The Pennsylvania State University The Graduate School

WIRELESS UNDERWATER-TO-AIR COMMUNICATIONS VIA WATER

SURFACE AND RADAR DETECTION

A Thesis in Electrical Engineering by Moniara R. Romero

© 2020 Moniara R. Romero

Submitted in Partial Fulfillment of the Requirements for the Degree of

Master of Science

August 2020 The thesis of Moniara R. Romero was reviewed and approved by the following:

Ram M. Narayanan Professor of Electrical Engineering Thesis Advisor

Timothy J. Kane Professor of Electrical Engineering Co-Chair

Daniel C. Brown Assistant Research Professor of Acoustics

Erik H. Lenzing Special Member The Applied Research Laboratory at Penn State

Kultegin Aydin Professor of Electrical Engineering Head of the Electrical Engineering Department

ii Abstract

This thesis explores a concept recently proposed for underwater-to-air communications using a combination of acoustic and electromagnetics modalities. The transmitter is an underwater acoustic projector that is used to vibrate the surface of the water whose vibrations are modulated by the communications message. The airborne sensor is a millimeter-wave radar hovering over the surface of the water which senses the water surface vibrations. The signal is used to determine the displacement of the water surface to the sub-millimeter level. From this information, the communications message can be demodulated. Prior research and our calculations suggest that acoustic frequencies in the range 100–300 Hz are ideal in terms of throughput and signal-to-noise ratio. In our measurements, a loudspeaker covered in copper tape is used as a surrogate water surface responding to the acoustic stimulus. A W-band Doppler radar operating at around 94-GHz using analog in-phase/quadrature (I/Q) demodulation is used as the radar sensor. The results are quite promising; while playing sinusoids at 100 and 300 Hz, the signal can clearly be observed in the radar output.

iii Table of Contents

List of Figures v

List of Tables vii

Acknowledgments viii

Chapter 1 Introduction 1

Chapter 2 Background 2 2.1 History of Communications between Underwater and Air-Based Platforms 2 2.2 MIT’s TARF ...... 3 2.3 Ocean Waves ...... 4

Chapter 3 Speaker as a Water Surface Surrogate 6 3.1 Displacement of Water by an Acoustic Source ...... 6 3.2 Benefits and Limitations of Using a Speaker ...... 8 3.3 Vibrometer Measurements of Speaker ...... 10

Chapter 4 Radar System Description 16 4.1 Radar Antennas ...... 18

Chapter 5 Experimental Results 22 5.1 Single Frequency Tones ...... 22 5.2 Humidifier ...... 27 5.3 PSK31 Modulated Signal ...... 28

Chapter 6 Conclusions and Future Work 31

References 32

iv List of Figures

2.1 P-M spectra...... 5

3.1 Source level needed to cause indicated water surface displacements from a depth of 3 meters...... 8

3.2 Frequency response of subwoofer...... 9

3.3 Vibrometer measurements of the displacement of the subwoofer while playing a 100-Hz sinusoid at 100-mV p-p voltage...... 11

3.4 Vibrometer measurements of the displacement of the subwoofer while playing a 100-Hz sinusoid at 1-V p-p voltage...... 11

3.5 Vibrometer measurements of the displacement of the subwoofer while playing a 100-Hz sinusoid at 3-V p-p voltage...... 12

3.6 Vibrometer measurements of the displacement of the subwoofer while playing a 300-Hz sinusoid at 1-V p-p voltage...... 12

3.7 Vibrometer measurements of the displacement of the subwoofer while playing a 300-Hz sinusoid at 3-V p-p voltage...... 13

3.8 Vibrometer measurements of the displacement of the subwoofer while playing a 300-Hz sinusoid at 4-V p-p voltage...... 13

3.9 Vibrometer measurements of the displacement of the subwoofer while playing a 2-kHz sinusoid at 500-mV p-p voltage...... 14

3.10 Vibrometer measurements of the displacement of the subwoofer while playing a 2-kHz sinusoid at 1-V p-p voltage...... 14

v 3.11 Vibrometer measurements of the displacement of the subwoofer while playing a 2-kHz sinusoid at 4-V p-p voltage...... 15

4.1 Radar block diagram with the frequency of the main signal component in each section of the radar...... 17

4.2 Radar diagram with power levels...... 19

4.3 Overall system in operation with the radar system on the left and the vibrating loudspeaker on the right...... 19

4.4 Top view of the radar system showing the millimeter-wave components on the top tray...... 20

4.5 Normalized antenna patterns for the standard gain horn antenna, (a) E-Plane, (b) H-Plane...... 21

5.1 The time domain signals captured during the three 100-Hz trials. The peak-to-peak voltage for each is (a) 100 mV, (b) 1 V, and (c) 3 V. . . . . 23

5.2 The time domain signals captured during the three 300-Hz trials. The peak-to-peak voltage for each is (a) 1 V, (b) 3 V, and (c) 4 V...... 24

