Abstract: A system is disclosed for locating swimmers who could be in danger of drowning, as determined by preset parameters such as the time duration that they remain at a depth where breathing is impossible. In this system, each individual swimmer wears a Swimmer Location Monitor (SLM). This is a miniature electronic device that not only determines the extent of the swimmer's danger but also emits an electromagnetic alarm signal whenever such a danger exists. At each occurrence, an alarm, which may be audible and/or visual, is sounded on the surface while the swimmer's location is tracked in real time from the signals received on an antenna network deployed along the bottom of the swimming area. These signals are combined in a central processor and the computed real-time location is displayed graphically on a monitor to guide the guard personnel in rescue operations. The invention is intended for use in pools, beaches, water parks and other swimming areas.

Abstract: A ferrite core is suspended from the base of a borehole in a tunnel roof structure and is positioned within coil turns located at the roof surface such that movement of the roof structure causes displacement of the core with respect to the coil turns thus producing a change in inductance. The coil and core are incorporated as a frequency-determining element in an LR oscillator. Therefore, movement of the tunnel roof structure is reflected as a change in the frequency of the oscillator. This frequency is monitored and, if the change exceeds a predetermined threshold, an alarm is given. Several methods of suspending the core in the coil are disclosed, one of which allows for a non-verticle borehole.

Abstract: A Frequency Shift Keying (FSK) system to transmit and receive data as a sequence of dual frequency short duration tone bursts. Normally the data is transmitted from an underground location, as in a mine, to the surface or to another underground location. At the receiving end the transmitted data is detected as marks or spaces that are used to clock the appropriate data bits into a storage shift register. Erroneous data from wideband noise transients at the receiver are eliminated by the use of an exclusive OR circuit which permits shift pulses to appear at the data register only when one or the other of the two shift pulses are present but not both. To further insure accurate copy of the digital data at the receiver, a receiver alert code is transmitted prior to the telemetering of the actual data. Only after recognition of the receiver alert code is the receiver able to receive the transmitted data bit stream.