A portable device, for example an offender monitor, can utilize a location detector, such as a GPS receiver, to provide location information for the device. The portable device can utilize geofences in connection with managing device location and can communicate over a cellular network, for example using a radio to transmit location readings over the network. The location detector and the radio can draw power from an onboard battery. To conserve battery life, the rate of acquiring location readings can be adjusted according to distance between the device and a geofence of interest. For example, a GPS data acquisition rate can increase as the device approaches a geofence and decrease as the device moves away from the geofence.