5.3 FFT of the three 100-Hz trials. The peak-to-peak voltage for each is (a) 100 mV, (b) 1 V, and (c) 3 V...... 25

5.4 FFT of the three 300-Hz trials. The peak-to-peak voltage for each is (a) 1 V, (b) 3 V, and (c) 4V...... 26

5.5 Comparison of captured radar signal with (right) and without (left) a fine mist while the speaker was playing a 100-Hz sinusoid at 1-V p-p voltage. 28

5.6 Comparison of captured radar signal with (right) and without (left) a fine mist while the speaker was playing a 300-Hz sinusoid at 4-V p-p voltage. 28

5.7 100 ms segment of transmitted PKS31 QPSK signal...... 30

5.8 100 ms segment of received PKS31 QPSK signal, after pre-processing. . . 30

vi List of Tables

3.1 Maximum displacement and velocity of the Dayton Audio Subwoofer as measured by a laser vibrometer...... 15

5.1 Velocity and displacement of the center of the speaker as measured by the Polytech laser vibrometer and the magnitude of the projected frequency component of the radar output for the six 100 and 300 Hz trials...... 27

vii Acknowledgments

I would like to thank the PSU Applied Research Laboratory (ARL) for supporting me via a Graduate Assistantship. I am also grateful to Erik Lenzing, Dan Brown, and Kris Greenert for guiding and helping me with my thesis research. I want to thank the others within ARL, and particularly those in the Antennas & Electromagnetics department (AKA: Electromagnetic Sensors and Antennas), who helped me out tremendously. Thanks are also due to my adviser, Dr. Narayanan. Finally, thanks to my husband for his invaluable love and support.

viii Chapter 1 | Introduction

The current methods of communications from submersed objects leave much to be desired. Currently, autonomous underwater vehicles (AUVs) and submarines need to either surface or communicate via a relay. A wireless method of communication from beneath the sea surface would be a major breakthrough. This paper explores a concept for an underwater-to-air communication system first proposed by the MIT Media Lab in 2018 [1]. They called the prototype they developed translational acoustic frequency (TARF) communication. The transmitter is an underwater acoustic projector that transmits sound signals which vibrate the surface of the water. Communications intended for an airborne platform are encoded within the water surface vibrations. A millimeter-wave radar over the surface of the water measures the disturbance and is thus able to demodulate the communications. In our work, we have designed and fabricated a prototype millimeter-wave radar system for this purpose and furthermore have validated the concept by the use of a loudspeaker as a substitute for a calm water surface. This thesis is structured as follows. Chapter 2 presents a history of underwater-to- air communications, describes MIT’s TARF in more detail, and briefly discusses the ocean surface. Chapter 3 discusses the theory behind the displacement of water surfaces by an acoustic transmitter and shows results justifying our use of a loudspeaker as a surrogate for a vibration modulated water surface. Chapter 4 provided details of the millimeter-wave radar system that was constructed for performing the validation tests. Chapter 5 presents our experiments with the previously described radar and speaker. Finally, Chapter 6 provides conclusions and recommendations for continuing research on this topic.

1 Chapter 2 | Background

2.1 History of Communications between Underwater and Air-Based Platforms

Communication from a submerged object to above the surface has always been a challenge. Acoustic signals are used for underwater communications [2], but completely reflect off the water-air interface. It has been known for decades RF frequencies are poorly suited for underwater communications, because they decay rapidly and attenuation increases exponentially with frequency. They decay rapidly because seawater is a good conductor. Moore [3] states that at 10 kHz attenuation over a 25 meter distance is 87 dB. Even at 100 Hz the distance to achieve 87-dB attenuation is only 250 meters. ELF (3-300 Hz) must be used to cover any sizable distance. At such low frequencies the data rate is exceptionally low. During the cold war, the US and the Soviet Union developed ELF communication systems that could penetrate the air-water interface and last several hundreds of meters underwater. One antenna could theoretically cover the entire world, and lightning was the only atmospheric cause of noise. However, their large wavelengths required antennas that were thousands to hundreds of thousands of kilometers long [4]. The US’s ELF air-to-underwater communications installation (creatively called Project ELF) was comprised of wires leading the current into the ground at two sites 148 miles (238 km) apart. The idea was to create a large dipole antenna. The carrier frequency was 76 Hz, which means the wavelength was 3,950 km. To increase the efficiency of the antenna the sites were in Michigan and Wisconsin above the Laurentian Shield. The abnormally low electrical conductivity of the formation forced the current to travel a longer path, thereby increasing the effective length of the antenna. Project ELF only allowed for one

2 way communications. Both sites were shut down in 2014 [5]. High powered VLF (3-30 kHz) signals can last a few hundred feet in sea water. This means the submarine doesn’t have to surface, but it is still be closer to the surface than is ideal. VLF has the same issue ELF does: It requires dipole antennas over a km long. Bannister [6] proposed an interesting solution for ELF/VLF: a giant space-based antenna. The antenna would take the form of a space tether made of metal wire. A space tether is a tether connecting an orbiting space vehicle to another, or possibly to the Earth. Sadly, a space tether has never been deployed, other than a demonstration on a space shuttle. Workarounds have included buoys on the surface of the water that can relay com- munications from a submarine or underwater autonomous vehicle. In some of these systems the link between the relay node and the underwater vehicle is acoustic and the link between the node and the air- or space-based receiver is RF [7]. In others, both are electromagnetic [8], [9]. In [8] the link between the submerged object and the buoy are in VLF or ELF, and the link between the buoy and the air is a higher RF frequency or optical. In [9] both are optical. There is ongoing research into free space optical communications through the sea-air interface. The roughness of the seas causes phase aberrations and other distortions. Overcoming that is the primary focus of the current research [10], [11].

