Current Endeavors and Future Ramifications of Global Positioning Systems

Authors: Dr. Steve Hallman, Dr. (LtCol) Michael L.Thomas, Dr. Al Stalh, Dr. Michel Plaisent, Dr. Prosper Bernard, and James Thorpe

What is GPS?

GPS (Global Positioning Systems) it is a satellite navigation system that uses “trilateration” to calculate the coordinates of positions at or near the Earth’s surface.Trilateration refers to the trigonometric law by which the interior angles of a triangle can be determined if the lengths of all three triangle sides are known.. Trilateration is done by using 3 or more orbiting satellites whose postions are known to fix a location in latitiude and longitude on the earth’s surface. If four or more satellites are within the receiver’s view (also called it’s “horizon”) the receiver is able to calculate the elevation and it’s velocity if it is in motion. To trilaterate, a GPS receiver measures distance by using the travel time of standard radio signals. To measure the travel time, a GPS needs very accurate timing. Along with distance, you need to know exactly where the satellites are in space. You must also correct for any delays and distortions the GPS signal undergoes as it travels through the atmosphere. is the method. This is done by the usage of 4 satellites to find a location down on the surface.

In 2004, the “constellation” of satellites that make up the system was declared fully operational. theThe system is composed of re were 28 satellites in 6 different orbits/plans20,20011,000 km above the earth in MEO (Medium Earth Orbit) that allows each satellite to complete 1 revolution in 12 hours.The system architecture is best described by speaking of different “segments’ – the user segment, the control segment and the space segment. The user segment is made up of the GPS receivers and people who use them to measure positions on the surface. The other two segments of the sytems are the space segment (the constellation itself) and a control segment composed of ground stations.

It took about $10 billion to build over 16 years and fully deploy. There are alternative technologies in use or planned as well. Russia maintains a similar positioning satellite system called GLONASS ( Member nations of the European Union have plans to deploy a similar system of their own, called Galileo by 2010. The first EU GPS GIOVE-A satellite began transmitting Galileo signals in January 2006. If the efforts are successful to make Galileo, GLONASS, and the U.S. Global Positioning System interoperable the end result will be a Global Navigation Satellite System (GNSS) that provides more than twice the signal-in-space resource that is available via US GPS alone.

The U.S. Coast Guard's Navigation Center publishes status reports on the GPS satellite constellation. Its report of November 9, 2005, for example, listed 30 satellites, four to seven in each of the six orbits planes (A-F). You can look up the current status of the constellation at Scientitists at NASA have created an implemented an interactive, three-dimensional model of the Earth and the orbits of the more than 500 man-made satellites that surround it. The model is a Java applet called J-Track 3D located at : ). Your browser must have Java enabled to view the applet. Instructions at the site describe how you can zoom in and out, and drag to rotate the model. To view orbits of particular satellites, choose Select from the Satellite menu. The Block IIA and R series are the most current generation of NAVSTAR satellites.

Uses of GPS

GPS is currently used in for many applications. These applications include tracking vehicles, cars, ships, airplanes, military targeting use and many others. GPS is currently applied to many aspects of things in everyday life. The most common might entail positioning such as being in the desert, on the ocean, cars navigation, ships navigation, cell phones calls, watches (for Alzheimer patents), bracelets, and airlines for navigation.

In military use, it is an effective navigation tool for soldiers, military aircraft, and even naval warships. It guides soldiers in difficult terrain that would normally be impossible to navigate such as sandstorms or where there is limited mapping data available. The latest generation of guided and gravity munitions are GPS guided. This means that they use GPS technology to pinpoint and find their target from great distances. . In Iraq, the Iraqi military burned tires and rubbish in the opening days of the war knowing it would block the laser guided munitions – the US forces had implemented the newer GPS based guidance systems and were unaffected by the heavy smoke over Bagdhad.

In civilian use, it helps guide the police, fire, and emergency services. Surveying, mapping, and construction companies heavily depend on GPS. Mining even depends on GPS to navigate their equipment when there is limited visibility and the receivers have become sensitive enough to even track the movements of tectonic plates.. In transportation use, it helps guide cars, delivery trucks, courier services, as well as public transportation find ways to their destinations.

There are two primary levels of service that GPS is used for. The first one is the standard positioning service (SPS) which is used for general purposes (GPS Basics). The second is precise positioning service (PPS), which is used by the Department of Defense as well as US allies to the US (GPS Basics).

The time element is important. GPS uses “universal time” or sometimes called “truth time.” Most of us know it by Zulu or Greenwich Mean Time. This is used for synchronization operation, agriculture, mapping, shipping lines, time stamping ATM transactions, networks, and a vast number of other applications (Wolfstead).

