Elf Communications

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  • DONALD McCLENON

    ELF COMMUNICATIONS THE AUTHOR INTRODUCTION

    received his BS in Electrical Engineering .porn the University of Maryland in 1939. He worked in the fields oj. Communication Techniques. Receiver Design, and VLF Satellite Propagation Studies at the Naval Re- search Laboratory . h m 1939-1961, and was a Satellite Tracking Station Director jor NASA in Quito, Ecuador fTom 1961-1964. He then was Head oj* the Bureau of Ships Com m u n ication Transmitter Procu rem en t Unit jrom'1964-1966, and next moved to the areas of the VLF VERDIN System, General Communications. and the ELF SANGUINE System in the Naval Electronic Systems Command @om 1966-1967. Since then he has been in the ELF Division. Electronics Branch of Special Communications PME 117 in the Naval Electronic Systems Command. He is a Registered Professional Engineer and a Senior Member of Institute of Electrical and Electronic Engineers.

    Editor's Note: This is an updated version o f the paper presented by the Author at the Joint Technical Symposium held 26 September 1975 in Washington, D. C.

    ABSTRACT

    A desired military capability is continuous communi- cations to all world wide deployed submarines, from one continental United States location, with minimum depth, speed, or heading constraints. Extremely Low Frequency (ELF) is the only usable portion of the elec- tromagnetic spectrum which penetrates seawater to sufficient depths to meet these requirements. A prac- tical ELF System can be hardened to withstand enormous enemy attacks, it can operate against jammers much larger than itself, and it is not seriously degraded by natural or nuclear induced ionospheric disturbances.

    A brief theoretical description of system operation is presented, showing how a vertical E-field wave is launched and propagated in the earth-ionosphere cavity, and propagates downward into the world's oceans. The required relation is shown between natural parameters such as deep earth conductivity, ionospheric height, ELF receiver site noise level, and engineering parameters such as frequency, antenna length, antenna arrangement, and antenna current. Practical system information is provided including transmitting antenna systems, modulation techniques, receiving sensors, and receiver design. Some actual message test results are shown for a communication path from the Navy Wisconsin Test Facility to a distant submerged submarine receiver.

    O U R POLARIS/POSEIDON NUCLEAR MISSILE sm- MARINES are relatively invulnerable to detection. Therefore, they form a vital part of our national war deterrent strategy. This invulnerability must be preserved against future advances in anti- submarine warfare, using every possible means to minimize any operating restrictions.

    Submarines must expose an antenna above the sea surface for most high frequency reception, and must float a wire or a tethered buoy near the surface for Very Low Frequency (VLF) reception (Figure 1). There are appreciable speed, depth, and heading constraints in this VLF Posture. Extremely Low Frequency (ELF) is the only usable portion of the electromagnetic' spectrum which penetrates seawater to sufficient depths to remove these restrictions. Neither the submarine nor its antenna must approach the surface, and the speed need not be reduced. Removal of the heading restriction now also looks promising.

    Propagation of an ELF signal in the earth- ionosphere cavity occurs with such extremely low attenuation, that essentially worldwide coverage is feasible from a single U.S. based transmitter site. This denies a distant jammer most of the range advantage he gets at higher frequencies. Addi- tional anti-jam protection is obtained by signal processing. Distances over which variable iono- spheric layer densities cause HF radio "black- outs". are such a small perceniage of an ELF wavelength, that there is a relatively minor effect on ELF signal transmission. This is true, whether

    I SUBMARINE RECEIVING CAPABILITIES I

    I, PRESENT C A P A B I L I T I f S

    Figure 1.

    Naval Engineers Journal, August 1976 33

  • ELF COMMUNICATIONS McCLENON

    the changes are caused by solar disturbances or high-altitude nuclear bursts.

    The presently planned ELF range of 45 to 80 Hz includes the frequencies used by most of the worlds electric power systems. Most communica- tion transmitting antennas are an appreciable fraction of a wavelength long. At 45 Hz, the signal w a v e m h is over four thousand miles, as shown in Figure 2. We obviously need a drastic length

    F = 4 5 Hz

    reduction before such a system could be practical, and fortunately this is feasible.

    The ELF transmitter complex can be hardened to withstand any postulated enemy attack, it can operate against jammers much larger than itself, and the system is not seriously degraded by natural or nuclear induced ionospheric disturbances. Speed, depth and range restrictions are expected to be far less than at present.