2.2 MIT’s TARF

MIT’s TARF is an underwater to air communications system [1]. TARF’s transmitter is an off-the-shelf underwater pool speaker. The projected signal displaces the surface of the water when it reflects off of it. The water surface is displaced only a few µm. The acoustic signal is a 100- to 200-Hz orthogonal frequency division multiplexing (OFDM) signal with 64 sub-carriers. They successfully tested it with binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), and 16-QAM sub-carriers and proposed using a mix chosen based on individual channel SNR. The receiver is a 60-GHz frequency- modulated continuous-wave (FMCW) radar, with a bandwidth of 3 GHz. The radar waveform uses linear frequency modulated pulse compression for ranging. My work posits that ranging is unnecessary. They radiated non-stop, rather than pulsing. They also used a different communication scheme than is typical: they treated the unwrapped and filtered phase of the signal as the communication signal. They tested TARF in a pool. It worked with waves up to 16 cm in amplitude. When the waves exceeded that threshold the phase wrapped too quickly for the unwrapping

3 stage to reliably unwrap it. The acoustic source was 70 cm from the surface, and the radar was 30 cm away. TARF is one-way, and horizontal mis-alignment leads to an exponential drop in SNR. A paper exploring the same method, except with an optical laser as the uplink, was published about a decade earlier. That system was tested in a water tunnel. The receiver was a red laser Doppler vibrometer, 27 cm above the surface. This implementation also tracked the vertical movement of the water’s surface in order to find a decent spot to transmit through [12]. TARF is similar in principle to what is known as a "laser microphone" or "laser eavesdropper". Conversation causes windows to vibrate slightly. A laser, or even sunlight, bounced off the outside of a window can pick up those vibrations [13].

2.3 Ocean Waves

Before this system can actually be implemented, the behavior of ocean waves must be understood. The ocean surface is usually described by its spectral density. The spectral density of the sea surface is the Fourier transform of the temporal correlation function of the surface displacement:

1 Z ∞ Φ(Ω) = h2C(τ)e+iΩτ dτ (2.1) 2π −∞ where Ω is an angular frequency of the wave, h is the rms height of the surface, and C(τ) is the temporal correlation as a function of the time lag. It follows that h2 is the average of the spectrum over all the frequencies. Equation (2.1) comes from the Wiener-Khinchine theorem [14, Section 13.1.1]. The most basic and widely accepted spectrum of ocean wave heights is the Pierson- Moscowitz (P-M) spectrum. It describes a fully developed wind driven sea surface with infinite fetch (extent of the sea) and assumes that the sea depth is great enough that the sea bottom doesn’t affect the waves. The P-M density spectrum is given by

  2 !4 αg Ω0 Φ(Ω) = exp −β  (2.2) Ω5 Ω where α and β are empirically determined constants; α = 8.1 x 10−3 and β = 0.74. 2 g is the acceleration due to gravity (9.81 m/s ) and Ω0 = g/U, where U is the wind velocity [15]. The P-M spectrum doesn’t hold well for low frequencies. In the original

4 work the fitted data was taken with wind speeds (measured 19.5 meters above the surface) of 10.29 to 20.58 m/s, and so may not work well with wind speeds outside of that range. Figure 2.1 is a plot of P-M spectra for four different wind speeds. Note that wind speed makes a major difference in wave height and frequency. This should be taken into account as this concept is further matured.

Figure 2.1: P-M spectra.

5 Chapter 3 | Speaker as a Water Surface Surrogate

3.1 Displacement of Water by an Acoustic Source

Before we can use a speaker as a model water surface, we must first understand how much an acoustic projector would disturb the surface of the water. The boundary between air and water is what is know as a pressure release surface. A fluid has a characteristic acoustic impedance r = ρv. ρ is the equilibrium density of the fluid (that is, the density when no pressure waves are present) and v is the speed of sound within the fluid. The reflection coefficient is the ratio of acoustic pressure reflected from a boundary, R = Pr/Pi. Similarly, the transmission coefficient is the ratio of transmitted acoustic pressure over incident,T = Pt/Pi. For a plane wave normally incident on a planar boundary, the reflection coefficient is given by

r /r − 1 R = 2 1 (3.1) r2/r1 + 1 and the transmission coefficient is

2r /r T = 2 1 (3.2) r2/r1 + 1

See sections 6.1 and 6.2 of [16] for the derivation. For this research, medium 1 is fresh water and medium 2 is air. Both are at sea level ◦ 3 6 and 20 C. r1 = 1480 m/s X 1000 kg/m = 1.48 X 10 Pa·s/m and r2 = 343 m/s X 1.20 kg/m3 = 413 Pa·s/m. So when an underwater acoustic wave is incident on the surface, r2/r1 is effectively 0. That means R ≈ −1 and T ≈ 0. It is called a pressure release

