The Global Positioning System (GPS) is a constellation of satellites that is primarily used to determine an exact location anywhere on the Earth. This is extensively used by the United States military, and the technology has more recently been used for the navigation of civilian cars, trucks, and planes. Furthermore, the GPS technology has been used by businesses to help in the tracking of shipments that are crucial to the organization. This overview is intended to give a general understanding of the global positioning system as it is used for navigational purposes in all aspects of life. In addition to an explanation about the workings of this system, several examples of GPS technology will be given to demonstrate the civilian uses for this navigational system.

Several years ago, the United States Department of Defense decided that the military needed a super-precise worldwide positioning system. With an initial cost of $12 billion, a constellation of 24 satellites was launched to make the system operational. The current satellites that are used in this constellation are called NAVSTAR and are manufactured by Rockwell International. These satellites, which are located at an elevation of 10,900 nautical miles from the Earth, weigh approximately 1,900 pounds when in orbit. Furthermore, these satellites have an orbital period of 12 hours, and they are located in an orbital plane that is 55 degrees to the equatorial plane. The planned lifespan of each individual satellite in the 24-satellite constellation is 7.5 years.

Diagram of GPS constellation

The satellite system maintains its accuracy through five ground stations that constantly monitor the GPS. These ground stations, which are located in Hawaii, Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs, continually check the operational health of the system and generate an exact position in space for each satellite.

In order for the system to accurately locate a specific place on the Earth, the GPS uses triangulation from three different satellites. In order to accomplish this, a GPS receiver measures the distance using the travel time of radio signals by a very accurate timing mechanism. Along with distance, the GPS needs to know exactly where each satellite is currently located in space. This is accomplished through high orbits and careful monitoring. However, the final step of getting this system to work is that it must be corrected for any delays that the signal experiences as it travels through the atmosphere.

For this to occur, three separate satellites generate a distance of a particular receiver from the individual satellite. After obtaining these three measurements, it becomes apparent to the system that the location is one of two points on the Earth. A fourth measurement would solve this problem, but the system is usually able to throw one of the two possible sites out because it is either too far from the Earth or it is moving at an impossible velocity. To measure the distance of something that is floating around in space, the satellites will then utilize travel time.

The measuring of travel time, which requires extremely precise clocks, is done by using what is called "Pseudo Random Code." The Pseudo Random Code is a complicated sequence of on and off pulses that resembles random electric noise. The GPS satellites use one of their two carrier frequencies to transmit this code. The first frequency (the L1 carrier) operates at 1,575.42 MHz and carries both the status message and the Pseudo Random Code. The second frequency (the L2 carrier) operates at 1,227.60 MHz and is used for the more precise military Pseudo Random Code.

The satellite system is capable of generating two types of Pseudo Random Code. The first type, which modulates the L1 carrier, is called the Coarse/Acquisition (C/A) code. This repeats every 1,023 bits and it modulates at a 1MHz rate. Each satellite has a unique Pseudo Random Code, and the C/A code is the basis for civilian GPS. The second type of this code is called the precise (P) code. This repeats on a seven-day cycle and modulates both the L1 and L2 carriers at a 10 MHz rate. Because this code can be encrypted, it is intended for military users (It is called ‘Y’ code when it is encrypted).

The satellite is able to measure travel time because it can make the assumption that both the satellite and the receiver are generating the same Pseudo Random Code at exactly the same time. The receiver then compares how late the satellite’s code appears to be to its’ own code, and it can then determine how long it took for the signal to reach the receiver. The travel time is then multiplied by the speed of light, and the distance is discovered.

With the measuring of travel time key to the operation of the GPS satellites, it is extremely important that the onboard clocks are almost perfect. This is possible because the satellites have incredibly precise atomic clocks. These clocks, which use the oscillations of a particular atom as a metronome, are the most stable and accurate reference that has ever been developed. However, these atomic clocks, which cost between $50,000 and $100,000, are too expensive to be used in the system’s receivers. Because of this, each receiver must use a trick to get by with a much less accurate clock.

This is possible through the addition of a fourth satellite to the measurement. The idea is that if three perfect measurements can locate a point in 3-dimensional space, then four imperfect measurements can do the same. If the receiver’s clocks were perfect, for example, then all satellite ranges would intersect at a single point (the position of the receiver). However, with an imperfect clock, the fourth measurement will not intersect with the first three. The satellite then realizes the discrepancy and begins to look for a single correction factor that it can subtract from all its timing measurements that would cause them to all intersect. This would then allow for the satellite and receiver to both be in sync with universal time.

The constellation of satellites that make up GPS, which was completed with the launch of the 24^th satellite in March of 1994, are placed in a very specific orbit by the United States Air Force that has satellites spaced so that a minimum of five satellites are in view from every point on the Earth. Each of the ground GPS receivers has an almanac programmed into its computer that tells it where each satellite is moment by moment.

Although the basic orbit of the GPS satellites are very exact, the five ground stations (each controlled by the U.S. Department of Defense) constantly use very precise radar to monitor the satellites exact altitude, position, and speed. The most prevalent error that these ground stations are checking for are ephemeris errors. These errors are caused by the gravitational pulls from the moon, and sun, and by the pressure of solar radiation on the satellites.

After the ground station measures the exact position of the satellite, it is then transmitted up to the individual satellite. This allows for the satellite to broadcast its corrected position information in the timing signals that it sends out.

