GPS (Global Position System) - pages 1 & 2
(Located at http://library.advanced.org/27887/gather/advanced_technology/gps.shtml)



How often have you gotten lost on a road trip, or tried to guess what kind of weather was headed your way. Well, a fairly new technology called GPS has made it possible to tackle some of life’s most ancient problems. GPS (Global Positioning System) is a constellation of 24 satellites owned and operated by the Department of Defense (DOD). Scientist and military personal have largely used the system, but in recent years has become a way for ordinary people to pinpoint their geographic location and make life a bit easier.

The location accuracy of GPS can be anywhere from 100 to 10 meters depending on your equipment. Military-approved equipment however can pinpoint accuracy within one meter. In Advanced forms of GPS you can make measurements to better than a centimeter, literally giving each space on earth a specific address. Currently, GPS satellites and 3 spares orbit above the earth about 10,600 miles. Each satellite contains a computer, an atomic clock, and a radio. The satellite is enabled to continually broadcast its changing position. Each day at a designated time, the GPS satellites check their own systems with a ground station to make minor corrections. The satellites are spaced in equal distance from each other in a way that four satellites will always be above the horizon.


The Space Segment

The space segment consists of the GPS satellites. The system has 24 operational satellites that orbit around the earth in about 12 hours. Depending upon satellite conditions, and possible repair needs, additional satellites may be launched on a needed basis. There are 6 orbital planes, with 4 satellites per plane. They exist equally apart at 60 degrees, and are inclined 55 degrees with respect to the plane. This way, from any point on earth, there are always 5 to 8 satellites visible.


The Control Segment

The GPS consists of several ground control stations, with the main control station located in Schriever Air Force Base in Colorado. Each individual monitor station measures signals from the space vehicles (SV’s) located inside each satellite. The main station in Colorado uploads ephemeral and clock data to the SV’s. The SV’s then send subsets of orbital data to PPS receivers. They use very precise radar to check each satellite's exact altitude, position and speed. GPS satellites are so high up their orbits is very predictable. The errors they're checking for are called "ephemeris errors" because they affect the satellite's orbit or "ephemeris." These errors are caused by gravitational pulls from the moon and sun and by the pressure of solar radiation on the satellites. The errors are usually insignificant, but for the most part, if you want to have good accuracy you will want to make sure you have no errors coming in. Once the DoD has measured a satellite's exact position, they relay that information back up to the satellite itself. The satellite then includes this new corrected position information in the timing signals it's broadcasting. It also contains a navigation message with ephemeris information as well.

DoD intentionally sends noise into each signal to create inaccuracy. The purpose of this is to ensure no hostile force uses GPS as means to exploit a terrorist agenda. Military personal can use access codes to decrypt the errors; therefore they are still able to use GPS with great accuracy.


The User Segment

The user segment consists of the GPS receivers and the users themselves. Navigation is the primary function of GPS. Navigation receivers are made for various ships, vehicles, and people. Time and Frequency have also become a popular use for GPS, as laboratory standards can be set to precise time signals, and within well-referenced points. With special purpose GPS receivers, precise positioning is possible using receivers at reference locations providing correction and relative positioning data for remote receivers. Examples of this include surveying, geological studies, and plate tectonic studies.

GPS at Work

The first and most obvious application of GPS is the simple determination of a "position" or location. GPS is the first positioning system to offer highly precise location data for any point on the planet, in any weather. That alone would be enough to qualify it as a major utility, but the accuracy of GPS and the creativity of its users are pushing it into some surprising realms. Knowing the precise location of something, or someone, is especially critical when the consequences of inaccurate data are measured in human terms. For example, when a stranded motorist was lost on in South Dakota for two days. GPS helped rescuers find her.

Sometimes an exact reference locator is needed for extremely precise scientific work. Just getting to the world's tallest mountain was tricky, but GPS made measuring the growth of Mount Everest easy. The data collected strengthened past work, but also revealed that the mountain itself is getting taller.

On the Water

It's interesting that the sea, one of our oldest channels of transportation, has been revolutionized by GPS, one of the newer navigation technologies. And as you would expect, navigating the world's oceans and waterways is more precise than ever. Today, receivers exist on vessels the world over, from hardworking fishing boats and long-haul container ships, to elegant luxury cruise ships and recreational boaters. A New Zealand fishing company uses GPS so they can return to their best fishing holes without wandering into the wrong waters in the process.

In the Air

By providing more precise navigation tools and accurate landing systems, GPS not only makes flying safer, but also more efficient. With precise point-to-point navigation, GPS saves fuel and extend an aircraft's range by ensuring pilots don't stray from the most direct routes to their destinations. GPS accuracy will also allow closer aircraft separations on more direct routes, which in turn means more planes can occupy our limited airspace. The moon and stars have always been a guiding light to many of navigators in the past. Tools like the compass and good memory for landmarks helped you get from point A to point B. However, the situation has never been perfect, and knowing ones location and path have never been that accurate. Today’s outdoorsman are using GPS to remedy the age-old science of finding ones way.

