**
Phase Ambiguity Convergence Analysis
for GNSS Precise Point Positioning (PPP) **

By Simon Banville, Dr. Rock Santerre & Dr. Marc Cocard (Laval University, Quebec)

**Actually,
precise positioning with GNSS systems (GPS and Glonass) is mostly achieved
through relative modes, which means that at least two receivers are required in
order to cancel common errors at both observation sites. Recent improvements in
satellite orbits and clock corrections modelling have led to the development of
an alternative solution to relative positioning, namely Precise Point
Positioning (PPP). This technique proposes to use a single receiver and to model
the errors degrading the positioning accuracy, simplifying the logistics and
costs of GNSS surveys.**

**The main
drawbacks of this method are that carrier phase ambiguities are not integers due
to a non-zero initial phase offset for each satellite-receiver pair and that the
residual errors could often exceed the threshold required to usually fix the
phase ambiguities to integers. Thus, traditional ambiguity resolution techniques
do not apply and one must wait several minutes for the phase ambiguities to
converge to obtain centimetre level accuracy. Some efforts have been made in the
last few years in order to reduce this convergence time, but instantaneous
centimetre accuracy is still a goal to reach.**

**This
research focuses on three important aspects that could help in reducing the
convergence time. The first one concerns the modelling of the receiver clock
error, which is a nuisance parameter for positioning purposes. The second part
of this research implies a study of the decorrelation techniques that can be
applied to a constant non-integer ambiguity solution, which would improve the
search speed and reliability of the estimation. Finally, the impacts of GNSS
modernization (third frequency for GPS and Galileo) will be assessed from a
geometrical point of view and by analyzing the different signal combinations.**