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Comparative Study of Satellite Navigation Systems
Vlad-Cosmin Vasile Communications Dept.
Technical University of Cluj-Napoca Sibiu, Romania
Corina Nafornita Communications Dept.
Politehnica University of Timisoara Timisoara, Romania
Monica Borda Communications Dept.
Technical University of Cluj-Napoca Cluj-Napoca, Romania
Abstract—This paper describes a comparative study of existing satellite navigation systems, highlighting their main features, as well as the advantages or disadvantages of each of them. Comparisons have also been made between the coverage of these navigation systems and the positioning accuracy offered by them.
Keywords—satellite navigation, sky plot, positioning accuracy
I. INTRODUCTION Global Navigation Satellite Systems (GNSS) are systems
that calculate the geospatial position of objects on the ground or in space using the signals received from the navigation satellites. These systems send users signals containing positioning and time information, requiring at least four satellites in the area of visibility. The most common navigation system is GPS (Global Positioning System), a global navigation system used by users around the world, belonging to the United States Army. A GNSS (such as GPS) can be even more accurate with satellite based augmentation systems (SBAS). These systems take into account environmental factors such as errors due to ionosphere crossing or solar bursts to improve the data provided by GNSS. [1]
The paper is structured as follows: Section 2 describes the global and regional satellite navigation systems, starting with GPS; Section 3 gives some notions about the satellite based augmentation systems (SBAS); in Section 4 are made simulations for the ground footprint of the satellites for different satellite navigation systems; Section 5 gives sky plots of the satellites of the four global satellite navigation systems, taking a point of observation in Romania, but also in other locations, for 24 hours; the last section draws conclusions.
II. GLOBAL AND REGIONAL SATELLITE NAVIGATION SYSTEMS
At present, the most widely known GNSS is the GPS. GPS is a system that uses a constellation of 32 satellites that orbit around the Earth [2], offering two different positioning services: SPS (Standard Positioning Service), using a L1 (1575.42 MHz) and PPS (Precise Positioning Service) using two signals in the L1 (1575.42 MHz) and L2 (1227.6 MHz) bands. Satellites launched in orbit since 2005 also emit in the L5 band (1176.45 MHz), this signal being interoperable with those issued by other navigation systems (European, Japanese and Indian).
GLONASS (GLObal NAvigation Satellite System) is the global navigation satellite system of the Russian Federation, using signals in the L1 (1602 MHz), L2 (1246 MHz) bands and, starting with 2011, L3 band (1202 MHz).
The Galileo system is a product of the collaboration between ESA (European Space Agency) and EC (European Commission). The first services were provided at the end of 2016. The system is still under construction, while new services will be tested and made available, with the completion of the system being estimated for 2020 and providing a constellation of 30 satellites, using three frequency bands: E1 (1575.42 MHz, the same as L1 of GPS), E5 (1191.795 MHz and E6 (1278.75 MHz).
The Chinese system called BeiDou originates in a regional system initially formed in 2003 from 3 geostationary satellites. At the moment, the system has become global, but it is still developing. BeiDou has become fully operational for China and neighboring areas in 2011 and will ensure global coverage in 2020. The system uses three frequency bands - B1 (1561.098 MHz), B2 (1207.14 MHz) and B3 (1268.52 MHz) - offering two-level positioning services: one available to the general public and one for the military and government agencies.
In addition to these global satellite navigation systems, two regional positioning systems are worth mentioning. The Quasi-Zenith Satellite System (QZSS) is a regional positioning system in development in Japan. It will use satellites placed in inclined geo-synchronous orbits so one of them is always in zenith in Japan, which is necessary due to the high buildings that easily block the satellite signal, being expected to become fully operational in 2018. QZSS satellites will transmit signals compatible with those of modernized GPS satellites and Galileo satellites in the L1 (1575.42 MHz), L2 (1227.6 MHz) and L5 (1176.45 MHz) frequency bands. In addition, the satellites will also use an experimental high-precision signal in the LEX band (L-band Experiment, 1278.75 MHz), similar to the Galileo E6 band.
