EDUCATOR’S PAGE GPS - Just a Matter of Time Michael J.Urban, MEM-1910
An interesting connection between the geosciences and modern physics can be made through an examination of the Global Positioning System (GPS). The precision with which measurements of location on the Earth are made via GPS is extremely valuable for geologi- cal mapping efforts. A number of factors contribute error to the determination of location, but for most, significant error corrections can be applied. Our ability to minimize errors in GPS measurements continues to be enhanced through tech- nological advancement.
What is GPS?
The Global Positioning System, devel- oped by the United States Department of Defense in 1978, is a means for deter- mining and providing specific data about locations on Earth in terms of point posi- tion (latitude, longitude, and elevation) and relative position (vector location). Initially designed for military purposes, it is now widely utilized for a variety of applications in both government and civil initiatives, including navigation, surveying and mapping, mining, road construction and maintenance, agricul- ture, and for instances when exact time with atomic clock accuracy is needed. The Global Positioning System consists of three components: satellites, ground stations, and receivers. Satellites trans- mit signals which are picked up and used by ground-based receivers to determine details about latitude, longitude, and elevation of its location. Ground stations verify the position of satellites in their orbits, a necessary “ground-truthing” action for the confirmation of data accu- racy. Receivers use satellite transmis- sions to calculate satellite distance and determine locations on the Earth. The information about location can then be given numerically, or translated into
maps through comparison to archived position and map data in the receiver device.
The accuracy of GPS technology has steadily increased since its inception and the degree of measurement error has evolved from meters to centime- ters (or less). Prior to the year 2000, the Department of Defense intention- ally prohibited maximum accuracy for civil use, termed Selective Availability, as a precautionary measure related to national security. Since then, the best signal has been made accessible, enabling anyone capable of purchas- ing the most sophisticated receivers to enjoy unsurpassed location resolution. Today, even relatively inexpensive GPS receiver units provide reasonably valid position information (error on the order of meters).
The U.S. Air Force controls the Global Positioning System of the United States, which is funded through taxpayer dol- lars mostly out of the Department of Defense budget. A network of 31 sat- ellites currently comprises the GPS constellation (United States Naval Observatory, 2015). A baseline configu- ration of at least 24 satellites is needed for total coverage, and extra satellites may be rotated into the baseline when a satellite is damaged or requires ser- vicing (National Coordination Office, 2015). The solar-powered satellites, weighing several thousand pounds each and encompassing various generations of technology since the 1990s, circle the globe twice each day at a distance of 20,000 km above the Earth. [See the references for details about the types and launch dates of the GPS constellation satellites.] The satellites are arranged into six planes spaced 60 degrees apart and inclined approximately 55 degrees with the equatorial plane (NAVSTAR
1. Using lengths of the sides of triangles instead of angles, which would be triangulation.
GPS User, 1996). Each orbital plane contains at least four satellites in specific locations. The arrangement permits at least four satellites to be detectable by a receiver at almost any point on the Earth in any moment. The constellation baseline configuration was expanded in the year 2011 by repositioning six satellites to accommodate the addition of three more; the resulting operational baseline constellation of 27 satellites has improved coverage globally (National Coordination Office).
How does GPS work (basic)?
In order to determine an exact posi- tion via GPS, an Earth-based receiver uses data from four orbiting satellites simultaneously. All of the satellites regularly broadcast information about their position and the exact time (to an accuracy of nanoseconds). Time is a criti- cal component of GPS, because despite the rapidity of the speed of light (i.e., radio signal), there is a delay between signal transmission from the satellite above and reception of the signal on the Earth below. It is this time lag that is actually used to determine the satellite distances from the receiver at the time of signal transmission. A clock in the receiver records the time the signal is received and computes the distance by comparing the reception time to the transmission time – embedded in the code of the satellite signal – to determine the time lapse in relation to the known speed of light. The time measured by the satellites and receiver must be in sync. However, this is not the case, so a correction must be applied.
Ideally, only three satellites would be required to trilaterate1 the position of the receiver on the Earth, because three overlapping spheres provide two possible
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