Interchangeable Integration of GPS and GLONASS by Using a Common System Clock in PPP

Combining GPS and GLONASS in PPP (Precise Point Positioning) has several advantages compared to using GPS only. The increased number of satellites strengthens the geometry, improves convergence time, enhances vertical position accuracy (in particular at high latitudes), and increases the solution availability. The latter is important in situations when part of the sky is blocked by obstructions in which case signals from several satellites are not received. This can for instance happen near structures, in urban areas, under tree foliage, during ionospheric scintillations or close to offshore oil installations. In such situations adding the GLONASS constellation to the GPS constellation significantly increases the chances of being able to calculate a reliable position (Melgard et al 2009). The common way of achieving a combined GPS and GLONASS PPP user solution, whether in real-time or post-mission, is to solve for two separate receiver clocks, i.e. one for GPS and one for GLONASS. This means that a minimum of five satellites is needed to calculate a threedimensional position and solving for the two clocks. It may be argued that this is acceptable since the total number of satellites is almost doubled by adding GLONASS to GPS. Still the need for solving for two clocks has an impact when signal blocking reduces the number of available satellites down to the minimum. In signal blocking situations it will typically be a mix of GPS and GLONASS signals that are received. If one common system clock can be used, for both GPS and GLONASS, then it will have the same effect as always receiving one more satellite due to the additional degree of freedom. And for all users to always receive one more satellite on a global scale, four to six additional satellites in orbit are needed, depending on what cut-off elevation is assumed for this hypothesis. From this perspective the