India has a different regional positioning system in development, originally called the Indian Regional Navigation Satellite System (IRNSS) and then renamed NAVIC (Navigation with Indian Constellation). The first satellite was launched in 2013, with the construction comprising three geostationary satellites and four satellites in inclined geo-synchronous orbits. It is assumed that the system will become operational at the end of 2018, after a full check. NAVIC satellites emit positioning signals in the L (1176.45 MHz) and S (2492.028 MHz) bands.
III. SATELLITE BASED AUGMENTATION SYSTEMS Positioning accuracy of a navigation system can be
greatly enhanced by satellite-based augmentation systems. Thus, EGNOS (European Geostationary Navigation Overlay Service) is part of the SBAS category, improving the accuracy of the US satellite navigation system (US GPS) using geostationary satellites. This system is considered to be the predecessor of Galileo, the European satellite navigation system.
Globally, there are several satellite based augmentation systems: U.S. has WAAS (Wide Area Augmentation System), Japan has MSAS (Multi-Function Satellite Augmentation System), and India has GAGAN (GPS and GEO Augmented Navigation System). The Russian Federation is also upgrading its GLONASS system through its own SBAS - SDCM (System for Differential Corrections and Monitoring); similarly, China is modernizing its navigation system called BEIDOU. The main difference between SDCM and other SBAS is that it is designed as a SBAS that uses both GPS and GLONASS signals, while other SBASs provide corrections and integrity messages based only on GPS satellites. All these systems have a terrestrial component containing monitoring stations. The layout of these monitoring stations determines the coverage of the systems they belong to.
With EGNOS, the accuracy of current GPS signals is improved to less than 2m, with an availability of over 99%, covering most European countries.
The WAAS is a SBAS developed by the FAA (Federal Aviation Administration) that uses GPS signals along with a network of reference stations and three geostationary satellites to achieve a better positioning accuracy.
This system covers the entire North American continent, providing a positioning accuracy of about 1m horizontally, and 1.5m vertically. As for the terrestrial component, the US system has a reference station in almost every US state, as well as other locations in Canada, Mexico and Puerto Rico. The spatial component consists of 3 geostationary satellites.
MSAS (Multi-functional Satellite Augmentation System), the Japanese satellite navigation enhancing system, consists of 6 reference stations located in Japan, one in Hawaii and one in Australia and two geostationary satellites.
The Indian Navigation Enhancement System, GAGAN (GPS and GEO Augmented Navigation), has 15 reference stations located in India and 3 geostationary satellites. The coverage of this system includes India, and the accuracy of the system is slightly weaker than that offered by EGNOS and WAAS because it is closer to the Equator and massive scintillation is taking place in this area.
At this moment, the Russian Federation is modernizing its GLONASS system through its SBAS - SDCM (System for Differential Corrections and Monitoring). In orbit there are already 3 geostationary satellites, and the other components of the system are being implemented. Interestingly, this system can provide corrections based on both GLONASS and GPS data.
IV. GROUND FOOTPRINT OF THE SATELLITES To make a comparison between existing navigation
systems, we used “GNSS Mission Planning simulator” [6], offered free by Navmatix.
The observation point was chosen in Timisoara, Romania (Fig. 1) and several simulations were performed to highlight the differences between the existing satellite navigation systems. Simulations have been set for May 15, 2018 (24 hours).
Fig. 2 shows the ground footprint for 24 hours for 5 satellites belonging to different navigation systems: GPS (green), GLONASS (red), Galileo (blue), BeiDou (pink) and
QZSS (brown). It can be seen that the ground footprint for the QZSS satellite covers a narrow geographical area. This is because it is a regional system, and the satellite's orbit is an inclined geosynchronous orbit, calculated to cover Japan's area of interest.
Fig. 1. Selecting the observation point using “GNSS Mission Planning simulator”.
Fig. 2. Ground footprint for satellites belonging to different systems. GPS is marked with green color, GLONASS with red, Galileo with blue, BeiDou with pink and QZSS with brown.
The following figures (Fig. 3 – Fig. 7) show the ground footprints for 24 hour, for each of the navigation systems with all their satellites.
