Submariners possess a basic understanding of sound propagation in water and its utility for sound navigation and ranging ( ONAR). Underwater acoustics is the environment in which the submariner lives and excels. The submariner also lives in an electromagnetic environment which provides information just as important and valuable as that provided by acoustics.
Some submariners view low-frequency communications, that is, extremely low frequency (ELF), very low frequency (VLF}, and low frequency (LF), as akin to black magic, which, on the basis of operating experience, appears to randomly succeed or fail. A basic understanding of the physical principles of lowfrequency communications can provide the submariner with the knowledge to retain both reliable low-frequency communications and maximum operational flexibility.
Sound propagation in water and low-frequency electromagnetic propagation in air are based on similar physical principles. In both cases energy is transmitted from a source to a receiver, but, the submarine’s use of each is very different SONAR is used to detect, localize, track, and classify contacts within the submarine’s ocean area of interest, whereas low-frequency communications are used to transfer information to the submarine.
One approach to developing a basic understanding of the physical principles of low-frequency communications is to build upon the submariner’s existing knowledge of acoustics and SONAR. The purpose of this article is to examine the similarities and unique properties of sound propagation in water and low-frequency radio-wave propagation to provide a basic understanding of low-frequency communications.
General
Sound propagates in water via an acoustic pressure wave bounded by the ocean’s floor and surface. Acoustic frequencies of interest are detectable in the hertz (Hz) to kilohertz (kHz) frequency ranges. Acoustic paths are characterized by spherical spreading at short ranges, and cylindrical spreading at long ranges.
In comparison, low-frequency electromagnetic waves propagate at the speed of light bounded by the earth’s surface and the bottom of the ionosphere (i.e., 70 to 90 km). Lowfrequency signals are also specified in Hz and kHz. Short-range low-frequency electromagnetic paths are also characterized by spherical spreading and long-range electromagnetic paths by cylindrical spreading. Excellent signal stability at long ranges and seawater depth penetration ability (Note: The earth’s surface is not a perfect boundary) are the primary reasons lowfrequency electromagnetic signals are used for submarine communications.
Passive SONAR Equation
A good place to start a comparison between SONAR and low-frequency communications is with the well-known Passive SONAR equation
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The above equation can be called the “Passive Communications Equation”. The same SONAR terms calculated from the Passive SONAR Equation can also be developed for the Passive Communications Equation.
Recognition Differential
A very useful term is the SONAR recognition differential (NRD) defined as the SNR required to detect a contact with the desired probability. As an example, the SONAR operator will detect the contact SO% of the time when the SNR is equal to the NRDSO%· In low-frequency communications, the equivalent term is SNRPCMR defined as the SNR required to provide the desired probability of correct message receipt (PCMR), for example, SNRpCMRSO’J(o provides a 50% PCMR.
Figure Of Merit Range
Another useful term is the SONAR Figure of Merit(NFM)· NFM is the allowable propagation loss to achieve the desired NRD. NFM is converted to theFigure of Merit Range (i.e., the range where NRD occurs) with the appropriate acoustic propagation loss curves. Likewise, a low-frequency Figure of Merit Range (i.e., the range where SNRpcMR occurs) can be calculated from the appropriate electromagnetic propagation loss curves. The low-frequency Figure of Merit Range is usually displayed in the form of coverage contours from a transmitter. The respective Figure of Merit equations are
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Signal Processing Gain
The ability to distinguish the desired signal in the presence of background noise or interference is related to the length of time the desired signal is averaged prior to a discrete measurement The NRD or SNRPCMR decreases as the time allocated for signal processing (or averaging) increases. For passive SONAR detection, the signal processing time is called integration time, e.g., short time average (STA), intermediate time average (ITA), and long time average (LTA). For low-frequency communications, the time allocated for signal processing is called the bit duration. Note: Communications engineers attempt to confuse the lay person by defining the binary one’s and zero’s (which are usually referred to as a bit) as a chip. Multiple chips are algebraically combined (averaged) to provide a bit decision, that is, an overall one or zero in a communications code. The process of averaging multiple chips for each bit decision is called spreading (i.e., the bit decision is spread over time). The simple equation to calculate the theoretical signal processing gain of a low-frequency communications mode is: Theoretical signal processing gain (in dB) = 10 log(# chips per bit).
