1. A receiver unit in a wireless communication system, the receiver unit comprising: a first receiver configured to receive and process first signals from a first system; a second receiver configured to receive and process second signals from a second 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 using at least one of the first signals or at least one of the second signals , or at least one of the first signals and at least one of the second signals, 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.