Cycle Slip Detection and Correction for Precise Point Positioning

Over the last years precise point positioning (PPP) has been receiving more attention as it provides a cheaper way to achieve decimetre and centimetre precision levels when compared to the typical differential techniques. While differential techniques rely on two or more receivers to differentiate their observations and cancel out most errors, PPP only uses undifferenced observations from a single receiver, leading to a more complex and difficult error modelling. One of the major sources of error in the observations, besides the atmospheric and relativistic effects, are the satellite ephemerides and clocks broadcast by the Global Positioning Service (GPS) system. This, however, can be corrected by using precise ephemerides and clocks made available by International GNSS Service (IGS). PPP then relies not only on precise products but also on the millimetre precise but ambiguous carrier phase measurements. Despite the presence of an ambiguity term, carrier phase measurements are also susceptible to other effects such as cycle slips. Cycle slips are a discontinuity in carrier phase that can cause a momentary loss of lock. Detection and correction of cycle slips will allow the estimation process to continue outputting precise solutions, without having to be reinitialised. It is then important to look out for these discontinuities in the carrier phase measurements before using them to acquire a solution, something that can be done through several methods that provide with a certain confidence level a detection mechanism for such discontinuities. Using inertial measurement unit (IMU) can also help understand in some cases if a cycle slip occurred or not, by knowing how much the carrier phase measurements should have changed due to the receiver’s dynamics. This work aims to discuss the contribution of IGS’s precise products in the increase of performance of the GPS system. Besides that it provides a solution to detect and correct cycle slips using predicted Doppler measurements, to which IMUs are essential in order to estimate the receiver’s velocity and displacement in between GPS measurements. This information is also used in order to provide a data regeneration mechanism to minimise the impact of constellation changes throughout the observation period.