Generally discussed herein are systems and apparatuses for determining a location of a device. Also discussed herein are methods of using the systems and apparatuses. According to an example a mobile device may include processing circuitry including a Measurement Engine (ME) and a Position Engine (PE), wherein the PE is configured to provide a location estimate of the device using a location function, generate an estimate location function that approximates the location function to within a specified error, and provide the estimate location function to the ME, and wherein the ME is configured 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, and determine, 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.