Today, underwater sensors cannot share data with those on land, as both use different wireless signals that only work in their respective mediums. Radio signals that travel through air die very rapidly in water. Acoustic signals, or sonar, sent by underwater devices mostly reflect off the surface without ever breaking through. This causes inefficiencies and other issues for a variety of applications, such as ocean exploration and submarine-to-plane communication.
To achieve high data rates, the system transmits multiple frequencies at the same time, building on a modulation scheme used in wireless communication, called orthogonal frequency-division multiplexing. This lets the researchers transmit hundreds of bits at once.
Submarines communicate via multiple, complementary RF systems, covering nearly all the military communications frequencies. No one communications system or frequency band can support all submarine communications requirements. Submarine shipboard communications systems consist of RF antennas and radio room equipment, both RF transmitters/receivers and baseband suites. Submarines require a suite of antennas to provide the necessary communications, navigation, and Identification, Friend or Foe (IFF) capabilities. Submarine antennas, as compared to surface ship antennas, are unique in design, shape, materials, and performance due to a submarine’s space and weight limitations, extreme environmental conditions, and stealth considerations. UHF SATCOM provides a relatively high data rate but requires the submarine to expose a detectable mast-mounted antenna, degrading its primary attribute – stealth. Conversely, extremely low frequency (ELF) and VLF broadcast communications provide submarines a high degree of stealth and flexibility in speed and depth, but are low data rate, submarine-unique and shore-to-submarine only.
The US Navy is investing in new and previously demonstrated techniques for communicating with submarines at speed and depth for coordinated ASW operations. These techniques most commonly use either trailing wires or towed buoys for submarine communications, which impose limitations on the submarine’s maneuverability and stealth, and therefore negatively impact the submarine’s ability to fully conduct ASW operations. An airborne laser which could penetrate shallow water would permit submarine communications without the restrictions of floating wires or buoys.
ELF [Extremely Low Frequency 30 Hz – 300 Hz 10,000 Km – 1,000 Km wavelength] – This is the only band that can penetrate hundreds of meters below the surface of the ocean. The US Navy transmits ELF messages using a huge antenna in Wisconsin and Michigan created by several miles of cable on towers in conjunction with the underlying bedrock. This band is used to send short coded “phonetic letter spelled out” (PLSO) messages to deeply submerged submarines that are trailing long antenna wires. The communication is only one way, therefore it is used primarily for prearranged signals or to direct the submarine to come closer to the surface for faster communications. Environmental factors do not have a strong influence on changing the signal and therefore it is quite reliable.
VLF [Very low frequency 3 kHz – 30 kHz 100 Km – 10 Km ] This band can penetrate several meters below seawater and can transmit much more information than ELF, therefore it is useful for submarine communications when the submarine cannot surface, but can come close to the surface. It can be affected by salinity gradients in the ocean, but these usually do not present problems for near-surface submarines. There are natural sources of VLF radiation, but in general, like ELF, it is not strongly influenced by changes in environmental conditions therefore it is useful for reliable global communications. The transmission antennas need to be large, therefore it is primarily used for one-way communications from shore-based command centers to surface ships and submarines. It can also be used to broadcast to several satellites at once, which can in turn relay messages to the surface. The Navy’s VLF systems serve as a back-up for global communication use during hostilities when nuclear explosions may disrupt higher frequencies or satellites are destroyed by enemy actions. VLF is also used for aircraft and vessel navigation beacons and for transmitting standard frequencies and time signals.
“The radar reflection is going to vary a little bit whenever you have any form of displacement like on the surface of the water,” Adib says. “By picking up these tiny angle changes, we can pick up these variations that correspond to the sonar signal.”
One of the immediate tasks delineated by the Navy in “From the Sea” is to continue the full integration of SSNs into expeditionary task forces. To be effective units of a Naval Task Group within a joint, Tailored Forward Element (TFE), submarines must be fully interoperable with both Naval and Joint communication systems. Submarines must be capable of tailoring on-board capabilities to optimize their support for the Joint Task Force (JTF) and Naval Component Commanders.
Coordination between multiple assets such as aircraft, surface ships, and submarines is critical to an effective ASW campaign. Integration of submarines into an overall ASW effort, arguably the most effective platform for wide area search and tracking, has traditionally been hampered by lack of or minimal communications to the submarine while deep.
As submarines continue to conduct a variety of missions to include intelligence collection, Indications and Warning (I & W), anti-submarine warfare, anti-surface warfare, strike warfare, and mine warfare, they will have to be an integral part of networked sensors and platforms.
The development of these advanced communications has already begun with the incorporation of Narrowband based systems that are IP architecture based. Following this is development of a higher data rate antenna and wideband based communications and ultimately a buoyant cable antenna that allows two-way communications at depth and speed.
Ultimately submerged data exchange and communications capabilities will be a key enabler for employing off-board vehicles, sensors, and distributed networks of UUVs, sensors and other payloads.
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