Vibration imaging for the characterization of extended, non-cooperative targets

Description:

TECHNOLOGY AREA(S): SENSORS

OBJECTIVE: Develop a vibration imaging approach that meets the spatial and temporal requirements needed to perform high-fidelity characterizations of extended, non-cooperative targets (aka combat identification) at extended standoffs with an aperture that has dual purpose for both directed-energy (DE) and intelligence, surveillance, and reconnaissance (ISR) missions.

DESCRIPTION:Vibration imaging offers a distinct way forward with respect to combat identification at extended standoffs for both DE and ISR missions.In practice, vibration imaging offers many advantages over a single-pixel laser Doppler vibrometer [1].This is said because vibration imaging simultaneously acquires and resolves velocity data over an extended spatial area.Given a single-pixel laser Doppler vibrometer, speckle is a dominant noise source, since the signal fades caused by speckle lead to velocity estimates which have spikes and an elevated noise floor.Via simultaneous data acquisition with a focal-plane array, vibration imaging offers the ability to spatially average which ultimately enables speckle-noise mitigation.Vibration imaging works via the use of doublet-pulse vibrometry [2].Here, the collection of multiplexed digital-holography data enables us to simultaneously measure the complex-optical field associated with two laser pulses separated in time [3, 4].By estimating the phase difference between the two received pulses, we can then measure the target’s velocity [2].In turn, vibration imaging enables us to perform target identification at extended standoffs. Such functionality offers promise for both DE and ISR missions, where it is important to know the characteristics of extended, non-cooperative targets.This added functionality will ultimately enable future DE and ISR assets to determine, for example, whether the engine is running or not (at a distance that is safe for inspection).It will also enable us to tell how fast the speckle is changing, which is currently an unknown.With this in mind, many DE and ISR solutions assume that the received speckle is either correlated or uncorrelated frame to frame; thus, it is important that we characterize this phenomena in the near future, so that we can move forward with the development of future DE and ISR assets.The end goal of this STTR topic is to design (Phase I and II) and demonstrate (Phase III) a vibration imaging approach that meets the spatial and temporal requirements needed to perform high-fidelity characterizations of extended, non-cooperative targets at extended standoffs.As such, during a Phase I effort, a detailed theoretical and numerical analysis shall be performed to explicitly verify wave-optics calculations for a variety of ranges and resolutions.A Phase II effort shall then develop experiments that verify the wave-optics calculations.For this purpose, facilities at AFRL could provide the scaled-laboratory environment needed to explore a variety of ranges and resolutions.A Phase III effort could then demonstrate vibration imaging at distances greater than 1 km in a field environment with moving targets.Such testing shall ensure commercialization of the developed approach. PHASE I: To achieve the identified Phase II objectives, a Phase I effort shall focus on the following deliverables.• Performing wave-optics calculations for a variety of ranges and resolutions.These calculations shall identify scalability and include the relationship between the aperture and the extended, non-cooperative targets of interest while at extended standoffs.This step shall ensure that the developed approach is ready for a Phase II effort.

PHASE II: To achieve the identified Phase III objectives, a Phase II effort shall focus on the following deliverables.• Performing scaled-laboratory experiments (potentially at AFRL) in order to verify the wave-optics calculations performed in a Phase I effort.This step shall ensure that the developed approach is ready for a Phase III effort.

PHASE III: Military application: Demonstrating the developed approach in a field environment at distances greater than 1 km with moving targets.This step shall ensure that the developed approach is ready for both DE and ISR missions.Commercial Application: The successfully demonstrated vibration imaging approach shall translate into a high-fidelity solution that is available to the DoD.

REFERENCES: 

1. P. Castellini, G. M. Revel, and E. P. Tomasini, “Laser Doppler Vibrometry,” An Introduction to Optoelectronic Sensors, 216-229 (2009);

2. P. Gatt et al., “Phased Array Science and Engineering Research (PHASER) Program Final Report,” TR CDRL-A001-01, Lockheed Martin Coherent Technologies (2015) [Dist. C, Export Controlled].;

3. S. T. Thurman and A. Bratcher, “Multiplexed synthetic-aperture digital holography,” App. Opt. 54(3), 559-568 (2015).;

4. M. F. Spencer, “Spatial Heterodyne,” Encyclopedia of Modern Optics II Volume 4, 369-400 (2018).

KEYWORDS:vibration imaging, vibrometry, combat identification, beam control, digital holography, spatial heterodyne

CONTACT(S):MarkSpencer AFRL/RDLTS 5058531607 mark.spencer.6@us.af.mil

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