Getting a Fix on GPS

Dec. 28, 2000
A brief explanation of what GPS is, how it works and its applications in the construction industry
Throughout the history of hu-man endeavors, there have been inventions and discoveries which have altered the complexion of society and the course of history. From the simplest technological advances, such as the wheel and fire, through the industrial advances of the electric light and the internal combustion engine, up to the present day development of personal computers, technology has played a major part in the development and progress of the human race. As we near the end of the 20th century, a new technology is poised to join these ranks and influence and change our lives.

Global Positioning System (GPS) technology is already revolutionizing many fields.

Japanese scientist are using it to track whales, while their Geographical Survey Institute is working with GPS to monitor the movement of the earth's crust in order to improve earthquake predictions. It has been used to map the ozone layer above the Arctic Ocean and Antarctica.

Soviet and Canadian explorers employed GPS to pinpoint the resting place of the Titanic at the bottom of the Atlantic Ocean. The U.S. Army is conducting tests in the hopes that GPS can guide artillery shells in flight.

Archaeologists have designed a reconnaissance model for Mayan archaeological dig sites with the aid of GPS. The U.S. National Park Service is using the technology to locate, map and study desert tortoises in California's Joshua Tree National Monument.

These are but a few of the more exotic applications. In its more traditional uses GPS is fast becoming the preferred method for mapping, navigating, tracking, guiding, positioning and locating anything.

New technologies have a habit of mystifying and intimidating people. The blinking 12:00s on VCRs across the country is just a small example of people's unwillingness to fully embrace new technologies. Therefore, a brief explanation of what GPS is and how it works will help us to better understand its applications in the highway industry.

In the 1970s, the U.S. Department of Defense (DOD) began work on a positioning and navigation system to aid U.S. military forces. What they developed was GPS, a constellation of satellites which orbit the earth twice a day, at an altitude of 10,900 miles, transmitting precise time and position information to receivers on earth.

In 1974, Rockwell International was awarded a contract to build satellites and receivers for the government's GPS program. In 1978 they launched the first four satellites and since then have gone on to build 40 GPS satellites for the U.S. government, including the 24 Block II and Block IIA satellites which presently orbit the earth, providing 24-hour coverage. The 5 ft wide, 17.5 ft long-including wing span-satellites are designed to last seven-and-a-half years.

A ground system is required to monitor the satellites. This system consists of five monitor stations and four ground antennas located around the world. The stations use GPS receivers to track the navigation signals on the satellites. The information gathered from the stations is processed at a master control station operated by the 2nd Satellite Control Squadron at Falcon AFB, Colo., and used to update the satellites' navigation messages. The ground antennas use an S-band signal to transmit commands to the satellites and to receive the satellites' state-of-health data.

The system is based on timing radio waves transmitted by three or four of the satellites. Each satellite broadcasts two signals known as L1 and L2. The L1 frequency contains the Course/Acquisition (C/A) code which provides Standard Positioning Service (SPS) for civilian use. The Precise or Protected (P) code is transmitted on both the L1 and L2 signals. The P code provides Precise Positioning Service (PPS) for military use. This is the most accurate form of GPS.

Naturally, to guarantee the U.S. military's exclusive use, the P code is encrypted and impossible to jam. The SPS signal provides an accuracy of better than 25 m; however, sometimes the DOD subjects it to Selective Availability (SA) interference. When activated, SA inserts random errors in the data transmitted by the satellites. This reduces the accuracy to 100 m. However, a method known as Differential GPS (DGPS) has been devised which can improve the accuracy of the SPS signal to 1 cm.

GPS uses a simple mathematical formula, velocity times travel-time equals distance, in calculating positions. Since radio signals travel at the speed of light (186,000 miles per second) we know the velocity. Next we note at what time the satellite began the radio signal transmission and the time it arrived at the receiver. This will give us the travel-time. By plugging these numbers into the equation we have a rough idea of where we are from a given satellite. By taking readings from other satellites and applying the formula, we can narrow down our position possibilities. At least three satellites must be used to calculate latitude and longitude fixes. A fourth satellite is used to obtain altitude positioning and to compensate for any inaccuracies in the GPS receiver's clock.

