GPS Precise Point Positioning as a Method to Evaluate Global TanDEM-X Digital Elevation Model

Kurzfassung

The TerraSAR-X add-on for Digital Elevation Measurements (TanDEM-X) mission of the German Space Agency (DLR) will derive a global digital elevation model (DEM) using satellite SAR interferometry. Two radar satellites (TerraSAR-X and Tandem-X) will map the earth in a resolution and accuracy, which was never possible in earlier missions: an absolute height error of 10m respectively a relative height error of 2m for 90% of the data are aimed for. One method to evaluate the accuracy is the use of GPS measurements. To get a high amount of data kinematic GPS data from all over the world will be used. To reach the required accuracy of around 0.5 m, phase data has to be evaluated, because code solutions will not provide such an accuracy. Since data has to be acquired all over the world, the procedure has to be easy and time and money economizing. This is the reason that first investigations propose the Phase Precise Point PositioninG (phase PPP) method that neither need any local reference station nor any regional reference station network. The solutions are independent of these kinds of infrastructure. As explained in this paper precise GPS orbits and clocks, e.g. delivered by the International GNSS service (IGS) are used to deliver coordinates with the required accuracy. These first investigations were concentrated in an area around Munich, Germany. They have been initiated by German Remote Sensing Data Center (DFD) of DLR. FIG Commission 5 has supported the search for kinematic-GPS-data-acquisiteurs all over the world. The evaluation of the tracks is carried through using the software GIPSY 5.0 of the Jet Propulsion Laboratory (JPL), USA. For comparison a second PPP-solution is generated using the online service of the Natural Resources of Canada named CSRS-PPP. Both results a combined thus defining the final solution. The conditions are rather difficult, since the data are acquired with velocities up to 120 km /h and the tracks show a lot of masking by shadowing trees, buildings and bridges. First results will be presented in this paper for Europe. These PPP results cover a track from Munich, Germany, to Sao Martinho, Portugal. The final average RMS is 0.48 m and the availability rate is calculated to 59 %. By this way the defined requirements are fulfilled. The tracks will be expanded all over the world. Up to now further tracks are acquired in Europe and South America. Plans for Africa, Ukraine, North America and Australia are on the way.