RELATIONSHIPS BETWEEN GPS-SIGNAL PROPAGATION ERRORS AND EISCAT OBSERVATIONS

When travelling through the ionosphere the signals of space-based radi o navigation systems such as the Global Positioning System (GPS) are s ubject to modifications in amplitude, phase and polarization. In parti cular, phase changes due to refraction lead to propagation errors of u p to 50 m for single-frequency GPS users. If both the L1 and the L2 fr equencies transmitted by the GPS satellites are measured, first-order range error contributions of the ionosphere can be determined and remo ved by difference methods. The ionospheric contribution is proportiona l to the total electron content (TEC) along the ray path between satel lite and receiver. Using about ten European GPS receiving stations of the International GPS Service for Geodynamics (IGS), the TEC over Euro pe is estimated within the geographic ranges -20 degrees less than or equal to lambda less than or equal to 40 degrees E and 32.5 degrees le ss than or equal to phi less than or equal to 70 degrees N in longitud e and latitude, respectively. The derived TEC maps over Europe contrib ute to the study of horizontal coupling and transport processes during significant ionospheric events. Due to their comprehensive informatio n about the high-latitude ionosphere, EISCAT observations may help to study the influence of ionospheric phenomena upon propagation errors i n GPS navigation systems. Since there are still some accuracy limiting problems to be solved in TEC determination using GPS, data comparison of TEC with vertical electron density profiles derived from EISCAT ob servations is valuable to enhance the accuracy of propagation-error es timations. This is evident both for absolute TEC calibration as well a s for the conversion of ray-path-related observations to vertical TEC. The combination of EISCAT data and GPS-derived TEC data enables a bet ter understanding of large-scale ionospheric processes.