GLOBAL POSITIONING SYSTEM

S.RAMJI A.RAMKUMAR

06341A0465 06341A0403

ramjisimhadri.369@gmail.com ram_raja54@yahoo.com

ABSTRACT “GPS for all reasons. Did you know GPS could be used for paying bills, remote controlling your car, tracking busses and containers and even for sport? Find out about its out-of-the-box Applications “

The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense(US Defense).GPS is funded by and controlled by the U. S. Department of Defense (DOD).

GPS Satellite is about 2000 pounds, 17 feet with solar panels extended and it requires about less than 50 watts. This works with solar energy.

GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time. Four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver clock.

Applications: Location- determining a basic position.

Navigation - getting from one location to another.Tracking - monitoring the movement of people and things.

Mapping- creating maps.

Timing - providing precise timing

Introduction: The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense (US Defense).GPS is used for tracking or finding the position of any object on the earth.

GPS is funded by and controlled by the U. S. Department of Defense (DOD).

GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time.

Four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver clock.

NAVSTAR GPS - Navigation Signal Timing and Ranging G P S.

GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use. GPS works in

any weather conditions, anywhere in the world, 24 hours a day. Over fifty GPS satellites such as this NAVSTAR have been launched since 1978. this was funded by U.S. DOD at a cost of about US$13 billion. There are no subscription fees or setup charges to use GPS; access is free to all users, including those in other countries.About GPS Satellites: The GPS satellite orbiting Earth at 11,000 miles. The system consists of a "constellation" of at least 24 satellites in 6 orbital planes. The GPS satellites were initially manufactured by Rockwell; the first was launched in February 1978, and the most recent was launched November 6, 2004(Delta II rocket). The first 10 satellites were development satellites, called Block I. From 1989 to 1993, 23 production satellites, called Block II were launched. The launch of the 24th satellite in 1994 completed the system. The DOD keeps 4 satellites in reserve to replace any destroyed or defective satellites. The satellites are positioned so that signals from six of them can be received nearly 100 percent of the time at any point on earth. Each satellite circles the Earth twice every day at an altitude of 20,200 kilometers (12,600 miles). With military accuracy restrictions partially lifted in March 1996 and fully lifted in May 2000, GPS can now pinpoint the location of objects as small as a penny anywhere on the earth’s surface. This GPS satellites are also named as Space Vehicles (SV`s) by U.S.Defence.Equipments used in GPS: This consists of a GPS receiver, GPS equipped device and also the Space vehicles. GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time.

Basically GPS works by using four GPS satellite signals to compute positions in three dimensions (and the time offset) in the receiver clock. So by very accurately measuring our distance from these satellites a user can triangulate their position anywhere on earth. GPS receivers have been miniaturized to just a few integrated circuits and so are becoming very economical. And that makes the technology accessible to virtually everyone. These days GPS is finding its way into cars, boats, planes, construction equipment, movie making gear, farm machinery, even laptop computers. This report shows the various features of GPS and the reasons why it may soon become almost as basic as the telephone.GPS Satellite Constellation:The satellite constellation consists of the nominal 24-satellite constellation. They transmit signals (at 1575.42 MHz) that can be detected by receivers on the ground. The satellites are positioned in six Earth-centered orbital planes with four satellites in each plane. This means that signals from six of them can be received 100 percent of the time at any point on earth. The nominal orbital period of a GPS satellite is one half of a sidereal day or 11 hr 58 min. The orbits are nearly circular and equally spaced about the equator at a 60° degree separation with an inclination relative to the equator of nominally 55° degrees. The orbital radius is approximately 26,600 km (i.e., distance from satellite to centre of mass of the earth).

GPS satellites transmit two low power radio signals, designated L1 and L2. Civilian GPS uses the L1 frequency of 1575.42 MHz in the UHF band. A GPS signal contains three different bits of information a pseudo-random code ephemeris data and almanac data. The

pseudo-random code is simply an I.D. code that identifies which satellite is transmitting information. Several different notations are used to refer to the satellites in their orbits. One particular notation assigns a letter to each orbital plane (i.e., A, B, C, D, E, and F) with each satellite within a plane assigned a number from 1 to 4. Thus, a satellite referenced as B3 refers to satellite number 3 in orbital plane B. A second notation used is a NAVSTAR satellite number assigned by the U.S. Air Force. This notation is in the form of a space vehicle number (SVN) 11 to refer to NAVSTAR satellite 11.

