Technical Report No. 217 036040-10-T THE MIMI FIELD TEST OF JULY 1970 by Richard M. Heitmeyer Approved by, c 4oc< q>I/ 2Theodore G. Birdsall for COOLEY ELECTRONICS LABORATORY Department of Electrical and Computer Engineering The University.of, Michigan Ann Arbor, Michigan Contract No. N00014-67-A-0181-0032 Office of Naval Research Department of the Navy Arlington, Virginia 22217 June 1972 Approved for public release; distribution unlimited.

ABSTRACT In the summer of 1970, a field test was conducted to determine if a portable signal processing unit built around a small digital computer could be used to run specific underwater propagation experiments in the Straits of Florida. A periodic broadband signal modulating a 420 Hz carrier was transmitted continuously across the Straits of Florida for nine days. At the receiving site, the power and phase angle of the carrier, the power in the signal sidebands, and the noise power in the signal band were measured. In addition, the total power and the power spectrum in a narrow band about the carrier line were determined as a measure of the modulation due to the forwardscattered surface reverberation. Finally, the correlation of the received signal with a stored reference was computed to measure the multipath structure and its stability. This report presents a brief description of the acoustical range, the experiments, and the preliminary analysis of the data. iii

ACKNOWLEDGMENTS The field test described in this report was conducted as part of Project MIMI (The University of Michigan, University of Miami) propagation and signal processing research sponsored by Code 468 of the Office of Naval Research. The field test was jointly conducted by the Acoustics Group of the Rosenstiel School of Marine and Atmospheric Sciences (RSMAS), University of Miami, under the direction of Dr. John Steinberg, and by the Signal Processing Group of Cooley Electronics Laboratory (CEL), The University of Michigan, under the direction of Dr. Theodore G. Birdsall. The Acoustics Group at RqMAS as a whole deserves special credit for not only maintaining the Miami-Bimini range but also for providing invaluable background information of the propagation conditions in the Straits of Florida. Special thanks go to Mr. Theodore Crabtree who did an excellent job of maintaining the equipment at the Bimini receiving site. The signal processing techniques used in the experiment were developed by Dr. Birdsall. Messrs. Kurt Metzger, Gerald Cederquist, Peter Wood and Brian Barton also made major contributions to the signal processing effort. iv

TABLE OF CONTENTS Page ABSTRACT iii ACKNOWLEDGMENTS iv LIST OF ILLUSTRATIONS vii CHAPTER 1: THE MIMI FIELD TEST OF JULY 1970 1 1. 1 The Miami-Bimini Range 2 1. 2 The Transmitted Signal 4 1.3 The Quantities Measured 10 1. 3. 1 Powers and Angle Experiment 10 1. 3. 2 Surface Reverberation Measurements 12 1. 3. 3 Multipath Structure Measurements 13 1. 4 The Equipment Configuration 14 CHAPTER 2: THE PRELIMINARY ANALYSIS OF THE JULY 1970 FIELD TRIP 20 2. 1 The Power and Angle Measurements 20 2. 2 The Surface Reverberation Experiment 29 2. 3 The Multipath Structure Displays 29 2. 4 Summary and Conclusions 34 DISTRIBUTION LIST 36 v

LIST OF ILLUSTRATIONS Figure Title Page 1 The Miami-Bimini range: (a) The Straits of Florida, (b) bottom profile 3 2 Sound speed vs depth, Miami to Cat Cay, 26-27 April 1961 5 3 Sound ray paths along 25~ 44' on 26-27 April 1961 6 4 A complement-phase modulated signal (a) a portion of the modulating waveform (b) the resulting transmission 7 5 The RMS spectrum of a complement-phase modulated signal for L = 15 and D = 8 9 6 The frequency responses of the digital processing filters 11 7 A multipath display for two transmission paths 15 8 Equipment configuration at RSMAS (Miami) 16 9 Equipment configuration at Bimini 17 10 Block diagram of the digital processing implementation 19 11 Power measurements from the Bimini hydrophone 22 12 Power measurements from the 7-mile hydrophone 23 13 Unity-gain plots of carrier power and sideband power, (a) Bimini hydrophone, (b) 7- mile hydrophone 25 vii

