GLOBAL POSITIONING SYSTEM

What is GPS? GPS, which stands for Global Positioning System, is the only system today that provides an answer to the question, Where on Earth am I ? GPS is able to show you your exact position on the Earth anytime, in any weather, anywhere.

Global Positioning System was originally established by the U. S Global Positioning System was originally established by the U.S.Department of Defence (DOD)for Military purpose at a cost of US$13billion. It was made free to all users from May 2000. GPS is a short form of NAVSTAR GPS – (Navigation System with Time And Ranging).

GPS Segments The three parts of GPS are: The Space segment - Satellites The Control segment- Ground stations The User segment - Receivers

Space Segment There are quite a number of satellites in space. They are used for a wide range of purposes: satellite TV, cellular phones, military purposes Remote Sensing etc. Satellites can also be used by GPS receivers

GPS Constellation The GPS Operational Constellation consists of 24 satellites that orbit the Earth at 20200km altitude in very precise orbits twice a day. GPS satellites emit continuous navigation signals. There are 6 orbital paths with 4 satellites in each.

Control Segment It is a system of tracking stations located around the world near the equator. One Master control station at Colarado Springs. Four Monitor stations - Hawaii, Kwajalein, Ascension Island and Diego Gracia. The path of the satellites are tracked by the monitoring stations.

The control stations- Tracks the GPS satellites Updates their orbiting position Calibrates and synchronizes their clocks. Predict its path for the next 24 hours.

User Segment GPS receivers come in a variety of formats, from devices integrated into cars, phones, and watches, to hand held devices. The GPS receiver is the user segment (US) of the GPS. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly-stable clock). They may also include a display for providing location and speed information to the user.

Receivers and Satellites GPS units are made to communicate with GPS satellites (which have a much better view of the Earth) to find out exactly where they are on the global scale of things

Signal Refraction Signals from satellites can be like light. When they hit some interference (air patterns in the atmosphere, uneven geography, etc.) they sometimes bend a little.

Light Refraction Refraction is the bending of light as it travels through one media to another. Sometimes the GPS signal from the satellite doesn’t follow a straight line.

Line of Sight Transmissions Line of sight is the ability to draw a straight line between two objects without any other objects getting in the way. GPS transmission are line-of-sight transmissions. Obstructions such as trees, buildings, or natural formations may prevent clear line of sight.

Signal Interference Sometimes the signals bounce off things before they hit the receivers

Atomic clocks Each GPS Satellite has several accurate Atomic Clocks. The clocks operate at a frequency of 10.23 MHz. They generate signals that are broadcast from the satellites. They broadcast two Carrier waves in L-band which travel at the speed of light.

L 1 Carrier Wave Transmits two codes: The C/A Code or Coarse/Acquisition Code and The P-Code or the Precision Code. The L-2 Carrier transmits one P- Code

GPS Signals Each GPS satellite transmits data that indicates its location and the current time. All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant. The signal is just a complicated digital code, i.e. a sequence of “on” and “off” pulses.

GPS Signals A GPS signal contains three different bits of information : A pseudorandom code-or an I.D. code that identifies which satellite is transmitting information. Ephemeris data which tells the GPS receiver where each GPS satellite should be at any time throughout the day. Almanac data, contains important information about the status of the satellite (healthy or unhealthy), current date and time..

Time Difference The GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is.

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude). With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.

Signal Errors Ionosphere and troposphere delays — The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct this type of error. Signal multipath — This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors. Receiver clock errors — A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.

Orbital errors —these are inaccuracies of the satellite's reported location. Number of satellites visible — The more satellites a GPS receiver can "see," the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground. Satellite geometry/shading — This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping.

Selective Availability (SA): is an intentional degradation of the signal once imposed by the U.S. Department of Defense. Users of Differential GPS are not much affected. Anti Spoofing (A-S): Denies civilians and hostile powers the access to the P-code

Geometric Principle of Triangulation You can find one location if you know its distance from other, already-known locations

Triangulation

Triangulation

GPS satellites use Atomic Clocks for accuracy 1.The dashed lines show the actual intersection point, and the gray bands indicate the area of uncertainty. 2.The solid lines indicate where the GPS receiver "thinks" the spheres are located. Because of errors in the receiver's internal clock, these spheres do not intersect at one point. 3.The GPS receiver must change the size of the spheres until the intersection point is determined. The relative size of each sphere has already been calculated, so if the size of one sphere is changed, the other spheres must be adjusted by exactly the same amount.

Latitude and Longitude GPS uses Latitudes and Longitudes to determine locations.

GPS Receiver There are number of pages on the GPS Receiver : Satellite Page Trip Computer Page Map Page Compass Page Main Menu Page The Find Page Waypoint Information Page

Recievers

What You can do with a Receiver? Mark Waypoints Create and view Tracks Create a Route

Waypoints Waypoints are locations or landmarks that can be stored in your GPS. waypoints may be entered directly by taking a reading with the unit at the location itself, giving it a name, and then saving the point Latitude and Longitude Your location Direction of waypoint Date and Time Waypoint

Tracks Tracks are like footprints. They show you exactly where you have moved. Tracks appear as small dotted lines on you receivers Map page. The GPS receiver automatically creates tracks as you move and displays the dotted lines called as Track log.

Routes A Route gives you straight line direction from one point to another or many other points.

How accurate is GPS? today GPS receivers are extremely accurate. Certain atmospheric factors, dense foliage or tall buildings can affect the accuracy of GPS receivers. Garmin GPS receivers are accurate to within 15 meters on average. GPS receivers with WAAS (Wide Area Augmentation System) capability can improve accuracy to less than three meters on average.

Differential Correction Differential correction is a technique that greatly increases the accuracy of the collected GPS data. It involves using a receiver at a known location - the "base station“- and comparing that data with GPS positions collected from unknown locations with "roving receivers."

Advantages of GPS Intervisibility between points is not required. Can be used at any time of day or in any weather. Has high geodetic accuracy. More work can be accomplished in less time with fewer people.

Uses of GPS There are five main uses of GPS : 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 of GPS Agriculture Aviation Environment Road rail and ocean Transport Military Public safety Recreation Space Surveying Timing

Competitors of NAVSTAR GPS GLONASS – Russia GALILEO - European Union