2. Introduction to Satellite Navigation

Index2.1 Introduction to Global Navigation Satellite Systems . . . . . . . . . . . . . . . 9

2.1.1 Navstar GPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.1.2 GLONASS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.1.3 Galileo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.1.4 BeiDou . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.2 Availability and Applications of Multi-Frequency GNSS . . . . . . . . . . . . . 112.2.1 GNSS Frequencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.2.2 Properties of Multi-Frequency Receivers . . . . . . . . . . . . . . . . . 112.2.3 Current Availability . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.1. Introduction to Global Navigation Satellite Systems

2.1.1. Navstar GPS

The US Navstar GPS system was introduced in the 1970’s and full operational capabilities werereached by 1995. GPS has become the embodiment of satellite navigation and is the currentleader in the commercial satellite navigation market with its civil L1 coarse/acquisition (C/A)signal being the de facto standard for commercial receivers. The GPS L1 band uses a carrierfrequency of 1575.42 MHz. The L1 C/A signal is currently the only operational GPS civil signaland has been so since the first satellite generation. The system is currently in a modernizationphase with newer satellite generations transmitting new signals especially targeted at civil users.All satellites launched since 2005 have included a transmitter for the second civil signal L2C.The L2C signal is designed for commercial needs and is currently in a pre-operational statewith full 24/7 global coverage expected in 2018. The L2C uses a frequency range that waspreviously only used by military GPS signals with a carrier frequency of 1227.6 MHz.The third civil signal L5 is included in all new satellites since 2010 and is expected to bedeclared fully operational in 2024. The L5 signal is in a new frequency band and speciallydesigned for high-performance applications. Its carrier frequency is 1176.45 MHz.L1C is the fourth civil signal and will operate in the same frequency band as the legacy L1 C/Asignal. L1C transmitters will be deployed for the first time in the newest satellite generationexpected to start launching in 2017. The L1C signal will enable interoperability between GPSand other GNSS. It also uses a more modern signal design than the L1 C/A signal.

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2.1. Introduction to Global Navigation Satellite Systems

2.1.2. GLONASS

The Russian GLONASS system was developed in the 1970’s by the Soviet Union. It wascompleted in 1995 but suffered from too few satellites during the late 1990’s. The systemreturned to full global coverage at the end of 2011 resulting in a wider adoption by civilusers. The GLONASS satellites transmit two civil and two military signals in similar frequencychannels as used by the US GPS system. The GLONASS channels are named with G1 and G2in this thesis to differentiate them from the similar GPS channels L1 and L2. The G1 frequencyband starts at 1593 MHz and ends at 1610 MHz while the G2 band reaches from 1237 MHz to1254 MHz.After having restored the GLONASS system to full operational capabilities the priority forRussia has shifted to modernization. A few newer satellites have already a transmitter onboard for a new signal officially called L3OC but referenced as G3 in this thesis. A slew of newsignals is planned but none of the new signals will be fully operational until the late 2020’s.

2.1.3. Galileo

Galileo is a GNSS currently being deployed by the European Union. One of the aims ofthe program is to provide a high-precision positioning system to European nations that isindependent from the other global systems run by the US, Russia and China. In comparison tothe programs of the mentioned nations the EU Galileo program is run by a civilian organizationwithout direct military involvement. Besides freely available civil signals Galileo also plans toprovide improved commercial services to paying customers and a regulated service for membernations. The system is expected to be fully operational by 2020.The Galileo system transmits civil signals in the E1 frequency band that also includes the USL1 band with a common carrier frequency of 1575.42 MHz. The Galileo signals where designedfor interoperability with the existing GPS signals.The Galileo system also transmits in a frequency band called E6 at 1278.75 MHz but the signalsin that band are only available for commercial and governmental users.Similar to the modern GPS signal L5, Galileo also transmits signals with a carrier called E5aat 1176.45 MHz but adds another carrier at 1207.14 MHz called E5b. The E5ab band is onlyused for civilian signals and is targeted at high-precision applications.

