Visibility of Satellites and Their Geometry for Different Numbers of Satellites of Global Navigation Systems

Abstract

At the time of this writing (January 2017) two global satellite navigation systems (SNS), GPS and GLONASS, are fully operational, two next, Galileo and BeiDou, are under construction. The number of operational satellites in nominal constellation of these four SNSs is equal 31, 24, 24 and 27 (three MEO orbits only), respectively. As the user’s position accuracy is a function of DOP (Dilution of Precision) coefficient and UERE (User Equivalent Range Error) the decreasing of the number of satellites visible by the user at different latitudes and the increasing of DOP coefficient in the case of one or two satellites out of service are analysed for each SNS in this paper. The additional analysis were made for GPS system and its UERE because this error depends among other things on the number of civil frequencies transmitted by the satellites; because some GPS satellites transmit two civil frequencies, some only one. The knowledge of the consequences of less than nominal number of satellites is for each SNS necessary for the users in all modes of transport in restricted area in particular.