In every wireless transfer, it is necessary to take into account the errors that might occur from different atmospheric conditions. Because of this, a good GPS receiver should always realize that there is a wide amount of different errors that could occur. For instance, in calculating distance to a satellite, a receiver uses the speed of light, but this measurement is only exact in a vacuum. To account for the errors that can occur when the signal passes through the ionosphere and the troposphere, the GPS satellites and receivers must compare the relative speeds of two different signals. This dual frequency measurement, however, is only possible with advanced receivers.

Another error that may occur for the GPS receivers is that the signal may bounce off of various structures before it reaches a certain receiver. However, good receivers use sophisticated signal rejection techniques to minimize this problem. As far as the satellites are concerned, there can also be errors with the precise atomic clocks that are on board. To correct this, good receivers are able to make geometric measurements that allow for it to pick the satellites that will give the lower margin of error.

In addition to these errors that occur from certain atmospheric conditions, there are also intentional errors that occur with the GPS satellites. These intentional errors, which are mandated by the U.S. Government (this is up for review in the year 2000), are called Selective Availability (SA). The entire point of this is so that the government can protect against the use of GPS by a hostile force or terrorist group to make accurate weapons. This is accomplished when the Department of Defense introduces noise into the satellite’s clock data. Furthermore, the DoD can also send incorrect orbital data to the satellites that they transmit back to the receivers on the ground as part of their status message. The military, however, uses a decryption key to remove the SA errors from the signal.

Now that the basic operation of the GPS system is understood, this report will focus on several civilian examples of location heading systems that use this technology. It is important to understand that although this technology is beginning to be used by the public, the United States Government still maintains control of its resources and is a massive user of the technology.

Recently, especially in the past year, the GPS technology has become very available to the general public and other non-military personnel. For instance, currently the GPS satellites are used in activities from keeping a hiker on the correct path with a GPS compass to getting an ambulance at a residence quicker. After viewing this next section, it will become obvious that GPS technology is gaining in popularity and is used in many important functions.

The Alpine Car Navigation System, which uses the global positioning system to operate, is used primarily for navigation while travelling in an automobile within the contiguous United States. CarNav, the company that offers this service out of Marina Del Rey, California, states that their system "offers solutions that make life easier for drivers by automatically selecting the right route and guiding the vehicle reliably to the desired destination."

The navigation system works by using GPS and a CD-ROM with a digital road map that holds all the information the system needs to show the user the way. The technical aspects of the system (as far as the satellites are concerned) work as described in the above sections.

According to CarNav’s informational packet, the system always knows which route is best for a user by using the GPS satellites and state-of-the-art sensors that record every movement. In fact, the system can know when an individual makes even the slightest change in direction. Before the user moves her vehicle, she simply enters her destination into the control unit (the GPS receiver) which is mounted on the dashboard of the vehicle. The destination is then located by accessing the stored maps from the CD-ROM that is mounted in the back of the vehicle, and the automobile is directed from start to finish.

According to CarNav, there are many benefits that come with this system. These are:

The CarNav System offers users many advantages that can be seen above, and this demonstrates the impressive capabilities that are available because of the GPS technology. The cost of the system, which is expensive, can not be exactly calculated for this report because there are many influencing factors that vary between users. However, systems such as this will most likely become more mainstream as the GPS technology is used more in civilian life.

The Silva GPS Compass, which is manufactured by Silva Sweden AB, is one of the first handheld satellite navigators with an integral electronic compass. Once again, this device uses the GPS satellites that were explained earlier to allow for an individual to receive navigational coordinates while beneath even the densest forest canopy. The company that manufactures this product states that it has so many features that is makes many ‘cheap’ GPS receivers obsolete. For instance:

The features of this product show yet another use of the GPS technology that has been discussed in this paper. With the Silva GPS/Compass there are many navigational tasks that can be accomplished from boating to hiking to driving.

Trimble Navigation Limited offers many different GPS products and services to customers that demonstrate the wide variety of GPS uses. For instance, the technology that has been discussed is used in end-to-end fleet tracking, management, and radio communication systems from Trimble. Some of the products offered by this company are as follows:

GPS/AVL Subsystem

The GPS satellite constellation that is operated by the United States Department of Defense is becoming more and more important to the civilian population of this country and the world. Although it was originally designed for military usage, it has been shown in this overview that there are numerous non-military applications that are currently available involving GPS technology. It should be realized that this report has only skimmed the surface of all of the GPS products that are available, but it has covered the general basics of this service. For instance, the main use of GPS technology is the determination of a certain location. Whether this is used for public service (ambulance and fire personnel navigation) or for recreational purposes (hiking and boating navigation), the main operating characteristics of the technology do not change.

In the future, possibly after this year, the reliability and availability of GPS may change in the non-military world, and this could possibly bring about even more products than are already available. However, for now, the products that are available today (although expensive) are making certain actions much easier for many different people.

Sources:

1.) Car Nav Systems – Alpine Car Navigation

http://www.carnav.com/

2.) CarNav Product Catalog (888) 928-8000

3.) Current GPS Constellation

http://tycho.usno.navy.mil/gpscurr.html

4.) Global Positioning System Overview

http://www.utexas.edu/depts/grg/gcraft/notes/gps/gps.html

5.) GPS Time Series

http://sideshow.jpl.nasa.gov/mbh/series.html

6.) Leick’s Homepage

http://www.spatial.maine.edu/~leick/gpshome.htm

7.) Newton’s Telecom Dictionary (15^th Edition)

8.) Sam Wormley’s Global Positioning System Resources

http://www.cnde.iastate.edu/staff/swormley/gps/gps.html

9.) Silva’s GPS Compass

http://www.macson.com.au/gps.html

10.) Trimble Navigation

http://www.trimble.com/

11.) The United States Naval Observatory

http://www.usno.navy.mil/