LIST OF ILLUSTRATIONS (Cont. ) Figure Title Page 14 Mean power plots (1/2 hour averaging time) 27 15 Carrier angle plot, (a) Bimini hydrophone, (b) 7-mile hydrophone 28 16 Multipath displays from the Bimini hydrophone taken on 4 July 1970 31 17 Multipath displays from the Bimini hydrophone taken on 5 July 1970 32 18 Multipath displays from the Bimini hydrophone taken on 5-6 July 1970 33 viii

Chapter 1 THE MIMI FIELD TEST OF JULY 1970 In the summer of 1970, a field test was conducted to determine if a portable signal processing unit built around a small digital computer could be used to run specific underwater propagation experiments in the Straits of Florida. A periodic, broadband signal, modulating a 420 Hz carrier was transmitted continuously across the Straits of Florida for nine days. At the receiving site, the power and phase angle of the carrier, the power in the signal sidebands, and the noise power in the signal band were measured. In addition, the total power and the power spectrum in a narrow band about the carrier frequency were determined as a measure of the modulation due to the forwardscattered surface reverberation. Finally, the correlation of the received signal with a stored reference was computed to measure the multipath structure of the channel and its stability. During the course of the experiment approximately 2 million digital words of the reception were recorded for later processing. This report, however, contains only the description of the different experiments and the preliminary analysis of the on-line results. A more complete report on the results of additional processing of the data is planned.

2 1. 1 The Miami-Bimini Range Location and Facilities. The Miami-Bimini range, illustrated in Fig. 1, is part of the facilities of the Acoustics Group of the Rosenstiel School of Marine and Atmospheric Sciences (RSMAS) of the University of Miami. It extends across the Straits of Florida from Miami to Bimini, Bahamas. The transmitting site is located at Fowey Rocks [point 2, Fig. l(a)] approximately 12 miles from the RSMAS laboratory. At the transmitting site, which is connected to the RSMAS laboratory by telephone lines (point 1, Fig. 1), a bottom mounted projector is located in 72 feet of water at the focal point of a 24 ft. compliant tube, parabolic reflector. It has a maximum output level of 120 dB/ /ibar at one meter with a nominal bandwidth of 100 Hz. The 30 beamwidth is directed toward Bimini, a distance of 43 nautical miles. Two receiving sites were used in the experiment. At the first of these, a bottom mounted hydrophone is located in 1000 feet of water at point 4 in Fig. l(a) approximately 7 miles from the source. The reception from this hydrophone is transmitted to the RSMAS laboratory by marine cable. This site is hereafter referred to as the 7-mile hydrophone site. The second receiving site is located off Bimini at point 3 (Fig. 1). There, a bottom mounted hydrophone J. C. Steinberg and T. G. Birdsall, "Underwater Sound Propagation in the Straits of Florida, " J. Acoust. Soc. Am., 39, 301-315 (1966).

5 Dpt h in fathomsMARINE.A 20A 20', | 7; -. - 6, - - - tMARINE LAB' 4s VIRGINIA KEY'41 *,, 406 6: ~~. S.BIMI406 00 S. \ - - 4 VERTA d t9 37 T L A N Fo EY ROCKS dSI -50, 440 ZCATCAY 730 0?3. 24M0 I"' OCEAN T 20~~~~~~" A 142 IT Tr 473, (to0OESA'NE tO 800 50' 40' 30' 20' 10 Fig0 ~. lONr 200 4 100 0O 4O 0 30' O' 1 2 600 C400 ^ V /, a. 800 (VERTICAL SCALE' 37x HORIZONTAL) 0 10 20 30 40 N.MLES (ONE TO ONE SCALE) Fig. 1. The Miami-Bimini range: (a) the Straits of Florida, (b) bottom profile

4 is located at a depth of 1200 feet and cable-connected to Lerner Marine Laboratory on Bimini, a distance of about 2 miles. This site is referred to as the Bimini hydrophone site. Only one of the two receiving sites was used at a time. Oceanographic and Acoustical Characteristics. The bottom profile of the Miami-Bimini range is illustrated in Fig. l(b). A shelf extends out from Miami about 15 miles to a depth of 400 meters followed by a sharp drop-off to a depth of 800 meters. Thirty miles beyond, the Grand Bahamas bank rises abruptly from this depth. A velocity profile, obtained during April 1961 but which is also typical of the summer conditions, appears in Fig. 2. This profile is characterized by a mixed layer which extends to the relatively constant depth of 100 meters followed by a region of negative velocity gradient. It is noted that the velocity gradient becomes increasingly negative as the Florida shore is approached. The ray diagram corresponding to the velocity profile of Fig. 2 is shown in Fig. 3. This diagram shows sound being propagated by reflections from surface and bottom and by refraction and reflection from the bottom. 1. 2 The Transmitted Signal The transmitted signal consisted of a linear-maximal, pseudorandom sequence used to complement-phase modulate (CM) a carrier.