6 surface because there is zero net acoustic pressure at the boundary: the reflected wave cancels out the incident. The surface perturbation δ(ω, t) is given by [1]

P (ω, t) δ(ω, t) = (3.3) ρωvw where P is the pressure at the surface, ω is the angular frequency of the acoustic wave, ρ is the density of the water, and vw is the speed of sound in water. Assuming a spherical wavefront, the pressure decreases by a factor of 1/R at a distance R away from the source. The power level of sonar devices is typically given in dB re 1 µPa. That is the decibel ratio of intensity (power per area) 1 meter from the source relative to the intensity of an acoustic wave having an rms pressure of of 1 µPa [2]. The final equation to calculate the needed source level in dB re 1 µPa, for a given maximum displacement, δ0, at a depth, D, (both in meters) is 6 P0 = 20 log(2πfρDδ0vw10 ) (3.4)

3 where f is the frequency in hertz, ρ is the water density in kg/m , and vw is the speed of sound in water in m/s. The factor of 106 is there to convert the units from Pa to µPa. A first in-water test would start in a freshwater tank about 3 meters deep. For fresh water, ρ = 1000 kg/m3. The speed of sound in water varies with temperature, depth, and salinity. For shallow freshwater at room temperature, vw = 1480 m/s [2]. There will be some reflection off the walls of the tank, but the greatest contribution will be from the line-of-sight, so we will use Equation (3.4) as an approximation. According to Urick [2], the absorption of pure water induces a loss of 1.6 X 10−5 dB at 300 Hz over a 1-km distance. Therefore, transmission losses can be safely ignored. We calculated the necessary source level to cause four different levels of displacement, namely, 3 µm, 30 µm, 300 µm, and 3 mm, over the frequency range 100 Hz – 2 kHz. The result is plotted in figure Figure 3.1.

7 Figure 3.1: Source level needed to cause indicated water surface displacements from a depth of 3 meters.

3.2 Benefits and Limitations of Using a Speaker

There are several benefits to using a speaker as a surrogate water surface. • Within a tank (or pool), there will be reflections from the walls of the tank. A speaker eliminates that. • A speaker is portable and allows for bench top validation experiments. • It allows for more control over the experiment. That is great for these early days while this method of communications is being fleshed out. A downside to using a speaker is the individual speaker response must be known and accounted for. Initial experiments started with an inexpensive automobile speaker. It was a small two-way cone speaker. In order to find out how far the surface displaced, it was measured with a Polytec PSV-400 scanning vibrometer. It was realized that after a certain frequency the sound did not come entirely from the cone, but also from the

8 “tweeter” in the center. That’s standard for a full range speaker. That was not good for my experiments, and the cone shape was less than ideal. This research needed a single vibrating surface, and preferably a flat one, so we bought a subwoofer. A subwoofer is meant to play frequencies up to 300 Hz. That is ideal, because water surface displacement is inversely proportional to the acoustic frequency (see (3.3)). We chose a Dayton Audio model LS12-44 12" Low Profile Subwoofer Dual 4 Ohm. This speaker was chosen for its flat surface and its relatively flat frequency response. The moving surface is circular and 8.5 inches (21.6 cm) in diameter. That is much larger than the projected beam from the radar antennas, meaning that a lot of the power out of the transmit antenna is reflected back to the receive antenna. A drawback of this approach is the frequency response of a speaker is quite different form that of a sea surface. The frequency response of the speaker is shown in Figure 3.2. As stated previously, this speaker had the flattest response we could find. Yet, you’ll note it is not monotonic, not even within its intended frequency range, 28-300 Hz. This will be obnoxious when projecting a modulated signal.

Figure 3.2: Frequency response of subwoofer. Data from vendor [17].

Another limitation is that you couldn’t simulate ocean waves on a speaker. The speaker used here proudly claims to have 1 cm maximum displacement. However, other

9 varieties of noise or attenuation can be added.

