Suppressed Feature Waveform Sonar Modulation
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Solution Overview
Problem
Traditional sonar systems are easily detectable by adversaries due to the regular nature of their modulation, which can be detected even at low power levels using feature detectors, posing a risk in covert operations such as mine hunting or naval warfare.
Innovation Solution
A sonar transmission method that modulates a carrier wave with a baseband waveform generated from a repeating pulse sequence, filtered to resemble noise, making it difficult for independent detectors to identify features within the pulse, while preserving complementary properties for information retrieval through cross-correlation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional linear frequency modulated sonar transmissions are used, then the sonar system can effectively collect information about objects, but the regular nature of the modulation makes the transmission easily detectable by adversaries using feature detectors
Solution Approach 1:
The patent applies parameter changes by transforming the baseband waveform from a regular linear frequency ramp to a noise-like waveform through spectral shaping. This is achieved by passing the modulating signal through a bandpass filter with specific frequency characteristics, thereby changing the spectral parameters of the modulation while preserving the essential sonar functionality. The filtered waveform maintains the necessary bandwidth for object detection but eliminates the regular features that make traditional sonar detectable.
2Object-affected harmful factors
If the baseband waveform is filtered to resemble noise and suppress features, then the sonar transmission becomes undetectable to independent feature detectors, but the pulse width increases and frequency band is limited
Solution Approach 1:
The patent resolves this contradiction by carefully selecting the bandpass filter parameters. The filter is designed with a center frequency and bandwidth that allow the suppressed feature waveform to achieve noise-like characteristics while maintaining an acceptable pulse width. The filter parameters are optimized to balance feature suppression with pulse duration requirements for effective sonar operation.
Solution Approach 2:
The patent employs dynamic pulse compression techniques where the transmitted waveform has a longer duration but the received signal is processed to achieve effective pulse compression. This allows the transmitted pulse to have the necessary extended width for feature suppression while the processed output maintains the resolution characteristics of shorter pulses.
3Object-affected harmful factors
If a repeating pulse sequence is filtered through a bandpass filter to create a noise-like baseband waveform, then the sonar transmission becomes difficult to detect, but the regular structure needed for information retrieval is suppressed
Solution Approach 1:
The patent implements a matched filtering or correlation detection system where the receiver uses a reference waveform that matches the transmitted suppressed feature waveform. This feedback mechanism allows the receiver to correlate the returned signal with the known transmitted pattern, thereby recovering object information even though the waveform appears noise-like to external detectors. The correlation process extracts the embedded information while being insensitive to the noise-like appearance.
Solution Approach 2:
The patent uses correlation detection as an intermediary process between the transmitted noise-like waveform and the extracted object information. The correlation operation serves as a mediator that can extract structured information from what appears to be random noise, enabling information retrieval without requiring the waveform to have obvious regular features.
Data Source
AI summary
A suppressed feature waveform sonar transmission is produced by modulating a carrier wave with a baseband waveform that is generated from a known pulse sequence. A waveform embodiment of the pulse sequence is modified to create interference among the primary lobes of the constituent waveforms that are representative of the individual pulses. The baseband waveform so created appears as noise, making the baseband waveform modulation of the sonar transmission difficult to detect without knowledge of the pulse sequence or baseband waveform structure. The sonar transmission can be analyzed by cross-correlating the received signal with the baseband waveform or the pulse sequence waveform. The pulse sequence is preferably a complementary sequence. Modification of the pulse sequence may be obtained by passing an embodiment of the pulse sequence through a bandpass filter. In such a modification, the bandpass filter is preferably overdriven by a pulse sequence waveform having a frequency that is about eight to about ten times the bandwidth of the filter.