11.3.4: Global Navigation Satellite Systems (GNSS)
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Global Navigation Satellite Systems (GNSS): GNSS are global systems that use a medium high constellation of satellites describing quasi-circular orbits inclined with respect to the terrestrial equator. Currently, just two systems are in practice active: The american GPS and the Russian GLONASS. Some other constellations are in development, such as the European Union Galileo positioning system, the Chinese Compass navigation system, and Indian Regional Navigational Satellite System.
GNSS systems are based on the transmission of an electromagnetic wave by the satellite that is captured and de-codified by the receiver (the aircraft). The basic information that we can obtain is the time that the signal takes while traveling. This time provides a so-called pseudo-distance. This is due to the fact that there exists a synchrony error between the time of the aircraft and the time of the satellites, and therefore, we can not know with certainty the real distance. These systems provide location and time information anywhere on or near the Earth where there is a line of sight to four or more GPS satellites.15
Figure 11.9: Service Areas of Satellite Based Augmentation Systems (SBAS). © User:Persimplex / Wikimedia Commons / CC-BY-SA-3.0.
This system is intended to offer higher precision (with an error of about 10m in determining the position), global coverage, and continuos navigation. However, one of the fundamental drawbacks that have made so far these systems impractical for air navigation is their strategic character in terms of national security. GLONASS was only open to limited civilian use in 2007. The GPS is maintained by the United States government and is freely accessible by anyone with a GPS receiver. However, its reliability is not complete in terms of precision and continuity in the coverage, i.e., the signal has no integrity due to its military character.
Therefore, in order GNSS to be used (still in a limited way and always with back-up systems) in some phases of the flight, a first generation of GNSS (the so-called GNSS-1) was conceived as a combination between the existing satellite navigation systems, i.e., GPS and GLONASS, and some type of augmentation system. Augmentation of a global navigation satellite system (GNSS) is a method of improving the navigation system’s attributes, such as accuracy, reliability, and availability, through the integration of external information into the calculation process. These additional information can be for example about sources of satellite error (such as clock drift, ephemeris, or ionospheric delay), or about additional aircraft information to be integrated in the calculation process. There are three types of augmentation systems, namely:
- The Satellite-Based Augmentation System (SBAS): supports wide-area or regional augmentation through the use of additional satellite-broadcast messages. SBAS systems are composed of multiple, strategically located ground stations. The ground stations take measurements of GNSS satellite signals are used to generate information messages that are sent back to the satellite constellation, which finally broadcasts the messages to the end users (aircraft). Regional SBAS include WAAS (US), EGNOS (EU), SDCM (Russia), MSAS (Japan), and GAGAN (India).
- The Ground-Based Augmentation System (GBAS): a system that supports augmentation through the use of terrestrial radio messages. As for SBAS, terrestrial stations take GNSS signal measurements and generate information messages, but in this case these messages are directly transmitted to the end user (the aircraft). GBAS include, for instance, the LAAS (US).
- The Airborne Based Augmentation System (ABAS): in this augmentation system the ground stations analyze only information coming from the aircraft. This information is transmitted back to the aircraft.
GNSS-2 is the second generation of satellite systems that will provide a full civilian satellite navigation system. These systems will provide the accuracy and integrity necessary for air navigation. These fully civil satellite systems include the European Galileo, which is expected to be fully operative in 2020. Also a civil GPS version is under development.
15. Notice that in order to determine the position of the receptor (the aircraft) we need in this case 4 satellites. This is because an extra satellite is needed to determine the synchrony error between the time of the aircraft and the time of the satellites.