Underwater acoustic communication

FSK is the earliest form of modulation used for acoustic modems. UAC prior to modems was by percussion of different objects underwater. This method was also used to measure the speed of sound in water.

FSK usually employs two distinct frequencies to modulate data; for example, frequency F1 to indicate bit 0 and frequency F2 to indicate bit 1. Hence a binary string can be transmitted by alternating these two frequencies depending on whether it is a 0 or 1. The receiver can be as simple as having analogue matched filters to the two frequencies and a level detector to decide if a 1 or 0 was received. This is a relatively easy form of modulation and therefore used in the earliest acoustic modems. However more sophisticated demodulator using digital signal processors (DSP) can be used in the present day.

The biggest challenge FSK faces in the UAC is multi-path reflections. With multi-path (particularly in UAC) several strong reflections can be present at the receiving hydrophone and the threshold detectors become confused, thus severely limiting the use of this type of UAC to vertical channels. Adaptive equalization methods have been tried with limited success. Adaptive equalization tries to model the highly reflective UAC channel and subtract the effects from the received signal. The success has been limited due to the rapidly varying conditions and the difficulty to adapt in time.

Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing (modulating) the phase of a reference signal (the carrier wave). The signal is impressed into the magnetic field x,y area by varying the sine and cosine inputs at a precise time. It is widely used for wireless LANs , RFID and Bluetooth communication.

Orthogonal frequency-division multiplexing (OFDM) is a digital multi-carrier modulation scheme. OFDM conveys data on several parallel data channels by incorporating closely spaced orthogonal sub-carrier signals.

OFDM is a favorable communication scheme in underwater acoustic communications thanks to its resilience against frequency selective channels with long delay spreads.[3][4][5]

The underwater telephone, also known as UQC, AN/WQC-2, or Gertrude, was used by the U.S. Navy in 1945 [13] after in Kiel, Germany, in 1935 different realizations at sea were demonstrated.[14] The terms UQC and AN/WQC-2 follow the nomenclature of the Joint Electronics Type Designation System.[15] The type designation "UQC" stands for General Utility (multi use), Sonar and Underwater Sound and Communications (Receiving/Transmitting, two way). The "W" in WQC stands for Water Surface and Underwater combined. The underwater telephone is used on all crewed submersibles and many Naval surface ships in operation. Voice or an audio tone (morse code) communicated through the UQC are heterodyned to a high pitch for acoustic transmission through water.[16]

In April 2017, NATO's Centre for Maritime Research and Experimentation announced[17] the approval of JANUS, a standardized protocol to transmit digital information underwater using acoustic sound (like modems and fax machines do over telephone lines).[18] Documented in STANAG 4748, it uses 900 Hz to 60 kHz frequencies at distances of up to 28 kilometres (17 mi).[19][20] It is available for use with military and civilian, NATO and non-NATO devices; it was named after the Roman god of gateways, openings, etc.

The JANUS specification (ANEP-87) provides for a flexible plug-in-based payload scheme. A baseline JANUS packet consists of 64 bits to which further arbitrary data (Cargo) can be appended.[21] This enables multiple different applications such as Emergency location, Underwater AIS (Automatic Identification System), and Chat. An example of an Emergency Position and Status message is the following JSON representation[22][23]:

{
   "ClassUserID":0,
   "ApplicationType":3,
   "Nationality":"PT",
   "Latitude":"38.386547",
   "Longitude":"-9.055858",
   "Depth":"16",
   "Speed":"1.400000",
   "Heading":"0.000000",
   "O2":"17.799999",
   "CO2":"5.000000",
   "CO":"76.000000",
   "H2":"3.500000",
   "Pressure":"45.000000",
   "Temperature":"21.000000",
   "Survivors":"43",
   "MobilityFlag":"1",
   "ForwardingCapability":"1",
   "TxRxFlag":"0",
   "ScheduleFlag":"0"
}

This Emergency Position and Status Message (Class ID 0 Application 3 Plug-in) message shows a Portuguese submarine at 38.386547 latitude -9.055858 longitude at a depth of 16 meters. It is moving north at 1.4 meters per second, and has 43 survivors on board and shows the environmental conditions.

Commercial hardware products have been designed to enable two-way underwater messaging between scuba divers.[24][25] These support sending from a list of pre-defined messages from a dive computer using acoustic communication.

Research efforts have also explored the use of smartphones in water-proof cases for underwater communication, using acoustic modem hardware as phone attachments[26] as well as using a software app without any additional hardware.[27] The Android software app, AquaApp, from University of Washington uses the microphones and speakers on existing smartphones and smart watches to enable underwater acoustic communication.[28] It had been tested to send digital messages using smartphones between divers at distances of up to 100 m.[27]