3.3 Vibrometer Measurements of Speaker

We covered the surface of the Dayton Audio subwoofer with copper tape to increase its radar reflectivity, making sure that the surface was flat without bumps or kinks. At W-band frequencies, water is about as reflective as copper. The signals were fed by a Realistic MPA-40 35-watt public address (PA) amplifier, which was fed by an Agilent 33250A 80-MHz function/arbitrary waveform generator. We measured the displacement over the surface of the speaker using a Polytec PSV-400 scanning vibrometer for a few different voltage and frequency configurations. Figures 3.3 to 3.11 contain color-scale plots of the magnitude and phase of the displacement over the surface of the speaker. What these plots show us is that different frequencies distort the speaker’s surface in different ways. For 100 and 300 Hz, the max displacement occurred in the center. The displacement at 100 Hz is not uniform (Figures 3.3 to 3.5), so it probably isn’t the best choice for a surrogate water surface.∗At 300 Hz, the displacement was greater in the center and fell off towards the edges (Figures 3.6 to 3.8). That is how one would expect the water’s surface to react, so it’s a good frequency to play on this speaker. For 2 kHz, the max displacement was near the edges (Figures 3.9 to 3.11). We would not expect water to respond that way to an acoustic wave, so 2 kHz is not a good frequency to use. Table 3.1 contains the maximum displacement and velocity for each trial. The listed voltage is the peak-to-peak voltage from the waveform generator. The amplifier gain was maintained at the same level for all trials. Table 3.1 also includes the source level required by an underwater projector in order to displace the surface of a tank of room temperature fresh water by the same amount from a depth of 3 meters, as calculated from (3.4) for P0.

∗Regarding the phase color-scales: deep red is 180◦ and deep blue is -180◦. The presence of both in the 100 Hz plots may lead one to believe that the displacements are all in phase, but they are not. Note that there are other colors in between the deep blue and deep red areas.

10 Figure 3.3: Vibrometer measurements of the displacement of the subwoofer while playing a 100-Hz sinusoid at 100-mV p-p voltage.

Figure 3.4: Vibrometer measurements of the displacement of the subwoofer while playing a 100-Hz sinusoid at 1-V p-p voltage.

11 Figure 3.5: Vibrometer measurements of the displacement of the subwoofer while playing a 100-Hz sinusoid at 3-V p-p voltage.

Figure 3.6: Vibrometer measurements of the displacement of the subwoofer while playing a 300-Hz sinusoid at 1-V p-p voltage.

12 Figure 3.7: Vibrometer measurements of the displacement of the subwoofer while playing a 300-Hz sinusoid at 3-V p-p voltage.

Figure 3.8: Vibrometer measurements of the displacement of the subwoofer while playing a 300-Hz sinusoid at 4-V p-p voltage.

13 Figure 3.9: Vibrometer measurements of the displacement of the subwoofer while playing a 2-kHz sinusoid at 500-mV p-p voltage.

Figure 3.10: Vibrometer measurements of the displacement of the subwoofer while playing a 2-kHz sinusoid at 1-V p-p voltage.

14 Figure 3.11: Vibrometer measurements of the displacement of the subwoofer while playing a 2-kHz sinusoid at 4-V p-p voltage.

Table 3.1: Maximum displacement and velocity of the Dayton Audio Subwoofer as measured by a laser vibrometer. The last column is the source level an underwater projector would need to displace the surface of a tank of room temperature fresh water by the same amount from a depth of 3 meters

15 Chapter 4 | Radar System Description

The radar used to measure the displacement on the surface of the loudspeaker (emu- lating a water surface) was a custom-built W-band coherent Doppler radar operating at approximately 94 GHz using analog in-phase/quadrature (I/Q) demodulation. To capture the displacements of the water surface to the micron level, mm-wave is best. 94 GHz was chosen because a partially assembled 94-GHz radar was readily available. Figure 4.1 shows a block diagram that indicates the signal frequency in each section of the radar. The L-band source is a voltage controlled oscillator which generates a 1.3-GHz CW signal at the appropriate tuning voltage. Its output is 6.4 dBm. The output signal is low pass filtered to suppress harmonics and amplified in a 14-dB gain amplifier. The output of the amplifier is connected to the input of a power splitter. One output of the power splitter, containing one half of the amplifier power output, is connected to the IF port of the millimeter-wave upconverter, whose LO port if fed by a 92.7-GHz millimeter-wave signal at a power level of 11.5-dBm fed by a 92.7-GHz oscillator and a 20-dB coupler. The other half is directed towards the I/Q detector in the receiver. The received millimeter-wave signal, reflected from the surface of interest containing the Doppler , is amplified by a 12.5 dB gain LNA and downconverted back to 1.3 GHz and fed to the I/Q detector. Separate horn antennas are used for transmit and receive functions. For more detail on the signal paths in the radar, see Figure 4.2 which contains the signal power in and out of every component and other parameters. The phase of the signal needs to be known, because the communications are phase modulated onto the radar signal. In the initial version of the radar, the upconverter was not a part of the system; the coupler connected directly to the transmit antenna and the received signal was immediately downmixed to baseband. Using a Hilbert filter to obtain the phase was considered, but a Hilbert filter acts as either a high pass or band pass filter [18, Sec. 5.5.2 and Table 5.2], meaning that the desired

16 signals would be filtered out. Analog demodulation is preferable instead of direct digital demodulation because a far lower sampling rate can be used when the signal is digitized at baseband. L-band was chosen for the intermediate frequency, and a frequency of 1.3 GHz specifically selected because the phase imbalance of the 90◦ hybrid was closest to 90◦ at that frequency. The MIT group used a continuously radiating linear FM compressed waveform. The distance bin with the highest cross section corresponded to the water’s surface, so they calculated the phase of the signal within that bin. Our radar does not use pulse compression. Pulse compression is most useful when high resolution is desired, but we were only interested in finding the change in distance in the form of surface undulations induced by the acoustic signal. Meaning, the W- and L-band signals can be signals. This will result in a little noise due to reflections off water particles and other sorts of clutter in between the radar and the water, but we believe that these will be far less significant compared to the reflection off the water. Figure 4.3 shows a photograph of the system in operation and Figure 4.4 shows a top view of the radar with the millimeter-wave components marked.

