N141-004 TITLE: Fully Integrated Low Size, Weight, and Power (SWaP) and Cost Magnetometers for Air and In-Water Anti-Submarine Warfare (ASW)

TECHNOLOGY AREAS: Sensors

ACQUISITION PROGRAM: PMA 290

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OBJECTIVE: Develop a low Size, Weight and Power (SWaP) and low cost total field scalar magnetometer with all control electronics fully integrated within the sensor package. The magnetometer is targeted for use in Unmanned Aerial Vehicles (UAVs), Unmanned Undersea Vehicles (UUVs), buoys, in-water arrays, Unmanned Ground Vehicles (UGV), as well as manned platforms.

DESCRIPTION: Recent work to reduce the SWaP and cost of magnetometers under SBIRs and commercial company Independent Research and Development (IR&D) funding has resulted in a vast improvement over the current production military and commercial systems, but still falls short of the optimum level for UAVs and in-water arrays and other remote sensor applications.

All current atomic scalar magnetometers consist of a sensor head (or physics package) that senses the magnetic field and a control circuit module located some distance away to drive the sensor and convert the analog sensor signals to useable digital data. The separation between the sensor head and control module is required so the electro-magnetic effects of the circuit card(s) will not interfere with the measurements of the sensor and create additional noise. While a significant amount of work has been focused on improved performance of the sensor head, the control module has been neglected and is still a design with separate analog, digital, and discrete components. The control module is also the major source of power consumption for the overall system. This configuration is not optimal for the Navy�s current interests in unmanned systems, where size, power, and available volume are at a premium or other noise sources exist. For example, in a UAV or rotary wing platform there are many inherent noise sources such as motors, servos and avionics that can add noise to the magnetic measurements and thus reduce the effectiveness of the magnetometer. One possible solution for noise mitigation, borrowed from helicopter Magnetic Anomaly Detection (MAD) systems, is to tow the sensor in a non-magnetic tow body far enough away from the aircraft so that it no longer interferes with the sensor measurements. Current systems have multiple wires and transmit analog signals between the sensor and electronics which requires a fairly large cable and additionally the analog signals are susceptible to electromagnetic interference (EMI) noise. A fully integrated magnetometer would reduce the size and weight in such a tow cable by putting the control electronics in the towed body and also reduce the power required. Also, in remote sensor applications, this separation requires larger sensor packages to be developed which creates additional complications when deploying the systems.

Low SWaP and cost expendable magnetometers are desired for the High Altitude Anti-Submarine Warfare (HAASW) mission, geomagnetic noise reduction, in-water detection, and land-based target detection such as buried weapons caches and improvised explosive devices (IEDs). Proposed solutions should provide innovative design concepts for a total field scalar magnetometer able to operate in all Earth's field orientations and magnitudes. Scalar in this usage defines a magnetometer that produces a total field magnetic measurement that is insensitive to motion of the sensor in the Earth's magnetic field, except for the atomic physics phenomena related to heading error. A vector magnetometer that only measures the magnetic field along a sensitive axis, or combinations of multiple vector magnetometers are not acceptable for this effort and will not be considered for this application. Vector magnetometer(s) in a moving platform cannot attain the noise level of a true scalar magnetometer due to motion induced measurement errors.

SWaP and cost goals are driven by intended small platform applications, which in many cases are expendable systems. The cost objective should be less than $5,000 in small quantities with a goal of less than $2,000 in volume production (100 - 500 units/year). Proposed designs should be small. The target volume threshold is equal to or less than 400 cm3 with an objective of equal to or less than 100 cm3 for the complete magnetometer. Also low-power (< 5 Watt total threshold, <1 Watt objective), and low-weight (< 2 lb. total) are required. The primary use is in airborne applications. The noise floor threshold of the magnetometer should be equal to or less than 10 pT/rt Hz from 0.015 to 100 Hz with a objective of equal to or less than 2 pT/rt Hz from 0.015 to 20 Hz and a raw heading error of < 300 pT (threshold) and compensated heading error <10 pT (objective). The secondary use is in-water applications which require 0.001 to 20 Hz bandwidth at the same noise levels. The threshold is the airborne frequency band and the objective is the in-water band. External master control units or processors for noise compensation or MAD detection algorithms need not be included in the SWaP requirements.

The external interface to the magnetometer must be at the minimum power and digital data. The power input can be assumed to be low voltage regulated DC or battery power but not necessarily clean voltage on the power line input to the magnetometer, meaning power conditioning will be required at the magnetometer. In order to measure the sensor noise level in the field and compensate for common environmental noise, the accepted procedure is to take measurements from two relatively close magnetometers and coherently subtract the total field measurement. This technique requires, at the minimum, to know the latency of the measurement and the timing must be consistent in order to synchronize the data streams between the two sensors. The bi-directional digital data interface can be any commercial standard, but the data packet transmission must be deterministic. Non-standard digital interfaces or power other than DC will be considered if it brings additional capability to the design such as reduced wire count or shown to reduce noise effects. The power and digital data must traverse 25 Meters (Threshold, 100 Meters Objective) of cable to the master control unit or processor. Additional inputs or outputs such as a 1 Pulse per Second (1-PPS) or shared frequency standard input or synchronization outputs are desirable but left up to the designer. Additionally, in order to reduce the inherent magnetic noise effects of the platform, additional sensors are usually used to include 3-axis vector magnetometers, 3-axis accelerometers, GPS inputs and other analog sensors. It is highly desirable to include 24 bit analog to digital converters in the design to accommodate these external sensors. The computer intensive computations such as heading error correction, noise suppression, and MAD algorithms need not be done in the magnetometer and can be done in an external master control unit or processor.

PHASE I: Design and develop an innovative concept for a low SWaP and cost fully integrated compact magnetometer that can achieve the described size, weight, power, performance, and cost requirements. Demonstrate the feasibility of the design relevant to the design requirements

PHASE II: From the Phase I design, fabricate two fully integrated compact magnetometer laboratory prototypes. Demonstrate the specified noise floor in a laboratory and field environment within the above-specified parameters.

PHASE III: Transition the compact magnetometer for use in appropriate platforms.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Miniature high-performance magnetometers will find application in UAVs for geologic applications including mineral and petroleum exploration. Also a fully integrated magnetometer has applications in the medical imaging field.

REFERENCES:
1. Happer, W. (1972). Optical Pumping. Reviews of Modern Physics 44. 169 - 249.

2. Kominis, I.K., Kornak, T.W., Allred, J.C., and Romalis, M.V. (2003). A Sub-femptotesia Multichannel Atomic Magnetometer." Nature 422. 596 - 598.

3. Shah, V., Knappe, S., Schwindt, P.D.D., and Kitching, J. (2007). Subpicotesla Atomic Magnetometry with a Micro-Fabricated Vapor Cell." Nature Photonics 1. 649 - 652.

4. Smullin, S.J., Savukov, I., Vasilakis, G., Ghosh, R.K., and Romalis, M.V. (24 July 2009). A Low-Noise High-Density Alkali Metal Scalar Magnetometer. arXiv:physics/0611085. Web.

5. Balabas, M.V., Karaulanov, T., Ledbetter, M.P., and Budker, T. (2010) Polarized alkali vapor with minute-long transverse spin-relaxation time. Phys. Rev. Lett. 105, 070801, arXiv:1005.1617

KEYWORDS: Magnetic Anomaly Detection; Magnetometers; Airborne ASW; Unmanned Air Vehicles (UAVs); Vertical Takeoff UAVs (VTUAV); Improvised Explosive Devices (IEDs)

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