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Most people know that GPS is a satellite-based system that was created by the Department of Defense. It was developed for targeting defense systems, precisely locating ships and aircraft ae systems, precisely locating ships and aircraft and other military purposes. Over several years, the Department of Defense installed a constellation of 24 GPS satellites that orbit the earth twice every 24 hours. Each satellite weighs about 1900 pounds and is about 17 feet in diameter.

This system of precise positioning was quickly utilized by the private sector. It has been welcomed with an explosive consumer demand by outdoor enthusiasts, boaters, the trucking industry and car owners. Technology, miniaturization and cost reductions now make it possible to have a wristwatch sized GPS receiver costing about $400 that accomplishes the same positioning functions that, only ten years ago, required a suitcase sized receiver weighing 50 pounds.

HOW GPS WORKS

It is interesting to note that the GPS satellites don’t actually broadcast a position signal to earth. Instead, using onboard atomic clocks, the GPS satellites broadcast extremely precise time information. The speed that the broadcast information travels is well established (the spavels is well established (the speed of light - 186,000 miles/second). Therefore, by knowing the exact altitude and position of at least four GPS satellites at any given time, it is possible to collect time data from these satellites simultaneously and calculate a fixed position on earth. There are many other variables that can affect the positioning calculations such as ionospheric and tropospheric noise and distortion. Computer programs evaluate these conditions and make corrections when processing GPS data.

 

HOW ACCURATE IS GPS?

Tens of thousands of people currently own recreational hand-held GPS receivers. Prior to President Clinton’s repeal of the government’s application of Selective Availability (SA) in May, 2000, these inexpensive units yielded positional accuracies ranging anywhere from the diameter of a putting green to the length of a football field. This information can be very valuable for activities such as hiking, boating, hunting and providing directions in automobiles and trucks.

WHAT WAS SELECTIVE AVAILABILITY?

For National Security reasons, the US government introduced a signal scramble, or noise, into the broadcast GPS signal that prevented GPS receivers from acquiring real-time positions that were more accurate than the length of a football field. This signal scramble could be removed for military purposes, such as during Operation Desert Storm. The signal noise was known as Selective Availability (SA).

Manufacturer’s of GPS equipment were continually developing software and methods to overcome SA and the government decided three years ago to discontinue SA in the year 2000. With SA discontinued, handheld GPS units are now able to obtain accuracies of about 30 to 50 feet of the true position. The elimination of SA will greatly increase the effectiveness of inexpensive handheld GPS.

 

SURVEYING GRADE GPS SYSTEMS

Land Surveyors have a special application for this technology. By using very high grade GPS receivers and software, accuracies ranging as close as 1/8 of an inch can be obtained.  These accuracies not only determine horizontal positions on the earth’s surface (latitude and  longitude or other surveying coordinate systems) but can also determine altitude with a precision of less than one inch. This equipment, software and new techniques are revolutionizing surveying and engineering.

 

DIFFERENT TYPES OF GPS

The three most common GPS surveying methods used today are: Static, Real-Time

Kinematic and DGPS (Differential GPS). Each method uses a different type of equipment and software producing different levels of accuracies. Static GPS surveying is the most precise producing millimeter accuracy. Real-Time Kinematic is also very precise typically producing centimeter accuracy, while DGPS offers sub-meter accuracy. DGPS receivers typically have an accuracy of three to five meters.

PRACTICAL USES OF GPS SURVEYING SYSTEMS

DGPS is the least accurate (sub-meter) type of GPS used by Surveyors. However, DGPS can be used in wooded areas where Real-Time Kinematic and Static GPS falter because trees block GPS satellite signals. Another advantage of DGPS is that it only requires one person to operate it. DGPS is well suited for mapping wetlands or utility easements, staking out clearing limits for construction projects and logging, staking oil well locations and other simil utility easements, staking out clearing limits for construction projects and logging, staking oil well locations and other similar projects. Another advantage of DGPS is that the surveys can be completed without the cutting of trees and brush that is typically associated with clearing sight lines for surveying done by traditional methods. Construction projects routinely need accurate topographical surveys. They require precise topographical data for: drainage facilities, roads, culverts, curbs, ground elevations, flow lines and utilities. Environmental projects require monitoring well locations and elevations. These projects lend themselves to applications of Real-Time Kinematic GPS technology. Real-Time GPS data from the field imports directly into all of the industry standard drafting programs such as AutoCAD® and Microstation® and GIS/LIS applications.

Real-Time GPS has also streamlined staking for construction projects. Survey crews can upload construction staking information into electronic data collectors which are then connected to the field GPS instruments. In certain situations, it is possible for one person to accurately stake out an entire site reducing staking costs.