WORKING:

GPS works in five logical steps:

1. The basis of GPS is "triangulation" from satellites.

2. To "triangulate," a GPS receiver measures distance using the travel time of radio signals.

3. To measure travel time, GPS needs very accurate timing which it achieves with some tricks.

4. Along with distance, you need to know exactly where the satellites are in space. High orbits and careful monitoring are the secret.

5. Finally you must correct for any delays the signal experiences as it travels through the atmosphere.

Triangulation???

We're using the word "triangulation" very loosely here because it's a word

most people can understand, but purists would not call what GPS does "triangulation" because no angles are involved. It's really "trilateration." (Meaning of the word “TRILATERAL: of, on or with 3 sides & 3 parties”)That's right, by very, very accurately measuring our distance from three satellites we can triangulation our position anywhere on earth.The receiver does not need a precise clock, as in the onboard clock of satellite but does need a clock with good short-term stability and the ability to receive signals from four satellites in order to determine its own latitude, longitude, elevation, and the precise time.

Fig : GPS clock

The receiver computes the distance to each of the four satellites from the difference between local time and the time the satellite signals were sent (this distance is called a pseudorange) . It then decodes the satellites' locations from their radio signals and an internal database. The receiver should now be located at the intersection of four spheres, one around each satellite, with a radius equal to the time delay between the satellite and the receiver multiplied by the speed of the radio signals.

Determining Your Position: Suppose we measure our distance from a satellite and find it to be 11,000 miles (how it is measured is covered later). Knowing that we're 11,000 miles from a particular satellite narrows down all the possible locations we could be in

the whole universe to the surface of a sphere that is centered on this satellite and has a radius of 11,000 miles.

Next, say we measure our distance to a second satellite and find out that it's 12,000 miles away. That tells us that we're not only on the first sphere but we're also on a sphere that's 12,000 miles from the second satellite, i.e. somewhere on the circle where these two spheres intersect. If we then make a measurement from a third satellite and find that we're 13,000 miles from that one, that narrows our position down even further, to the two points where the 13,000 mile sphere cuts through the circle that's the intersection of the first two spheres. So by ranging from three satellites we can narrow our position to just two points in space. To decide which one is our true location we could make a fourth measurement. But usually one of the two points is a ridiculous answer (either too far from Earth or moving at an impossible velocity) and therefore can be rejected without a measurement.

Measuring Your Distance:

How the satellites actually measure the distance is quite different from determining your position and

essentially involves using the travel time of a radio message from the satellite to a ground receiver. To make the measurement we assume that both the satellite and our receiver are generating the same psedo-random code at exactly the same time.

In the case of GPS we're measuring a radio signal so the velocity is going to be the speed of light or roughly 186,000 miles per second.

The Pseudo Random Code is a fundamental part of GPS.Since each satellite has its own unique Pseudo-Random Code this complexity also guarantees that the receiver won't accidentally pick up another satellite's signal. So all the satellites can use the same frequency without jamming each other

By comparing how late the satellite's pseudo-random code appears compared to our receiver's code, we determine how long it took to reach us. Multiply that travel time by the speed of light and you obtain the distance between the receiver and the satellite. However this calls for precise timing to determine the interval between the code being generated at the receiver and received from space. On the satellite side, timing is almost perfect due to their atomic clocks installed within each satellite. However as it would be extremely uneconomical for receiver to use atomic clocks a different method must be found. GPS solves this problem by using an extra satellite measurement for the following reason: If our receiver's clocks were perfect, then all our satellite ranges would intersect at a single point - our position. But with imperfect clocks, a fourth measurement, will not intersect

with the first three satellite ranges. So the receiver's computer will then calculate a single correction factor that it can subtract from all its timing measurements that would cause them all to intersect at a single point. That correction brings the receiver's clock back into sync with universal time , ensuring (once the correction is applied to all the rest of the receivers’ measurements) precise positioning.

