Alex Jones



Alex Jones

Geog 339

The GPS Experience

What is GPS?

There are many new complex technologies in the world today. Many of which help make jobs easier and more precise. Many different professions use a new technology that is often referred to as GPS. GPS or Global Positioning Systems is a complex system that was first used by the government, more specifically the U.S. Department of Defense. This wide spread technology is now used in surveying, aviation, and many other professional uses, not to mention private uses like finding your way to grandmother’s house. It is used for the most part to let people know where they are and how to get to where they want to be. The Military first started using GPS for similar purposes like navigating there way through sandstorms in Desert Storm. Even though private companies have made large advancements to bring GPS to the public, the Military still has large controls over the use of GPS.

So the real question on everyone’s mind is, How does GPS work? It is a very complex system that the military devised, but knowing the basics helps one understand the great technology. A main key to understanding GPS as a whole is the use of satellites. The first satellite was launched in 1978, with 9 more being launched within the next ten years. These first satellites were used in development. Then in 1989 the launching of the new line Block I satellites began. Within 5 years the government deployed 24 Block I satellites completing it’s system. Today there are about thirty satellites orbiting, of which not all are always transmitting data. Each satellite is in a specific orbit at a very high altitude. The system uses triangulation of satellites to calculate the basis of GPS. Complex mathematical equations are used in determining the correct information. The satellites use radio signals to transmit information to the receiver. A satellite also has an accurate clock to measure time of travel through space.

Knowing all of the information about the satellites is the backbone to GPS. With the help of the GPS receiver a triangulation of distances from satellites can be calculated. Taking the exact distance in space, measuring distance through travel time and Radio message position can be determined. Knowing that you are a certain distance from each satellite you can determine a position to a certain area in space. Using three satellites you can determine that you are positioned in space at one of two points. Using a fourth satellite you can narrow that position to the exact place in space you are. Mariners can also use the earth’s radius as a point because they are at sea level. The angle in which you receive each signal from the satellites is also important. If all angles are located in the approximate area in the sky then data maybe affected.

Other aspects that may affect the data include military jamming. The military doesn’t really jam the frequency, but instead gives out the wrong position for the satellites. Other little aspects of the system can be slightly flawed as well. Minor delays in the signal can occur when the signal travels through the atmosphere and ionosphere, difference in radio signal speed and multipath error due to buildings. The accuracy of the receiver also needs to be taken into consideration, although with most receivers having nanosecond accuracy that shouldn’t be a problem in most cases. Frequency is not a problem between satellites and receivers because all satellites use the same frequency to transmit data. Even though there are the imperfections in the system it can be and usually is very accurate.

What is Differential GPS?

Differential GPS is similar to normal GPS, but it uses another piece of equipment to become even more accurate then GPS. Differential GPS uses 2 receivers tracking the same satellite signals. When doing this one receiver is put into a known location. This allows for the calculation of combined error. So both sets of data can be used together to eliminate any errors that may occur. This process gets ride of most man made errors as well.

So with the use of a reference receiver, the satellite, and the wandering receiver error is located and destroyed. This process can be completed in the field instantly or it can be corrected later. The whole key to the differential correction is the reference or stationary receiver. This receiver is used to monitor and transmit errors and corrections. It then compares data to wandering receiver and the satellite data and obliterates bad data. It does all of this by using its known position and timing and plugging it into a mathematical formula to receive the delay error. The formula may be similar to this Theoretical time –Actual time = Error of Delay. That is just a general summary of part of an equation that may be used in practice. There isn’t a lot to differential GPS that is significantly different to that of GPS but that little bit can make a large difference.

Where in the world would someone ever use this GPS thing anyway?

This general question can really take you a long way. GPS is becoming one of the most widely used fancy new gadgets. Many people have these new contraptions in all of their different shapes and sizes. The machine can be used in many different fields for many different purposes. Businesses could use GPS for tracking a shipment of goods and locating a new building. Farmers can use GPS to have more precise planting of crops. The military uses GPS to plan attacks, locate troops and move those troops to new locations. It can also be used in Avionics to locate and track planes. People in general also like to use simpler forms of GPS to find their location and locate a position that they would like to go to.

You can use some Receivers to do much more than that though. A receiver can be used in surveying or even simpler mapping of a certain feature. Many scientists can use GPS record data more efficiently, by knowing the exact location of an event. If a scientist wanted to map out the frequency of a certain species of tree to a given area than he would have to got to that area and measure out each tree. Using GPS that same scientist could GPS each tree, label each tree and gather data for each tree using the receiver. When he had the data it would already be in the computer where he could manipulate the data as wanted. When talking about manipulating data one could not leave out the military. The military does manipulate data, for their protection, not to hinder the use for everyone else. They also use it for many things that are good for them selves. In a storm the military can use GPS to maneuver troops to new locations. They can also use GPS to locate their own tanks and equipment, so their own troops are safer.

There are many other things that people can do with GPS; the possibilities are almost endless. One could take GPS units to almost anywhere in the world and still receive information. GPS can now be used at sea, in the air or by land. Someday GPS will probably be used on every surface known to mankind, in all types of climate and all types of conditions. Someday GPS may even be used on other planets. GPS can and will be used for many different purposes and over time will only get more accurate and wide spread.

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