5 NAUTI(4.AL (MLI, ( s 20.0 40 i i I i I I I I i i i I Wi $(.' ~ -7C / 4' 7::' _, o o540 I 20 4 0 0 - -| SOUND SPEED S- / S C of - CPM sigl0l is.shown i/ /Fig. 4 where: - are00r C d = duration of the sequence digit STATION POSITION /ocri p' sqe c i"". c D = number of cycles of carrier per sequence digit

NAUTICAL MILES FROM MIAMI1 0-0 20 40 200-~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~~~~~~~~~~~~&N: -~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~~~~~~~~*-0 Fig. 3. Sound ray paths along 250 441 on 26- 27 April 1961

+1 d=D/f -r - - 1 I I ------- (a) H 1- 1/fc (b) Fig. 4. A complement-phase modulated signal (a) a portion of the modulating waveform (b) the resulting transmission Evidently d = D/fc Furthermore, when the number of cycles of carrier per digit D is integer-valued, the signal is periodic with period T= Ld LD/f c where L is the number of digits in one period of the modulating sequence.

8 The RMS power spectrum of a typical (CPM) signal is shown in Fig. 5 for the special case L = 15 and D = 8. It is seen that this spectrum has a sin(x)/x envelope except at the carrier frequency. It can easily be shown that approximately half of the total power is contained at the carrier frequency with the other half contained in the sideband frequencies. For the specific signal used in the experiment, f = 420 Hz c D = 8 cycles/digit and L = 63 digits with the result that the digit duration and the period were d = 0. 019 seconds and T = 0.59 seconds Furthermore, the frequency line spacing Af and the effective signal band B (frequency spread between the spectral zeros on either side of the carrier) were f = 1/T = 1.69 Hz B = 2/d = 105 Hz Finally, it can be shown that there are 61 spectral lines lying within the signal band.

9 1_2~~~ ~ 1 [S(02A Af - - Fd 5 t Fig. 5. The RMS spectrum of a complement-phase modulated signal for L = 15 and D = 8

10 1. 3 The Quantities Measured Three different experiments were conducted during the July 1970 field test: (1) a signal power, noise power and carrier angle experiment, (2) a forward-scattered surface reverberation experiment, and (3) a multipath structure experiment. The measurements in each of the experiments were obtained using digital processing techniques implemented by a digital computer located at each receiving site (Section 1. 4). Each of the measurements is computed and recorded every 75 seconds on the basis of the full 75 seconds of data except for the reverb spectrum (Section 1. 3. 2), which is computed every 125 seconds. 1. 3. 1 Powers and Angle Experiment. C Power. The C power measurement is a measure of the power present in the carrier of the received signal. It is determined as that power passing through a digital processing filter with a frequency 2 response, Hc(f), as illustrated in Fig. 6(a). Note that due to the 3 narrow bandwidth, (13. 24 MHz), Hc(f) passes only the carrier frequency and none of the sideband frequencies present in the spectrum of the transmitted signal. Only the main lobes of the frequency responses of the processing filters in Fig. 6 are illustrated since these lobes contain almost all of the total power. By bandwidth we mean one-half the frequency spread between the two zeros on either side of the center frequency of the main lobe.

~H ~(fi -~~l~~-A 13.24MHz - f 420 Hz f (a) di5f) -*| K13.24 MHz - ff= 1.69 Hz -- -I~ I i~ A1 A I A A111 AI-A~ f Af f C (b) C W(t.9 13.24 MHz - Af = 1.69Hz NY T A A A A A ~~ ~~AfAA AfAAA >af ~+ (2 (c) eT HR(f) k-.846 fY C (6i Fig. 6. The frequency responses of the digital processing filters