Fig. 3. Ground footprint for GPS satellites (USA).
Fig. 4. Ground footprint for GLONASS satellites (Russian Federation).
Fig. 5. Ground footprint for Galileo satellites (Europe).
Fig. 6. Ground footprint for BeiDou satellites (China).
Fig. 7. Ground footprint for QZSS satellites (Japan).
From the analysis of these figures it can be seen that the satellites belonging to the Russian system have a better coverage in the extreme latitudes (both in the north and in the south), see Fig. 4. It can also be seen that the Chinese system (Fig. 6) is made of satellites that are orbiting at medium altitude, similar to those of the other 3 systems with global coverage, as well as satellites with inclined geosynchronous orbits, similar to those of the Japanese system (Fig. 7).
V. SKY PLOT AND NUMBER OF SATELLITES The position of a satellite can be described by azimuth
and elevation. The azimuth of the satellite represents the horizontal angle between the North direction and a line from the observation point to the point where the satellite is located projected on the horizontal plane. The elevation represents the vertical angle between the line joining the observation point with the point where the satellite is located and the horizontal projection of this line.
The following figures show the azimuth and elevation (sky plot) of the satellites of the four global systems relative to the direction of the celestial North Pole taking a point of observation in Timisoara, Romania for 24 hours (May 15, 2018).
From these charts, it can be seen that the American (Fig. 8) and Russian systems (Fig. 9) offer better coverage in Romania than the European (Fig. 10) and Chinese (Fig. 11) systems. This is explained by the fact that the last two are under development.
Fig. 8. Sky plot for GPS satellites for 24 hours (May 15, 2018) relative to the direction of the celestial North Pole. The observation point is in Timisoara, Romania.
Fig. 9. Sky plot for GLONASS satellites for 24 hours (May 15, 2018) relative to the direction of the celestial North Pole. The observation point is in Timisoara, Romania.
Fig. 10. Sky plot for Galileo satellites for 24 hours (May 15, 2018) relative to the direction of the celestial North Pole. The observation point is in Timisoara, Romania.
Fig. 11. Sky plot for BeiDou satellites for 24 hours (May 15, 2018) relative to the direction of the celestial North Pole. The observation point is in Timisoara, Romania.
For a better comparison between GPS and GLONASS, we obtain the graph in Fig. 12. From this graph we can see that, due to the fact that the GLONASS satellites rotate with a higher inclination, this system offers slightly better coverage. In fact, there is a cone around the celestial North Pole in which the satellites are never visible and this cone is smaller for GLONASS satellites.
Fig. 12. Sky plot for GPS and GLONASS satellites, superimposed for 24 hours (May 15, 2018) relative to the direction of the celestial North Pole. The observation point is in Timisoara, Romania.
An important factor in achieving the most accurate positioning is played by the number of satellites in the area of visibility at one time. Fig. 13 and 14 represent two graphs showing the number of satellites in the area of visibility, for 24 hours, respectively the period when each satellite was in this area, the observation point being also chosen in Timisoara, Romania.
Fig. 13. Number of satellites visible within 24 hours (May 15, 2018). On the horizontal axis we have the time and on the vertical axis we have the number of satellites. The visible satellites from each system are represented with different colors (GPS-green, GLONASS-red, Galileo-blue, BeiDou-pink). The observation point is in Timisoara, Romania.
Fig. 14. Satellites visibility for 24 hours (May 15, 2018). On the horizontal axis we have the time and on the vertical axis we have each visible satellite. Similarly, the visible satellites from each system are represented with different colors (GPS-green, GLONASS-red, Galileo-blue, BeiDou-pink). The observation point is in Timisoara, Romania.
These two charts show that the European system has, on average, the fewest satellites present in the area of visibility, which clearly leads to a much lower positioning accuracy. This time, the US system has a slight superiority to the Russian one, providing a larger number of satellites in the area of visibility over the 24 hours.
When calculating the position of a user, the navigation system combines distance measurements with different satellites in the area of visibility. The errors that affect the measurements are combined, resulting in a positional error. The relationship between the mean distance error and the positioning error is represented by a factor named DOP (Dilution of Precision) or Geometric Dilution of Precision (GDOP) [5].
