Aspecto ético

The most commonly used GNSS is GPS. Its design started in the late 1960s and reached Full Operational Capability (FOC) in 1995 (Kaplan and Hegarty, 2006). The satellites belonging to the Blocks II-A and IIR broadcast the Coarse/Acquisition (C/A) code at the L1 (1575.42 MHz) frequency and encrypted Precision (P) code (P(Y) code) at the L1 (1575.42 MHz) and L2 (1227.6 MHz) frequencies (GPS, 2014). The P code signals are designed for the U.S. military authorized use only and L1 C/A signal is available for everyone.

The P(Y) signals are broadcast in two different frequencies to enable the elimination of first order ionospheric errors (Grimes, 2007). Methods to eliminate ionospheric delays are

19 discussed in more detail in Section 3.4.4. In the case of the original GPS constellation, the C/A civilian signal is broadcast only on one frequency. This is an issue for civil users who want to eliminate the second order ionospheric delays. However, modern geodetic quality receivers can receive also the P code signals using semi-codeless techniques (GPS, 2014).

Enabling civilians to receive GPS signals on multiple frequencies is one reason why the USA started to modernise GPS (GPS, 2013b). Additional motivations are providing signals with improved noise and multipath properties, a signal which is suitable for safety-of-life applications and signals with better jamming resistance for military use (GPS, 2013b).

The first step in the modernisation programme was the addition of the M-code military and L2C civilian signals starting with the GPS Block-IIR(M) satellites (GPS, 2013b). The L2C signal (GPS, 2010a) is broadcast on the L2 (1227.6 MHz) frequency and has improved signal properties compared to the L1 C/A signal (GPS, 2013b). The GPS L5 signal (1176.45 MHz) is broadcast on Block II-F satellites (GPS, 2010b). The GPS L5 signal is primarily designed for safety-of-life and other high performance applications (GPS, 2013b).

In the future, the USA will launch Block-III satellites, which will broadcast the GPS L1C signal (GPS, 2010c). The L1C signal is primarily designed for consumer and other civilian applications and Multiplexed Binary Offset Carrier (MBOC) modulation enables improved signal receiving in urban canyon-type environments (GPS, 2013b).

The originally designed number of satellites in the GPS constellation was 24 (Kaplan and Hegarty, 2006). It was selected to provide sufficient availability with sufficiently high Dilution Of Precision (DOP). Nowadays, the number of available satellites is even larger, for example, there were 31 available satellite in August 2013 (IAC, 2013). The number of available satellites has increased mainly because the satellites have lasted longer than their designed operational life.

3.1.2 GLONASS

GLONASS is a satellite navigation system developed by the Soviet Union at approximately the same time as GPS (Chebotarev, 2007). GLONASS first achieved FOC in 1995 (IAC, 2014).

However, the number of satellites in the constellation started to decrease, because of

20 funding problems, and there were only six to eight satellites in 2001 (Chebotarev, 2007).

Thereafter, Russia managed to restore the constellation and there were 24 operational satellites restoring FOC in May 2013 (IAC, 2013). The originally designed total number of satellites in the GLONASS system is 24, which is selected based on the need for global coverage (IAC, 2014).

The main difference between GPS and GLONASS is their signal structure: GLONASS uses Frequency Division Multiple Access (FDMA) while GPS uses Code Division Multiple Access (CDMA) signals (IAC, 2013). When using CDMA, signals from all satellites are transmitted on the same frequency, but using different Pseudo Random Noise (PRN) codes (IAC, 2013). On the other hand, when using FDMA, satellites on the same side of the Earth broadcast signals on slightly different frequencies, but using the same PRN code (IAC, 2013).

Using FDMA compared to CDMA makes receiver implementation more complex, because receivers must handle multiple frequencies (Kaplan and Hegarty, 2006). In addition, using FDMA causes frequency dependent biases to signals. On the other hand, FDMA signals are less vulnerable for narrow-band interference than CDMA signals and there is no cross-correlation between signals transmitted from different satellites as in the case of CDMA signals (Kaplan and Hegarty, 2006).

