OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Integrated Sensing and Cyber The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), 22 CFR Parts 120-130, which controls the export and import of defense-related material and services, including export of sensitive technical data, or the Export Administration Regulation (EAR), 15 CFR Parts 730-774, which controls dual use items. Offerors must disclose any proposed use of foreign nationals (FNs), their country(ies) of origin, the type of visa or work permit possessed, and the statement of work (SOW) tasks intended for accomplishment by the FN(s) in accordance with the Announcement. Offerors are advised foreign nationals proposed to perform on this topic may be restricted due to the technical data under US Export Control Laws. OBJECTIVE: High altitude balloons occupy the middle ground' between the Earth's surface and space because they can capture signatures that are difficult to record from either ground stations or satellites. Over the last decade, acoustic sensors on high altitude balloons have captured a diverse set of phenomena, including rocket launches, aircraft, spacecraft reentry, ground explosions, earthquakes, thunder, wind turbines, and even freight trains. However, these sensors have consisted of repurposed ground micro barometers that have relatively high intrinsic noise levels. Currently there are no high quality, low size, weight, power and cost (SWaP-C) commercially available acoustic sensors with the required sensitivity and environmental resilience for use in the low atmospheric pressures and extreme temperatures found at high altitudes. Adding an acoustic sensor to a HAB sensor suite will enable the detection, tracking and identification of ships and aircraft, even when they maintain radio silence, because their acoustic signatures are detectable at long ranges in the stratosphere. Currently, most acoustic recorders used on HABs were designed for use at the Earth's surface, where wind noise is much greater and therefore intrinsic sensor noise is less important. In contrast, background noise levels is extremely low on free floating balloons, far lower than would ever be realized on the ground. This effort is related to existing infrasound detectors deployed around the world at ground level for the detection of explosions (chemical and nuclear) as well as seismic events. DESCRIPTION: The work that would be accomplished in this specific topic would result in a new class of sensor that when deployed on a high altitude balloon (HAB) would enable the detection and tracking of aircraft, ships and other sources with acoustic signatures such as explosions, rocket launches, space object re-entry. The proposed work would result in a combined acoustic sensor/accelerometer package meant for high altitude balloons. The sensor would be capable of recording faint sound waves emanating from distant sources and utilizing the accelerometer to determine the direction of origin of the signal. This would permit the detection, tracking, and identification of human and natural phenomena such as chemical and nuclear explosions, aircraft, ships, hypersonic objects, meteors, volcanic eruptions, and earthquakes. Additional diagnostics such as the existence and state of health of infrastructure such as bridges, dams, and industrial facilities could be collected also. These acoustic sensors are expected to be lightweight with low power requirements. Most of the sensor data processing can be performed at the sensor reducing the data bandwidth requirements. These sensors could be an important part of the growing interest in HABs for surveillance, electronic warfare, and communications support. Because of their small size, low weight, low power requirements and low cost, combined with their ability to provide information on a wide range of activity and threats, they could be used on almost all HAB platforms. The intent is to make it simple to add the sensor to existing HABs with larger payloads, thereby developing an acoustic network from already-extant constellations. The sensor package could be flown on small, dedicated balloons as well. PHASE I: In Phase I, awardee(s) will produce a preliminary design with resource requirements, performance, and cost estimates. Included in the resource requirements are size, weight and power for both the sensor and any on-board processing. To reduce data communication requirements most signal processing will need to be done on board. The performance estimates include sensitivity and noise level across from infrasound (0.1 Hz) to the lower-mid range of human hearing (1 kHz), as well as the accuracy of direction of arrival measurements. Cost estimates should include low volume initial production costs as well as estimates for a commercial product. PHASE II: During Phase II the awardee(s) would complete its preliminary designs from Phase I, develop prototype that can be evaluated in altitude chambers and on short duration high altitude balloon flights and incorporate the results of these tests into low-rate initial production articles for testing. The proposed high altitude platform is the Sandia heliotrope solar hot air balloon, which can deliver a payload of up to 2 kg at the target altitudes for several hours of level flight. This is a standard platform for high altitude infrasound sensing. Multi-day high altitude balloon flights would be used for operational testing of the test articles. The results of the operational testing would be incorporated into designs that would become commercial products under a Phase III award. Because these devices should be relatively inexpensive and fairly well understood it is reasonable to accomplish this during a Phase II contract. PHASE III DUAL USE APPLICATIONS: During Phase III the acoustic sensors would become a commercial product available for use by DoD and IC, and potentially our allies, on high altitude balloon flights to refine their concept of operational employment and tactics, techniques and procedures. The Army and Navy use of high altitude balloons would likely benefit from incorporating acoustic sensors on their platforms. Having acoustic sensors on multiple high altitude balloons in a region results in better geolocation of targets. Follow-on efforts to develop algorithms for exploiting acoustic data from multiple sources would enhance the utility of this data. REFERENCES: Silber, S. A. and Bowman, D. C. (2023). Detection of the Large Surface Explosion Coupling Experiment by a sparse network of balloon-borne infrasound sensors. MDPI Remote Sensing 15, 542. Bowman, D. C., Rouse, J. W., Krishnamoorthy, S. and Silber, S. A. (2022). Infrasound direction of arrival determination using a balloon-borne aeroseismometer. The Journal of the Acoustical Society of America Express Letters 2 (5) Garcia, R. F., Klotz, A., Hertzog, A., Martin, R., G rier, S., Kassarian, E., Bordereau, J., Venel, S. and Mimoun, D. (2022) Infrasound from large earthquakes recorded on a network balloons in the stratosphere. Geophysical Research Letters 49 (15), e2022GL098844 Bowman, D. C. and Albert, S. A. (2018). Acoustic Event Location and Background Noise Characterization on a Free Flying Infrasound Sensor Network in the Stratosphere. Geophysical Journal International 213, p. 1524-1535; KEYWORDS: high altitude balloons;stratospheric sensors;acoustic sensor; infrasound; microbarometer; remote sensing
