The GPS Toolkit
Another GPSTk application at ARL:UT involves a satellite in low-earth orbit that carries a GPS receiver. This satellite collects GPS data both for the satellites above it, referred to as top-side data, and those visible below it, or bottom-side data. The GPS signal for the bottom-side data has a long path length through the atmosphere, which is ideal for remote sensing of the atmosphere. The top-side data is used for computing the LEO satellite's rapidly changing position as it orbits Earth. A problem arises here as the top-side data is collected with a lower data rate (10 seconds) than on the bottom-side (1 second), yet the position of the LEO satellite is needed for processing the bottom-side data at the higher data rate. To solve the problem, a program was written, using only GPSTk, which reads the GPS data, computes the LEO position with the top-side data and then interpolates those positions to 1-second epochs. The result is shown in Figure 6, a plot of the position of the LEO satellite as it orbits Earth.
Open-source GPS processing, on the scale anticipated for the GPS Toolkit, is unprecedented; we are excited by the prospect of what could develop. GPSTk potentially has a broad range of audiences. Universities can use the GPSTk to process GPS data with open-source code. Embedded developers can develop software to perform GPS positioning and to read, write and edit RINEX data files. Researchers may find that this code is an excellent foundation for GPS receivers implemented entirely in software, called software receivers.
Although the growth of the GPSTk will depend strongly on user feedback and participation, changes also will be driven by shifts in the satellite navigation arena. In the near term, the first satellite to provide dual-frequency pseudoranges to civilians is scheduled for launch in 2005. Furthermore, the European community is creating Galileo, which will provide a public-regulated service compatible with GPS, essentially augmenting the current constellation with a new one. In the long term, GPS will have new signals in the L5 and M code. GPSTk, with its emphasis on fundamental observations, can provide the basis to explore and exploit these changes.
It is our hope that university students, laboratory researchers, system engineers and software developers will contribute to, as well as benefit from, the GPS Toolkit. We already have seen many benefits to using this code within our lab and believe that the GPSTk will inspire a number of innovative GPS applications.
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Dr Brian W. Tolman is a research scientist at Applied Research Laboratories, The University of Texas, with 18 years experience in GPS-related research, data analysis and software development. He holds a PhD in theoretical physics from The University of Texas at Austin.
Ben Harris is an Engineering Scientist at UT Austin. When he is not researching GPS, studying for a PhD or spending time with his beautiful family, he reprograms animatronic fishes to perform scenes from Pulp Fiction.