In many combined GPS/cellular radio receivers, the cellular radio segment is not used to aid the positioning process. Recently proposed Assisted-GPS systems use the communications channel to improve receiver time-to-first-fix and the GPS tracking sensitivity. However, the cellular radio signals can also be used for locating the receiver. Studies using the GSM mobile communication system have shown positional accuracy comparable to that of SA-degraded GPS. To achieve maximum performance (accuracy, availability, integrity and reliability), both sources of positioning information, GPS and cellular, should be regarded as sensor inputs to an integrated navigation system. In this way the benefits of both systems can be fully exploited to produce an optimal navigation solution.