S.А. Yakushenko – Ph.D. (Eng.), Associate Professor, Professor of the Department, FSPEI HPE Military Telecommunications Academy (St. Petersburg)
D.V. Sal'nikov – Ph.D. (Eng.), Senior Lecturer, FSPEI HPE Military Telecommunications Academy (St. Petersburg)
I.S. Meshkov – Adjunct, FSPEI HPE Military Telecommunications Academy (St. Petersburg)
А.N. Frolov – Deputy Technical Director, JSC «SPE «Radiosvyaz» (Krasnoyarsk)
A criterion for assessing the provision of navigation-time definitions of global navigation satellite systems with a given quality is the availability of GNSS. Service Availability means GNSS readiness to provide consumers with navigational definitions with the required accuracy characteristics.
In accordance with we have: «Accuracy. The degree to which the calculated or measured value corresponds to the true value. Note. The accuracy of a position measurement is generally expressed by the distance from the declared location within which, as determined with a certain degree of probability, the true location is located».
Thus, the accuracy characteristic of the location given in the table in accordance with this definition is the radius of the sphere or
circle. The probability of a random point falling into a sphere under the condition that the errors of its linear rectangular coordinates have a normal distribution with a mathematical expectation equal to 0 and equal root-mean-square errors.
Based on the above theoretical material, the authors of this article have developed a number of programs simulating the calculation of the position of GNSS GLONASS, GPS and Galileo navigation satellites based on the initial data of the almanacs.
These programs allow you to calculate the geometry factor of the specified GNSS for any point on the Earth's surface. Taking into ac-count the budget of errors, it is possible to predict the availability of the previously mentioned GNSS.
Various GNSS consumers, in order to meet their mission objectives, put forward their requirements for accessibility only in horizontal or vertical planes or in a fully positional service availability.
Positional Service Availability is the percentage of time in any 24-hour interval, during which, with a 95% probability, the spatial position error is smaller than the threshold value for any point in the systems working area.
Let us consider the results of calculations. Simulated date is May 20, 2017, simulation time is 24 hours in 4-minute increments. The investigated area of the Earth's surface is the entire Earth, represented by geodetic coordinates; height above the geoid 50 meters; 1 degree – step of moving along the investigated area of the Earth's surface; 5 degrees – the angle of the mask. According to actual almanacs for the analyzed date, GNSS was operational: GNSS GLONASS – 24 NCA; GPS – 31 NCA; Galileo – 27 NCA.
The shape of the Earth's surface, on which the required numerical values of the radius are provided, is detected and expanded from the poles of the Earth as the radius increases.
The results of calculating the availability of GNSS terrestrial GPS positioner are slightly different in comparison with GLONASS GNSS. A distinctive feature of the shape of the Earth's surface, which provides the required numerical values of the radius when reducing them from 40 m, for GPS GNSS is its meridional narrowing to the equatorial region.
The article presents the results of the calculation of GNSS accessibility by space, analyzes the shapes of the areas of the earth's surface on which different values of the accuracy requirements for position measurement are provided.
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