Extensible GPS receiver system

A positioning system, comprising a computer processor operable under the control of a real time operating system (RTOS) to perform (i) position signal processing in accordance with first computer-readable instructions embodied as a GPS process stored on a computer-readable medium communicatively coupled to the computer processor, and (ii) user application processing in accordance with second computer-readable instructions embodied as a user application process stored on the computer-readable medium, wherein the user application process has a lower priority than the GPS process for execution by the RTOS and access the RTOS through a firewall stored on the computer-readable medium and configured to prevent the user application process from interfering with data utilized by the GPS process.