An Introduction What is GPS? to GPS...An Introduction to GPS By: Corrie Neuens Geography 339 Dr....
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Figure 4: Time difference between pseudo-random codes of receiver and satellite Figure 3: Four GPS satellites intersecting Figure 1: GPS Control Figure 2: Triangulation of satellites to determine position on earth Figure 5: Differential GPS What is GPS? GPS or Global Positioning System was invented by the Department of Defense to keep track of where things are on the earth. It is comprised of 24 satellites orbiting the earth at a high enough altitude to avoid interference with land based systems. Data is transferred between a satellite and control station. Data is then sent from the satellite to the receiver or GPS unit (Figure 1). There are five basic steps on how a GPS works: 1.) Triangulation from satellites is the basis of the system 2.) To triangulate, GPS measures distance using the travel time of a radio message 3.) To measure travel time, GPS needs very accurate clocks 4.) Once you know distance to a satellite, you then need to know where the satellite is in space 5.) As the GPS signal travels though the ionosphere and the earth's atmosphere, it gets delayed T riangulation uses a minimum of three satellites to determine a rough location on the earth, but four will give an exact location. This location is pinpointed by marking the intersection of these imaginary spheres around each satellite (Figure 2-3). Satellite position is located by determining the distance to each satellite using the equation for velocity: D=speed of light * time Timing It is vital that the timing of the receiver and the satellite are the same otherwise the location could be off by several hundred feet. In order to make sure the timing is the same for both receiver and satellite an identical code is sent out by each at the same time. This code is known as the pseudo-random code. The time the code was sent by the satellite is compared to the time the code is received by the receiver to determine the time difference(Figure 4). Accurate timing is vital in determining the distance to satellites. The satellites are very accurate because they have high-tech atomic clocks in them, but the receivers clocks are not as good. Receivers clocks do not have to be as good because errors can be cancelled out using a fourth satellite to determine an exact location. How does Real Time DGPS of Trimble's ProXR's Work? Accuracy of global positioning systems can be improved even more using a technique called "differential GPS". This can reduce uncertainty of the measurments to within a meter. This is achieved by putting a GPS receiver on the ground in a known location to act as a reference point. It can transmit an error correction message to other receivers in the area, and use that error message to correct their positions. This works because the satellites are so high up that any errors measured on one receiver will be almost the same as on any other receiver in the same vicinity. Because the GPS signal is so simple, this correction factor takes care of all errors including: receiver clocks, satellite clocks, the satellite's position, or ionospheric and atmospheric delays (Figure 5). Figure 6: Geometric Dilution of Precision (GDOP) An Introduction to GPS By: Corrie Neuens Geography 339 Dr. Sean Hartnett 10-13-99 Ionospheric and Atmospheric Delays The GPS system is as accurate as it can be because many possible errors have been eliminated. Clocks on satellites are precise and an extra measurement can be taken to eliminate any error on a receiver clock. Satellites even broadcast corrections in their orbital positions every minute. Although many errors are eliminated, there are a few sources of error that remain. One of the main errors is located in the earth's ionosphere. The ionosphere is a blanket of electrically charged particles 80 to 120 miles above the earth. The particles affect the speed of light and therefore the speed of GPS radio signals. Light slows down and speeds up depending on the material it is traveling through, in this case a thick blanket of particles. There are a few ways we can lower the error caused by the ionosphere. We can predict what the typical speed variation is on an average day, but not every day is average. Another way is to compare two signals and look at their variation in speed. Although this kind of error correction is very sophisticated and requires the most advanced GPS receivers. After the GPS signals travel through the ionosphere they have to deal with atmospheric blockades, mainly weather. The amount of water vapor in the atmosphere can affect the signals, but this error is almost impossible to correct. Luckily, the error is small and accounts for less than the width of an average street. Other Errors Even though the atomic clocks on satellites are very accurate, they are still subject to small variations that are corrected by the Department of Defense. The receiver may also make mistakes by rounding mathematical operations or electrical interference. These can be small or big, and can throw a few feet of uncertainty into every measurement. There are many kinds of error that all add up depending on atmospheric and equipment conditions. The accuracy can also be lessened on purpose by the Department of Defense using an operational mode called "Selective Availability" or "S/A". If this is implemented it will be the source of the greatest error (Figure 7). Bibliography Hurn, Jeff. GPS: A Guide to the Next Utility.Trimble Navigation Ltd. Sunnyvale, CA. 1989 Figure 7: Error Budget Another type of error is called "multipath" error where a GPS signal bounces from satellite to satellite before reaching the receiver. Special antennae are used to minimize this problem, but it still adds some uncertainty to a GPS measurement. The geometry of the satellites or angles they send signals at can affect the accuracy of the GPS measurements. This is called "Geometric Dilution of Precision" or GDOP. The geometry of the satellites can lessen or magnify the GPS results. If two satellites are farther apart they have a better measurement then two that have a smaller angle (Figure 6).