For example, a communications mode with a 1000 chips per bit has a theoretical signal processing gain of 30 dB (i.e., SNRPCMR improves by 30 dB – a factor of 1000). In the same manner as the signal processing gain of the ITA and LTA displays provide the ability to detect a contact below the SNR where a sonarman can hear a contact. The signal processing gain of the special low-data-rate Minimum Essential Emergency Communications Network (MEECN) modes used for Emergency Action Message (EAM) transmission provide successful message reception below the SNR where a radioman can hear a signal.
The signal processing gain for low-frequency communications can also be improved by using error detection and correction (ED A C), which consists of transmitting extra bits of information (i.e., parity check bits inserted by the transmit system) to improve reception performance. There are numerous EDAC encoding techniques used to improve signal processing gain. The most efficient techniques provide several dB of additional gain. In other words, correct message copy can be achieved with up to 10% of the received bits in error.
The gain from SONAR and low frequency communications signal processing can be used either to
1. Improve the recognition differential (or PCMR) while maintaining the same range, or
2 Increase the range from the source (or transmitter) while maintaining the same recognition differential (or PCMR).
Attenuation
The useful range of an acoustic or low-frequency electromagnetic system is also a function of the attenuation rate (or propagation loss rate) of the desired signal. The desired signal can be detected until the received signal level falls below the sensitivity threshold of the receive system. Typical attenuation rates for acoustic and low-frequency electromagnetic signals are
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Acoustic systems are limited to useful ranges (in sea water) of tens or hundreds of nautical miles; whereas, low-frequency communications systems may provide useful ranges (in air) of thousands of nautical miles. The low frequency electromagnetic signal propagates in air from the transmitter to the ocean surface above the submarine. Then it propagates in sea water to the submarine’s submerged communications antenna which may be tens of feet below the ocean surface for VLFJLF reception or hundreds of feet below the ocean surface for ELF reception.
Background Noise/Interference
The sources of acoustic noise which compete with the desired acoustic signal originate from a variety of ambient and shipboard sources (e.g., biologics, shipping, weather, flow noise, rotating equipment, etc.). Likewise, the sources oflow-frequency electromagnetic noise which compete with the desired electromagnetic signal originate from both ambient and shipboard sources.
The total electromagnetic noise (or interference) power at the submarine receiver is the sum of the powers of all sources of electromagnetic interference. The possible sources of lowfrequency electromagnetic interference are
1. Atmospheric noise
2. Electromagnetic Interference (EMI)
3. External man-made interference
4. Receiver thermal noise
Atmospheric noise-Low-frequency electromagnetic radiation from lightning bolts during thunderstorms is the major source of low-frequency atmospheric noise. The level cf atmospheric noise at a submarine’s radio antenna is the power sum of all thunderstorm-generated low-frequency radiation which propagates to the submarine’s location.
The daily and seasonal variations in thunderstorm activity are the major sources of low-frequency atmospheric noise level variations. Nighttime atmospheric noise levels are usually higher than daytime atmospheric noise levels because nighttime attenuation is lower. In the northern hemisphere, winter atmospheric noise levels are less than summer atmospheric noise levels because thunderstorm activity occurs farther south resulting in greater propagation distances (and more attenuation) to northern areas.
Electromagnetic interference – The primary source of lowfrequency EMI on submarines is electromagnetic noise generated by rotating equipment (e.g., power generators, propeller shaft, etc.) and other electrical loads (flre control, SONAR, navigation, etc.). In the same manner as the submarine’s acoustic self-noise is controlled to maximize the acoustic SNR, EMI must be controlled to avoid degrading the submarine’s ability to receive low-frequency communications. A high EMI level on the desired signal’s frequency may prevent message reception in much the same way as a sound short may mask detection of a SONAR contact. EMI can be identified and minimized by conducting periodic EMI surveys. The generation of new sources of EMI can be minimized by using proper installation, maintenance, and repair procedures on all shipboard electrical and electronic systems.
External man-made interference -The presence of undesirable signals within the bandwidth of the communications receiver is a possible source of interference. External man-made interference may originate from unintentional sources (i.e., existing transmitters) and/or intentional sources (i.e., hostile jamming). The large bandwidth requirements of a low frequency communications system relative to the usable frequency band and the world-wide distribution of VLF/LF transmitters increase the possibility of external man-made interference. As an example, a VLF receiver with a 1-kHz bandwidth monitors over 5% of the usable VLF band (14 to 30kHz). With over two dozen VLF transmitters in operation world-wide, the possibility of external man-made interference exists. The effects of external man-made interference may be reduced by minimizing the bandwidth of the communications receiver, that is, the bandwidth of the communications receiver must be centered and matched to the bandwidth of the transmitted signal.
