A portable device, for example an offender monitor, can incorporate a location detector and an orientation detector. The orientation detector, for example an accelerometer, can provide orientation information. The location detector, for example a GPS receiver, can acquire location readings that the device transmits over a cellular network. The portable device can control the location detector based on the orientation information. For example, the device can acquire more location readings when the orientation information indicates that a person wearing the portable device is reclined than when the person is upright.