N15A-T004 TITLE: In-Air E-field Sensor for Airborne Applications

TECHNOLOGY AREAS: Air Platform, Sensors

OBJECTIVE: Develop an in-air, rotationally invariant Electric-field (E-field) sensor which can be used aboard a fixed wing aircraft or in a Tier 1 Unmanned Autonomous Vehicle (UAV).

DESCRIPTION: The Navy uses magnetometers for airborne submarine localization. One of the significant sources of noise for the magnetometers is geomagnetic noise (currents in the ionosphere). A second magnetometer/UAV can act as a reference sensor for the first system. The two systems must be separated sufficiently that only one would sense a target. The geomagnetic noise is coherent over tens of miles so that if the measured signals in the two systems are subtracted, the geomagnetic noise would be significantly reduced, but the target signature would remain. Since an electric field originating below the surface of the water will not significantly propagate into the atmosphere, an E-field sensor on the same platform could, if successful, provide a reference sensor for reduction of the geomagnetic noise without loss of target signal. This method requires less equipment and manpower, and could reduce the cost of the detection operation.

The first challenge to using an E-field sensor is that there are two possible ways to determine the sensitivity needed and they do not agree. A straightforward determination of the required E-field from the desired Magnetic field (B-field) obtained by multiplying by c (speed of light) indicates that the geomagnetic E-fields should be on the order of millivolts per meter (milliV/m). Yet ground based measurements of geomagnetic noise shows E-field values in the range of microvolts per meter (microV/m). The second challenge is designing a sensor with the required microvolt to millivolt sensitivity that is rotationally invariant and can be flown on an aircraft or Tier 1 UAV.

It is required that the E-field sensor output be insensitive to rotations, e.g., a scalar sensor or summed vector sensors would be possible approaches. Rotational rates are in the 0.1-1 degree/second range for platforms of interest. The E-field sensor must not create magnetic noise greater than 10 picoTesla per root Hertz (pT/RtHz) in the magnetometer when placed at a distance of one foot away. It is likely that there will be E-field noise arising from the platform itself, which will need to be mitigated. This should be achieved without any modification to the aircraft and without shielding that would remove the signal, so it is likely that a software solution will be required.

PHASE I: Determine the required sensitivity of the E-field sensor so it can correlate with magnetic noise at the 10 pT/RtHz level in the 10 milliHertz (mHz) - 100 Hertz (Hz) band. This may be done either by theory or experiment. Demonstrate the feasibility of the proposed E-field sensor design, which can be used aboard an airborne platform, with the required sensitivity.

PHASE II: Develop an E-field sensor prototype based on the results of the Phase I. Demonstrate the prototype sensor aboard an airborne platform jointly with a magnetometer of the required scalar sensitivity, to measure correlation and effectiveness of the E-field sensor in reducing geomagnetic noise. Demonstration can be done on any aircraft chosen/available to the proposer, but the prototype sensor must be able to fit in a Tier 1 UAV: this restricts the size to tens of cubic decimeters, weight to about a kilogram and power to tens of watts.

PHASE III: Productionize E-field sensor hardware and mature algorithms. Assist in obtaining flight clearance for use on NAVAIR UAV. Develop manufacturing and commercialization plans. Transition sensor to appropriate Navy Tier 1 UAV platforms.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Oil and mineral exploration over the ocean also have the problem of geomagnetic noise and would benefit from such an E-field sensor.

REFERENCES:
1. Renno, N., Kok, J., Kirkham, H., & Rogacki, S. (2008). A Miniature Sensor for Electrical Field Measurements in Dusty Atmospheres. Institute of Physics. Retrieved from: http://deepblue.lib.umich.edu/bitstream/handle/2027.42/64202/jpconf8_142_012075.pdf?sequence=1

2. Dodle, F., H. Fedder, T. Nöbauer, F. Rempp, G. Balasubramanian, T. Wolf, F. Reinhard, L.C.L Hollenberg, F. Jelezko, and J. Wrachtrup. (2011). Electric-field Sensing Using Single Diamond Spins. Nature Physics. Retrieved from: http://www.nature.com/nphys/journal/v7/n6/pdf/nphys1969.pdf

3. Zeng, R., Wang, B., Yu, Z., Niu, B., Hua, Y. (2011). Integrated Optical E?-field Sensor Based on Balanced Mach�Zehnder Interferometer. Optical Engineering. Retrieved from: http://opticalengineering.spiedigitallibrary.org/article.aspx?articleid=1158454%20&journalid=92

4. Bordovsky, M. (1998). Electrooptic Electric Field Sensor for DC and Extra-low-frequency Measurement. Brunel University Research Archive (BURA). Retrieved from http://bura.brunel.ac.uk/handle/2438/4957

5. Sedlacek, A., Schwettmann, A., Kübler, H., & Shaffer J.P. (2013). Atom-Based Vector Microwave Electrometry Using Rydberg Atoms in a Vapor Cell. University of Oklahoma, Department of Physics and Astronomy. Retrieved from: http://www.researchgate.net/publication/256097508_Atom-based_vector_microwave_electrometry_using_rubidium_Rydberg_atoms_in_a_vapor_cell

6. Griffiths, D. (1999, January 9). Introduction to Electrodynamics. Addison Wesley; 3rd Ed.

7. Kraichman, M. (1977). Electromagnetic Background Noise in the Ocean Due to Geomagnetic Activity in the Period Range 0.5 to 1000 Seconds. Naval Surface Weapons Center, White Oak Laboratory. Retrieved from: http://www.dtic.mil/dtic/tr/fulltext/u2/a040922.pdf

KEYWORDS: Airborne; Magnetometer; E-field; Anti-Submarine Warfare (ASW); Geomagnetic; Noise

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