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Title Page

Contents

Abstract 12

1. Introduction 14

1.1. Motivations and Contribution of This Dissertation 14

1.2. Organization of This Dissertation 17

2. Overview of an Underwater Active Sonar System 18

2.1. Category of Sonar 18

2.2. The Sonar Equation 22

2.2.1. Parameters of the Sonar Equation 22

2.2.2. Active Sonar Equation 24

2.3. Ambient Noise 25

2.3.1. Main Source of Ambient Noise in the Ocean 25

2.3.2. Typical Frequency Spectrum of Ambient Noise 27

2.3.3. Minimum Ambient Noise 30

2.4. Sources of Radiated Noise 30

2.4.1. Radiated Noise from Submarines 32

2.4.2. Radiated Noise from Surface Ships 32

2.4.3. Radiated Noise from Torpedo 33

2.4.4. Self-noise of Vessels 34

2.5. Volume Reverberation 36

2.5.1. Volume Scattering 37

2.5.2. Volume Backscattering Strength 37

2.5.3. Volume Reverberation Level 39

2.6. Reverberation Frequency Spread and Doppler Gain Potential 42

2.6.1. Environmental Frequency Spreading 42

2.6.2. Frequency Spreading Due to Transmitter and Receiver Motion 43

2.6.3. Frequency Spreading Due to Target 45

2.7. Generation of the Short-Range Reverberation 47

2.7.1. Sources of Reverberation 47

2.7.2. Short Distance Reverberation Theory 47

2.7.3. Statistical Characteristics of Reverberations 50

2.7.4. Definition of Signal to Reverberation Ratio (SRR) 51

3. Analysis of Continuous Wave Radar in Underwater Environment 53

3.1. Received Signal Reflecting the Geometric Characteristics 53

3.1.1. Delay Time According to the Movement of High Speed Underwater Vehicles. 54

3.1.2. Characteristics of Received Signal According to the Movement Scenario of Vehicles 59

3.2. Distance and Doppler Estimation of the FMCW Technique. 66

3.2.1. Signal Types of FMC W Radar 67

3.2.2. Block Diagram of FMCW Radar 67

3.2.3. Distance and Doppler Estimation Based on Beat-frequency 68

3.2.4. Characteristic of Beat-frequency in FMCW Sonar 71

4. Hopping-Frequency-Coding-Based Continuous-Wave 76

4.1. Three Frequency State based HFC-CW 76

4.1.1. Parameters of the HFC-CW 78

4.1.2. Generation of the HFC-CW 80

4.2. Random Cyclic Code Based HFC-CW Generation 84

5. Distance and Doppler Estimation for HFC-CW 86

5.1. GCAS for Three Frequency State based HFC-CW 86

5.1.1. Distributive property of matched filters 87

5.1.2. Group-Correlator Based Doppler Estimation 90

5.1.3. Accumulated-Sum Process 91

5.1.4. GCAS threshold for FHT detection 96

5.2. GCAS for Random Cyclic Code based HFC-CW 98

6. Performance Evaluation 105

6.1. Three frequency State based HFC-CW and GCAS 105

6.2. Random Cyclic Code Based HFC-CW and GCAS 108

6.2.1. GCAS with the orthogonality constraint 109

6.2.2. GCAS performance according to the reverberation 113

7. Conclusions and Future Works 118

References 120

요약 123

List of Tables

Table 2.1. WMO sea state code 29

Table 2.2. Examples of frequency spectra of surface ships 33

Table 2.3. Equivalent two-way beam widths 40

Table 3.1. Change rate of received signal length 65

Table 3.2. Doppler ratio of received signal 66

Table 4.1. Parameters related to the HFC-CW 79

Table 5.1. Simulation parameters 93

Table 6.1. Simulation parameters 105

Table 6.2. Parameters of simulation-01 109

Table 6.3. Parameters of simulation-02 109

Table 6.4. Parameters of simulation-03 111

Table 6.5. Parameters of simulation-04 113

Table 6.6. Parameters of simulation-05 115

Table 6.7. Parameters of simulation-06 116

Table 6.8. Parameters of simulation-07 116

List of Figures

Fig. 1.1. High-speed underwater vehicles and the HFC-CW. 14

Fig. 2.1. Design procedure 18

Fig. 2.2. Sonar classifications 19

Fig. 2.3. Classification of sonar by purpose. 20

Fig. 2.4. Classification of sonar by the wet end or installation position. 20

Fig. 2.5. Near-range active sonar mounted on an anti-torpedo torpedo. 21

Fig. 2.6. Derivation of sonar equation. 23

Fig. 2.7. Wenz curve. 26

Fig. 2.8. Typical spectrum of ambient noise. 27

Fig. 2.9. Examples of torpedo spectra. 34

Fig. 2.10. Self-noise of a submarine (at 1,000 ㎐). 35

Fig. 2.11. Self-noise of a destroyer (0.1 ~ 10 ㎑). 35

Fig. 2.12. Volume scattering strength versus depth, Eastern Pacific at 5 ㎑ showing diumal (Diel)... 38