12 N Power. The N power measurement is a measure of the noise power contained within the signal band: f - B/2 < If! < f + B/2. C -- -- C It is determined as that power passing through the digital processing filter with a frequency response, HN(f), as illustrated in Fig. 6(c). Such a filter is often referred to as a comb filter. Note that each "tooth" in HN(f) is centered midway between the spectral lines in the signal spectrum, and the bandwidth of each tooth is again 13. 24 MHz. S Power. The S power measurement is a measure of the signal power within the signal band, excluding the carrier power. It is determined as that power passing through the digital processing filter with frequency response Hs(f) as illustrated in Fig. 6(b). This filter is recognized as a comb filter with a tooth missing at the carrier frequency. The remaining teeth are aligned with the line frequencies of the transmitted signal. The bandwidth of each tooth is 13.24 MHz. Carrier Angle (A). The carrier angle is a measure of the relative difference between the phase of the output of the carrier processing filter and the phase of the local reference oscillator (Section 1. 4). 1.3. 2 Surface Reverberation Measurements. The effect of surface reverberation is to scatter some of the power of a transmitted

13 frequency into sidebands on either side of that frequency. It is felt that this scattered power should lie between 0. 1 Hz and 0. 4 Hz on either side of the transmitted frequency, depending on the spectrum of the surface waves. For the signal used in the experiment, the spacing between the lines in the power spectrum is Af = 1. 69 Hz so that the sideband line frequencies do not interfere with the power scattered from the carrier frequency. Thus, the effect of surface reverberation can be measured by computing the power spectrum at the output of the processing filter whose frequency spectrum, HR(f), is shown in Fig. 6(d). Note that the bandwidth of HR(f) is large enough to include the power scattered by surface reverberation but small enough to exclude the power in the signal sidebands. In addition to computing the reverberation or "reverb" spectra, the total power at the output of HR(f) minus the carrier power is also computed. This power, referred to as the reverb power or R power, is a measure of the power in the sidebands of the reverb spectrum. 1. 3. 3 Multipath Structure Measurements. The magnitude of the cross-correlation function between the demodulated reception and a pulse-compression reference is computed to measure the multipath structure of the channel. In the absence of noise, this function has a triangular peak of width, 2d = 0. 038 seconds, centered at the arrival time of each transmission path, mod(T), and an amplitude proportional

14 to the amplitude of the signal received over that path. For example, the cross-correlation function for the special case of only two transmission paths is shown in Fig. 7. Note that this is the same function as would be obtained if a single, large, one-digit pulse were transmitted once every T seconds and received through a matched filter. As a measure of the time stability of the channel multipath structure, the zero-delay complex cross-correlation coefficient between the demodulated received signal and a previously stored replica of the received signal is calculated for each data set. When this coefficient falls below a set value, indicating a marked change in the multipath structure, the current signal becomes the replica, and the new multipath picture is displayed. Otherwise, the previous replica is displayed and retained in storage. The changing magnitude of the correlation coefficient is an indication of how fast the multipath structure is changing. 1. 4 The Equipment Configuration The configuration of the equipment used in the experiment is shown in Figs. 8 and 9. Figure 8 illustrates the configuration used in the generation and transmission of the signals and in the processing of the reception from the 7-mile phone at Miami. Figure 9 shows the configuration used in the processing of the reception at Bimini. The operation of the equipment in Fig. 8 is as follows. The frequency standard (accurate to 1 part in 10 ) provides a 100 kHz

15 Time i I-2d 1 r 2d y~ -- T seconds -'|. Fig. 7. A multipath display for two transmission paths reference signal which is converted to a 1680 Hz (= 4x carrier frequency) clock signal by the frequency synthesizer and then passed through an isolation amplifier. One output of the isolation amplifier is used as a reference for the digital modulator whose output, after filtering, is the CPM signal described in the preceding section. This signal is amplified and then transmitted down the Miami-Bimini range. The reception at the 7-mile hydrophone is amplified and transmitted back to the RSMAS laboratory. There it is filtered and amplified and then processed by the FIELD-8 computer. The other output of the isolation amplifier provides a clock signal to the FIELD-8 computer for use in sampling the reception. The receiving equipment at Bimini (Fig. 9) operates in essentially the same way as the receiving equipment at Miami. The

