Doug J. Grimmett

Modeling Specular Occurrence in Distributed Multistatic Fields

The effective fusion and tracking of multistatic active sonar contacts is challenging, due to high levels of false alarm clutter present on all sonar nodes. Utilizing the occurrence of high strength detections generated by the specular geometric condition, a cueing approach can be used to selectively extract further data which is stored locally on the individual sonar nodes. This approach can significantly reduce the false alarm rate at the input to the fusion/tracking algorithm, and reduce node-to-fusion- center communication link throughput requirements. The cost of this approach is increased detection latency, while waiting for specular cues to occur. For this concept to be viable there must be sufficient statistical probability that specular detections will occur within the field to initiate the cueing process. The specular detection cues will also have to occur with acceptable latency, compared to the ASW surveillance missions operational time scale. This paper describes a new modeling methodology which has been developed, enabling the evaluation of distributed multistatic fields for specular detection occurrence. Metrics are presented which enable this evaluation, and applications to example multistatic fields are shown.

Adaptive ping control for track-holding in multistatic active sonar networks

Distributed multistatic active sonar networks provide an Anti-Submarine Warfare capability against small, quiet, threat submarines in the harsh clutter-saturated littoral and deeper ocean environments. Adaptive ping control techniques provide the potential to significantly increase the multistatic networks performance, by pinging (in an optimum sense) the right source, at the right time, with the right waveform. This paper describes an automatic, adaptive ping control algorithm. It specifically addresses the “trackhold” objective, which is to adapt multistatic sonar operations to maintain and hold one or more target tracks which have been previously initiated (detected). The approach is unique in that it includes both sonar performance modeling and multistatic tracker outputs, in a closed-loop control structure. The paper motivates the approach, describes the algorithm, and shows some validating results. The evaluation utilizes a simple sonar performance model, a ping contact simulator, and a multistatic target tracker. Results are shown for a simple simulated scenario, showing the advantages of this adaptive ping control algorithm compared to using a preplanned, non-adaptive ping transmission schedule.

Ocean heave cancellation for a maritime infrasound sensor

Infrasound is very low frequency airborne acoustic energy in the frequency band of 3.3 mHz to 20 Hz, that is inaudible to human beings. Natural sources of infrasound include earthquakes, meteors, volcanoes, tsunamis, auroras, and ocean swells. Among anthropogenic sources are atmospheric and underground nuclear explosions. Because of their low frequency, infrasound waves experience little attenuation, and can therefore propagate to, and be detectable from, very long distances. Land-based infrasound microbarometer sensors and networks have the capability of detecting these signals. In this paper, we investigate the fielding of microbarometer sensors in the maritime environment, on boats, buoys, or unmanned surface vehicles (USV). A particular problem to be overcome is the effect of ocean heave on sensor performance. Ocean heave induces interfering pressure fluctuations on the sensor which may obscure the infrasound signal of interest. In this paper, an ocean-heave cancellation method is developed and applied to infrasound data collected with a sensor deployed onboard a ship at sea. The results show the performance of this approach in cancelling ocean heave-induced interference on infrasound sensors fielded in the maritime environment.

Measuring Infrasound from the Maritime Environment

Worldwide infrasound coverage is obtained using fixed, land-based sensing networks. However, two-thirds of the earth’s surface is composed of oceans, and while islands in the ocean already host sensing stations, no capability yet exists to monitor infrasound from sensors fielded directly in the maritime environment. Deployment in the maritime would greatly enhance the ability to monitor natural and anthropogenic sources of infrasound around the world through improved event detection, localization, and classification. The additional sensing may also facilitate improved knowledge of global atmospheric environmental conditions. The advantages and challenges of infrasound sensing in the maritime environment are described, as are potential host platforms for fielding them. Some technical challenges for this concept include sensor motion, wind noise, composing arrays of sensors and survivability in the ocean environment. An in-depth analysis of one of these, the negative impact of ocean heave on performance, is described along with a potential solution for its mitigation.

SPECSweb tracking on a large, simulated multistatic field of the MSTWG

Effective fusion and tracking of multistatic active sonar contacts is challenging, due to high levels of false alarm clutter present on all sonar nodes. Such false alarms often overload the sensor-to-fusion-center communications links and fusion/tracking processes, producing too many false tracks. The Specular-Cued Surveillance Web (SPECSweb) multistatic tracker mitigates these problems by allowing track initiation to occur only when high-strength specular target detections are identified. Using these “specular” cues, and subsequent track state estimates, a selective data retrieval approach is used which significantly reduces the data rate at the input to the fusion/tracking algorithm, and reduces node to fusion-center communication link throughput requirements. This paper provides performance results of this tracking algorithm on a simulated multistatic data set from the Multistatic Tracking Working Group (MSTWG). The data set simulates a large multistatic field, and is characterized by low probability of detection (PD), high false alarm rate (FAR), and high measurement errors. The impacts of these challenging conditions on tracker performance are explained, including the degradation of association gating with large error in bearing measurements.