Figure 4.1: Radar block diagram with the frequency of the main signal component in each section of the radar. fw is the millimeter-band local oscillator frequency out of the W-band oscillator and fd is the Doppler shift frequency.

17 4.1 Radar Antennas

The radar has separate, but identical, receive and transmit antennas. The antennas are MI-Wave 261W/387, 25 dBi standard gain rectangular horn antennas. The antenna pattern was measured [19] and is plotted in Figure 4.5. The usual approximation for calculating the far field of an antenna is R ≥ (2D2)/λ, where λ is the wavelength and D is the largest dimension of the antenna [20, Equation 6-113]. The emitted signal is approximately at 94 GHz, which corresponds to 3.2 mm wavelength. The horn antenna aperture is 25.9 mm x 30.7 mm. This yields a far field distance of 590 mm = 23.2 in. Thus, all measurements were taken with the antennas about 2 ft (610 mm) away from the target in order to remain in the far field of the antennas. The beam projections of the antenna is [21, Equation 5.7]

θ ! φ ! A = πR2tan 3 tan 3 csc(δ) (4.1) 2 2 where θ3 and φ3 are the 3 dB beamwidths of the principle planes and δ is the grazing angle. In our case the 3 dB beamwidth in the E-plane is 8◦ and in the H-plane is 10◦ [19]. The grazing angle is 90◦. Thus, the projected beam area is 0.077 ft2. The speaker’s diameter is 8.5 in., so it’s area is 0.39 ft2. That is sufficient to reflect most of the transmitted power.

18 Figure 4.2: Radar diagram with power levels.

Figure 4.3: Overall system in operation with the radar system on the left and the vibrating loudspeaker on the right.

19 Figure 4.4: Top view of the radar system showing the millimeter-wave components on the top tray.

20 Figure 4.5: Normalized antenna patterns for the standard gain horn antenna, (a) E-Plane, (b) H-Plane [19].

21 Chapter 5 | Experimental Results

In all experiments, the radar antennas were about 2 ft (61 cm) away from the surface of the subwoofer and the sampling rate was 4 MHz. Data were collected on an Agilent Infiniium DS080804B oscilloscope.

5.1 Single Frequency Tones

For the pure sinusoids, the collection interval was 0.5 seconds. We were able to play the same tones we measured with the vibrometer. The radar could not pick up the 2-kHz tone at all, even at displacements greater than the displacements of the lower frequency tones that were detected. It possibly had to do with the asynchronous displacement of the subwoofer’s surface, as shown in Figures 3.9 to 3.11. The 100-Hz signal can be seen in the time-domain graphs of the two higher-powered trials, as shown in Figure 5.1. The 300-Hz signal (Figure 5.2) is not so obvious, but the FFT shows that it is present. For the spectral analysis, the signals of the two channels were combined into one complex vector. The first channel is the in-phase (I) component and the second is the quadrature-phase (Q) component, so the complete signal is s(t)=I(t)+jQ(t). In all 100-Hz and 300-Hz trials, the FFT shows that the sinusoid played by the speaker was captured. Figure 5.3 contains the plotted FFTs of all the 100-Hz trials, and Figure 5.4 contains the FFTs of all the 300-Hz trials. The large 3rd harmonic in Figure 5.3(c) is likely the result of the asynchronous distortions shown in Figure 3.5. Table 5.1 contains the FFT values for the transmitted frequencies.

22 Figure 5.1: The time domain signals captured during the three 100-Hz trials. The peak-to-peak voltage for each is (a) 100 mV, (b) 1 V, and (c) 3 V.

23 Figure 5.2: The time domain signals captured during the three 300-Hz trials. The peak-to-peak voltage for each is (a) 1 V, (b) 3 V, and (c) 4 V.

24 Figure 5.3: FFT of the three 100-Hz trials. The peak-to-peak voltage for each is (a) 100 mV, (b) 1 V, and (c) 3 V.

25 Figure 5.4: FFT of the three 300-Hz trials. The peak-to-peak voltage for each is (a) 1 V, (b) 3 V, and (c) 4V.

26 Table 5.1: Velocity and displacement of the center of the speaker as measured by the Polytech laser vibrometer and the magnitude of the projected frequency component of the radar output for the six 100 and 300 Hz trials.

