Compact, low-power-consumption magnetic-sensor units called "integrated sensor system" (ISS) units are being developed for detecting buried mines and arsenals without exposing human searchers to unnecessary danger. The fully developed ISS units would be mass produced at relatively low cost, so that they could be deployed in large numbers and regarded as disposable. Many ISS units would be dispersed onto a mine field from a low-flying aircraft. After a specified time, the ISS units would transmit magnetic-field- and attitude-measurement data to the same or different aircraft for processing. Analysis of the data would reveal local deformations of the Earth's magnetic field, indicative of the magnetic dipole moments of mines and other objects.
Each ISS unit (see figure) would contain a miniature three-axis flux-gate magnetometer, a Sun-angle sensor, a data subsystem, a battery power subsystem, a transmitter, and a patch antenna. The ISS unit would be packaged with nonmagnetic components. The entire unit would be designed with great care to ensure that the magnetometer readings would not be corrupted despite the proximity of the magnetometer to the electronic circuitry and packaging.
The three-axis magnetometer chosen for the ISS is one that was recently developed for use in an airborne unit. It includes low-noise ferromagnetic cores with windings for the three axes, plus drive and readout circuits made of commercially available components.
The Sun-angle sensor, designed previously for use on small satellites, is a wide-angle optoelectronic sensor module in a pyramidal housing. The angle of the Sun, relative to the unit, is deduced from differences among the currents generated by four solar photovoltaic cells.
The data subsystem would include a 22-bit analog-to-digital (A/D) converter. The flow of data and the general operation of the rest of the ISS unit would be controlled by a field-programmable gate array (FPGA). A 4-Mb static random-access memory (SRAM) would store data (typically about 7 minutes' worth) until transmission. Data would not be acquired during transmission because operation of the transmitter could corrupt the magnetometer readings.
The battery power subsystem would include six high-capacity Li/SOCl2cells, plus power-regulation circuitry that is part of the data subsystem. Once activated, the ISS unit would continue to operate until the battery runs down (nearly two hours).
The transmitter would operate at a center frequency of 2,250 MHz. It would contain a frequency synthesizer in the form of a voltage-controlled oscillator locked in phase with a temperature-controlled crystal oscillator (TCXO). The output of the frequency synthesizer would be phase-modulated with the data signal, then amplified, then fed to the patch antenna.
Future refinements of the ISS design would effect increases in magnetic sensitivity, making it possible to detect mines with smaller magnetic moments. Of course, the magnetic-field-based ISS units would not be capable of detecting mines constructed entirely of nonferrous materials. On the other hand, they can be expected to detect a variety of mines of a low-technology type that are built at clandestine factories and have large magnetic moments. As newer mines are made with ever weaker magnetic moments, the basic ISS design could be modified to incorporate electronic chemical sensors to detect explosive vapors.
This work was done by Hamid Javadi of Caltech for NASA's Jet Propulsion Laboratory. NPO-20471