This NAVSTAR User Segment's communications architecture consists of SV-to-user direct broadcast on fixed and known frequencies. Communications is unidirectional; there is no direct feedback from User Segment stations to other NAVSTAR components. The existing GPS signals, frequencies, and data formats form a standard, workable protocol for the NAVSTAR User Segment. While minor changes might (if technically feasible) be made to improve accuracy, any such modifications would need to be in the form of a backwards-compatible extensionto the existing GPS signals and data formats due to the large installed base of existing GPS receivers.
The existing hardware/software architecture of the NAVSTAR User Segment also cannot be feasibly redesigned at this point. There is already a large (multiple thousand unit) installed base of GPS receivers worldwide. The computing hardware/software used in the GPS User Segment is heterogeneous; it in fact forms an open environment based on published GPS standards. Indeed, the exact hardware/software used in the GPS user segment is immaterial except for radio portion (it must receive the established GPS signals at the standard frequencies and modulations).