5.2 Humidifier

To further simulate an aquatic environment, we ran a small humidifier in front of the speaker. It is a bifine brand mini bedroom humidifier. It has two nozzles pointed upwards at 45◦ angles. We operated it with just one nozzle on. The specifications state that the minimum spray rate is 30-40 ML/H. It covered almost all of the area immediately in front of the speaker with a fine mist. We played the two clearest signals (100 Hz at 1 V and 300 Hz at 4 V) with and without the humidifier on. The time-domain signal and FFTs are shown in Figures 5.5 and 5.6. The mist induced some noise, but there was no effect on the strength of the transmitted frequencies. The DC component is different, but the absolute mean voltage of the radar signal tends to drift over time, so that was probably not caused by the humidifier.

27 Figure 5.5: Comparison of captured radar signal with (right) and without (left) a fine mist while the speaker was playing a 100-Hz sinusoid at 1-V p-p voltage.

Figure 5.6: Comparison of captured radar signal with (right) and without (left) a fine mist while the speaker was playing a 300-Hz sinusoid at 4-V p-p voltage.

5.3 PSK31 Modulated Signal

PSK31 (phase-shift keying, 31 ) modulation was used to transmit an actual message. PSK31 is a radioteletype mode popular among users, particularly for HF

28 sky wave communications. It transmits text at 31 baud to match human typing speed and has a narrow bandwidth. The standard modulation is BPSK. The BPSK mode does not provide any error correction. The QPSK mode provides some forward error correcting capabilities; it employs a sliding window of five bits to chose the phase shift. Still, PSK31 is not considered suitable for applications that require very low error rates. The QPSK mode is also slower to decode than the BPSK mode. PSK31 works well at low SNRs, when voice is not possible. It is designed to be synthesized with a computer sound card with the computer’s audio jack connected to the audio port of a single side band radio transceiver [22]. The Simulink code, “PSK31 Model with Symbol Timing and Carrier Recover”, was downloaded from the Matlab Central file exchange [23]. It contains a model that creates and plays a BPSK or QPSK audio signal from a text message, sends it through a simulated channel, and then decodes it again. This model, “psk31C_1.mdl”, was modified to record WAV files. A single phrase was recorded in both BPSK and QPSK. The audio carrier frequency was chosen as 100 Hz so the spectrum of the modulated signal was within the frequency response of the sub-woofer. The code also includes a Simulink model containing only the receiver, “PSK31C_R1.mdl”. It is built to decode a live audio signal input to the audio jack of a computer. It was modified to accept a recorded WAV file or a binary file recorded with the Infiniium oscilloscope. The binary file required some processing before being passed to the existing receiver code: 1. Read single precision variables from binary file at the 4 MHz sampling rate 2. Convert the single precision variables to double 3. Pass the signal through a 300 Hz LPF (300 Hz is the maximum frequency of the subwoofer) 4. Down sample by a factor of 500, so that the sample rate of the signal is 8 kHz, which is what the model requires Attempts to decode the signal were almost successful. Not a single character was recovered correctly, but the received signal was very similar to the transmitted one. The problem is thought to lie in SNR or synchronization issues. The transmitted audio signal was 20 seconds long, and the recorded radar files were 10 seconds long. Figures 5.7 and 5.8 are time-domain plots of 100 ms segments.

29 Figure 5.7: 100 ms segment of transmitted PKS31 QPSK signal.

Figure 5.8: 100 ms segment of received PKS31 QPSK signal, after pre-processing.

30 Chapter 6 | Conclusions and Future Work

This work has further corroborated the communications system concept introduced by the MIT media lab and has validated the use of a speaker to perform more controlled testing. However, there is much research left to be done. More complicated scenarios can be simulated with the subwoofer, such as noise and multipath. An actual tank test should be conducted to validate the subwoofer results. There are also other issues that need to be explored before this method of commu- nications can be implemented. There needs to be a method to horizontally align the transmitter and receiver. The surface of a rough seas is often foamy: the impact of that on the accurate capture of the sea surface displacement by the radar should be investigated. The speed of sound in water is a function of depth, temperature, and salinity [2]. The last two factors vary over the vertical profile of the ocean. Meaning, that acoustic waves are bent as they travel toward the surface, which will lower the SNR. The biggest issue to solve before actual implementation is how to distinguish between naturally occurring waves and the displacement caused by the acoustic projector. This is an area of further studies.

31 References

[1] F. Tonolini and F. Adib, “Networking across boundaries: Enabling wireless Communication through the water-air interface,” in Proc. 2018 Conf. ACM Special Interest Group Data Communication, Budepest, Hungary, Aug. 20-25 2018, pp. 117–131, doi: 10.1145/3230543.3230580.

[2] R. J. Urick, Principles of Underwater Sound, 3rd ed. New York, NY, USA: McGraw-Hill Book, 1983.

[3] R. K. Moore, “Radio communication in the sea,” IEEE Spectrum, vol. 4, no. 11, pp. 42–51, 1967, doi: 10.1109/MSPEC.1967.5217169.