Satellite, Sector, and Signal Information

As stated earlier, the GPS Systmes Architecture is composed made up of 3 segments which are the space, control, and the user segment . Within the original space segment, there are about 24 primary satellites and 4 spares. or more satellites (Modernization). These satellites orbit the earth in 12 hours. The control segment is made up of the Master Control Station (MCS), six monitor stations, five antennas, and a backup MCS (Modernization). The user segment contains civil and military GPS which are used for air, land, sea, as well as space applications . The control segment of the Global Positioning System is a network of ground stations that monitors the shape and velocity of the satellites' orbits. The accuracy of GPS data depends on knowing the positions of all of the satellites at all times. The orbits of the satellites are sometimes disturbed by the interplay of gravitational forces and this must be corrected for. Monitor stations are very precise GPS receivers installed at known locations. They record discrepancies between known and calculated positions (ground truthing) caused by slight variations in satellite orbits due to the gravitational forces that occur between the earth and the moon. Data describing the orbits are produced at the Master Control Station at Colorado Springs, Colorado, uploaded to the satellites, and finally broadcast as part of the compete and corrected GPS positioning signal. GPS receivers use this satellite Navigation Message data to adjust the positions they measure. If necessary, the Master Control Center can modify satellite orbits by commands transmitted via the control segment's ground antennas.

Location of Function of Control Stations

NAVSTAR Block II satellites broadcast at two frequencies, 1575.42 MHz (L1) and 1227.6 MHz (L2). (For the sake of comparison, commercial FM stations typically broadcast between 88-108 MHz.) Only L1 was intended for civilian use. Single-frequency receivers produce horizontal coordinates at an accuracy of about 5 meters at a cost of about $100. Some units allow users to improve accuracy by filtering out errors identified by nearby stationary receivers, a post-process called "differential correction." $300-$500 single-frequency units that can also receive corrected L1 signals from the U.S. FAA's Wide Area Augmentation System (WAAS) network of ground stations and satellites can perform differential correction in "real-time." Differentially-corrected coordinates produced by single-frequency receivers can be as accurate as 2 meters. The signal broadcast at the L2 frequency is encrypted for military use only. Clever GPS receiver makers soon figured out, however, how to make dual-frequency models that can measure slight differences in arrival times of the two signals (these are called "carrier phase differential" receivers). Such differences can be used to make use of the L2 frequency to improve accuracy without decoding the encrypted military signal. Survey-grade carrier-phase receivers able to perform real-time kinematic (RTK) differential correction, can produce horizontal coordinates at sub-meter accuracy at a cost of $1000 to $2000. It is no wonder that GPS is replacing electro-optical instruments for many land surveying tasks. There are 2 different types of frequencies that are currently used known as L1 and L2 which are used by the satellites to broadcast timing and navigation signals (Modernization). L1 broadcasts at 1575.42 MHz and L2 broadcasts at 1227.60 MHz (GPS Basics). In the near future, 3 new signals will be added to GPS. These signals include L2C at 1227.6 MHz, L5 at 1176.45 MHz, and L1C at 1575.42 MHz. These signals will be used as civil signals and are designed to give better performance and precision to users compared to the old L1 and L2 signals.

GPS receivers calculate distances to satellites as a function of the amount of time it takes for satellites' signals to reach the ground. The receiver must be able to tell precisely when the signal was transmitted, and when it was received in order to make this calculation..The satellites are equipped with very accurate atomic clocks, so the timing of transmissions is always known. Receivers contain less accurate clocks, which add to the total sources of measurement error. The signals broadcast by satellites, called "pseudo-random codes," are accompanied by the broadcast ephemeris data that describes the shapes of satellite orbits.

The GPS constellation is configured so that a minimum of five satellites should always be "in view" anywhere on the surface of the Earth. If only one signal is available to a receiver, the set of possible positions would include the complete range sphere surrounding the satellite. If distances from three satellites are known, the receiver's position must be one of two known points at the intersection of three spherical ranges. GPS receivers are “smart” enough to choose the location nearest to the Earth's surface. At any rate, at a minimum, three satellites are required for a two-dimensional (horizontal) fix. Four ranges are needed for a three-dimensional fix (horizontal and vertical).

Challenges

Some of the challenges for GPS are the background noise of he earth, solar flares, program stability, and program budgets. Another part is the maintenance of the satellite once per year repositioning it in orbit, and annual maintenance on the atom clock is done. Additional, once per year for each activitysuch as maintenance of remotes antennas at sites around the world. The military operates the systems for about 5-10% of users worldwidewhile providing this service free of charge.

User Equivalent Range Errors (UERE) is an umbrella term for all of the error sources below, which are presented in order of their contributions to the total measurement error of a GPS receiver.

