Several hardware techniques have been developed in the civilian community to receive L2 range signals despite them being encrypted. This error is essentially a function of the weight of the atmosphere above the user. It is therefore easily modeled using the atmospheric pressure. The total value of this error looking straight up is 2.3 m. Therefore it is usually not a factor in position accuracy.
Receivers can model it to a few centimeters knowing only the altitude.
In order to navigate, the user needs to know the origin of the ranges he is measuring, that is the satellite position. The satellites broadcast this. However this information is just read out of a memory on the satellite. It is usually loaded in once a day from the computation center in Colorado Springs. Therefore this will be a prediction of where the satellite will be .