The prototype for a nitrogen-cooled high-T/sub c/ SQUID gradiometer has been developed and is being evaluated for magnetic anomaly detection of underwater targets in mobile surveys. The prototype's design is based on the concept of the Three-Sensor Gradiometer (TSG). In the TSG approach, balance of two independent SQUID magnetometers is more difficult to attain than for conventional low-T/sub c/ gradiometers in which signal subtraction occurs prior to a single SQUID stage. Experiments have been conducted using a platform-motion simulator to evaluate performance of this gradiometer for mobile operation. Sensor configuration, experimental procedures, approaches for improved performance, and empirical results are reported. Interesting results of predictions to estimate detection range obtained from matched-filter calculations are included. The paper concludes with description of current preparations for a sea test of this sensor and a perspective of future developments.
The Laser Scalar Gradiometer (LSG) is a sensitive passive magnetic sensor based on the opto-magnetic properties of helium-4 gas in accordance with the Zeeman effect. The LSG has attained increased sensitivity over comparable sensors by the use of a laser in place of incoherent light for optical pumping. It employs four helium sense cells configured in a volume-filling arrangement to measure four independent channels of information: the scalar field magnitude and three linearly independent admixtures of the three components of the scalar-field gradient vector. The LSG has now been integrated into the REMUS 600 and evaluated in land-based testing. This land-based testing has assured the proper functionality of this integrated system prototype and established the sensor's noise floor in the electromagnetic environment of the REMUS 600. Following this land-based testing, at-sea shakedown tests and experiments over target fields have been conducted to provide a more definitive measure of the LSG's performance under actual operational conditions and to evaluate its current capability to detect, classify, and localize (DCL) buried mines. The system configuration, the experiment design, associated test procedures, and results of data analysis from the underwater experiments conducted with the LSG onboard REMUS 600 are reported in this paper
During the 1980's the Superconducting Gradiometer/Magnetometer Sensor was demonstrated in the Magnetic and Acoustic Detection of Mines Advanced Technology Demonstration to provide effective mine detection, localization, and classification capabilities, especially against buried mines, and to reduce significantly acoustic false alarms arising from bottom clutter. This sensor utilized Superconducting Quantum Interference Devices manufactured using the low critical temperature (low Tc) superconductor niobium and liquid helium for sensor cooling. This sensor has most recently bee integrated into the Mobile Underwater Debris Survey System and has been demonstrated successfully in a survey to locate unexploded ordnance in coastal waters.
Optical identification, which can provide compelling confirmation that a proud sonar contact is a mine, is obviously not possible for fully buried mines. One interesting sensor-fusion concept under consideration to confirm buried sonar contacts is to reacquire the contacts at short range using an unmanned underwater vehicle (UUV) carrying a magnetic sensor and a bottom-looking sonar, such as the Buried Object Scanning Sonar (BOSS) developed by Florida Atlantic University. Two magnetic sensors are currently being developed by the Office of Naval Research (ONR) for buried minehunting. The Quantum Magnetics' Realtime Tracking Gradiometer (RTG) is a multichannel tensor gradiometer being developed using fluxgate technology. An existing RTG prototype has been integrated with an existing BOSS prototype onboard a tow body and underwater tow tests have been conducted over targets to demonstrate the effectiveness of the BOSS/RTG fusion concept. A new miniaturized version of the RTG is being developed for operation onboard relatively small unmanned underwater vehicles (UUVs). The Polatomic Laser Scalar Gradiometer (LSG) is a multichannel scalar magnetic sensor designed to operate on relatively small UUVs. It is based on the electron-spin resonance (ESR) properties of helium-4 gas in accordance with the Zeeman effect, very similar in concept to the US Navy's AN/ASQ-208. The LSG has attained increased sensitivity over the AN/ASQ-81 by the use of a laser in place of incoherent light for optical pumping. In this paper, the sensors and system configurations being pursued are discussed. Relevant tests that have been conducted for individual sensors and sensor combinations and test results are described. Current status of these developments and future plans to test and demonstrate these technologies and concepts are presented.
