August 30, 2008
Q: It has become common in the last year or two for manufacturers to offer GPS receivers, Glonass receivers and GNSS receivers. Are these receivers different? They seem to look the same.
A: As most people in surveying know, GPS stands for Global Positioning System, a system created and maintained by the United States government. Glonass stands for GLObal NAvigation Satellite System, the satellite navigation system originally created by the Soviet Union that is now under the control of the Russian Federation. GNSS also stands for Global Navigation Satellite System; it is used as a generic acronym for any of the satellite navigation systems. The use of the term “GNSS” has become popular since the European Union created its new system named Galileo. Galileo is not an acronym; this system is named in honor of the famous Italian astronomer and scientist born in the 17th century.
In the United States, most manufacturers have offered satellite-based surveying technology using only the GPS satellites, but a few have also offered receivers with support for Glonass satellites. Typically, these were referred to as GPS-Glonass receivers. Even though Glonass satellites were launched beginning in the 1970s, the system has never had the design constellation of 24 satellites (21 operating plus three in-orbit spares). The number of operational Glonass satellites near the middle of this year was 16. Thus many manufacturers and users have been watching the Glonass constellation to determine if or when they should plan on operating surveying receivers with GPS and Glonass signals.
The Galileo system has only recently put into orbit the second of its test satellites; operational satellites are not expected to be placed in orbit before 2010. However, as the information about the signals broadcast by Galileo (and Glonass) is available now, many manufacturers are building their receivers with the (sometimes optional) capability to receive, decode and use those signals, as well. This can provide added positioning certainty or increase the possibility to determine position by using a few satellites from each different constellation in cases where there is significant signal blockage, such as in artificial and natural canyons, forested areas, etc. If you purchase a GNSS receiver today, you will only be able to use its GPS and Glonass capabilities. Once Galileo satellites become “live” and there are a sufficient number to make the system usable (even before it is declared fully operational), that same GNSS receiver may be able to use those new satellites. You should check with each manufacturer to determine what “GNSS-capable” means since it is not currently possible for you to test its Galileo system capability.
Q: I understand that GPS uses a different coordinate system, even though surveyors develop coordinates on a plane coordinate system (State Plane, UTM, etc.) or a system devised by a local government agency--or even a completely arbitrary system developed for the conven-ience of the surveyor. Why not use the GPS coordinates in the future to enable easy conversion and use?
A: GPS actually uses two types of GPS coordinate systems. One type, called the satellite reference coordinate system, is unique to each satellite in the system because it is based on each individual satellite’s orbit parameters. The satellite reference coordinate system (remembering that all satellite orbits are elliptical) will not be described in detail here, but one of the axes connects the points of apogee and perigee (points farthest and closest to the Earth’s center of mass), and the origin of the system is at the center of mass of the Earth.
The other coordinate system, the one more familiar to most surveyors, is often referred to as Earth-centered Earth-fixed (ECEF). This system, also called the geocentric coordinate system, provides the terrestrial frame of reference so that points on Earth, whose positions have been determined by observing the satellites, can then be related to each other. In this system, the Earth’s center of mass is the origin, the Z axis coincides with the polar axis (though it is defined much more rigorously), the X axis passes through the Greenwich meridian (0o longitude) at the equator and the Y axis passes through the eastern 90th meridian (90o east longitude) at the equator. All satellite reference coordinates are converted to ECEF using four angular parameters that are defined or, more properly, measured for each satellite. Thus, when your GPS receiver determines your position (autonomously or using point positioning), it uses Cartesian coordinates but not the way you commonly use them with Z representing the “up” direction. Once the position has been determined, if desired, the X-Y-Z position in the ECEF system may be converted to latitude, longitude and height (i.e., spherical coordinates where “zero height” is the reference ellipsoid--WGS84, not “sea level”).
While the X-Y-Z coordinates of ECEF may be used to uniquely define a point on Earth, they are not convenient for surveying purposes for several reasons. The first reason is that the coordinate values are very large (we are a little under 4,000 miles, or approximately 21 million feet, from the center of mass). But the other two reasons are more important: ECEF coordinates don't coincide with our concepts of north, south, east and west, which makes visualizing points, lines and planes difficult; and the coordinates are also difficult to visualize with relative differences in heights. Thus, in practice, GPS coordinates typically require at least one more conversion to get to some type of geodetic or plane coordinate system.
GIAA is a trade association of manufacturers, suppliers and distribution partners encompassing the present and emerging technologies in surveying, GPS, engineering, construction, GIS/LIS and related fields.
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