    ELF SYSTEM DESCRIPTION

    The basic elements of an ELF System are shown in Figure 3. The transmitting antenna launches a vertically polarized E-field signal into the earth- ionosphere waveguide cavity. There is a wave tilt at the land or sea surface, sp a reduced horizontal E-field propagates downward into the worlds oceans. An E-field sensor deployed from a sub- marine can detect this signal. The associated H-field at right angles to the E-field propagates with it as it enters the ocean. Its detection by an H-field sensor can be verv useful as will be shown

    ELF COMMUNICATIONS SYSTEM OVERVIEW

    I

    F m 2. ELF Wave Length. later.

    -

    ELF PROPAGATION PATH

    RECEIVING SUBMARINE 0 RECEIVING ANTENNAS

    PERPEND. H-FIELD ALWAYS HORIZ. Figure 3.

    34 Naval Engineers Journal, August 1976

  • McCLENON ELF COMMUNICATIONS

    TRANSMITTING ANTENNA

    Several ELF transmitting antenna schemes were tried prior to 1963 including tuning a big NAVY VLF antenna at ELF. One scheme has proven so superior to all others, that its various forms are now the only ones seriously considered. The antenna consists of a conductor insulated from the earth, grounded at both ends, and fed with ELF power. Current flows from one ground terminal through the earth and back to the other ground. The lower the earth conductivity (the higher the resistivity) under the antenna, the deeper into the earth the current will be forced to flow to get back and the more effective the radiator becomes. A typical 75 Hz example gives an equivalent loop antenna depth into the earth of 3.5 kilometers. Nature gives us three sides of the loop, and all we have to do is close in the top. (Figure 4). The NAVYS Wisconsin experiments have demonstrated that for the same antenna current (30A in a 16 mile line; 450 kW ELF input, 1 watt radiated), the signal in Europe was identical whether the antenna was an insulated cable buried several feet deep in the ground or whether it was strung on telephone poles 30 feet in the air.

    A single long antenna would not give the desired global pattern because there is not suitable geology covering the required distance, and even if there were, such an antenna would be extremely vulner-

    able to attack. Placing intermediate grounds on the antenna and using several smaller in-phase intermediate transmitters, radiates the same signal if the antenna current is the same. It is also possible to place the four separate grounded antennas in parallel instead of in series and thus obtain the same signal if they are far enough apart to minimize mutual coupling. (Figure 5).

    A series of parallel East-West lines will give an East-West figure-of-eight pattern. A series of parallel North-South lines provides a North-South figure-of-eight pattern. Combining the two, with controllable phasing, gives a grid arrangement,

    ANTENNA CONFIGURATION

    I I

    Figure 5.

    ELF TRANSMITTING ANTENNA

    3.5 KM TYPICAL

    E Q U I V A L E N T , - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ , /

    / \ -- - - -- -- - - - - - --_a LOOP

    Figure 4.

    Naval Engineers Journal, August 1976 35

  • ELF COMMUNICATIONS McCLENON

    and thereby permits either an omni-directional or steered figure-of-eight pattern. This makes op- timum use of a given geologic area and is the basis for most ELF transmitter station designs. (Figure 6 ) .

    SYSTEM DESIGN

    System design must meet a set of performance requirements, such as a given message length in a

    R A D I A T I O N PATTERNS A N T E N N A A R R A Y R A D I A T I O N P A T T E R N

    L

    L G?

    L a I

    Figure 6.

    given time (data rate), with a specified certainty of receipt, a specified accuracy, a specified percent of the year, at the required locations, with required platform speeds, depths, and headings. To do this, a signal design (modulation/demodulation/coding scheme) is developed, using the best practical state-of-the-art techniques. The required worst- case receiver signal during maximum noise condi- tions is determined. Subtracting ocean depth and oceanlair interface loss allowances gives a required surface field strength. Knowledge of earth- ionosphere parameters then permits calculation of the required transmitting system Current-Length (IL) product. Figure 7 shows the relationship of these parameters. It is more accurate than similar expressions used at higher frequencies because there are no cavity mode interferences and other parameter variations have less effect. Since trans- mitter system cost is far higher than all the receivers in use, very sophisticated and expensive receivers are justified for this service.