[4] S. L. Bernstein et al., “Long-range communications at extremely low frequencies,” Proc. IEEE, vol. 62, no. 3, pp. 292–312, 1974, doi: 10.1109/PROC.1974.9426. [5] B. Villeneuve, “ELF station Republic, MI,” Oct. 15 2015, https://ss.sites.mtu.edu/ mhugl/2015/10/10/elf-sta-republic-mi/ (accessed: June 15, 2020).

[6] P. Bannister et al., “Orbiting transmitter and antenna for spaceborne communica- tions at ELF/VLF to submerged submarines,” In AGARD, ELF/VLF/LF Radio Propag. and Syst. Aspects 14 p (SEE N93-30727 11-32), 1993. [7] M. Rhodes, D. Wolfe, and B. Hyland, “Underwater communications system com- prising relay transceiver,” U.S. Patent 7,877,059, Jan. 25, 2011.

[8] R. Woodall, F. Garcia, and J. Sojdehei, “Magneto-inductive submarine communica- tions system and buoy,” U.S. Patent 6,058,071, May 2, 2000.

[9] J. J. Puschell, R. J. Giannaris, and L. Stotts, “The autonomous data optical relay experiment: first two way laser communication between an aircraft and submarine,” in NTC-92: National Telesystems Conf., Washington, DC, USA, May 19-20 1992, pp. 14–27, doi: 10.1109/NTC.1992.267865.

[10] A. K. Majumdar, J. Siegenthaler, and P. Land, “Analysis of optical communications through the random air-water interface: feasibility for under-water communications,” in Laser Communication and Propagation through the Atmosphere and Oceans, vol. 8517, San Diego, CA, USA, Oct. 24 2012, p. 85170T, doi: 10.1117/12.928999.

32 [11] O. Alharbi, W. Xia, M. Wang, P. Deng, and T. Kane, “Measuring and modeling the air-sea interface and its impact on FSO systems,” in Laser Communication and Propagation through the Atmosphere and Oceans VII, vol. 10770, San Diego, CA, USA, Sep. 18 2018, p. 1077002, doi:10.1117/12.2323301. [12] F. A. Blackmon and L. T. Antonelli, “Experimental detection and reception per- formance for uplink underwater acoustic communication using a remote, in-air, acousto-optic sensor,” IEEE J. of Ocean. Eng., vol. 31, no. 1, pp. 179–187, Jan. 2006, doi: 10.1109/JOE.2005.861248.

[13] F. M. Mims III, “Surreptitious interception of conversations with lasers,” Opt. News, vol. 11, no. 11, pp. 6–12, Nov. 1985, doi:10.1364/ON.11.11.000006.

[14] H. Medwin and C. S. Clay, Fundamentals of Acoustical Oceanography. San Diego, CA, USA: Academic, 1998. [15] W. J. Pierson Jr and L. Moskowitz, “A proposed spectral form for fully developed wind seas based on the similarity theory of S.A. Kitaigorodskii,” J. of geophysical res., vol. 69, no. 24, pp. 5181–5190, Dec. 1964, doi: 10.1029/JZ069i024p05181. [16] L. E. Kinsler, A. R. Frey, A. B. Coppens, and J. V. Sanders, Fundamentals of Acoustics, 4th ed. New York, NY, USA: Wiley, 2000. [17] “Dayton audio ls12-44 12" low profile subwoofer dual 4 ohm,” Dayton Audio, 2015, accessed: Jun. 13, 2020. [Online]. Available: https://www.parts-express.com/ dayton-audio-ls12-44-12-low-profile-subwoofer-dual-4-ohm--295-253

[18] R. J. Schilling and S. L. Harris, Digital Signal Processing: Using MATLAB®, 3rd ed. Boston, MA, USA: Cengage Learning, 2017. [19] K. A. Gallagher, “Simultaneous human detection and ranging using a millimeter- wave radar system transmitting wideband noise with an embedded tone,” Master’s thesis, Dept. Elect. Eng., Pennsylvania State Univ., State College, 2013.

[20] C. A. Balanis, Advanced Engineering Electromagnetics, 2nd ed. Hoboken, NJ, USA: Wiley, 2012.

[21] N. C. Currie, “Characteristics of clutter,” in Principles of Modern Radar: Basic Principles, M. A. Richards, J. A. Scheer, and W. A. Holmes, Eds. Raleigh, NC, USA: SciTech, 2010, ch. 5, pp. 165–210. [22] P. Martinez. (1998) PSK31: A new radio-teletype mode with a traditional philosophy. Accessed: June 6, 2020. [Online]. Available: https://web.archive.org/ web/20100621115142/http://det.bi.ehu.es/~jtpjatae/pdf/p31g3plx.pdf [23] D. Benson. (2014) PSK31 model with symbol timing and carrier recovery. Accessed: March 19, 2020. [Online]. Available: https://www.mathworks.com/matlabcentral/ fileexchange/2839-psk31-model-with-symbol-timing-and-carrier-recovery

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