Receiver thermal noise – Electromagnetic noise is generated within the communications receive system from the residual movement of charged particles in electrical components. The kinetic energy of the charged particles is proportional to the temperature of the electrical component. Any electrical component connected to (e.g., antennas, multicouplers and amplifiers) or within the communications receiver may be a source of thermal noise. Low-frequency communications receive systems are designed with high-quality components (with low thermal noise characteristics) to minimize receiver thermal noise levels. Thermal noise can be minimized by ensuring that proper maintenance and repair procedures are used to maintain all communications receive system components at design specifications.
Sensors/Antennas
The purpose of a sensor is to convert acoustic or electromagnetic energy into an electrical signal capable of being processed by a receive system to extract meaningful information. Acoustic pressure waves and electromagnetic waves are received by hull/sail mounted and tethered sensors.
There are two basic types of sensors used for the reception of low-frequency communications (i.e., loop antennas and wire antennas). A loop antenna receives the magnetic component of the electromagnetic field. The loop antennas used for submarine VLF/LF reception are the towed buoy antenna and various mast mounted VLF/LF magnetic antennas. Loop antennas are not used for ELF reception on submarines. The sensitivity of a loop antenna is proportional to its magnetic cross-sectional area. The effective cross-sectional area of a loop antenna increases with additional wound turns of conductor. A single-loop antenna has a figure-eight reception pattern with maximum reception in the direction of the plane of the loop. The output of two-loop antennas at right angles to each other (i.e., crossed loops) can be combined to provide an omnidirectional reception pattern. The loop antenna is normally operated several meters below the surface. The electromagnetic field available to the loop antenna may be reduced by the high VLF/LF attenuation of sea water directly above the loop antenna.
In comparison, a wire antenna receives the electric component of the electromagnetic field. The wire antennas used for submarine low-frequency reception are the floating wire antenna (FW A) and the auxiliary wire (i.e., pigtail) of the towed buoy antenna.
Terminators/Thermal Layers
The location of the ionospheric terminator {day-night interface) with respect to a low-frequency propagation path is one of the most important factors affecting the daily and seasonal variability of low-frequency electromagnetic signal propagation. The location of the terminator determines the ionospheric condition (day, night, day-night transition, night-day transition) through which a low-frequency electromagnetic signal must propagate. Propagation through a terminator may result in destructive interference (i.e., reduced signal levels) of lowfrequency electromagnetic signals. Figure 1 is a plot of signal strength over a 24-hour period from the VLF transmitter at Lualualei, Hawaii to a ground-based receiver at Laurel, Maryland.
The effect of a terminator in a low-frequency propagation path is similar to cross-layer detection of a SONAR contact on the other side of a thermal layer. Low-frequency communications reception (or SONAR detection) for cross terminator reception (or cross-layer detection) is more difficult because of the lower received signal level. Unlike thermal layers where the submarine can control which side of the thermal layer to operate, the submarine cannot control the ionospheric conditions affecting low-frequency communications. However, the presence of terminators is very predictable based on the solar zenith angle at the transmitter and submarine locations.
Summary
In the same manner as the submariner knows and controls his submarine’s acoustic environment, he must also know and control the electromagnetic environment. He should be aware of the actual received SNR at his communications receivers and the dominant source of interference, and should understand his ability to control these signal levels. When the received SNR is large, the depth of the communications antenna may be increased while successful low-frequency communications are maintained. The resulting, less stringent communications posture provides additional operational flexibility and may also reduce the submarine’s vulnerability.
There are, however, seVeral factors beyond the submariner’s control which may affect his ability to successfully copy lowfrequency communications from a particular transmitter. These factors are
1. Transmitter power and frequency,
2 Atmospheric noise and external interference levels on the transmitter’s frequency,
3. Signal propagation Joss based on the geographical distance from the transmitter,
4. Ionospheric conditions of the propagation path between the submarine and the transmitter, and
5. Signal processing gain of the transmitted communications mode.
In conclusion, the propagation of low-frequency electromagnetic signals is based on predictable physical principles. A basic knowledge of these principles provides the submariner with the ability to maintain reliable low-frequency communications while also retaining maximum operational flexibility
