Navigation employing Portable GPS and inertial sensors is a synergistic relationship. The integration of these two types of sensors not only overcomes performance issues found in each individual sensor, but also produces a system whose performance exceeds that of the individual sensors. GPS provides bounded accuracy, while inertial system accuracy degrades with time. Not only does the GPS sensor bound the navigation errors, but the GPS sensor calibrates the inertial sensor. In navigation systems, GPS receiver performance issues include susceptibility to interference from external sources, time to first fix (i.e., first position solution), interruption of the satellite signal due to blockage, integrity, and signal reacquisition capability. The issues related to inertial sensors are their poor long-term accuracy without calibration and cost. One primary concern with using GPS as a stand-alone source for navigation is signal interruption. Signal interruption can be caused by shading of the GPS antenna by terrain or manmade structures (e.g., buildings, vehicle structure, and tunnels) or by interference from an external source.
Each vertical line in this figure indicates a period of shading while driving in an urban environment. The periods of shading (i.e., less than three-satellite availability) are caused by buildings and are denoted by the black lines in the lower portion. When only three usable satellite signals are available, most receivers revert to a two dimensional navigation mode by utilizing either the last known height or a height obtained from an external source. If the number of usable satellites is less than three, some receivers have the option of not producing a solution or extrapolating the last position and velocity solution forward in what is called dead-reckoning (Wireless GPS Tracker ) navigation. Inertial navigation systems (INSs) can be used as a flywheel to provide navigation during shading outages.
The OS performance in terms of accuracy and availability will be competitive with existing GNSS systems and further planned evolutions; nonetheless, the service will be interoperable with those systems, in order to ease the provision of combined services. Most receivers will use both GALILEO and GPS signals: this will offer users seamless service performance in urban areas. The OS signals are separated in frequency to allow the correction of errors induced by ionosphere by differentiating the ranging measurements made at each frequency (GPS Personal Tracker ). The ionosphere correction at the receiver is based on a simple model in the single frequency case. Each navigation frequency will include two ranging code signals (in-phase and quadrature). Data are added to one of the ranging codes, whereas the other “pilot” ranging code is data-less for more precise and robust navigation measurements. Often, these applications are local in nature and require a service within a limited area of coverage. These high-performance requirements will be met by generating additional local signals (local component) to augment the satellite ones to provide additional performance in terms of accuracy, availability, continuity, and integrity. The GALILEO local component is part of the overall system definition and consists of GALILEO local elements. Both GOC and external service providers will deploy local elements, offering services to a large variety of users.
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