    The available bandwidth of ELF systems is of the order of 10 Hz, and so messages must be com- pressed to a maximum extent and bandwidth conserving signal designs must be employed. The most practical modulation scheme found to date is Minimum Shift Keying (MSK), now used on the

    ELF PROPAGATION

    TRANSMITTER RADIATION $- CONSTANT IZE ATTENUATION PATTERN FACTOR

    1 4 t

    FREQUENCY GEOMETRICAL EXClTATlON SPREADING

    FACTOR FACTOR

    = ELECTRIC FIELD STRENGTH, VIM = VERTICAL COMPONENT = HORIZONTAL COMPONENT = HORIZONTAL MAGNETIC FIELD STRENGTH = DISTANCE FROM TRANSMITTER, M = ATTENUATION RATE, dB/Mm

    = FREQUENCY Hz = 4n x 10-7 HENRIESIM

    c So = EXCITATION FACTOR

    0, = EFFECTIVE CONDUCTIVITY, MHOSlM a = EARTH RADIUS, M 0 = ANGLE OFF ANTENNA LINE I = ANTENNA CURRENT, A L = ANTENNA LENGTH, M

    = 3 x lo8 METERSlSEC

    hi = IONOSPHERE HEIGHT, M

    Flyre 7.

    36 Naval Engineers Journal, August 1976

  • McCLENON

    NAVY VLF VERDIN equipment. It is phase modulation with certain constraints placed on it to minimize transients and bandwidth. It can also be regarded as a special kind of frequency modulation (FM). A typical set of three transitions or chips, with the corresponding spectrum, is shown in Figure 8. Note the zero rate of change at the times when the frequency shift begins.

    The basic chips are controlled by a crypto- graphically secure pseudo-random bit stream. A very accurate time standard is required at both ends of the system to keep it synchronized. There are many chips per channel symbol, and several channel symbols per actual message intelligence bit. A larger number of individual chip errors can occur, and still cause no errors whatever in the final message. Convolutional message encoding/interleaving is done at the transmitter, and in effect, each message is repeated several times. This is done in such a way that the receiver can decode the message the first time if the signal-to-noise ratio is high. However, if this ratio is low, the receiver can continue processing through several repeats until decod- ing is successful.

    >

    = AND NOISE DECODER L FILTERING

    PROCESSING a J

    ELF COMMUNICATIONS

    T T Y

    r

    RECEIVER

    The basic receiver concept is shown in Figure 9. The analog front end is fairly conventional. It is usually provided with notch filters to remove local power line interference. It must cope with relatively enormous noise spikes produced by lightning storms up to thousands of miles distant.

    MINIMUM SHIFT KEYING I I 1 0 1

    HIGH F LOW f HIGH f

    Figure 8.

    ELF RECEIVER

    ANTENNA FRONT END

    A MPLl FlER AND FILTER A MPLl FlER AND FILTER

    HARDWIRED

    (HARDWARE)

    ANALOG

    SIGNAL SYMBOL DISPLAY

    . PROCESSING PROCESSING

    COMPUTER

    (SOFTWARE) 0 *

    DIGITAL

    SIGNAL AND NOISE

    DIGITIZED SIGNAL AND NOISE

    Figure 9.

    Naval Engineers Journal, August 1976 37

  • ELF COMMUNICATIONS McCLENON

    One of the most valuable receiver features is a non-linear device such as a hard limiter or clipper. By clipbing the lightning-generated noise spikes, it makes the remaining noise appear much more random and thus more tractable for further receiver processing. The ocean acts as a low-pass filter. Hence, as the depth increases, the high frequency components of noise spikes are attenuated more than the lows, and the noise is smeared. The benefits of clipping are then greatly reduced. To combat this, an inverse ocean filter is used to equalize response. It must auto- matically change characteristics as antenna depth increases.

    The resulting signal and noise is then digitalized and the pseudo random code synchronism and sequential decoding processing (the major receiver operations) are performed by a minicomputer. The final messagk output is printed on a teletypewriter as well as a metric or quality number related to the effective signal-to-noise ratio. It tells how certain the receiver is that no errors are present.

