Precise Point Positioning: Implementation of the constrained clock model and analysis of its effects in T/F transfer

In recent years, many national timing laboratories have collocated geodetic Global Positioning System receivers together with their traditional GPS/GLONASS Common View receivers and Two Way Satellite Time and Frequency Transfer equipment. Many of these geodetic receivers operate continuously within the International GNSS Service (IGS), and their data are regularly processed by IGS Analysis Centers. From its global network of over 350 stations and its Analysis Centers, the IGS generates precise combined GPS ephemerides and station and satellite clock time series referred to the IGS Time Scale. A processing method called Precise Point Positioning (PPP) is in use in the geodetic community allowing precise recovery of GPS antenna position, clock phase and atmospheric delays by taking advantage of these IGS precise products. Previous assessments, carried out at INRIM with a PPP implementation developed at NRCan, showed PPP clock solutions have better stability over short/medium term than GPS CV and GPS P3 methods and significantly reduce the day boundary discontinuities when used in multi-day continuous processing, allowing time-limited, campaign-style time-transfer experiments. This paper reports on follow-on work performed at INRIM and NRCan, to further investigate the effects of applying statistical constraints on station clock parameter in the PPP estimation process, specifically its impact on clock solution short term noise.