1. A receiver unit in a wireless communication system, the receiver unit comprising: a first receiver configured to receive and process a received signal to provide data for a first position determination system; a second receiver configured to receive and process the received signal to provide data for a second position determination system; and a processing unit coupled to the first and second receivers and configured to: obtain an initial position estimate for the receiver unit, obtain a set of measurements from the first or second position determination system, or both, determine measurement elements from the initial position estimate; obtain observation values for the set of measurements; and update the initial position estimate based on the measurement elements and the observation values to obtain an updated position estimate for the receiver unit.