The civilian C/A code signals is transmitted on the L1 frequency and military P-code signals are transmitted on the L1 and L2 frequencies by the GLONASS block one and two satellites, which were launched before 2003. In 2003, Russia started to launch GLONASS-M satellites, which also broadcast civilian C/A signal at the L2 frequency. In addition, the lifetime of the GLONASS-M satellites has increased to seven years, which is at least two times longer than the previous generation of GLONASS satellites.

Russia is adding CDMA signals to new GLONASS satellites primarily to improve interoperability with GPS and other GNSS systems (Revnivykh, 2012). It is easier to design receivers when both GPS and GLONASS use CDMA signals. Russia has added the L3OC test CDMA signal at the L3 (1202.025 MHz) frequency to the GLONASS-K1 satellites, launched starting in 2011 (Revnivykh, 2012). It is planned that Russia will add the L1OC, L2OC and L3OC civilian signals and L1SC and L2SC military signals to GLONASS-K2 satellites, which be

21 launched starting in 2014 (Revnivykh, 2012). In addition, there is a longer term future plan to add L1OCM and L5OCM civilian signals (Revnivykh, 2012).

3.1.3 Galileo

The European Space Agency (ESA) is developing the Galileo satellite navigation system (ESA, 2010). The primary motivation for its development is to make European GNSS users independent from the GPS and GLONASS systems (EU, 2013). Additional motivations are to improve positioning accuracy and availability through better signal design and increased number of satellites, as well as to provide business opportunities related to GNSS for the European private sector (EU, 2013).

There are four different service levels in the Galileo system: open service, safety-of-life service, public regulated service and commercial service (GSA, 2013). The open service is freely available to anyone. The safety-of-life service provides an integrity function, which will alarm users if the performance of the system does not meet the integrity requirements.

Thus, the safety-of-life service is suitable for safety critical applications such as aviation. The public regulated service is an encrypted service for government authorised users. The commercial service is an encrypted service for authorised commercial users.

The Galileo E1 signal at the L1 frequency and E5a, E5b and E5 signals at the L5 frequency can be used for the open service (ESA, 2010). The safety-of-life service uses the E1 and E5b signals (ESA, 2010). The E6 signal is used for the commercial service (ESA, 2010). The E6 service is at the frequency band between 1260 and 1300 MHz. The public regulated service uses the E1 and E6 signals (Palestini, 2014). All Galileo signals use CDMA modulation, which make it easily interoperable with GPS.

It was originally planned that Galileo would achieve FOC be fully in 2008 (DGMOVE, 2007).

However, the development of Galileo has been severely delayed and it is currently estimated that 18 Galileo satellites should be available in 2015 (initial operational capability) and the full constellation of 30 satellites should be available in 2020 realising FOC (ESA, 2013). In August 2013, the Galileo systems is still in the In-Orbit Validation (IOV) phase and there are only four satellites available (ESA, 2013). Therefore, tests on the performance of the Galileo using real GNSS are very limited.

22 3.1.4 BeiDou

BeiDou is a satellite navigation system developed by the People’s Republic of China (BeiDou, 2011). Currently, it is providing regional navigation services in China and the surrounding area. There are currently 14 operational satellites: five Geostationary Earth Orbit (GEO), five Inclined Geo-Synchronous Orbit (IGSO) and four Medium Earth Orbiting (MEO) satellites.

The current Phase-II BeiDou satellites broadcast civilian signals at the B1 (1561.098 MHz) and B2 (1207.14 MHz) frequencies (Gibbons, 2013). In addition, authorized (military) signals are broadcast at the B1, B2 and B3 (1268.52 MHz) frequencies (Gibbons, 2013). All of the BeiDou signals use the CDMA modulation. The full BeiDou Space Interface Control Document for the B1 civilian signal was published in December 2012.

The Chinese government is currently (in 2013) evaluating the performance of the current BeiDou constellation and it is not currently launching more satellites (Gibbons, 2013). It is planned that there will be five GEO, 27 MEO and three IGSO satellites in the full BeiDou constellation, which will provide global coverage (BeiDou, 2012b). The reason for using GEO satellites in the BeiDou constellation is that they can provide higher availability in China and the surrounding area and provide full BeiDou operability there, even though the full constellation of BeiDou is not ready yet. It is planned that the full constellation of BeiDou satellites will be ready in 2020 (BeiDou, 2013).