Calculating the DOP values, it results that in Romania the American and Russian navigation systems have a better positioning accuracy than the European and Chinese systems. As far as GPS and GLONASS performance are concerned, they are similar.
Other simulations have also been made, this time changing the location of the point, where the accuracy of the positioning systems has been checked. Regarding the northern hemisphere, the results were similar to those in the area of Romania, with the observation that in the areas where the Chinese system has satellites with inclined geosynchronous orbits, its performances show visible improvements. These areas can easily be seen in Fig. 6. Obviously, the BeiDou DOP values for a location in China are better compared to the values obtained in Romania.
In the equator area, there was a slight improvement in the performance of navigation systems, especially with regard to the American system. This is mainly due to the fact that in this area each satellite is for a longer period in the area of visibility and, in addition, the cone around the celestial pole in which no satellites were present disappears. Fig. 15 shows the sky plot of GPS satellites for a location on the Equator in Brazil and Fig. 16 illustrates their visibility chart.
Fig. 15. Sky plot for GPS satellites for 24 hours (May 15, 2018) relative to the direction of the celestial North Pole. The observation point is on the Equator, in Macapa, in Brazil.
Fig. 16. GPS satellites visiblity for 24 hours (May 15, 2018). On the horizontal axis we have the time and on the vertical axis we have each visible satellite. The observation point is on the Equator, in Macapa, Brazil.
In the southern hemisphere, simulation results were similar to those in the northern one. Obviously, this time, satellites are never visible in a cone around the southern celestial pole.
Fig.17 shows the sky plot of the GLONASS satellites for 24 hours (May 15, 2018), also relative to the direction of the celestial North Pole for a location in Cape Town, South Africa.
Fig. 17. Sky plot for GLONASS satellites for 24 hours (May 15, 2018) relative to the direction of the celestial North Pole. The observation point is in Cape Town, South Africa.
VI. CONCLUSIONS In this paper, a comparative study of satellite navigation
systems was achieved. From the comparative study, it was observed that the satellites belonging to the Russian system have a better coverage in the extreme latitudes due to the fact that they rotate around the Earth with a higher inclination. Because of this, the system belonging to the Russian Federation also offers slightly better coverage in other areas. In fact, for the northern hemisphere, there is a cone around the celestial North Pole in which the satellites are never visible and this cone is smaller for GLONASS satellites. Similarly, it also happens in the southern hemisphere with
the exclusion zone that appears around the celestial South Pole.
It has also been noticed that the US and Russian systems offer better coverage than European and Chinese systems, which is also explained by the fact that the last two mentioned systems are under development. From the performed simulations, it was seen that the European system has, on average, the fewest satellites present in the area of visibility, which clearly leads to a lower positioning accuracy. This time, the US system has a slight superiority to the Russian one, providing more satellites in the area of visibility.
It has been confirmed that US and Russian navigation systems have a better positioning accuracy than the European and Chinese systems, with the observation that in areas where the Chinese system has satellites with inclined geosynchronous orbits, its performance has made visible improvements. As far as GPS and GLONASS performance are concerned, they are similar.
In the equator area, there was a slight improvement in the performance of navigation systems, especially with regard to the American system. This is mainly due to the fact that in this area each satellite is present for a longer period in the area of visibility and, in addition, the cone around the celestial pole in which no satellites were present disappears.
REFERENCES [1] Springer handbook of global navigations systems, Springer Nature,
2017. [2] Global Positioning System SPS Performance Standard, 4th Edition,
September 2008. [3] European GNSS Agency, EGNOS Safety of Life (SoL) service
definition document, ISBN 978-92-9206-028-2, 2016. [4] Joint ACAC/ICAO MID workshop on GNSS, GAGAN - extension to
the Gulf Region, November 2017. [5] Wikipedia, Dilution of precision,
https://en.wikipedia.org/wiki/Dilution_of_precision_(navigation). [6] Navmatix, GNSS mission planning, http://gnssmissionplanning.com/.