2.1.4. BeiDou

The Chinese BeiDou Navigation Satellite System started with an experimental regional satellitenavigation system also known as BeiDou-1 that covers China and neighboring regions. BeiDou-1 is operational since 2000.The second phase of the BeiDou development consist of the first global navigation systemcalled BeiDou-2 or also known as COMPASS. BeiDou-2 is currently being deployed and isexpected to reach full global operation capabilities by 2020. The BeiDou-2 system transmitsa civil and encrypted signal at 1561.098 MHz in the Galileo E1 band and just below the GPSL1 band. Similar to the Galileo system BeiDou-2 also has an encrypted signal band called B3at 1268.52 MHz, just 10.23 MHz below the E6 band. A third BeiDou-2 signal band named B2has the same carrier frequency than the Galileo E5b signal with 1207.14 MHz.China is already deploying its third generation of BeiDou called BeiDou-3. BeiDou-3 sportsimproved signals in the same frequency bands as used by BeiDou-2. The BeiDou-3 B1 signalhas the same 1575.42 MHz carrier than the other interoperability signals L1C and E1. The B3band adds an additional encrypted signal and the B2 band adopts the same signal structurethan the Galileo E5ab signals. These BeiDou-3 signals are called B2ab and have two carriersat 1176.45 MHz and 1207.14 MHz.

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Chapter 2. Introduction to Satellite Navigation

2.2. Availability and Applications of Multi-Frequency GNSS

2.2.1. GNSS Frequencies

The individual frequency bands can be grouped into a lower frequency band that reaches from1164 MHz to 1300 MHz and a higher band that starts at 1559 MHz and ends at 1610 MHz.The frequency bands for GPS, GLONASS and Galileo are illustrated in Fig. 2.1 and the figureshows where the individual bands are situated. The BeiDou bands are not shown in the figurebut use the same lower and upper frequency ranges. The upper band with the GPS L1 C/Aand the GLONASS G1 signals is currently the most used civil band. The signals in the lowerbands are still being deployed and should be fully operational by 2020.

Figure 2.1.: GPS, GLONASS and Galileo navigation frequency bands [5].

2.2.2. Properties of Multi-Frequency Receivers

The use of multiple frequencies is the most effective way to correct the ionospheric error whichis frequency dependent and can therefore be derived from two signals with different frequencies.Also the modernized wideband signals in the L5/E5ab band provide better multipath mitigationand lower noise levels. All this leads to much improved positional accuracy. Better immunityto interference is also given with a multi-frequency receiver since even if one signal is notavailable the receiver is still able to track the satellites on alternative frequencies. Multi-frequency receivers also allow quick initialization and recovery of RTK tracking.

2.2.3. Current Availability

GLONASS is currently the only system that provides operational signals with uninterruptedglobal coverage on two frequencies. For GPS currently only the L1 C/A signal is unencryptedand provides uninterrupted positioning services to civil users. But thanks to some codelessprocessing tricks the encrypted L2 signals is already used by current high-performance receivers.The GPS modernization will make this trick obsolete with the introduction of the dedicatedcivil signal L2C in the currently encryption-only L2 band. The L2C deployment is already well

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2.2. Availability and Applications of Multi-Frequency GNSS

advanced and is expected to provide uninterrupted global coverage in 2018. The Fig. 2.2 showsthat the L2C signal is very close to be always available from four or more satellites. The figurecomes from a calculation made as part of this thesis that calculates the number of visiblesatellites for the different civil GPS signals. The figure only shows the visibility for 24 hoursbecause the GPS satellite orbital period of 12 hours and the earth rotation have a commonmultiple at 24 hours. This means that the availability does not change much from one day toanother and the pattern repeats. The figure also shows that the L5 signal is already availablefrom four or more visible satellites at certain times during the day. The "Operational" remarkin the legend indicated that all the GPS satellites are currently fully operational and usable forpositioning.

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Figure 2.2.: Availability of GPS civil signals (September 2016)

The Galileo system is currently busy deploying is first constellation and is like the ChineseBeiDou-2 system expected to provide full global uninterrupted positioning services in 2020.The availability of Galileo satellites was also calculated and plotted in Fig. 2.3. The Galileofigure shows the visibility of the current satellites for a time of ten days. During this time thesatellites perform exactly 17 orbits and the visibility pattern repeats. The figure also shows witha dashed line what the availability is when including the satellites that are already deployedbut still in the commissioning phase.

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Figure 2.3.: Availability of Galileo civil signals (September 2016)

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