Frequency Standard Frequency Synthesizer Isolation Amplifier 1680 Hz.i Fwey Transducer Hydophone 1ock __y, —------—' ^S^ASFowey Hydophone Digital Bandpass ISARocks 7- mile |~~~~~Modulat.or r —-I ~~~~Filtl~er Line Power channel Amplifier Amplifier 0 Telepone Lines Amplfter Bandpass RSMAS Marine Cable I Amplifie~r [ pFiltg r 8. Terminal Rocks ___ Fter Amplifier Amplifier' —-- Filtered reception Field- 8 Computer Fig. 8. Equipment configuration at RSMAS (Miami)

Hydrophone r A I _ ~Terminal _ Bandpass Isolation Amplifier Terminal L/ | "Amplifier Filter Amplifier Marine Filtered - Cable Reception Field-8 Frequency Frequency Standard Synthesizer Computer 1680 Hz Clock Signal Fig. 9. Equipment configuration at Bimini

18 reception from the hydrophone is transmitted to the Lerner Marine Laboratory at Bimini, amplified and filtered and then processed by the FIELD-8 computer. The coherent clock signal for the computer is obtained from a frequency standard and synthesizer identical to that employed at Miami. The FIELD-8 computers shown in Figs. 8 and 9 are used to implement the processing filters and to calculate the reverb spectrum and multipath displays as discussed in Section 1. 3. In this capacity, the FIELD-8 functions as indicated in Fig. 10. The FIELD-8 computer itself, consists of a modified PDP-8 with special peripherals to display and store the data. During the July 1970 field test only one of these units was available so that it was necessary to transport it from one receiving site to the other.

19 Filtered Reception 1680 Hz Clock -I4 - - --- - -- Complex l Demod. ( HN( N Power 8 Power I II - I | Correlator Multipath Display I —---- jHp(f) | --- - C Power, C Angle " R Power I II- |~~~I -— I_ ^tz~~~~~~~ — Reverb Spectrum.. l Analyzer FI_. -O -lock -iagram -f -he -igital._rocessingI Fig. 10. Block diagram of the digital processing implementation

Chapter 2 THE PRELIMINARY ANALYSIS OF THE JULY 1970 FIELD TRIP Both the power and angle experiment and the reverb experiment were run over the 7-mile hydrophone for a 24-hour period. At Bimini, all three experiments were conducted over an 8-day period. This chapter presents the preliminary analysis of the data from those experiments. 2. 1 The Power and Angle Measurements Examples of the power measurements, expressed in decibels relative to an arbitrary reference, have been plotted as continuous functions of time. (Continuity has been obtained by linear interpolation between the discrete data points.) Two different vertical scales have been used. In the one case, the vertical scaling is determined as if the gain of each of the processing filters in Fig. 6 is equal to unity. These plots are referred to as the unity-gain plots (UG). In the other case, the vertical scaling has been determined as if the product of the gain and the bandwidth of each of the processing filters is constant. These plots are referred to as the constant gainbandwidth (CGB) plots. The CGB plots have the property that the approximate signal-to-noise ration (in dB) at the output of the processing filters HC(F), Hs(f) and HR(f) can be obtained simply by 20

21 subtracting N power from C, S or R power respectively. Figure 11 illustrates a CGB plot of the power measurements from the Bimini hydrophone taken over a period of 9-1/2 hours. It is seen that the carrier power varies over a wide range with most of the variation appearing as sharp, deep fades occurring at a rate of about two fades per hour. The sideband power is considerably more stable than the carrier power with all of the variation limited to a 10 dB range. Note that there appears to be little correlation between the sideband power and the carrier power. Typical signalto-noise ratios for the carrier power and the sideband power are 30 dB and 15 dB respectively. The Bimini noise power in Fig. 11 varies over a range of 25 dB during the 9-1/2 hour period. Part of this variation appears as sharp isolated peaks that can be attributed to the local shipping noise. The remainder appears as slowly varying changes in the average level. The reverb power exhibits the same general behavior as the noise power with a signal-to-noise ratio of approximately 6 dB. Figure 12 illustrates a CGB plot of the power measurements from the 7-mile hydrophone. It is noted that although there are deep fades in the 7-mile carrier power, they occur at a considerably lower rate than in the Bimini carrier power. In the 9-1/2 hour period illustrated in Fig. 12 there are only 5 deep fades. The 7-mile sideband

22 dB Carrier Power 110 100 90 \Sidebalnd 0 er 80 70liReverb Power Noise Power 60 50 2300 0100 0200 0300 0400 0500 0600 0700 0800 0900 3-4 July 1970 HOUR Fig. 11. Power measurements from the Bimini hydrophone