    RECEIVING ANTENNA

    A typical trailing electrode pair E-field sub- marine antenna contacts two points in the ocean separated by about 300 meters. Insulated conduc- tors from each contact electrode are brought into the submarine, and the voltage between them is applied to the receiver. The nearest electrode to the submarine is kept far enough out so that it receives negligible submarine-generated ELF noise. This antenna provides a figure-of-eight horizontal pattern in the antenna direction as shown in Figure 10, with no signal normal to it. As was previously pointed out, there is a magnetic H-field at right angles to the E-field, and propagating with it. When the E-field is tilted 90 degrees to propagate downward into the seawater, the H-field likewise propagates downward, still horizontally, just shifted in direction of motion. A trailing long thin solenoid magnetic H-field antenna gives a figure-of-eight horizontal pattern normal to the antenna direction. If each antenna has the same sensitivity, the patterns can be combined by phasing to give either a circle or a steered figure-of-eight to null out noise 9r jamming, or to maximize a signal. Some R&D improvement is still required to bring the co-located ~ H-field antenna performance up to this level, but the prospects for sucess are good.

    The ultimate antenna would be located on the hull, so no trailing device would be required with its maneuvering restrictions. Such an antenna has been developed and demonstrated, but its per-

    formance is still limited by short effective length and locally generated submarine noise.

    INJECTION LINKS

    Strategic use of any communication system requires that the NCA (National Command Authority; President, Secretary of Defense, or their alternates) have immediate and continuous access to it. The NCA may be at various places, including a moving aircraft. Thus several redundant means (injection links) are required to connect him to the ELF transmitter. Soft, or peacetime, links include telephone or data landlines and LF or HF radio nets which are susceptible to jamming or blackouts. Hardened, or survivable, links in- clude airborne VLF, airborne line-of-sight UHF, and satellite links of various kinds. The pre-attack concept is an Input Message Processing Terminal that processes all incoming traffic, eliminates redundant messages, establishes priorities, ar- ranges interrupts for higher priorities, maintains the buffer store, and does all the crypt0 and status keeping chores. This soft terminal feeds the selected traffic simultaneously to all the trans- mitters.

    The post-attack concept for a hardened/ dispersed ELF transmitter complex is for each unmanned transmitter station to receive the message directly via survivable airborne or satellite injection links.

    TRANSMITTERS

    ELF power can be produced by conventional rotating machines, vacuum tube amplifiers, silicon controlled rectifier (SCR) inverters, transistors, and cyclo-converters. All of these can be MSK

    B I D I R E C T I O N A L

    *AmiEGzr------- T R A I L I N G f L C l R O D E P A I R I - F I E L D )

    B l D l R E C T l O N A L I 1 R A I L I N G M A G N E T I C A N l E N N A I H - F I E L D 1

    l R A l l l N G E L E C I R O D E PLUS M A G N E T I C O M N I D l R E C l I O N A L

    Figure 10. Antenna Patterns.

    38 Naval Engineers Journal, August 1976

  • McCLENON ELF COMMUNICATIONS

    modulated. The Wisconsin Test Facility uses SCRs. The present ELF development contractor proposes to use large vacuum tubes in a switching mode configuration.

    A soft transmitting configuration is expected to have a large antenna grid, with a relatively few high-power above-ground transmitter stations. The antennas will probably be buried for economic or esthetic reasons, which incidentally also hardens them.

    A hardened surface survivable transmitting configuration uses a buried antenna grid with many transmitters dispersed throughout in hard- ened buried capsules. Each capsule contains its own time standard, input link, message processing terminal, and emergency power source. Smart switches reconfigure the system to make maxi- mum use of the remaining transmitters and antenna lines as damage occurs. With this ar- rangement, any desired system margin can be designed in. Design includes solution of the control and stability problems. The system then has a graceful degradation under attack so that messages to maximum range can still be delivered, but it would take more time than for pre-attack conditions. Delivery times would be correspond- ingly less for shorter ranges. This situation is illus- trated in Figure 11.

    ENVIRONMENTAL COMPATIBILITY

    There has been considerable objection on the part of environmentalists and ELF System critics that the transmitting antenna fields may be harmful to local ecology, animals, or people. After extensive studies, standards have been established for the strongest permissible local E- and H-fields which will still not produce any undesirable effects. Several typical safe appliances, such as hair dryers and electric blankets, subject people to hundreds

    TRANSMITTER SURVIVABILITY

    1 M. AIIACU

    Figure 11.

    of times more harmful effects than that of the proposed ELF fields. Several years of research by competent experts has failed to uncover any situation where harmful effects were found at the specified ELF levels.