Fig. 2.13. Scattering volume for volume reverberation. 39

Fig. 2.14. Range dependence of signal, surface reverberation, and volume reverberation, assuming an... 40

Fig. 2.15. Reverberation and signal versus time for an area of the Philippine Sea in summer. 41

Fig. 2.16. Spread in reverberation due to own ship motion, at relative bearings. 44

Fig. 2.17. Spread in reverberation due to own ship motion, the environment, and pulse shape. 44

Fig. 2.18. Example of frequency spread (Doppler equivalent) of signal and reverberation. 46

Fig. 2.19. Relationship between noise, echo and reverberation levels. 48

Fig. 2.20. Illustration of the average intensity of volume and surface reverberation. 50

Fig. 2.21. Bandwidth for calculation of SRR. 52

Fig. 3.1. transmitted & received velocity vector of underwater vehicle in long and near range situation. 54

Fig. 3.2. Movement path and position vectors according to position and velocity of vehicles. 55

Fig. 3.3. Starting and ending time points of the transmitted signal and the received signal. 55

Fig. 3.4. Delay times of transmission - reflection section and reflection - reception section. 56

Fig. 3.5. The sampling point and the receiving point do not match. 58

Fig. 3.6. Oversampling at transmission and down-sampling at reception. 58

Fig. 3.7. RT vector and incident angle of received signal. 60

Fig. 3.8. Definition of transmit and receive signal's lengths. 60

Fig. 3.9. Length and phase of transmitted & received signal. 62

Fig. 3.10. The distance between the vehicle T and R. 63

Fig. 3.11. The length of the received signal. 64

Fig. 3.12. The Doppler frequency of the received signal. 64

Fig. 3.13. The incident angle of the received signal. 65

Fig. 3.14. Signal types of FMCW: (a) sawtooth, (b) triangular. 66

Fig. 3.15. Block diagram of FMCW radar. 67

Fig. 3.16. Simple block diagram to obtain the beat-frequency. 68

Fig. 3.17. A conceptual diagram for obtaining the beat-frequency in a sawtooth type FMCW. 69

Fig. 3.18. The beat-frequency of a triangular type FMCW with no Doppler. 70

Fig. 3.19. The beat-frequency of a triangular type FMCW with Doppler. 70

Fig. 3.20. When there is the Doppler, the up & down beat-frequency in the sawtooth type FMCW sonar. 71

Fig. 3.21. Beat-frequency of the FMCW sonar: (a) sawtooth type, (b) triangular type. 72

Fig. 3.22. Time-frequency graph and beat-frequency of sawtooth type FMCW in near-range situation. 72

Fig. 3.23. Change period of up & down beat-frequency in sawtooth type FMCW. 73

Fig. 3.24. Beat-frequency according to the bandwidth change of transmitted signal. 74

Fig. 3.25. The beat-frequency according to the speed of two underwater vehicles 75

Fig. 4.1. Details of the HFC-CW, (a) code & codeword, state, and hopping-frequency, (b) length 5 & 6... 77

Fig. 4.2. Three tone signals of duration Th.(이미지참조) 79

Fig. 4.3. State transition diagram, codeword length ＝ 3. 81

Fig. 4.4. State transition diagram, codeword length ＝ 5. 81

Fig. 4.5. Frequency hopping of HFC-CW generated by; (a) cw₁, (b) cw8.(이미지참조) 82

Fig. 4.6. Example of array size of random cyclic code. 84

Fig. 4.7. Example of random cyclic code total array with NF ＝ 4 and L ＝ 17.(이미지참조) 85

Fig. 5.1. Block diagram for the GCAS algorithm. 86

Fig. 5.2. Unit Doppler Correlators. 88

Fig. 5.3. Diagram of the Doppler group-correlators and accumulated-sum process. 90

Fig. 5.4. Example of transmitted HFC-CW with L ＝ 7. 91

Fig. 5.5. Received HFC-CW and outputs of the selected Doppler GC. 92

Fig. 5.6. GCAS outputs for state-sequences s2,58 and s2,68.(이미지참조) 93

Fig. 5.7. GCAS outputs for state-sequences s2,510 and s2,610.(이미지참조) 94

Fig. 5.8. All components of Wi,jM set to 1, and it satisfies the orthogonality constraint.(이미지참조) 96

Fig. 5.9. All components of Wi,jM set to 1, and it does not satisfy the orthogonality constraint.(이미지참조) 97

