Lecture 4: Global Positioning System (GPS)

What is GPS? What its purpose? Learning Outcomes At the end of this lecture, the student should be able to: Explain the basic working principle of GPS Describe the advantages of GPS Why do we need GPS? What is GPS? What its purpose? Who developed GPS? Who can use GPS? How GPS works?

Why do we need GPS? One drawback of using radio waves generated on the ground is that you have only two choices: A system that is very accurate but doesn’t cover a wide area; or A system that covers a wide area but is not very accurate

What is GPS & What the purpose of GPS? GPS is stands for Global Positioning System. Official name of GPS is Navigational Satellite Timing And Ranging Global Positioning System (NAVSTAR GPS) The purpose of GPS is to show you your exact position on the Earth anytime, in any weather, anywhere. 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.

Who developed GPS? First developed by the US DOD (United States Department of Defense) Feasibility studies begun in 1960’s.Pentagon appropriates funding in 1973. First satellite launched in 1978. System declared fully operational in April, 1995. Open to the public, 2000. Currently controlled by the United States Air Force. It costs about $750 million to manage and maintain the system per year

GPS Frequency GPS operates in the UHF band. There are two types of services available: For Civilian/Research Use  Standard Positioning System L1 (1575.42 MHz), L2 (1227.60 MHz), L5 (1176.45 MHz) – For Civilian L4 (1379.913 MHz) – For Research For Military Use  Precise Positioning System L3 (1381.05 MHz) Each navigation satellite transmits a unique UHF signal. The receiver pairs itself to this transmission and determines the time difference between the satellite clock and the receiver clock. The time difference multiplied by the speed of light gives the receiver distance from the satellite. Signals from multiple satellites are used to fix the receivers position in space.

Military Marine Automobile Individual Aircraft Navigation Who Can Use GPS Individual Automobiles are often equipped GPS receivers. They show moving maps and information about your position on the map, speed you are traveling, buildings, highways, exits etc. Some of the market leaders in this technology are Garmin and TomTom, not to mention the built in GPS navigational systems from automotive manufacturers. For aircraft, GPS provides Continuous, reliable, and accurate positioning information for all phases of flight on a global basis, freely available to all. Safe, flexible, and fuel-efficient routes for airspace service providers and airspace users. Increased safety for surface movement operations made possible by situational awareness. Marine applications GPS allows access to fast and accurate position, course, and speed information, saving navigators time and fuel through more efficient traffic routing. Provides precise navigation information to boaters. Enhances efficiency and economy for container management in port facilities. Other Applications not mentioned here include Railroad systems Heading information – replacing compasses now that the poles are shifting Weather Prediction Skydiving – taking into account winds, plane and dropzone location Many more! Aircraft Navigation

Who can use GPS? Automobiles: show moving maps (highways, buildings) Military: target detection. Aviation: for aircraft navigation Marine: Provides precise navigation information to boaters.

Who can use GPS? Handheld GPS Receivers Casio GPS wristwatch

Automobiles are often equipped GPS receivers. They show moving maps and information about your position on the map, speed you are traveling, buildings, highways, exits etc. Some of the market leaders in this technology are Garmin and TomTom, not to mention the built in GPS navigational systems from automotive manufacturers.

HOW GPS WORKS?

GPS: Global Positioning System Consists of two dozen GPS satellites in medium Earth orbit (The region of space between 2000km and 35,786 km) GPS uses satellites as reference points to calculate accurate positions. Each satellite orbits the earth every 12 hours (2 complete rotations every day). This ensures that every point on the Earth will always be in radio contact with at least 4 satellites. Ground stations are used to precisely track each satellite's orbit. 24 satellites in orbit dedicated to GPS that orbit the Earth in very precise orbits twice a day. 12,000 miles above Earth 6 satellites are within view of any location at one time. Satellites constantly transmit their location information and time data. There are quite a number of satellites out there in space. They are used for a wide range of purposes: satellite TV, cellular phones, military purposes and etc. Satellites can also be used by GPS receivers.