Error Correction

As would be expected, a variety of different errors can occur within the system, some of which are natural, whilst others are artificial. First of all, a basic assumption, the speed of light, is not constant as this value changes as the satellite signals travel through the atmosphere. As a GPS signal passes through the charged particles of the ionosphere and then through the water vapour of the troposphere it gets slowed down, and this creates the same kind of error as bad clocks. This problem is tackled by attempting to use modelling of the atmospheric conditions of the day, and using dual-frequency measurement, i.e. comparing the relative speeds of two different signals. Another problem is multipath error, this is when the signal may bounce off various local obstructions before it gets to our receiver. Sophisticated signal rejection techniques are used to minimize this problem.

Fig: Error due to the tall building.

Uses of GPS Technology

GPS technology has matured into a resource that goes far beyond its original design goals. These days people from a plethora of professions are using GPS in ways that make their work more productive, safer, and sometimes even easier. There are five main uses of GPS today:

Location- determining a basic position.

Navigation - getting from one location to another.

Tracking - monitoring the movement of people and things.

Mapping- creating maps. Timing - providing precise timing .

APPLICATIONS:

Astronomers, power companies, computer networks, communications systems, banks, and radio and television stations can benefit from this precise timing. One investment banking firm uses GPS to guarantee their transactions are recorded simultaneously at all offices around the world. And a major Pacific Northwest utility company makes sure

their power is distributed at just the right time along their 14,797 miles of transmission lines. Chicago developed a GPS tracking system to monitor emergency vehicles through their streets, saving precious time responding to 911 calls. And on the commercial front, two taxi companies in Australia track their cabs for better profit and improved safety.The primary military purposes are to allow improved command and control of forces through improved locational awareness, and to facilitate accurate targeting of smart bombs, cruise missiles, or other munitions.

GPS for private and commercial use

The GPS system is free for everyone to use, all that is needed is a GPS receiver, which costs about $90 and up (March 2005).

GPS on airplanes

Most airline companies allow private use of ordinary GPS units on their flights, except during landing and take-off

GPS for the visually impaired

The projects of the navigation system using GPS for the visually impaired have been conducted quite a few times. GPS was introduced in the late 80’s and since then there have been several research projects such as MoBIC, Drishti, and Brunel Navigation System for the Blind, NOPPA, BrailleNote GPS and Trekker.

"Where am I?"

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

On the water : A New Zealand commercial fishing company uses GPS so they can return to their best fishing holes without wandering into the wrong waters in the process.

GPS is very popular in fleet management. Many companies have fitted their trucks or busses with tracking devices which means thousands of vehicles can be monitored form a single computer.(time if bus stops, area, speed)

A GPS machine can be integrated with any type of machine. This can be used to make bills on gas, electricity & water etc.

GPS devices can be fitted to your car and can be used as remote to all parts of your car, U can now about the details of fuel & also the music run in your car.

MAINTANENCE

The cost of maintaining the system is approximately US$400 million per year, including the replacement of aging satellites. For normal civilian to use GPS he/she must have GPS receiver system which costs normally of about 90$.

European concern about the level of control over the GPS network and commercial issues has resulted in the planned GALILEO positioning system.

Russia already operates an independent system called GLONASS (global navigation system), although with only twelve active satellites as of 2004, the system is of limited usefulness.

Awards:

Two GPS developers have received the National Academy of Engineering Charles Stark Draper prize year 2003:

Ivan Getting, emeritus president of The Aerospace Corporation and engineer at the Massachusetts Institute of Technology established the basis for GPS, improving on the World War II land-based radio system called LORAN (Long-range Radio Aid to Navigation).

Bradford Parkinson, teacher of aeronautics and astronautics at Stanford University developed the system.

REFERENCES:[1] Muller N. J.; Desktop Encyclopedia of Telecommunications ; 1998[2] Kaplan E.D. ; Understanding GPS , Principles & Applications ; 1996[3] Lichtenegger B.H., Collins J. ; GPS: Theory and Practice ; 1994[4] United States Coast Guard Navigation Center. <http://www.navcen.uscg.gov/>[5] InfoTooth Knowledge Base <http://www.palowireless.com/infotooth/knowbase.asp>[6] Trimble GPS Tutorial http://www.trimble.com/gps/