Running Head: DIFFERENTIAL GLOBAL POSITIONING SYSTEM 1

Differential Global Positioning Systems
Clay Guida
Southern Illinois University

DIFFERENTIAL GLOBAL POSITIONING SYSTEM 2
Global Positioning Systems (GPS) have become one of the greatest innovations of the modern world. Simply put, your personal guide to anywhere in the world. It can accurately get a fix on your position to within 100 meters. This is where most people would say that their GPS gets them down to ten meters or less, and they would be right. But it is GPS in coordination with the Differential Global Positioning System (DGPS) that gets them that accurate of a location. To understand this we must start from the beginning and go over GPS’s history. From there we will take a look at what causes some of the inaccuracies that GPS can. Then we can see how DGPS counters these and gives you the most accurate reading you can have. Differential Global Positioning System, GPS’s right hand man for accuracy.
To understand how a DGPS helps a GPS we start at the beginning of it’s short history. Though made for use by the United States government, it was the Soviet Union that launched Sputnik in 1957 and gave us the idea. After the launch, Massachusetts Institute of Technology (MIT) researches observed that they could pick up the radio signal that Sputnik was producing. They also realized that the sound of this transmission varied with the proximity of Sputnik to its antenna receiver. As the satellite got closer to the receiver the strength of the signal got stronger. Likewise the satellite drawing farther away caused the signal received to decrease. This is called the Doppler shift or effect.
It was these increases and decreases that allowed the United States scientists to be able to track the satellite and its exact orbit. The proof that a satellite's orbit could be precisely determined from the ground was the first step in establishing that positions on
DIFFERENTIAL GLOBAL POSITIONING SYSTEM 3 the ground could be determined by homing in on the signals broadcast by satellites or “artificial stars”. The United States Navy saw big promise in this and started a program called Transit. Transit was conceived in the early 1960s to support the precise navigation requirements of the Navy's fleet ballistic missile submarines. There were at first four, and later added two more to make a total of six satellites that were used to locate and help navigate the fleet. Due to a lack of satellites the Navy would only be able to approximate their ships once every few hours, and then only to within 10 minutes of a degree. Though pretty advanced for its time, the Department of Defense (DOD) wanted a system that could provide a more accurate, continuous display of navigational information. The DOD coincided with Ivan Getting of the Raytheon Corporation on a project that was called NAVSTAR GPS. Later this was shortened to just GPS.
Getting, along with Air Force Colonel Brad Parkinson brainstormed that a constellation of satellites, or artificial stars, could be used as reference points to calculate positions accurately on earth to a matter of meters. The NAVSTAR project began in 1973 and only five years later launched its first satellite into orbit. The constellation of satellites was complete in 1995. These 24 satellites orbit the globe at 12,000 miles above the earth. Each one in its own orbit and are used to give precise information on the location of GPS units on the ground.
So here’s how it works. At any one given time there are at least three satellites orbiting the earth that are within range of the ground GPS receiver you may have in your car or plane. These three satellites are used to triangulate the position of the receiver that
DIFFERENTIAL GLOBAL POSITIONING SYSTEM 4 by sending a series of specially coded messages. The coded message is heard by the GPS receiver. From these, the receiver on the ground can determine when a timing signal has left the satellite, and also when that same signal arrives at its own antenna. The difference between the two is the travel time that the signal took. From this the receiver can calculate the distance it is away from the satellite. With that measurement the receiver can narrow down where on the surface of the earth it is to that satellite. Bringing in another reading from a different satellite helps to narrow it down even farther, being that it can only be so far from satellite one, and so far from satellite two. This creates a circle of points from which the receiver must be in. The measurement signal from the third satellite will intersect the circle formed by the other two and hone in to its position in that circle of points to within 100 meters of the receiver. So we went from 10 minutes of a degree every few hours, to within 100 meters of the receiver at any given time. Huge leap, but that can still be a pretty wide spectrum. This is where DGPS steps in. It doesn’t just give you the ability to navigate boats or planes but can help GPS to become a universal measurement system, capable of positioning things on a very precise scale. DGPS works to cancel out many of the man- made and natural errors that are the norm for GPS measurements. It does so by using two different receivers. The first receiver is the one that you have that is roving around the globe. This one is going to receive the inaccurate GPS signals. Now we put another GPS receiver in a position where we know exactly where it is and leave it there as a standard. This receiver will then be used as a reference and will continuously calculate its measurements which it knows to be true against that of the satellite data. The reference
DIFFERENTIAL GLOBAL POSITIONING SYSTEM 5 receiver compares its own data with that of the satellites and comes up with the difference. This difference is the error that the satellite is sending out. From here the reference receiver computes the errors that the GPS is sending out and calculates the corrections for these errors, sending these corrections to the roving receiver. The roving receiver can then apply these corrections and give a reading that is far more accurate than just the satellites alone. So let’s take a look at how these satellites can be sending erroneous signals. The first error is in the satellites themselves. The satellites are equipped with what is called an atomic clock, and though they are very accurate this type of clock is not perfect. Their drift in the amount of time might be minuscule, but even the slightest error can lead to our GPS’s measurement being off. This in turn gives us an inaccurate position. The satellites are also prone to drifting out of their known path of orbit which all of these calculations are based on which can also lead to inaccuracies. The earth’s atmosphere is another thing that has its hand in making the satellites signal inaccurate. As the signal travels through the atmosphere it is not just traveling through the air but is being influenced by the water vapor in the troposphere, and the highly charged particles in the ionosphere. This causes the signal to once again become delayed, further decreasing the accuracy that the receiver is going to read. Reflections off of obstructions can cause what is called multipath error. This is where the receiver reads the original signal, and then later reads the erroneous signal that

