Adaptive Algorithm and Software for Recognition of Ground-based, Airborne, Underground, and Underwater Low Frequency Events
information technology and software
Adaptive Algorithm and Software for Recognition of Ground-based, Airborne, Underground, and Underwater Low Frequency Events (LAR-TOPS-305)
Ground-based infrasonic array to track atmospheric turbulence
Overview
The innovative approach is to exploit modern signal processing methods. i.e. adaptive filtering, where the computer is trained on-line to recognize features of the low frequency events (on the ground, in air, underground, and underwater) to be detected. Modern instrument development techniques and computational capability has enabled the development of a vastly more powerful algorithm and detection techniques.
The Technology
Acoustical studies of atmospheric events like convective storms, shear-induced turbulence, acoustic gravity waves, microbursts, hurricanes, and clear air turbulence over the last forty-five years have established that these events are strong emitters of infrasound (sound at frequencies below 20 Hz). Over the years, NASA Langley has designed and developed a portable infrasonic detection system which can be used to make useful infrasound measurements at a location where it was not possible previously. The system comprises an electret condenser microphone, and a small, compact windscreen. The system has been modified to be used in the air, underground, as well as underwater (to determine man-made and precursor to tsunami). The system also features a data acquisition system that permits real-time detection, bearing, and signature of a low frequency source. However, to determine bearing of the received signals, the microphones are to be arranged as an equilateral triangle with a certain microphone spacing. The spacing depends upon location of the microphone array. For a ground-based array, the microphone spacing of 100 feet (30.48m) is desired to determine time delay for signals arriving at each microphone location. The microphone spacing depends upon speed of sound through the array medium. For underwater array, the spacing between microphones would be around 1500 feet. The data acquisition system provides data output in the infrasonic bandwidth which is then analyzed using an adaptive algorithm (least-mean-squares time-delay-estimation) using modern computational power to locate source by plotting source location hyperbolas on-line.
A smaller array size reduces the time resolution resulting in strong signal coherence. The innovation approach is to exploit modern signal processing methods, i.e. adaptive filtering, where computer is trained on-line to recognize features of the event to be detected. Modern computational capability permits the adaptive algorithm (least-mean-squares time-delay estimation or LMSTDE) which is vastly more powerful algorithm. This system has better resolution able to determine direction with arrived signals within five-degree accuracy.
Benefits
- The technology has better demonstrated accuracy compared to comparable technology
Applications
- Detecting and tracking atmospheric turbulence
- Detection of stealth submarines
- Detection of stealth submarines
- Detecting tsunamis
- Nuclear test ban detection
Similar Results
Extreme Low Frequency Hydrophone
The extreme low frequency hydrophone boasts unprecedented capability for precise detection as proven in testing, where it sensed wave range between surface waves and tidal surges with periods between .3 and 30 seconds, or 3 to .033 Hz.
The technology uses a back-electret microphone, inherently reducing noise, in a stainless steel body. The stainless steel diaphragm conducts infrasound well and the materials robust nature and internal configuration facilitates sub-freezing and deeply submerged sensing of sound down to .0001 Hz.
With an appropriately spaced array of three hydrophones it is possible to determine the direction of origin of a submerged infrasonic source, the addition of one more in another location will also enable determination of the precise location of origin.
The oil industry uses existing infrasound systems to locate undersea oil deposits and this technology could potentially improve the accuracy or reliability of current practices. It could also be used to give tsunami and earthquake warnings, monitor ships, and to generate electrical energy from infrasound. This technology has potential to unlock new industry uses not currently understood due to the unprecedented nature of its capabilities.
Low Frequency Portable Acoustic Measurement System
Langley has developed various technologies to enable the portable detection system, including:
- 3-inch electret condenser microphone - unprecedented sensitivity of -45 dB/Hz
- compact nonporous windscreen - suitable for replacing spatially demanding soaker hoses in current use
- infrasonic calibrator for field use - piston phone with a test signal of 110 dB at 14Hz.
- laboratory calibration apparatus - to very low frequencies
- vacuum isolation vessel - sufficiently anechoic to permit measurement of background noise in microphones at frequencies down to a few Hz
- mobile source for reference - a Helmholtz resonator that provides pure tone at 19 Hz
The NASA system uses a three-element array in the field to locate sources of infrasound and their direction. This information has been correlated with PIREPs available in real time via the Internet, with 10 examples of good correlation.
Infrasound Sensor Technology
Large aircraft can generate air vortices in their wake, turbulence that can prove hazardous to aircraft that follow too closely. Because wake vortices are invisible, all takeoffs at busy airports are spaced several minutes apart. This separation gives the vortices time to dissipate, even though they only occur 10% of the time, with resulting loss of operational efficiency. Similarly, clear air turbulence is invisible and can also be hazardous to aircraft. By detecting such disturbances through their infrasound emissions, precautions can be taken to avoid them.
Other phenomena can be detected through infrasound, including tornadoes, helicopters on the other side of mountains, underground nuclear explosions and digging tunnels. Through the unique properties of infrasound, many of these can be detected from hundreds of miles away. NASA's infrasound sensor is a highly refined microphone that is capable of detecting acoustic waves from 20 Hz down to dc, the infrasound range. The design is robust and compact, eliminating the bulk and weight found in other technologies. Where most alternative methods are restricted to certain weather conditions and locations,
the NASA sensor filters noise from wind and other sources, allowing its use under any weather or geographic conditions.
Analog Signal Correlating
Each of two analog signals (channels A and B) is converted to a digital bit stream by phase correcting it and comparing it to an average of itself at a sampling clock rate f. The hard-limited conversions of A and B are bitwise compared to measure the level of similarity between the two by the OBDC. This similarity measurement X is equal to the maximum possible Hamming distance (N bits in disagreement) minus the measured number of bits in disagreement.
The OBDC functions are embedded into a field programmable gate array (FPGA). The OBDC is made up of two shift registers containing the current sample values (of length N) from each of the two input channels (A and B). During each sample clock, a new sample from each A and B input is clocked into the input linear shift register for each respective channel; this input shifts the current values in the linear shift register. Once the inputs have been clocked in, the correlation routine can start. Once the correlation value has been calculated, this result is forwarded to compare with the max correlation value register. If the X value is greater than the current max correlation value, then the max correlation value becomes X, and the shift counter register is latched and put into the best correlation index register, providing the index of the current best correlation. This index is the number of sample clock periods difference between the two input signals and thus, for sample clock rate f, indicates the delay between the signals A and B.
This is an early-stage technology requiring additional development. Glenn welcomes co-development opportunities.
Robotic Inspection System for Fluid Infrastructures
The Robotic Inspection System improves the inspection of deep sea structures such as offshore storage cells/tanks, pipelines, and other subsea exploration applications. Generally, oil platforms are comprised of pipelines and/or subsea storage cells. These storage cells not only provide a stable base for the platform, they provide intermediate storage and separation capability for oil. Surveying these structures to examine the contents is often required when the platforms are being decommissioned. The Robotic Inspection System provides a device and method for imaging the inside of the cells, which includes hardware and software components. The device is able to move through interconnected pipes, even making 90 degree turns with minimal power. The Robotic Inspection System is able to display 3-dimentional range data from 2-dimensional information. This inspection method and device could significantly reduce the cost of decommissioning cells. The device has the capability to map interior volume, interrogate integrity of cell fill lines, display real-time video and sonar, and with future development possibly sample sediment or oil.
