Studying tropospheric turbulence with GNSS

Long-term variations of the tropospheric refractive index delay the carrier phase measurements from Global Navigation Satellite System (GNSS). This information is now operationally integrated in Weather prediction models. Random fluctuations of the refractive index correlate the phase measurements and induces non-stationary noise processes. The correlation structure and spectral properties of observation residuals from GNSS relative positioning provide a unique opportunity to study specific properties of the turbulent atmosphere. In this contribution, we will give a short overview on turbulent processes and their impact on GNSS carrier phase measurements. We will discuss our data analysis concepts to separate the tropospheric fluctuations from other temporally varying error sources such as GNSS receiver clock errors or multipath. The analysis is based on the power spectrum of single or double differences of carrier phase measurements. This approach enables a determination of the cut-off frequencies of the atmospheric noise and the associated power law processes with their typical slopes. The obtained values are compared with theoretical expectations. We will show results for GPS from the Seewinkel network (Austria), as well as from a small network at Physikalisch-Technische Bundesanstalt (PTB, Germany) where all receivers are connected to a common highly stable atomic clock. We show that (i) a two slopes power spectrum can be reliably determined and (ii) that the outer scale length can be taken to a constant value, close to the physically expected one and in relation with the size of the eddies at tropospheric height. The study of their dependencies with the satellite geometry, the Day of the Year (DOY) or the time of the day provides a new insight on the two- and three-dimensional atmospheric turbulence in the free atmosphere.