23 dB.,/.~~~~~~140 tCarrier Power 140 130' 120/ ~ jt \: Sideband Power 120 110 Reverb Power 80 70 Noise Power 2300 0100 0200 0300 0400 0500 0600 0700 0800 0900 29- 30 June 1970 HOUR Fig. 12. Power measurements from the 7-mile hydrophone

24 power, however, appears to be less stable than the Bimini sideband power. There are several periods during which the 7-mile sideband power varies more than 10 dB in less than an hour. Typical signalto-noise ratios for the 7-mile carrier power and sideband power are 60 dB and 50 dB respectively. The 7-mile noise power in Fig. 12 has the same general character as the Bimini noise power. Again there are several large shipping noise spikes and some slowly varying changes in the mean noise level. There does, however, seem to be a component of high frequency variation not contained in the Bimini noise power. The 7-mile reverb power in Fig. 12 differs radically from the Bimini reverb power. The reverb signal-to-noise ratio is over 20 dB and there appears to be little correlation with the noise power. Also there is a significant amount of high frequency variation in the 7-mile reverb power that is not present in the Bimini reverb power. The most striking distinction between the power measurements at the two receiving sites is the amount of correlation between the carrier power and the sideband power. The two Bimini powers show little if any correlation, whereas, the carrier power and the sideband power from the 7-mile hydrophone show a high degree of correlation. This distinction is most apparent in Fig. 13 which illustrates the two powers from both receiving sites plotted on a UG scale. The plot of the Bimini powers in Fig. 11 illustrates only a

25 dB Sideband P er 90 N., 80 70 Carrier Power 6o 0- 3-4 July 1970 [ I..... I..... I I I,, I..... 23 0 1 2 3 4 5 6 7 8 9 dB (a) Hour Sideband Power 1 2, I (, r, 90 I1.L I.i. 19 23 0 1 2 3 4 5 6 7 8 9 (b) Hour Fig. 13. Unity-gain plots of carrier power and sideband power (a) Bimini hydrophone, (b) 7-mile hydrophone

26 small portion of the total Bimini data. To illustrate the general character of the power measurements from all of the Bimini data, the slowly varying component of each power has been estimated by computing a sliding average based on a one-half hour averaging time. The plot of these "mean" powers appears oni a constant-gain bandwidth scale in Fig. 14. The most noticeable feature in Fig. 14 is the gradual increase in the levels of both the mean carrier power and the mean sideband power followed by a gradual decrease. Both powers reach the highest levels on the 6th and 7th of July when the signal-to-noise ratios for the carrier power and the sideband power exceed 50 dB and 30 dB respectively. A similar behavior also appears in the mean reverb power although to a lesser extent. The reverb signal-tonoise ratio reaches a maximum of 15 dB for a short period on 6 July. The mean noise power exhibits daily increases and decreases with the maxima occurring during midday and the minima occurring in the late evening and early morning hours. Examples of the carrier angle measurements from both receiving sites are plotted in cycles (relative to the local reference oscillator) against time in Fig. 15. The carrier angle from the Bimini hydrophone appears in Fig. 15(a). The rate of change of the Bimini carrier angle is typical of the "rapid" changes that occur elsewhere in the Bimini data. This rate of about 12 cycles in 9 hours

d'i 130 "-20.~ I Warn ^e r 120 h~i vA \/ Powerb 100 Sideband'~* lA. i/vP \ ^^ j v "/ 90 A I ~~~~~~~~~~~~~~Power 70 fy/J~f~~v ii l'Power 90~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~U i' y ~ 60 3 4 5 6 8 9 Fig. 14. Mean power plots (1/2 hour averaging time) Date: July 1970

28 Cycles 16 12 3-4 July 1970 4 2300 0100 0200 0300 0400 0500 0600 0700 0800 0o00 Hour Cycles (a) 16 12 8 29- 30 June 1970 2300 0100 0200 0300 0400 0500 0600 0700 0800 0900 Hour (b) Fig. 15. Carrier angle plot (a) Bimini hydrophone (b) 7-mile hydrophone

