Geography 85
Applications on GeoSpatial Technology
Lecture 6 - GPS (Global Positioning Systems)
You have probably heard of GPS technology, perhaps even confused it with GIS. GPS stands for Global Positioning Systems (GPS), and today it is used for a host of applications far beyond its intended uses. Originally developed for the military, GPS units could help a soldier locate one's position in featureless landscape, such as the flat deserts of Kuwait and used during the Gulf War. And this endeavor was a huge expense to the American tax payer, to build and launch all those satellites. But today it has paid off immeasurably for scientific research, business applications and best of all -- it's free for public use.
I've been told consumer GPS units were first sold to anglers who wanted to recall sizable fish school locations and where they last caught "the big one." Today GPS applications span a broad range -- from GPS microchips implanted in your cell phone to GPS units installed in vehicles to assist with directions and emergencies, such as the On-Star service (which incidentally include automatic notification of air-bag deployment, stolen vehicle tracking, emergency services, roadside assistance with location, remote door unlock, remote diagnostics, route support, concierge and convenience services and more). And perhaps the best known application is the commonly used amateur GPS unit used for hiking and "wayfinding" in the backcountry. Incidentally wayfinding is a GPS feature that will bookmark an exact location in the field (or as with the case with anglers… in the ocean) that one can navigate back to in the future.In addition to consumer uses, GPS has become an essential part of GIS. GPS units are often used to locate landscape features that are difficult or impossible to locate with remote sensing, such as maintenance holes, fire hydrants, utility and public works locations, fish and wildlife observations, archaeological finds, park trails, and the list goes on. Anything you can map, you can use a GPS to accurately locate in a geographic coordinate system, such as latitude and longitude or Universal Transverse Mercator (UTM), a simplified metric coordinate system.
Prior to the 1990's, finding an accurate field location was not so easy. Maritime and aviation systems, such as LORAN and VOR (Very high frequency Omnidirectional Range), were used to determine one's location and compass bearing, but they served a specialized audience. To determine precise locations, one could hire a survey crew but the process required a crew armed with high-tech equipment and a great deal of skill to operate the equipment. Finally there was the simple compass methods of triangulation, using an arrangement of at least three (3) prominent landscape features in conjunction with compass bearings to roughly determine one's location. Nothing was inexpensive or accessible, however, to the general public like GPS technology is today.
How the GPS Works - a summary
A GPS location is determined by satellites (24 active ones) floating in space that measure the distance to you. This is based on a time differential, which must be exact (using atomic clocks and even accounting for minor delays from atmospheric delays and the gravitational pull of the sun & moon). But nevertheless a GPS unit can determine your distance within about 10 to 15 meters (+/- 50 feet). Then by using "trilateration," generally known as the triangulation, several satellites can pinpoint your location.
Triangulation - more than 1 satellite is needed
While a satellite can determine your distance to it based on a time differential, you could be anywhere along an encompassing sphere of that distance (see your book, p.9 for clarification if needed). That is, you could be anywhere along an arc that intersects the Earth. Thus more than one satellite is needed, in fact, a total of four (4) other satellites (minimum) must intersect your location to accurately determine your location. If the configuration of these satellites is not spread out well enough, this can slow you down (due to poor satellite distribution). But there are at least four visible satellites in the sky at all times to provide theoretical coverage at all times. I might add, however, this is just theoretical… and if you're in a deep canyon or next to a tall building, you might as well give up. (More on positional dilution of precision [PDOP] and off-sets when the class meets.)
Selective Availability (S/A) - security measures
Add to complexity of achieving an accurate time differential and satellite configuration, the U.S. Government in the past intentionally caused positional errors for security reasons. Selective Availability (S/A), as it is called, could put your location off by as much as 100 meters. Fortunately this features was turned off during the Clinton administration to allow more consumer uses of GPS technology. This could be turned back on, however, if the U.S. Government chose to do so.
Differential Correction - a method for precision
One "work around" to S/A is "differential correction," essentially matching your GPS position to a known position and then adjusting your position accordingly. Let's say a base station with a known position continuously receives a GPS signal that is 50 feet to the NE of its true position; then most likely your position will also be off by 50 feet the NE too (assuming you're within a 100 miles of the base station.) Knowing this then, you can post-process the GPS data to snap the position back 50 feet in the opposite direction (SW). This works for points, lines and even polygons, and still is needed today to assure complete accuracy within feet. High-end units that cost tens of thousands of dollars can be within inches, and many expensive GPS units can perform real-time differential correction using nearby base stations, Coast Guard beacons, and the like.
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