Barometric Assistance Service for Assisted GNSS Receivers
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In the age of information the ability to navigate persons and equipment has become increasingly important. A rising number of applications and services depend on the precise positioning that is provided by global satellite positioning systems such as the GPS. However, most people using satellite-based positioning services are living in the most challenging surroundings for the satellite positioning systems - densely populated cities. Fundamentally, satellite navigation is based on distance measurements from the receiver to satellite vehicles in orbit of the Earth. The receiver determines its location - latitude, longitude, and elevation - and the system time using the satellite positioning system. The determined location is only an estimate: residual errors induce inaccuracies to the determination process. At worst, the receiver may not be able to determine its position if not enough signals could be acquired. The performance of the receiver could be greatly improved if one or more of the geographic coordinates or the precise time could be obtained from another source with smaller error. One such source is Earth's atmospheric pressure which is relative to the altitude and from which a receiver can deduce its altitude if a reference pressure level is known. To address the problem of unavailability and inaccuracy of positioning in urban environment, a barometric assistance service was designed and a respective software application was implemented. The implemented assistance service generates continuously time and location-dependent reference pressure data from high resolution weather forecasts that are calculated by the Finnish Meteorological Institute. Receivers with the barometric sensor can download the assistance data from the service and utilize it to determine the current barometric altitude consistently and more accurately than a conventional receiver. Barometric altitude measurements improve the availability of the positioning service and reduce the time required for the first position estimate by decreasing the number of required satellite measurements.