GNSS and selected basics of signal propagation

The term Global Navigation Satellite System, or GNSS, has recently been introduced. It incorporates all navigation systems with a space segment: the United States of Amer-ica’s Global Positioning System (GPS), the Russian Federation’s GLObal’naya NAvi-gatzionnaya Sputnikovaya Sistema (from Russian - Global Navigation Satellite System, GLONASS), the European Union - Galileo, the People’s Republic of China - BeiDou and the regional Japanese Quasi-Zenith Satellite System (QZSS) along with the Indian Regional Navigation Satellite System (IRNSS). These systems share similar technical parameters, such as L-band carrier frequencies and multiple inclined Medium Earth Or-bits (MEO) (combined with Geostationary OrOr-bits (GEO) for the regional systems), of their space segments. All of these systems provide a spectrum of capabilities for the whole GNSS constellation, which cannot be achieved by any of these systems separately (Hoffmann-Wellenhof et al., 2008).

Although this work is entitled "Derivation and analysis of atmospheric water vapour and soil moisture from ground-based GNSS stations", most of the presented results are performed using the GPS system only. Since the system was developed and available before the other competing GNSS, environmental measurements are historically clustered around GPS data. The details in the development of this system’s signals is important for the better understanding of the results and used methods in this work.

The first GPS satellite was launched in 1978. Together with the following 9 satellites they are from the first generation of GPS, commonly known as "Block I". The Block I satellites are using slightly different orbits from the following generations at inclination angle of 63^o. They are transmitting signals in the L1 (f = 1575.42 M Hz, λ ≈ 19 cm) and L2 (f = 1227.60 M Hz, λ ≈ 24.4 cm). The satellites are controlled using S-band communications and are powered by a solar array, outputting over 400W. The initial Block I constellation was transmitting L1 signals in a Coarse/Acquisition (C/A) coding,

which is freely available to the public and L2 signals in a special Precision, or P-code, which is only available to the US military (Parkinson et al., 2005).

Figure 3.1: Timeline of the introduction of GPS signals. Since the lifetime of GPS satellites is over 10 years, new codes and frequencies have gradually been implemented throughout the system.

The second generation of GPS satellites, also known as Block II saw a major upgrade over the Block I with higher power of the solar array, more precise atomic clocks and higher power of the output signals. They are launched in several upgraded versions from 1989, until 2016. During the development of the satellites new frequencies and new encodings are developed for civilian use. In 2005 the first satellite of the Block IIR-M was launched, which transmits L2C (C for civilian) signals. These signals are freely available for civilians, but are also transmitted with higher power, compared to the L2P signals. L5 (f = 1176.45M Hz, λ≈25.5cm) signals are first transmitted by a Block IIF satellite in 2009 and are designed for search and rescue, as well as for better ionospheric corrections (Hofmann-Wellenhof et al., 2012;Teunissen and Montenbruck, 2017).

As stated above, the GNSS satellites continuously broadcast microwave L-band signals towards the Earth with ground-based GNSS antennas passively capturing the incoming signal. GNSS signals can be received at any time (day and night), and at any environmen-tal condition, including through clouds and during heavy precipitation events (Gleason and Gebre-Egziabher, 2009). Every satellite has its own space vehicle number (or SVN), which is serial numbers assigned to each GPS satellite. Each satellite is recognized by the unique "pseudo-random noise" sequences (PRN’s), or Gold codes, associated with the specific position of the satellite in the constellation. Thus over time when a new satellite replaces an old one in the constellation, it has a new, unique SVN, but it inherits the PRN of the satellite it replaces.

The GNSS signals on their way through the atmosphere are affected in several different ways by the atmosphere. The higher layers of the atmosphere, between 60 and 600 km above ground contain significant amounts of ionized gases and free electrons, compared to

3.1 GNSS and selected basics of signal propagation 27 the neutral atmosphere. In total 0.1% of the mass of this layer is ionized. The GNSS sig-nals, like any electromagnetic wave, are bent when passing through the Ionosphere. The bending of the signals is frequency-dependent, so the effect of the Ionosphere on the sig-nal bending can be calculated using the difference between the GNSS sigsig-nal’s frequencies (Petrie et al., 2010). Thus GNSS signals can be used for Total Electron Content (TEC) measurements in the higher atmosphere (Arras et al., 2008). Secondly the GNSS signals are being delayed due to the changing optical density of the atmosphere with altitude (Tralli and Lichten, 1990). This delay is used for atmospheric water vapour observa-tion, a method which is described in more details in chapter 3.4. The space-based GNSS applications are used in radio occultation missions for ionospheric and tropospheric re-trievals (CHAMP (Wickert et al., 2001), FORMOSAT-3/COSMIC (Wickert et al., 2009), gravimetry missions (GRACE, GOCE, GRACE-FO (Flury and Rummel, 2005)), reflec-tometry missions (UK-DMC (Gleason, 2006), TDS-1, CYGNSS, G-TERN GEROS-ISS (Wickert et al., 2016)) for sea ice coverage (Zhu et al., 2017;Cardellach et al., 2018), wind speed retrievals and rain effects (Foti et al., 2015; Asgarimehr et al., 2018). In this work ground-based geodetic stations are exclusively used for the monitoring of atmospheric water vapour and soil moisture (Bevis et al., 1992; Guerova et al., 2016a;Georgiadou and Kleusberg, 1988).

Figure 3.2: GNSS signals and the layers of the atmosphere.

In chapter 5, which is devoted to GPS reflectometry, L1, L2C and L5 signals are used for the estimation of the soil moisture and in chapter 7 for snow height observa-tion from the reflected GPS signals. Unlike GLONASS, Galileo and BeiDou, GPS or-bits are chosen at such altitude, that the satellites repeat their position every side-real day (23h 56m 4s). This orbit period ensures that each GPS satellite rises and sets from the same direction in regards to a static GNSS receiver every day consis-tently. Thus the GPS orbits enable ground reflections from each satellite to be located in the same area continuously over long periods of time, enabling daily observa-tions. GLONASS satellite orbits provide

such continuity not on a daily basis, but every 8 days. Galileo provides orbit repeatabil-ity every 10 days and BeiDou - every 7 days. Thus creating reflections time series over the same reflection points from Galileo, GLONASS and BeiDou can be performed at worse than weekly data rate, compared to the daily rate from GPS.