Transcript of An Introduction What is GPS? to GPS...An Introduction to GPS By: Corrie Neuens Geography 339 Dr....
Figure 4: Time difference between pseudo-random codes of receiver and satellite
Figure 3: Four GPS satellites intersecting
Figure 1: GPS Control
Figure 2: Triangulation of satellites to determine position on earth
Figure 5: Differential GPS
What is GPS?GPS or Global Positioning System was invented by the Department of Defense to keep track of where things are onthe earth. It is comprised of 24 satellites orbiting the earth at a high enough altitude to avoid interference with land based systems. Data is transferred between a satellite and control station. Data is then sent from the satellite to the receiver or GPS unit (Figure 1).
There are five basic steps on how a GPS works:1.) Triangulation from satellites is the basis of the system2.) To triangulate, GPS measures distance using the travel time of a radio message3.) To measure travel time, GPS needs very accurate clocks4.) Once you know distance to a satellite, you then need toknow where the satellite is in space5.) As the GPS signal travels though the ionosphere and the earth's atmosphere, it gets delayed
T riangulation uses a minimum of three satellites to determine a rough location on the earth, but four will give an exact location. This location is pinpointed by marking the intersection of these imaginary spheres around each satellite (Figure 2-3).Satellite position is located by determining the distance to each satellite using the equation for velocity: D=speed of light * time
TimingIt is vital that the timing of the receiver and the satellite are the same otherwise the location could be off by several hundred feet. In order to make sure the timing is the same for both receiver and satellite an identical code is sent out by each at the same time. This code is known as the pseudo-random code. The time the code was sent by the satellite is compared to the time the code is received by the receiver to determine the time difference(Figure 4).
Accurate timing is vital in determining the distance to satellites. The satellites are very accurate because they have high-tech atomic clocks in them, but the receivers clocks are not as good. Receivers clocks do not have to be as good because errors can be cancelled out using a fourth satellite to determine an exact location. How does Real Time DGPS of Trimble's ProXR's Work? Accuracy of global positioning systems can be improved even more using a technique called "differential GPS". This can reduce uncertainty of the measurments to within a meter. This is achieved by putting a GPS receiver on the ground in a known location to act as a reference point. It can transmit an error correction message to other receivers in the area, and use that error message to correct their positions. This works because the satellites are so high up that any errors measured on one receiver will bealmost the same as on any other receiver in the same vicinity. Because the GPS signal is so simple, this correction factor takes care of all errors including: receiver clocks, satellite clocks, the satellite's position, or ionospheric and atmospheric delays (Figure 5).
Figure 6: Geometric Dilution of Precision (GDOP)
An Introductionto
GPS
By: Corrie Neuens
Geography 339Dr. Sean Hartnett
10-13-99
Ionospheric and Atmospheric Delays The GPS system is as accurate as it can be because many possible errors have been eliminated. Clocks on satellites are precise and an extra measurement can be taken to eliminate any error on a receiver clock. Satellites even broadcast corrections in their orbital positions every minute.Although many errors are eliminated, there are a few sources of error that remain. One of the main errors is located in the earth's ionosphere. The ionosphere is a blanket of electrically charged particles 80 to 120 miles above the earth. The particles affect the speed of light and therefore the speed ofGPS radio signals. Light slows down and speeds up depending on the material it is traveling through, in this casea thick blanket of particles.There are a few ways we can lower the error caused by the ionosphere. We can predict what the typical speed variation is on an average day, but not every day is average. Another way is to compare two signals and look at their variation in speed. Although this kind of error correction is very sophisticated and requires the most advanced GPS receivers.After the GPS signals travel through the ionosphere they have to deal with atmospheric blockades, mainly weather. The amount of water vapor in the atmosphere can affect the signals, but this error is almost impossible to correct. Luckily, the error is small and accounts for less than the width of an average street. Other Errors Even though the atomic clocks on satellites are very accurate, they are still subject to small variations that are corrected by the Department of Defense. The receiver may also make mistakes by rounding mathematical operations or electrical interference. These can be small or big, and can throw a few feet of uncertainty into every measurement.
There are many kinds of error that all add up depending on atmospheric and equipment conditions. The accuracy can also be lessened on purpose by theDepartment of Defense using an operational mode called"Selective Availability" or "S/A". If this is implemented it will be the source of the greatest error (Figure 7).
BibliographyHurn, Jeff. GPS: A Guide to the Next Utility.Trimble Navigation Ltd. Sunnyvale, CA. 1989
Figure 7: Error Budget
Another type of error is called "multipath" error where a GPS signal bounces from satellite to satellite before reaching the receiver. Special antennae are used to minimize this problem, but it still adds some uncertainty to a GPS measurement.The geometry of the satellites or angles they send signals at can affect the accuracy of the GPS measurements. This is called "Geometric Dilution of Precision" or GDOP. The geometry of the satellites can lessen or magnify the GPS results. If two satellites arefarther apart they have a better measurement then two that have a smaller angle (Figure 6).
