GPS Q&A
By Frank van Diggelen

Q.What is GPS+GLONASS technology, its intended uses, and its merits? J. Hughes Seattle, Washington

A.GPS+GLONASS technology combines measurements from both the U.S. GPS and Russian GLONASS satellite systems, and is accessible with Magellan's specially-designed receivers that obtain and process signals from both constellations. The benefits of GPS+GLONASS technology are threefold: increased satellite availability, improved accuracy, and enhanced integrity monitoring. These benefits are particularly significant in obstructed environments.

Like GPS, GLONASS functions using the principals of trilateration; that is, a receiver determines the distance from itself to each of several satellites, and because the positions of the satellites are known as a result of the satellite's broadcast information, the receiver's position can be calculated. A GPS-only or GLONASS-only receiver must receive signals from at least four satellites to calculate latitude, longitude, altitude, and time. For real-time centimeter-level accuracy, five or more satellites are required. In unobstructed locations, the typical receiver can expect seven satellites visible 10 degrees or more above the horizon. However, when typical environmental obstructions are considered, such as near a mountain or skyscraper, the num ber of visible satellites may drop to three or fewer and a GPS-only or GLONASS-only receiver will probably be unable to provide accurate position measurements. However, using both satellite systems with GPS+GLONASS technology, more satellites are visible than with either GPS or GLONASS alone, permitting measurements in blind environments.
    Increased satellite visibility using GPS+GLONASS technology provides higher availability of valid position computations. In instances where large obstacles such as skyscrapers block portions of the sky, real-time precision (centimeter- and decimeter-level) availability is more than doubled by the augmentation of GPS with GLONASS. (Martin & van Diggelen, GIM Magazine, April 1997. Vol 11: No. 4. pp. 73-75.) GPS+GLONASS technology also increases confidence in the accuracy of measurements with integrity monitoring. A navigation system bears "integrity" when it can warn a user that a position fix is in error. A GPS receiver must use five or more satellites to be able to detect an integrity problem. However, with the availability of additional GLONASS satellites, a system's integrity can be greatly increased using a GPS+GLONASS receiver. Thus, GPS+GLONASS technology is the preferred choice in cities, mountainous areas, dense wilderness, or other areas where much of the sky and many satellites are potentially blocked.
    The use of GLONASS also significantly improves accuracy in autonomous measurements. The GLONASS satellites do not intentionally introduce measurement errors like Selective Availability (SA), which is a major GPS error source. Therefore, a GPS+GLONASS receiver is able to mitigate the errors introduced by SA, providing higher accuracy to the user. Magellan has further improved the GPS+GLONASS advantage using Real-Time Kinematic (RTK) technology to provide centimeter- and decimeter-level measurements with a single-frequency receiver.
    With greater signal availability (especially in obstructed environments), improved accuracy, and enhanced integrity monitoring, GPS+GLONASS provides significant advantages over GPS-only receivers. As a result, GPS+GLONASS receivers are becoming an important part of the professional's equipment arsenal, and the only technology currently available that provides the extra satellites many users require for applications with a limited view of the sky. Open-pit mine operators, for example, can survey points consistently and reliably using GPS+GLONASS technology - a task impossible to perform with GPS alone - since the technology allows their receivers to use enough satellites to generate accurate measurements. In fact, in real world applications all over the world, professionals are realizing significant productivity gains using GPS+GLONASS. Many GPS+GLONASS users are reporting paybacks in their investment in less than a year and in some cases a few months, as the utilization of GPS+GLONASS is proving to be both an effective and economical solution. GPS+GLONASS is the only practical alternative to GPS-only technology to-date that provides an increasingly powerful solution to the global navigation and positioning needs of users worldwide.

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