GNSS processing software

All software and techniques, mentioned in this section relate to processing GNSS data for positioning and tropospheric parameters. The bespoke software, developed for reflectom-etry is discussed in section 5.1.

There are two approaches to processing GNSS observables: Precise Point Positioning (PPP) and Differential Processing (DGNSS). The DGNSS technique is the older approach

3.3 GNSS processing software 33 of the two. DGNSS relies on the availability of many GNSS stations, which are processed together. From the direct pseudorange and carrier-phase observations each station is producing, position differences between the stations (usually referred to as baseline coor-dinates) are estimated (Hatch, 1989). The pseudorange differences are used in standard DGNSS processing, while the more accurate carrier-phase differential observations enable Real-Time Kinematic solutions (RTK). Satellite parameters, such as frequency devia-tions, clock offsets, orbits can be calculated with higher precision when large networks of stations with long baselines are processed together in differential mode (Teunissen and Montenbruck, 2017).

A recent development in GNSS processing is use of the PPP strategy (Zumberge et al., 1997). In contrast to DGNSS, PPP uses data only from the station of interest, as well as GNSS satellite orbits and clocks, products of DGNSS processing. PPP is preferable for individual stations, or small dense network, since it uses preprocessed clocks. Since 2013, the International GNSS Service (IGS, Dow et al. (2007); Caissy et al. (2012)) provides ultra-fast or real-time precise satellite orbit and clock corrections in support of PPP processing (Douša and Vaclavovic, 2014;Li et al., 2015b; Yuan et al., 2014;Ahmed et al., 2016). The PPP strategy has the advantage of being computationally much more efficient than DGNSS and hence can provide estimates for large networks of stations with high temporal resolution (every 5 min). This task can be achieved by the conventional DGNSS strategies only by using superior IT infrastructure.

3.3.1 NAPEOS

NAPEOS (NAvigation Package for Earth Orbiting Satellites, http://www.esa.int/Ou r_Activities/Operations/NAPEOS) is developed by the European Space Agency (ESA) for the processing of GNSS data. NAPEOS is developed and maintained by the European Space Operations Centre (ESOC) of the European Space Agency (ESA). NAPEOS is used at ESOC since January 2008. NAPEOS has several key features:

• Multi-GNSS processing, incorporating GPS, GLONASS and the European GALILEO,

• Can be used both for network solution undifferenced processing, as well as for PPP,

• User-friendly interface,

• The license is free of charge for institutions in ESA member countries.

The NAPEOS software requires stations coordinates and dynamic parameters (station speed, ocean loading displacement) in order to process the GNSS data. The processing sequence of NAPEOS in PPP mode is shown in the scheme on figure 3.5:

The input files for processing in PPP mode are:

• Uncompressed RINEX files

RINEXf ile

↓↓P recise coordinates T ropospheric products

Figure 3.5: NAPEOS processing scheme for PPP processing, used in this thesis in section 4.3. The inputs to the software are the RINEX files, together with orbit and clock files.

In the intermediate steps of the processing files are generated with synchronized orbits (.rcb), clocks (.tcb) ambiguities fixing (.fix), normal equations (.neq, .fneq) as well as catalogue files for internal use.

• Orbit files (.sp3), obtained from IGS

• Clock files (.clk), obtained from IGS

• Dynamic parameters (station speed, ocean loading displacement)

The NAPEOS version 3.3.1 is used for the processing in this study. The processing is performed using the GMF and 10^◦ elevation cut-off angle. The data is processed using the PPP strategy with fixed ambiguities and employing IGS satellite orbits and clocks.

The computed ZTDs are with a temporal resolution of 300s (5min).

3.3.2 EPOS

Earth Parameter and Orbit System (EPOS) is a GNSS processing software, developed in GFZ. The development of the software was started in the 1990’s by the IGS group in GFZ

3.3 GNSS processing software 35 (Gendt et al., 2004). The software has since been developed into several generations, the latest being EPOS8. As a first step in the processing (the "Base cluster analysis" in figure 3.6), EPOS employs a network solution least squares adjusted undifferenced strategy for calculating precise satellite orbits and clocks. In the second step the processed network or stations are divided into clusters and handled in PPP. The ZTD’s and gradients can be produced with very high temporal resolution. The clustering and the PPP strategy allow for processing of vast networks, since every station is handled independently.

IGShourly RINEX

The Bernese GNSS Software (BSW) is developed in the Astronomical Institute of the University of Bern (AIUB), Switzerland. Bernese is one of the most widely used GNSS processing software packages in the world, especially in Europe. The software operates using a double differencing network solution processing approach, where multiple ground-based GNSS stations with long baselines are necessary for accurate positioning solutions.

The software can also use a PPP strategy. Bernese in its current version 5.2 supports fully both GPS and GLONASS. The analysis of dual–frequency data for the upcoming new systems, like European Galileo, Chinese BeiDou, or Japanese Quasi-Zenith Satellite System (QZSS), is prepared but not yet fully developed for an operational processing (Dach and Walser, 2015).

Bernese is a software with wide range of scientific applications, including positioning, monitoring of earth crust movements, estimation of tropospheric delays and gradients and ionospheric effects for both GNSS and Satellite Laser Ranging (SLR) applications. The software can also be used for correction of receiver and antenna biases. Both GMF and VMF can be used for the estimation of the tropospheric delays.

Figure 3.7: Evolution of the Bernese GNSS processing software.