On the Ground

Tracking is the process of monitoring the user as he or she moves from location to the next. Commerce relies on fleets of vehicles to deliver goods and services either through such areas as crowed cities. So, effective fleet management has direct bottom-line implications, such as telling a customer when a package will arrive, spacing buses for the best scheduled service, directing the nearest ambulance to an accident, or helping tankers avoid hazards. GPS used in conjunction with communication links and computers can provide the backbone for systems tailored to applications in agriculture, mass transit, urban delivery, public safety, and vessel and vehicle tracking. Cities like Chicago have developed systems using GPS that track down ambulances, making it easier to save lives in some of today’s fastest growing metropolitan areas. Many of today’s emergency units are now finding it easier to respond to 911 calls.

GPS technology has made surveying easier than ever as well. GPS can pinpoint positions and routes. Mountains, rivers, forests, roads, routes, and city streets can all be surveyed using GPS. The city of Modesto California improved their efficiency and job performance by using GPS and mountain bikes to create a precise map of its network of water resources and utilities.

Timing is Everything

Although GPS is well known for navigation, tracking, and mapping, it's also used to disseminate precise time, time intervals, and frequency. Time is a powerful commodity, and exact time is even better. Knowing that a group of timed events is perfectly synchronized is often very important. There are three fundamental ways we use time. As a universal marker, time tells us when things happened or when they will. As a way to synchronize people, events, even other types of signals, time helps keep the world on schedule. GPS satellites carry highly accurate atomic clocks. And in order for the system to work, our GPS receivers here on the ground synchronize themselves to these clocks. That means that every GPS receiver is, in essence, an atomic accuracy clock. Astronomers, power companies, computer networks, communications systems, banks, and radio and television stations can benefit from this precise timing.


Differential GPS

GPS is the most advanced navigational application made for the average users. For most of its uses, including location and weather, it is extremely accurate. So why would there be a need for anything else? Well, taking human nature into account, it’s always been a passion to want bigger and better tools. So some crafty engineers came up with Differential GPS, a means to correct the various inaccuracies and built in imperfections of the GPS system.

How it Works

Differential GPS or "DGPS" works a bit differently than regular GPS. It can yield measurements well into to a couple of meters in moving applications and even better in stationary situations. This improved accuracy has made a significant improvement in the GPS system. With it, GPS becomes more than just a system for navigating stations and satellites. In a sense it becomes a universal measurement system capable of positioning things on a very precise scale, mapping you down to your exact coordinates. Differential GPS involves the cooperation of two receivers, one that's stationary and another that's roving around making position measurements.

The stationary receiver is the key to DGPS, it allows for the satellite measurements to be tied into a single reference. GPS receivers use timing signals from at least four satellites to establish a position. Each of those timing signals is going to have some error or delay depending on weather conditions or other atmospheric chaos as the signal is sent from the satellite the a receiver. Since each of the timing signals that go into a position calculation has some error, that calculation then becomes compounded as the system goes from one point to the next.

We are so far away from the satellites, that even a few hundred kilometers is a short distance. The little distances we travel here on earth are insignificant when compared to the vastness of space. So if two receivers are fairly close to each other, say within a this hundred kilometers, the signals that reach both of them will have traveled through virtually the same space in the atmosphere, and so will have virtually the same errors. In essence, that's the idea behind differential GPS. One receiver measures the timing errors and then provides correction information to the other receivers that are roving around. That way virtually all errors can be eliminated from the system, as well as the Selective Availability error that the DOD puts into the signal for national security reasons.

Basically what you do is ground the reference receiver in a place that has been well surveyed and keep do not allow it to move. Basically it calculates the normal GPS equation in reverse, and uses its position to calculate time. DGPD uses its known position to calculate exact timing. It figures out what the travel time of the GPS signals should theoretically be, and then compares the data to the actual statistics. One more difference includes the correction of errors. That is, the receiver transmits the normal error information to the roving receiver so it can use it to correct the measurements in everything else.

The receiver basically contacts all the satellites to fix the errors. It also encodes the information into a standardized format, and eventually sends it to the roving satellite. Basically it’s similar to synchronizing watches, or tuning into the same channel on a network of radio receivers. The U.S. Coast Guard, as well as other agencies, are trying to put up receiver stations around busy harbors and bays. Anyone in the local harbor or bay area can receive DGPS corrections because the U.S. Coat Guard uses the radio frequencies already in use in the area for Maritime vessels. Many new GPS receivers are being designed to accept corrections, and some are even equipped with built-in radio receivers.

The possibilities for DGPS seem endless. One can monitor better tracking by enabling ambulances with GPS receivers so that operators and other emergency personal can monitor a situation with ease and precision. You can easily use a radio frequency for this, and it would be running off the same GPS system as normal, accept now it would be subject to the local corrections, making it more accurate.