1. Satellite clock: GPS receivers calculate their distances from satellites as a function of the difference in time between when a signal is transmitted by a satellite and when it is received on the ground. The atomic clocks on board NAVSTAR satellites are extremely accurate. They do tend to stray up to one millisecond of standard GPS time (which is calibrated to, but not identical with Coordinated Universal Time; (see ). The Control Segment monitoring stations calculate the amount of clock drift associated with each satellite. GPS receivers that are able to make use of the clock correction data that accompanies GPS signals can use this to reduce clock error significantly.

2. Upper atmosphere (ionosphere): Space is nearly a vacuum, but the signals must pass thru earth’s atmosphere. GPS signals are deflected and delayed as they pass through the ionosphere, the layer of the atmosphere that extends from roughly 50 to 1,000 km above the Earth's surface. Signals transmitted by satellites close to the horizon take a longer route through the ionosphere than signals from satellites overhead, and are thus experience greater interference. The ionosphere's density varies by latitude, by season, and by time of day, in response to the Sun's ultraviolet radiation, solar storms and the stratification of the ionosphere itself. The GPS Control Segment is able to model these ionospheric variations. Monitoring stations transmit corrections to the NAVSTAR satellites, and these are broadcast as corrections along with the GPS signal. Such corrections eliminate only about three-quarters of the variations, however, leaving the ionosphere the second largest contributor to the GPS total measurement error.

3. Receiver clock: GPS receivers are equipped with quartz crystal clocks that are less accurate than the atomic clocks used in the NAVSTARs. Receiver clock error can be eliminated by comparing times of arrival of signals from two satellites whose transmission times are known exactly.

4. Satellite orbit: GPS receivers calculate coordinates relative to the known locations of satellites in space. Knowing where satellites are at any moment involves knowing the shapes of their orbits, their heights as well as the orbital velocities. The gravitational attractions of the Earth, Sun, and Moon all contribute to variations in the satellite orbits. The Control Segment continually monitors satellite locations, calculates the orbit eccentricities, and compiles these deviations in published tables called “ephemerides”. An ephemeris is compiled for each satellite and broadcast with the GPS signal. Receivers that are able to process ephemerides can thus compensate for some orbital variations.

5. Lower atmosphere layers: (troposphere, tropopause, and stratosphere) The three lower layers of atmosphere encapsulate the Earth from the surface to about 50 km. The lower atmosphere creates a delay in GPS signals, adding slightly to the calculated distances between satellites and receivers. Signals from satellites close to the horizon experience the greatest delay, since they pass through more atmosphere than signals from satellites directly overhead.

6. Multipath: Ideally, GPS signals travel from satellites through the atmosphere directly to GPS receivers. In reality, GPS receivers must discriminate between signals received directly from satellites and other signals that have been reflected from surrounding objects, such as buildings and trees. Some reflected signals are identified automatically and accommodated for. Antennas are designed to minimize interference from signals reflected from below, but signals reflected from above are more difficult to eliminate. Multipath is caused when a GPS signal bounces from some other object before reaching your receiver. Since it now takes longer to reach you, it calculates a greater distance, causing a decrease in accuracy. Multipath must be avoided, as it cannot be corrected by DGPS. One technique for minimizing multipath error is to track only those satellites that are at least 15° above the horizon, a window called the "mask angle."

The arrangement of satellites in the sky also affects the accuracy of GPS positioning. The ideal arrangement (a minimum four satellites) is one satellite directly overhead and the other three equally spaced near the horizon. GPS coordinates calculated when satellites are clustered close together in the sky suffer from "dilution of precision" (spatial collinearity also called DOP), a factor that multiplies the uncertainty associated with UERE. The DOP associated with an ideal arrangement of the satellite constellation is approximately 1, which does not magnify UERE. The lowest DOP encountered in practice is about 2, which doubles the uncertainty associated with UERE. GPS receivers report several components of DOP, including Vertical Dilution of Precision (VDOP) and Horizontal Dilution of Precision (HDOP). The combination of both of these components is call position dilution of precision (PDOP). A key element of GPS mission planning is to identify the time of day when PDOP is minimal. Since satellite orbits are known, PDOP can be predicted for a given location and time.

A variety of factors such as the clocks in satellites and receivers, the atmosphere, satellite orbits, and reflective surfaces near the receiver, degrade the quality of GPS coordinates and contribute to overall measurement error. PDOP also makes matters worse. Various techniques have been developed to filter out and correct for positioning errors. Random errors can be partially overcome by simply averaging repeated fixes at the same location, although this is often not very efficient. Systematic errors can be compensated for by modeling the phenomenon that causes the error and predicting the amount of variation (think common and special cause variations). Some errors such as multipath errors vary in magnitude and direction from location to location. Other factors, including clocks, the atmosphere, and orbital eccentricities, tend to produce similar errors over large areas of the Earth's surface at the same time. Errors of this kind can be corrected using a method called differential correction.