1. A mobile device comprising: processing circuitry including a Measurement Engine (ME) and a Position Engine (PE), the PE to: provide a location estimate of the device using a location function, the location estimate estimating the location of the device to within a first error, determine the PE is going into a sleep mode, in response to a determination that the PE is going into the sleep mode, generate an estimate location function that approximates the location function to within a specified error and generate a corresponding expiration time that indicates a time at which the estimate location function is no longer valid, the location of the device as determined by the estimate location function including a second error, the first error less than the second error, and provide the estimate location function to the ME, and the ME to: receive a geo fence boundary defining a boundary of a geographical region, determine a plurality of pseudo ranges between the device and a plurality of navigation satellites, determine, while the PE is in the sleep mode and using the estimate location function and the plurality of pseudo ranges, whether the device is within the geographical region defined by the geo fence boundary and whether the specified time has elapsed, and in response to determining at least one of (1) the device is not within the geographical region defined by the geo fence boundary and (2) the expiration time has elapsed, provide a signal that causes the PE to wake up and provide an updated estimate location function to the ME.