This paper is concerned with the early stage efforts to augment the AUV (autonomous underwater vehicle) with an enhanced magnetic detection/localization capability that is inherently unaffected by environmental conditions found in the shallow water environment. Prior to the placement of a sensitive magnetic sensor system on an actual AUV, the magnetic characteristics of the vehicle itself must be measured and their detrimental effects on the sensor mitigated. In particular, this paper discusses techniques for the magnetic characterization of several AUVs and a comparison of these results. It then briefly discusses proposed methods of mitigation and some of the surprising results obtained from candidate platforms. Detailed mitigation techniques and results are presented in a companion paper.
A dc superconducting quantum interference device (SQUID) modulation and feedback circuit operating at a modulation frequency of 16 MHz has been constructed. Using a novel wide band superconducting thin film transformer to impedance match the SQUID to a rf amplifier allows the system to operate at the SQUID noise level for most types of low-TC SQUIDs. This system has a closed loop bandwidth exceeding 2.5 MHz and a slew rate greater than 1×106Φ0/s at frequencies up to 1 MHz. This greatly improved performance compared to existing modulation methods can be obtained without enhancing the transfer function of the SQUID. The system allows low- and high-TC SQUID magnetometers and gradiometers to be operated totally unshielded without unlocking in the dc, 60 Hz, and radio frequency electromagnetic fields present in most SQUID applications.
Fusion concepts using a magnetic sensor in combination with acoustic and optical sensors operating onboard an unmanned underwater vehicle (UUV) are under consideration to reacquire and confirm buried contacts detected in an initial sonar search. Two magnetic sensors are currently being developed by the Office of Naval Research for buried minehunting (BMH): Polatomic's Laser Scalar Gradiometer (LSG) and Quantum Magnetics' Realtime Tracking Gradiometer (RTG). The LSG is a multi-channel electron-spin resonance scalar magnetometer/gradiometer, while the RTG is a multi-channel tensor gradiometer using fluxgate technology. In this paper, we will describe progress in the development and testing of the LSG. The operation of the LSG is based on the opto-magnetic properties of helium-4 gas in accordance with the Zeeman effect. The LSG and its predecessor, the P-2000, have attained increased sensitivity over comparable sensors by the use of a laser in place of incoherent light for optical pumping. The LSG employs four helium sense cells configured in a volume-filling arrangement to measure four independent channels of information: the scalar field magnitude and admixtures of the three components of the scalar-field gradient vector. The P-2000 electronics consists of a mixture of discrete analog and digital circuits that requires three rack-mounted units. To satisfy the size and power requirements for operation in small UUVs and the requirement for autonomous sensor operation, the LSG electronics features integrated digital surface mount technology. In initial testing, preliminary LSG performance has been measured by acquiring data with the sensor stationary and in motion in a magnetically quiet environment. In addition, a tracking experiment has been conducted to validate algorithms for target detection, classification and localization (DCL). Work is in progress to optimize the performance of the LSG. When this work is completed, the LSG will be integrated into the REMUS 600 developed by Woods Hole Oceanographic Institute (WHOI) and experiments over target fields at sea will be conducted. This paper reviews the LSG design, presents results from the initial land-based testing, and describes plans for system integration and underwater experiments conducted with the LSG onboard REMUS 600.
Polatomic, Inc. has developed a He-4 electron-spin resonance (ESR) magnetometer/gradiometer, the Polatomic 2000 (P-2000), for anti-submarine warfare. The P-2000 features a laser in place of an incoherent light source for optical pumping in order to increase sensitivity, and a magnetometer/gradiometer configuration to provide more target information than possible with a magnetometer alone. In this paper, the sensor's configuration, the experimental setup and the test procedures are described. Results presenting spectral gradient sensitivity are reported for the experiments conducted. Predictions to estimate detection range obtained from matched-filter calculations are also included.
A test has been conducted to demonstrate the detection, classification, and localization (DCL) of targets in the very shallow water (VSW) and surf-zone (SZ) regions using high performance magnetic sensors towed on the surface at high speeds. Targets in as little as 5 feet of water were detected and accurately localized at speeds as high a 30 knots. In particular, one low-ferrous target was detected at ranges exceeding 6 m. This performance can equate to a high area coverage rate for some mine hunting operations against both proud bottom, buried, and volume targets.
