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Introduction to MSAS
Available Global Navigation Satellite Systems (GNSS), such as the familiar GPS system or the Russian GLONASS system, offer positional accuracies of about 15 meters on an average. While that could be good enough for everyday car navigation use, better accuracies are required for many applications. The instrument landing systems (ILS) that helps aircrafts land in low visibility or even navigate can be simplified by using GPS. ILS category 1 (Cat 1) demands vertical positional accuracies of 4 meters (13 feet) or better. Survey calls for accuracies in the range of 10 centimeters! This article talks about accuracies required for precision farming. So, corrections are required. Multi-functional Satellite Augmentation Systems, or MSAS, supply the necessary corrections to receivers equipped to receive them and make the corrections. Some accuracy issues are discussed here. Read "What is SBAS? Understanding Satellite Based Augmentation System" for another view into these systems.
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The FAA of US introduced the Wide Area Augmentation System (WAAS). The system aims to augment the accuracy of the GNSS signals so that they can be used for aircraft navigation. Several other augmentations systems such as The European Geostationary Navigation Overlay service (EGNOS) is operated by the European Space agency. A Wide Area GPS Enhancement (WAGE) network is operated by the US Department of Defense. A Multi-functional Satellite Augmentation System (MSAS) is operated by the Japanese Ministry of Land, Infrastructure and Transport. There are a few commercial networks too. These include Starfire Navigation System operated by John Deere and Starfix DGPS and OmniSTAR systems by Fugro. The latter two are precise enough to be useful in survey applications where accuracies better than 10 centimeters or 4 inches are required.
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The Way MSAS Works
The augmentation networks depend on a set of reference stations that have their positions very accurately surveyed. These stations keep reading their own positions from the GNSS system and compare the readings with the known good position data they already have.
The difference is the error in the system and is fed to a master ground station. The master station computer calculates corrections and assesses integrity of GNSS system too. The data is then up linked to a geostationary communication satellite. This communication satellite forwards the correction signal to the aircraft’s GPS receiver as well as other satellites for relay over a wider coverage area.
These correction details are sent over in a message format defined by the FAA. The aircraft receiver then makes necessary adjustments to provide improved positioning accuracy. The communication satellites also act as GPS navigation satellites That increases the number of satellites in the GPS constellation that can be used by receivers to make position readings. Read this article to get a feel of some real GPS products for aircraft use. Due to the nature of the variations in the factors that introduce errors, there is a slow changing error and a quick change, instantaneous errors. These augmentations systems are generally known as Multi-Functional Satellite Augmentation Systems (MSAS) as they depend on distribution of correction data over a set of satellites. There are Ground Based Augmentation Systems or GBAS that depend on a terrestrial distribution of correction messages.