GPS components The airborne GPS equipment (GPS receiver) receives messages from the space segment of the GPS system (24 satellites) to determine the position of the aircraft in space in terms of longitude, latitude and altitude. The GPS receiver is capable of simultaneous reception of digital messages from more satellites orbiting around the earth. These digital messages contain information regarding satellite position and its unique code, satellite atomic clock (extremely precise), information on ionospheric conditions and supplementary information (usually future positions of the satellite).

How GPS works GPS receivers are generally composed of an antenna, tuned to the frequencies transmitted by the GPS satellites. Each GPS satellites then transmit signals to the GPS receivers . The signals, moving at the speed of light, arrive at a GPS receiver at slightly different times because some satellites are farther away than others. These signals indicates satellite’s location and the current time. Each GPS satellite has special clocks to provide very accurate time reference (atomic clocks). GPS receivers require an unobstructed view of the sky, so they are used only outdoors and they often do not perform well within forested areas or near tall buildings. 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. This can be done through TRIANGULATION method. To triangulate, GPS receiver measures distance using the travel time of radio signals.

How GPS works The GPS receiver measures the time taken for a signal to travel from satellite to receiver. The distance to each GPS satellites can be determined by multiplication of speed of light and time. Distance = Speed of Light x Time Knowing the distance from at least 4 GPS satellites, the GPS receiver can calculate their position in ground or in air (for aircraft). The position can be described in terms of latitude and longitude. The receiver measures the amount of time it takes a signal to travel from satellite to receiver (it actually subtracts the satellite time received in the message from its own time when the message was received), and the time is then converted into distance from the satellite (multiplication of the time with the speed of light). Knowing the satellite position and the distance from it, the receiver can construct (calculate) the sphere of possible position of the aircraft in space. Since these messages are received from more than one satellite simultaneously, the same calculations are performed for all of them. Then these spheres of possible positions are compared and the fix in space is constructed in the following way. The intersection of two spheres will give a circular position line. The introduction of a third sphere will produce two positions several thousand miles apart. The space position is rejected as impossible and a fix is produced by intersection of the three spheres. Thus, three satellites would be enough to fix a position in space in terms of longitude, latitude and altitude. However, a fourth satellite is used in addition to compensate for the errors of the receiver’s time. All the receivers are equipped with precise crystal oscillators to provide time, but the accuracy does not compare with the accuracy of the satellite atomic clock. The result of this receiver’s clock errors is thick spheres of possible positions.

Latitude and Longitude Latitude and Longitude are spherical coordinates on the surface of the earth. Latitude is measured North or South of the Equator. Longitude is measured East or West of Greenwich. GPS uses Latitudes and Longitudes to reference locations.

How GPS works GPS also can tell you What direction you are heading How fast you are going Your altitude A map to help you arrive at a destination How far you have traveled How long you have been traveling Estimated time of arrival

GPS in AVIATION

GPS receiver in the aircraft’s cockpit Can provide the pilot with navigational information in terms of longitude, latitude and altitude. In addition, by observing the change in position and the rate of change, GPS calculates actual track, ground speed and wind.

Using GPS, aircraft can fly the most direct routes between airports

A GPS receiver in the cockpit provides the pilot with accurate position data and helps him keep the airplane on course.

Aircraft Navigation using GPS Civil aircraft typically fly from one waypoint to another. With GPS, an aircraft's computers can be programmed to fly a direct route to a destination. This can save fuel and time GPS also can simplify and improve the method of guiding planes to a safe landing, especially in poor weather. With advanced GPS systems, airplanes can be guided to touchdown even when visibility is poor.

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. Once entered and saved, a waypoint remains unchanged in the receiver’s memory until edited or deleted. A waypoint is based on coordinates entered into a GPS receiver’s memory. It can be either a saved position fix, or user entered coordinates. It can be created for any remote point on earth. It must have a receiver designated code or number, or a user supplied name. Once entered and saved, a waypoint remains unchanged in the receiver’s memory until edited or deleted.

Waypoints Latitude and Longitude Your location Direction of waypoint Date and Time Waypoint