GPS, or Global Positioning System, is a key technology for both the civilian and scientific research markets. GPS provides users with precise geodetic positioning and time determination. Most GPS receivers require specialized hardware and complex software to operate. One receiver, known as GPSBuilder-2 originally designed by GEC Plessy, takes a novel approach. This receiver uses a minimal amount of hardware, thus its operation relies extensively upon software. The software runs on a Pentium class machine under DOS and interfaces an ISA card. In an effort to modernize this receiver, we migrated the code to RTLinux. The major aspects of the migrated code are an Interrupt Service Routine, multiple threads, and TCP connectivity. Following the migration we noticed that not only did receiver stability increase dramatically, but also errors in positioning decreased significantly. To provide insight into code migration from DOS to RTLinux, we will focus on the specific difficulties, discoveries, and enlightenments encountered.

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