DIFFERENTIAL GLOBAL POSITIONING SYSTEM 6 has bounced off of the obstruction. A good way to explain this is to think about how TV’s use to get a ghost like image on it from the same sort of effect. The two signals may come just nanoseconds from one another but this can lead to signal error, and thus more of inaccuracies in the roving GPS receiver. As the receivers themselves have their own internal clock, it is important to understand that they too can be faulty and cause their own inaccuracies. Once again, no clock is perfect. The receiver is also affected by its own internal noise. All errors but the last can be solved by the use of DGPS. I feel it is a strong point to put here that before May 2, 2000 there was another source that was known as Selective Ability (SA). This was basically the United States government causing intentional error that would lead to inaccuracies. The reasoning they implemented this was because of national security. Almost instantly as they got rid of SA, GPS started to boom to the common house hold name status that they hold of today. So now that we know what kind of errors we know are being sent we get back to DGPS and what it does. The key to this is the relationship between the two ground based receivers. One that is put in place and held as a standard and the other that the consumer uses that is free to wander around. The one that is set as a standard receives the signals from the satellites and instead of using timing signals to calculate its position, it uses its position to calculate the timing. From this it uses the difference between the two signals to calculate the error of the satellite, it will then calculate the corrections for these satellite signals and simultaneously send these corrected signals to any mobile receiver
DIFFERENTIAL GLOBAL POSITIONING SYSTEM 7 that is within its area. The mobile receiver uses this information and makes corrections giving you a very precise and accurate reading. The reading can be as close as a few meters in moving applications, and in still applications it is good down to inches. That’s what makes DGPS so great, it takes the already advanced system of GPS and brings it to a more accurate level than ever before. So though DGPS doesn’t get the recognition it should, that is simply because most people don’t even know that it is already working with their GPS to make it even better. We started with the history of the GPS, which had Transit bring us in the ball park. Then it wasn’t long before NAVSTAR brought us close. To get the most accurate reading possible though it took DGPS and its ability to sense, calculate and correct GPS’s errors that have brought us to inches of wherever we want to be. GPS aided by its unseen hero, DGPS.

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REFERENCES
Aeronautics and Space Engineering Board, National Research Council. (1995) The Global Positioning System: A Shared National Asset (49-51,99,102)
James, R. (2009) A Brief History of GPS. Time Magazine. This article was retrieved from http://www.time.com/time/nation/article/0,8599,1900862,00.html
Hurn, J. (1993) Differential GPS Explained. Sunnyvale, CA, Trimble Navigation, Ltd., (3-7, 9-15, 32-45)
Steede-Terry, K. (2000,) Integrating GIS and the Global Positioning System. Redlands, CA, ESRI Publications (13-18)

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All Poems - All Soapbox | | | Fly-by-wireFly-by-wire describes a system where no direct mechanical connections exist between the steering instruments of an aircraft and the aerodynamic control surfaces such as flaps and rudder. Under the traditional mechanical control system, which is still used on light aircraft, the pilot adjusts the aerodynamic control surfaces via a system of levers, wires, and pushrods. Often, servo-tabs and aerodynamic balances are used to reduce the amount of force required to operate the controls. On larger or higher performance aircraft, the control forces the pilot must apply increase rapidly and even with servo-tabs it is usually necessary to power the control system. This operates much as an automotive power steering system does: it converts the pilot's control inputs into a hydraulic impulse that is transferred via a system of small pipes to servos that drive the control surfaces. (Alternatively, the system may be pnuematic or use electric motors, but the principle remains the same.) Feedback to the pilot is generated artificially. There is still, however, a direct (although not necessarily linear) relationship between the pilot's control inputs and the movement of the control surface. In a fly-by-wire system the pilot's control inputs are fed to a computer which decides, by consulting the rules with which it has been programmed, which control surfaces to operate, and by how much. Normally the control surfaces will move as the pilot commands, but in some cases the fly-by-wire system may modify the response depending on the particular circumstances. If the pilot's control input is considered unsafe it may not be carried out at all. In this respect fly-by-wire represents a major departure from all that has gone before, since it takes a measure of ultimate authority from the pilot and gives it to the system designers. The first fly-by-wire aircraft were military jet fighters in which the system was intended to give the fighters greater maneuverability. In many modern fighters, the aircraft is designed to be aerodynamically unstable and cannot be controlled without computer monitoring. In 1988, Airbus was the first company to introduce fly-by-wire in a commercial aircraft, the A320. On June 26 1988 an A320 crashed at an airshow at Mulhouse-Habsheim Airport in France killing three passengers. The pilot claimed that the fly-by-wire system prevented him from flying safely, but the official inquiry returned a finding of pilot error. The A320 went on to be one of the most successful aircraft ever made. Many other modern large commercial aircraft now use fly-by-wire, other Airbus designs to begin with, and more recently newer models from Boeing. | |