1. A method for a portable device having a Global Navigational Satellite System (GNSS) receiver, comprising: when a dead zone is entered, determining whether a dead zone database has a record for the dead zone; if a dead zone database record exists for the dead zone: retrieving polynomial coefficients corresponding to the dead zone from the dead zone database; calculating one or more synthetic position solutions using the polynomial coefficients while in the dead zone; and using at least one of the one or more synthetic position solutions to calculate one or more position solutions when exiting the dead zone, wherein the dead zone comprises an area where either no or insufficient GNSS satellite signals are received by the portable device.