A simulation model is proposed to represent the channel of the Global Positioning System (GPS). Initially, a digital elevation model, building databases, and a vegetation model are processed to generate azimuth-elevation maps of path states (clear, shadowed, and blocked) for a large number of observers. At each simulation step, satellite positions are updated, and azimuths and elevations of paths from observers to satellites are calculated. Signal strengths and range errors are assigned to paths with the aid of random number generators for each path state. This information is processed to determine the statistics of the channel. It will be shown that model predictions are able to display good agreement with the results from an experimental campaign carried out for validation purposes. The simulation model will then be applied to a large number of observers deployed along two routes in densely urbanized areas in the city of Rio de Janeiro, Brazil (22.8° S, 43.3° W), with buildings displaying different height distributions to quantitatively show how the most probable number of available satellites, the probability that four or more satellites are simultaneously available (that is, that positions can be fixed), and the position errors change with the average building height and for vehicles in static and kinematic modes. In combination with the comparison between measurement and prediction results, this indicates that the simulation model is an efficient and flexible tool for studying and planning satellite-based location and navigation applications with accuracy and sensitivity, which will be used in future developments of mitigation techniques of multipath effects, leading to improved performance of intelligent transportation systems.

Ionospheric scintillation effects encountered in the equatorial anomaly crest, polar cap, and auroral regions have been contrasted to provide information for the design and evaluation of the performance of multifrequency satellite communication links in these regions. The equatorial anomaly region is identified as the most disturbed irregularity environment where the amplitude and phase structures of 250 MHz andL-band scintillations are primarily dictated by the strength of scattering rather than ionospheric motion. In the anomaly region, the spectra of intense amplitude scintillations at these frequencies are characterized by uniform power spectral density from the lowest frequency (10 MHz) to 4 Hz at 257 MHz and to 1 Hz atL-band (1541 MHz) and steep rolloff at higher fluctuation frequencies with power law indexes of -5 to -7. Such structures are compatible with intensity decorrelation times of 0.1 and 0.3 s at 257 and 1541 MHz, respectively. The phase spectra at 244 MHz are described by power law variation of psd with frequency with typical spectral indexes of -2.4. The strong scattering at VHF induces extreme phase rates of 200° in 0.1 s. The 90th percentile values of rms phase deviation at 244 MHz with 100 s detrend are found to be 16 rad in the early evening hours, whereas amplitude scintillation can cover the entire dynamic range of 30 dB not only in the 250 MHz band but atL-band as well. In the polar cap, the 50th and 90th percentile values of rms phase deviation at 250 MHz for 82 s detrend are 3 and 12 rad, respectively, with comparable values being obtained in the auroral oval. The corresponding values for the S4index of scintillation are 0.5 and 0.8 in the polar cap, which are slightly higher than those recorded in the auroral oval. The power law index of phase scintillation at high latitudes is in the vicinity of -2.3, which is not a result of very strong turbulence as in the equatorial region but is considered to be a consequence of shallow irregularity spectral indexes. The phase rates at auroral locations are an order of magnitude smaller than in the equatorial region and attain values of 100° in 0.5 s. The extreme variability of ionospheric motion in the auroral oval sensitively controls the structure of scintillations. The long-term morphology (period 1979-1984) of intensity scintillations at 250 MHz in the polar cap shows that, in addition to the absence of diurnal variation of scintillations, and the presence of an annual variation with a pronounced minimum during local summer, there exists a marked solar control of scintillation activity such that it abruptly decreases when the solar activity falls below a threshold level.