Post Processing Service

Transcription

1 Post Processing Service The delay of propagation of the signal due to the ionosphere is the main source of generation of positioning errors. This problem can be bypassed using a dual-frequency receivers but this raise considerably the cost of GPS devices, and increase its complexity. In addition, the device would increase time used to complete the reading of position. The alternative solution that allows you to circumvent the delay of propagation without increasing the complexity of the GPS device is to use a single frequency receivers (L1) and make the correction of the data with a post-processing system. In addition, through the readings of real-time data on the GPS constellation is possible to generate a (almost) real-time ionospheric model (the delay is related to time processing of data) more precise model commonly offered by the GPS receiver. GPS System Errors The GPS system is subject to different types of errors: error in the satellite position; error in the Offset time in the satellite; error in the propagation (ionospheric and tropospheric); multipath error; error in the user clock receiver. In addition, to worsen system performance for civilians, the DoD has decided to degrade the GPS measures by introducing errors random nature, known as Selective Availability (SA); Selective Availability (SA).

2 Satellites Errors In addition to error SA, there are other errors in the space segment influencing the position accuracy. Among the most significant errors we should remember the drifting of clocks of the satellites and those is deriving from imperfect knowledge of the orbital parameters of the satellites, these errors are similar to SA but smaller (1-3 m).

3 Propagation Errors The GPS signal transmitted by satellites spreads through the atmospheric layers. In general for the GPS are considered only two layers: the ionospheric layer that extends from 70 to 100 km above the Earth and the ozone layer that extends from the surface up to 70 km. When the signal passes through the ionspheric layer suffer, as a result of refraction (both the carrier that the phase of signal), changes since the actual distance is different from the minimum between satellite and receiver. The ionospheric delay depends on the density of electrons in the ionosphere and the frequency signal. The error in distance may be considered an average of between 5 to 30 m. This error can be eliminated using a dual-frequency, and this is why receivers using double frequencies (L1, L2) reduce the ionospheric error. The experimental measures have shown that between ionospheric delay and frequency L there is the following relation: with k coefficient of proportionality which depends on the square of the density of electrons per meter along the signal path and DR. Ionospheric delay in units of length. So, if we indicate with R the real distance from the satellite receiver and Ri the pseudo distance measured on the frequency fi you have the following relations: That can be grouped: So, measured two pseudorange is possible to reduce the effect of the ionosphere on the extent of pseudo distance. The GPS receivers that use a single frequency eliminate partially the ionospheric effect through a model included in the internal software of the receiver, known as a model of Klobuchar; parameters for this model are contained in the message transmitted by satellite navigation.

4 Ionospheric Model Ionospheric delay is due to variation in the speed group of electromagnetic waves through the ionosphere. The delay caused by the variation in the speed group may be determined by the relation: As the index of refraction in L band is given by: the delay caused by the variation in the speed group is given by: In the which represents the TEC content of electrons per meter of the signal path from the satellite to the GPS receiver. The content of electrons along the path is therefore responsible for the ionospheric delay variability. The single-frequency (L1) receivers use an algorithm known as a model of Klobuchar to reduce this delay. This model of ionospheric correction is based on geographical coordinates of the receiver and high azimuthal satellite coordinates (h, Az) used to the extent of pseudorange. The algorithm uses the coefficients na and nb message navigation and provides the error in seconds produced by the variation in the speed group signal through the ionosphere. Increasing positional information with higher performance system As mentioned, one of the actual problem, talking about satellite positioning systems, is the precision of this kind of information; there s an increasing demand of precision because, especially thinking about delivery network. Again, technology become in aid to solve this problem. In the future we ll have a free wide solution, introducing new commercial satellite (GALILEO), but today is possible to reduce GPS error with many solutions. The presented solution use a post processing module to recalculate positional information given from vehicle and affected of error. This module collect a large amount of information coming from different correction systems like local ground stations (providing i.e. ionospheric correction) or EGNOS system and correct stored vehicle GPS data. This architecture obviously require an high performance system (usually expensive) but this solution use a new NVIDIA technology named CUDA. We re talking about parallel processing based on GPUs that provide, as show on picture below (from NVIDIA CUDA Programming Guide), a power-full computing solution to increase performances of the system.

5 CUDA stands for Compute Unified Device Architecture and is a new hardware and software architecture for issuing and managing computations on the GPU as a data-parallel computing device without the need of mapping them to a graphics API. The tracking solution presented use CUDA to provide a cheap but functional solution to correct positional information and provide a full tracking solution with precise positioning system, even using low cost GPS receiver (SIRF Star III). About Security Last but not least we think about security. In a very competitive business sector, all information must be kept on the inside of the company; on the contrary source information coming from dislocated areas and also users typically need to connect to the system everywhere. That s why this tracking solution communicate with higher reliable encryption algorithm AES at 256 bits key.