an operator using a coastal/ship station, located on shore or on board a ship.

The underwater part of RACK-CS is connected to the base coastal/ship station via an underwater cable. The base station receives information from a hydroacoustic receiver, sends commanding signals, and supplies 24V DC voltage to charge the power supply batteries.

The switching of the jammer into the active mode of operation is performed on the command of an operator and sent by a cable. Switching it back into standby (listening) mode is done in the same manner.

By command given by the operator the jammer is activated and starts to emit for a certain period of time (about 3 minutes) a pseudo continuous wave consisting of a set of short pulses with acoustic power of no less than 1600 – 1900 Pa/m (180-185 dB re 1μPa at 1m), which are filled with random frequency from the spectrum of the selected frequency (sub) range.

Description of the Jammer
RACK-CS consists of:

• a coastal/ship station containing an indicating device and software for analysis of the received signals, a system managing the operating modes (emitting, checking the parameters etc.), a converter for charging the batteries (supply voltage from 100 to 230 V AC, frequency 50-60 Hz);
• an underwater jammer consisting of an anchor and a floating body connected to each other by a cable rope (fig. 6). There is a housing at the center of the upper part of the anchor for the positioning of the floating body during transportation (Fig.2). After positioning of the anchor at the seabed, the floating body floats up at a distance of 2.0 – 2.5 m above it.
• an underwater cable connecting the coastal/ship station and the anchor with a length of 100 m, wound on a reel.

COASTAL/SHIP (BASE) STATION
The base station (BS) is designed to:

• charge the batteries located inside the body of the anchor;
• charge the battery located in the BS which powers the BS during a temporary interruption of the mains power supply;
• display on an indicating device (laptop) the marine noises coming from the hydroacoustic receiver;
• control the modes of operation and the frequency ranges of the active part of the device;

The BS elements are assembled in a special shock-resistant and hermetically sealed case.

In the upper part of the case is mounted a plate on which are located:

• a laptop, bottom of the laptop is firmly attached to a Velcro tape with screws and can be detached and removed if necessary;
• handles for lifting of the plate to provide access to the elements inside the body of the case;
• an illuminated button for switching on the power supply (of 110 to 230 V AC);
• a switch for switching the 12V battery and the power supply for the command and control unit;
• a button for switching a cooling fan;
• LEDs indicating charging of the batteries in the anchor and in the BS (24V and 12V correspondingly); they glow in green when fully charged and in red during charging;
• a keypad of the control unit, used to set the operating modes, the emitting frequency ranges, etc.;
• a control unit display that indicates which functions are currently being executed;
• a male coupling for connecting to the mains voltage (100V to 230V AC). The connection is made with a power cord provided with a female coupling;
• a female plug for connection to the underwater cable by means of an intermediate cable.

Underwater Jammer
The underwater part contains a hydro acoustic transceiver with a control unit that is executed in the form of a single-PCB microprocessor controller.

Operating principle
а. After connecting the sealed coupling to the underwater jammer (before putting it in position on the sea bed) the batteries start to charge and the hydroacoustic receiver is activated. The jammer goes into standby mode.
b. The monitor of the coastal/ship station displays the noise picture in the surrounding marine area.
c. In the main mode of operation, the operator analyzes the noise situation and classifies signals that may be due to underwater communication.
d. If the classification under section c. is affirmative, the operator determines the frequency range of the communication, the sub bands and the bandwidth.
e. Using the laptop or the keypad of the control unit, the operator selects the corresponding combinations that define the parameters of the interfering signal emitted by the jammer.
f. At the command by the operator, the jammer starts to emit a jamming signal within the specified frequency within the specified frequency sub-band for a period of 3 minutes. During this time, the hydro acoustic receiver is blocked;
g. After the expiration of the above time interval, the hydroacoustic receiver is switched on and the sequence of operation described in sections b. – g. is repeated.

Visualization and analysis of the noise situation
The information from the hydroacoustic receiver is visualized and analyzed by the software of the laptop, on basis of which is decided if there is a communication signal and what is its frequency range. The name of the software for visualization and analysis of the noise situations is Underwater Jammer Commander (UJC).

UJC consists of a settings line and three embedded windows marked with green numbers on Fig. 6. It is developed with the purpose to perform jamming with chosen frequencies. A hydro acoustic serial port and a commands serial port can be connected using the settings menu. The window marked with 1 represents the raw signal and the power spectrum. The window marked with 2 and named RS 485 COMMUNICATION is used to choose the jamming frequency range. In the frequency (axis X)-time (axis Y) plot window, marked with 3, is shown in color the power of the received hydroacoustic signal (red for high values and blue for low values).