29 corresponds to a stability of 1 part in 1. 134 x 10 or a frequency shift of 0. 00037 Hz. The carrier angle from the 7-mile hydrophone is illustrated in Fig. 15(b). It is seen that there is only about 1/3 the rate of change in the 7-mile carrier angle although it is not possible to tell if this is typical of 7-mile hydrophone data since no other measurements are available. 2. 2 The Surface Reverberation Experiment The measurements comprising the surface reverberation experiment consist of the reverb power measurement and the reverb spectra. In the preceding section, it was seen that the Bimini reverb signal-to-noise ratio varied between 6 dB and 15 dB, whereas the 7-mile reverb signal-to-noise ratio was approximately 20 dB. Nevertheless, the reverb spectra from both receiving sites did not show a noticeable amount of power in the reverb sidebands as had been expected. It might be mentioned, however, that the seas were unusually calm during the field test, and it is felt that more data are needed before any conclusions can be reached. 2. 3 The Multipath Structure Displays The displays of the multipath structure are obtained as the magnitude of the cross-correlation between the demodulated reception and a pulse-compression reference (Section 1. 3). The length

30 of the time delay axis in these displays is equal to the period of the transmission, 0. 59 seconds, and the duration of a single peak in the display is 2d = 0. 038 seconds. (The duration of a single peak is indicated on the top display of each column in each figure. ) The vertical scaling for the multipath displays is arbitrary so that care must be used in comparing the relative strengths of arrival peaks in different displays. Figures 16, 17 and 18 illustrate sequences of multipath displays taken on three successive days from the Bimini data. The time at which each display was taken and the value of the sideband power at that time are indicated next to each display. The multipath displays in Fig. 11 are associated with the lowest values of the sideband power, while those in Fig. 12 are associated with the intermediate values and those in Fig. 13 are associated with the highest values. An examination of these figures indicates a correlation between the number of strong arrival peaks and the value of the sideband power, with the large number of strong arrival peaks being associated with the smaller values of the sideband power. Also note that the spread of the major arrival peaks is greatest when the sideband power is low.. Finally, it is seen by examining successive displays in each sequence that the location and amplitudes of the arrival peaks are changing from display to display indicating that the multipath stability is maintained only for periods of the order of minutes.

31 1631 hrs. 2324 hrs. 4 July 4 July S = 86 = 91 - - 2d -1 2d 1638 hrs. 2337 hrs. 4 July 4 July S 87 S 87 1645 hrs. 2350 hrs. 4 Jy \ 4 uly S 86 = 90 1-2 sec 1.2 sec1.2 se Fig. 16. Multipath displays from the Bimini hydrophone taken on 4 July 1970

32 0733 hrs. 0737 hrs. 5 July 5 July S 90 \ A 92 - ^ 2d - - 2d 0738 hrs. I A 0801 hrs. 5 July 5 July S = 90 S = 93 0748 hrs. A l 0806 hrs. 5 July 1\ \ 5 July S = 92 1 \ / \ S = 91 1.2 sec 1 2 sec1.2 Fig. 17. Multipath displays from the Bimini hydrophone taken on 5 July 1970

33 2156 hrs. 0918 hrs. 5 July 6 July = 94 i\S = 99 -A K 2d - k-2d 2211 hrs.. 0936 hrs. 5 July 6 July S = 94 t\ = 99 2220 hrs. 0948 hrs. 5 July I \ 6 July S = 94 S = 95 -1.2 sec ----- 1.2 sec.| Fig. 18. Multipath displays from the Bimini hydrophone taken on 5-6 July 1970

34 The technique used to reduce the multipath display data by recomputing new displays only when the correlation coefficient exceeded a threshold, worked as intended with one shortcoming that was not anticipated. Namely, large bursts of shipping noise tended to cause the threshold to be exceeded and a "noisy" multipath display to be stored. In future multipath experiments, it will also be necessary to take into account the current value of the sideband signal-to-noise ratio when thresholding the correlation coefficient. 2. 4 Summary and Conclusions The main purpose of the July 1970 field test was to determine whether or not a portable signal processing unit (the FIELD-8) could be used to run underwater propagation experiments. As a result of the field test, it was seen that this could indeed be done successfully. The FIELD-8 was transported by automobile from Ann Arbor, Michigan to Miami, Florida, set up in the RSMAS Laboratory and run for one day. Then it was transported by charter plane to Bimini, Bahamas, set up and run for eight days and finally returned to Ann Arbor. All of this was done without suffering any equipment failures even though on several occasions the equipment received fairly rough handling. The operation of the equipment proved even easier than had been anticipated since the only task left to the operator was the scheduling of the experiments. Thus, it was concluded that a digital processing unit of this type could be transported and installed with a