Fig. 5.10. Required correlator for different frequency hopping time. 99

Fig. 5.11. Set the total codewords with length 6. 99

Fig. 5.12. Set the total codewords with length 14. 100

Fig. 5.13. Entire correlators for time, Doppler, and codeword axis. 100

Fig. 5.14. Structure of GCAS. 101

Fig. 5.15. # of multiplications according to the length M of correlators. 102

Fig. 5.16. Doppler and distance estimation with GCAS. 103

Fig. 5.17. The final output of GCAS is a map of time & codeword. 103

Fig. 6.1. RMSE of the STFT-based method with Ns ＝ 10.(이미지참조) 106

Fig. 6.2. RMSE of the STFT-based method with Ns ＝ 50.(이미지참조) 106

Fig. 6.3. RMSEs of the GCAS and STFT-based methods. 107

Fig. 6.4. RMSE of the GCAS with M ＝ 5 and 10. 107

Fig. 6.5. GCAS output (time-codeword map) for simulation-01. 110

Fig. 6.6. GCAS output (time-codeword map) for simulation-02 with Bw ＝ 4.4㎑.(이미지참조) 110

Fig. 6.7. GCAS output (time-codeword map) for simulation-02 with Bw ＝ 5.2㎑.(이미지참조) 111

Fig. 6.8. Signal and reverberation of 0㏈ SRR for simulation-03. 112

Fig. 6.9. GCAS output (time-codeword map) for simulation-03. 112

Fig. 6.10. GCAS output (time-codeword map) for simulation-04 with M ＝ 15. 113

Fig. 6.11. GCAS output (time-codeword map) for simulation-04 with M ＝ 22. 114

Fig. 6.12. GCAS output (time-codeword map) for simulation-04 with M ＝ 29. 114

Fig. 6.13. Distance RMSE according to the length M of correlator. 115

Fig. 6.14. RMSE comparison between Simulation-06 and Simulation-07 116

초록보기

연속파 기 반의 능동 소나(CAS : continuously active sonar)는 펄스 기반의 능동 소나(PAS : pulsed active sonar)보다 짧은 주기로 추정치를 갱신하여 뛰어난 표적 추적 성능을 확보할 수 있으며, 연속파에 대한 긴 수집시간을 적극 활용하여 탐지 거리를 향상 시킬 수 있다. 뿐만 아니라, CAS는 PAS보다 낮은 저피탐 확률을 가지는 장점이 있다.

연속파를 송신하면 높은 파워 레벨의 체적 잔향음과 수표면 및 해저면 잔향음이 지속적으로 발생한다. 표적 반사신호에 충분한 도플러가 발생하지 않는다면, 잔향음과 표적 반사신호는 주파수 대역을 공유하게 된다. 뿐만 아니라, 표적의 거리가 멀수록 신호대 잔향음 비(SRR : signal to reverberation ratio)가 감소하기 때문에 탐지 과정에서 잔향음은 오탐지의 발생의 원인이 된다. 따라서, 원거리 CAS에서는 안정적으로 표적 탐지를 수행하기 위해서 잔향음을 억제할 수 있는 신호 모델과 신호처리 기법은 매우 중요한 요소이다. 여러 논문에서 잔향음의 영향을 억제하기 위해서 분석되고 있는 대표적인 신호 모델은 Costas code와 linear frequency modulation(LFM)으로 설계된 frequency coded continuous wave (FCCW)이다[20][21].

고속으로 기동하는 표적에 대한 정보를 연속적으로 추정하고 갱신하기 위해서 CAS를 활용할 때, 고려해야 될 문제점은 원거리 CAS와 차이가 있다. 근거리 상황에서는 표적과의 거리가 가까울수록 SRR이 개선된다. 또한, 고속으로 기동하는 표적에 의해 충분한 도플러가 발생한다면, 잔향음과 수신신호가 서로 다른 주파수 대역에 위치할 수 있다. 이러한 상황이라면, 근거리 CAS에 적용될 신호 모델과 신호처리 기법은 잔향음 억제보다는 추정치의 갱신 주기와 추정 기법의 안정성을 강화하는 방향으로 설계되는 것이 적합하다.

본 논문에서는 고속 수중운동체의 정보를 지속적으로 추정 및 갱신하기 위한 CAS에 활용할 목적으로 신호모델인 hopping-frequency-coding-based continuous-wave(HFC-CW)와 수신신호처리 알고리즘인 group-correlators and accumulated sum(GCAS)를 제안한다. HFC-CW는 잔향음과 수신신호의 주파수 대역이 분리되는 대역폭을 사용할 때, 적절한 잔향음 억제 성능을 확보하고 정보 갱신 주기를 최소화하는 방향으로 설계되었다. GCAS는 잔향음과 수신신호의 주파수 대역이 분리되지 않는 상황에서도 안정적으로 정보를 추정하기 위한 방향으로 설계하였다. 이를 위해서 GCAS는 HFC-CW의 생성방법과 관계없이 상관기의 길이를 조절하여 정보 추정의 안정성을 확보할 수 있다.

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