Abstract: A method and apparatus for combining pressure information from pressure sensors, motion information from dead reckoning or other motion sensors, such as accelerometers, gyroscopes, and geomagnetic sensors, and temperature information from temperature sensors, to separate a change in altitude from a change in environmental pressure or temperature. A change in measured pressure without any motion or change in temperature must be a weather-related barometric pressure change. If there is an associated temperature change, but no motion, and the rate of change is too fast for a normal weather change, the measured change represents a change in sensitivity of the pressure sensor with temperature, and the sensor calibration can be adjusted. A rapid change in pressure, associated with horizontal motion, but no measurable vertical motion represents a movement into a different pressure environment. Only changes in pressure that are associated with measurable vertical motion are true changes in altitude.

Abstract: Reserve power is provided in a portable cellular transceiver comprising a Global Positioning System (GPS) receiver by monitoring a power level of a battery of the portable cellular transceiver using a power detection circuit. A comparator circuit determines whether the power level of the battery is greater than a predetermined threshold. Based on the available power level, a power control circuit automatically places the transceiver into at least one low power mode when the power level reaches a corresponding threshold level. The low power mode selectively powers down circuitry of the portable cellular transceiver. The corresponding threshold level reserves power for an emergency telephone call. Power is provided to the circuitry of the portable cellular transceiver when the available power level is greater than the predetermined threshold.

Abstract: Reserve power is provided in a portable cellular transceiver comprising a Global Positioning System (GPS) receiver by monitoring a power level of a battery of the portable cellular transceiver using a power detection circuit. A comparator circuit determines whether the power level of the battery is greater than a predetermined threshold. Based on the available power level, a power control circuit automatically places the transceiver into at least one low power mode when the power level reaches a corresponding threshold level. The low power mode selectively powers down circuitry of the portable cellular transceiver. The corresponding threshold level reserves power for an emergency telephone call. Power is provided to the circuitry of the portable cellular transceiver when the available power level is greater than the predetermined threshold.

Abstract: Reserve power is provided in a portable cellular transceiver comprising a Global Positioning System (GPS) receiver by monitoring a power level of a battery of the portable cellular transceiver using a power detection circuit. A comparator circuit determines whether the power level of the battery is greater than a predetermined threshold. Based on the available power level, a power control circuit automatically places the transceiver into at least one low power mode when the power level reaches a corresponding threshold level. The low power mode selectively powers down circuitry of the portable cellular transceiver. The corresponding threshold level reserves power for an emergency telephone call. Power is provided to the circuitry of the portable cellular transceiver when the available power level is greater than the predetermined threshold.