35 high degree of reliability and run with a minimum of operator attention. The three propagation experiments ran much as had been expected. The processing gains obtained in the FIELD-8 proved sufficient for achieving the necessary signal-to-noise ratios. The amount of variation in the data over the 9-day period, however, suggests that additional experiments be run to determine the long-term behavior of the power and angle measurements. Also, more data are needed to reach a conclusion about the structure of the reverb spectra and the multipath stability. In the future field tests it is felt that the FIELD-8 software package should be rewritten to allow all three experiments to run simultaneously.

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37 DISTRIBUTION LIST (Cont.) No. of Copies Commander Naval Ordnance Laboratory 1 Acoustics Division White Oak, Silver Spring, Maryland 20907 Attn: Dr. Zaka Slawsky Commanding Officer 1 Naval Ship Research & Development Center Annapolis, Maryland 21401 Commander 2 Naval Undersea Research & Development Center San Diego, California 92132 Attn: Dr. Dan Andrews Mr. Henry Aurand Chief Scientist 1 Navy Underwater Sound Reference Division P. O. Box 8337 Orlando, Florida 32800 Commanding Officer and Director Navy Underwater Systems Center Fort Trumbull New London, Connecticut 06321 Commander 1 Naval Air Development Center Johnsville, Warminster, Pennsylvania 18974 Commanding Officer and Director 1 Naval Ship Research and Development Center Washington, D. C. 20007 Superintendent 1 Naval Postgraduate School Monterey, California 93940 Commanding Officer & Director 1 Naval Ship Research & Development Center* Panama City, Florida 32402 *. Formerly Mine Defense Lab.

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,cqiltt ClI11srwtt irn! DOCUMENT CONTROL DATA. R & D f.tErtwltv rI*4lltfeltlon nf itle, hrf er l o hrftrt eAnd trndet#felrd noalflon m.el. her ennterd wh.e the overall repott In cIewtIflIed QRIOINA INO AC T I V I T V (COFpOrtlfe (thar) 29. REPORT SECURITY CLASSIFICATION Cooley Electronics Laboratory Unclassified The University of Michigan lb. ROUP Ann Arbor, Michigan 48105 /" PrO T tI TLK The MIMI Field Test of July 1970. OEScA P Tvr nOt TEy ap of trpo;t and Incuc.leve d.r.e) Technical Report No. 217 - 036040- 10-T AU T1.4iO1t (Firat man., mfddle tIiltal, *tet nome) Richard M. Heitmeyer REPORTa OATS 76. TOTAL NO. OF PAGCES 7b. NO. OF EFI June 1972 54 0. CnNTRACT OR GRANT NO. oe. ORIGINATOR'S REPORT NUMBER(S) N00014-67-A-0181-0032 036040-10-T b. PROJICT NO. c. Ob. OTHER REPORT NOTS) (Any other numbers thlt may be assltnod this report) TR 217 0. 0DISTRIUTION STATEMENT Approved for public release; distribution unlimited. I. SUPPLKEMEXTArY NoteS (2. *PONSORING MILITARY ACTI VITY Office of Naval Research Department of the Navy Arlington, Virginia 22217. AS TRACT In the summer of 1970, a field test was conducted to determine if a portable signal processing unit built around a small digital computer could be used to run specific underwater propagation experiments in the Straits of Florida. A periodic broadband signal modulating a 420 Hz carrier was transmitted continuously across the Straits of Florida for 9 days. At the receiving site, the power and phase angle of the carrier, the power in the signal sidebands and the noise power in the signal band were measured. In addition, the total power and the power spectrum in a narrow band about the carrier line were determined as a measure of the modulation due to the forward-scattered surface reverberation. Finally, the correlation of the received signal with a stored reference was computed to measure the multipath structure and its stability. This report presents a brief description of the acoustical range, the experiments and the preliminary analysis of the data. D *~"o..14 73 tTsI ato - GSecurity Classification

Security Claniitcation _ 14. LINK A LINK a LINK C KKV WOR, ROLET WT ROL E T fW,r wT Signal processing Underwater sound Multipath Surface reverberation

UNIVERSITY OF MICHIGAN 3 9015 03026 7804