    Long electrical conductors, such as fences, power lines, and telephone lines, parallel to an ELF antenna can have sufficient voltage induced in them to interfere with normal operation, or to be hazardous to people or animals. The Wisconsin Test Facility was built primarily to investigate these problems. Satisfactory solutions have now been found for all of them as was required before system development could proceed. There will be no more physical impact on the environment than is now caused by direct-buried utility cables.

    PRESENT ELF PROGRAMS

    There are several variations or configurations of ELF systems being considered, studied, or pro- posed. The major ones are briefly noted:

    1) Propagation Validation System (PVS) - This is an experimental program to use the Wisconsin Test Facility to transmit to 10 sub- marine-installed receivers. The submarines are performing their normal duties, and the ELF receivers gather signal and noise data on magnetic tape for later processing. Very simple coded messages will also be printed on the submarine teletypes when sent by proper authorities. First installations are planned for mid-1976. 2) Seafarer - This is a soft transmitter

    complex which is to communicate with all strategic submarines, attack submarines, and other stra- tegic forces. It is now in the Design Validation stage.

    3) Sanguine - Originally a more or less generic name for all ELF systems. Now, specifically, it is applied to a surface survivable system with a buried antenna grid and transmitters with emer- gency power buried in hardened capsules offset from each antenna intersection. Seafarer could be upgraded to Sanguine, but there are no present plans to do so.

    4) Shelf (Super Hardened ELF) - An ELF transmitter, antenna, emergency power system, and Input Message Processing Terminal, located thousands of feet below ground in rock tunnels and cavities. It is now in the Concept Validation stage.

    CONCLUSIONS

    A Decisive Experiment was performed by the NAVY in 1%3 to verify that an ELF system would

    Naval Engineers Journal, August 1976 39

  • ELF COMMUNICATIONS McCLENON

    actually work as predicted. A 109-mile long antenna in the Appalachian Mountains, radiating half a watt of ELF power, sent a message to a submerged submarine 2,500 miles away.

    In 1971 and 1972 a submerged submarine thousands of miles away and a land-based receiver in Norway, received the Naval Academy Motto, Ex Scientiu Tridens (From Knowledge, Sea- power) on teletypewriters at the operational posture and communication rate, The Wisconsin Test Facility sent the signal from its 30 miles of antennas using less than one watt of radiated power:

    There is no longer any doubt that ELF works and that it can be made compatible with its en- vironment. It can be made as survivable as military needs require, and it can provide a service not now available from any other system.

    Bibliography

    [ l ] Sunde. E.D.. Earth Conduction Effects in Transmission Systems. Dover I%&

    [2] IEEE Transactions on Communications. Vol. COM-22, No. 4 (April 1974). Special Issue on Extremely Low Fre- quency (ELF) Communications.

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    [3] Papers selected to illustrate major ELF system features:

    HISTORICAL (A) Merrill, J., Some Early Historical Aspects of

    ANTENNAS (B)

    (C)

    PROPAGATION (D) Jones, D. L., Extremely Low Frequency (ELF)

    Ionospheric Radio Propagation Studies Using Natural Sources.

    (E) Pappert, R. A. and W. F. Moler, Propagation Theory and Calculations of Lower Extremely Low Frequencies (ELF).

    (F) White, D. P. and D. K. Willim. ProDaeation

    Project SANGUINE.

    Fessenden, C. T. and D. H. Cheng, Development of a Trailing Wire E-Field Submarine Antenna for Exfremely Low Frequency (ELF) Reception. Keaer, B. E.. Early Development of the Project SANGUINE Radiating System.

    Measurements in the Extremely Low FieGency (ELF) Band.

    RECEIVER DESIGN (GI Bernstein. S . L. et al. A Signalling Scheme and

    Experimental Receiver for Extremely Low Fre- quency (ELF) Communication.

    ENVIRONMENTAL IMPACT (H) Benedick. M. H. and B. Greenberg, The SANGUINE

    Biological-Ecological Research Program. (I) Valentino. A. R., et of. Project SANGUINE Inter-

    ference Mitigation Research.

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