For example, the reading from our first satellite places us at a distance of 13,000 miles. That means we can be anywhere on an imaginary sphere with a radius of 13,000 miles. The reading from our second satellite will narrow it down even more. If that distance is 12,000 miles then we are located somewhere on a small circle where the two imaginary spheres intersect. A third satellite, at a distance of 11,000 miles will give us another sphere. This third sphere will intersect with the circle formed by the other two spheres at two points. Our position will be one of these points. One point will be eliminated because it will be an impossible location. It may be located some where in space not even close to earth. You can also eliminate one of the points if you know your altitude. A sailor, for instance, knows he is at sea level, so he can discard any point which does not appear at sea level.

In the early days of GPS, before there were 24 satellites in position orbiting the earth, readings were usually made with just three satellites. Now that more satellites are available the majority of readings taken use four. A fourth satellite eliminates one of the final two points without the need for guess work. It also provides a three dimensional reading and greatly improved accuracy. This is important in mapping and geographic information system (GIS) data collection.

Using the velocity times travel-time equation requires a clock with high accuracies at measuring short time spans. A radio wave broadcasted by a GPS satellite can travel as fast as 6¦100th of a second. If the clocks in the satellite and the receiver were out of sync the distance measurement could be off by thousands of miles.

To alleviate the satellite portion of the clock problem the satellites are equipped with atomic clocks. These clocks do not use atomic energy; however, their name is derived from their method of using the oscillations of a particular atom as a metronome for keeping time. They are accurate to within one second every 70,000 years. Naturally they are very expensive, costing over $100,000 and each satellite has four to ensure that one is working at all times. Unfortunately the receivers can not use atomic clocks because they would be too expensive for practical use.

To over come the potential for clock error in the receiver a fourth satellite is used. By taking a measurement from a fourth satellite compensations can be made for any time error. So while it is theoretically possible to take a position with three satellites it is best done with four. If, after measuring the distances from the satellites the computer in the receiver notices that there is no intersecting points it will assume there is a time error and begin compensating to find the correct distance. In a very short time the computer will find a point where all four satellite ranges intersect. This will give you the position you are looking for.

As mentioned earlier, GPS has many uses. It is often used in fleet management for locating and tracking vehicles locally or worldwide. Their position can be measured down to a street address. This is important when dispatching vehicles. With persicion tracking, delivery schedules can be tighten leading to improved productivity. GPS can also be used to locate a specific vehicle on a large crowded work site. This can save time when equipment is in need of repair or service.

The accuracy and speed of surveying has improved greatly through the use of GPS. It has helped cut the costs of establishing local control points as well as eliminating the need for line of sight between points. In 1986, using traditional survey techniques to set a single control point cost almost $10,000. By using GPS, a two-person survey crew can do the same work for $200. A GPS-equipped survey team on foot can survey hundreds of points in an hour. Using a vehicle they can measure thousands of points in an hour. A survey now can even be performed by a single person, working in all types of weather any time, day or night. This may sound like an opportunity to overwork some one but it does have great safety-improving implications. Road surveying crews no longer need to be exposed to the dangers of heavy day time traffic. Work can now be conducted at night when traffic is lighter.

The use of GPS receivers in aerial cameras has greatly improved photogrammetry by reducing the amount of ground control thus allowing for faster aerial mapping. Accuracy also is increased when used with digital cameras. It is predicted that as more digital camera systems are used in conjunction with GPS the need for ground control will be totally eliminated.

Studies also are being conducted to use GPS in robotics applications for construction equipment. The Japanese have been experimenting with GPS to aid in the remote control of equipment such as dozers, graders and dump trucks. By using laser guidance and GPS positioning they can operate a vehicle from a safe distance away. This will allow machines to be used in unsafe environments which may pose a hazard to a human operator.

Lasers and GPS also can be used to improve the performance of manned machinery. While work is being done to position the blades of graders to better than 1 in., engineers at Ohio State University's Center for Mapping have been studying methods for using GPS in the guiding of bulldozer blades. They have found that by mounting a receiver-topped pole onto the dozer's blade they can achieve accuracies to within centimeters. There are also attempts to incorporate GPS into pavers; however, this is still at an experimental stage.

GPS also has applications to the bridge construction industry and is presently being used in building a bridge over Tokyo Bay. Its precision helps in the positioning of girders and other bridge components.

The use of GPS in construction has many benefits which translate into reduced construction costs, improved crew safety and increased quality of work. As more construction applications for this technology are discovered and perfected we will see even more benefits for our industry.

About the Author

David Banasiak

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