The Global Positioning System

TheGlobal Positioning System ()GPS

Abdallah M. Samy Entomology Department Faculty of science Ain Shams University

?What is GPS Global Navigation Satellite System developed by US Department of defense.  uses a constellation of between 24 and 32 Medium Earth Orbit satellites that transmit precise microwave signals, that enable GPS receivers to determine their current location, the time, and their velocity. Satellite

Microwave signal

GPS receiver Location Time Velocity Sometimes; Environmen tal conditions

Why GPS is a useful tool in ?the study of Ecology GPS has become a widely used aid to navigation worldwide. useful tool for map-making, land surveying, commerce, scientific uses, and hobbies such as geocaching. the precise time reference is used in many applications including scientific study of earthquakes, and synchronization of telecommunications networks.  Traffic signal timing

How a GPS receiver ?calculates its position A GPS receiver calculates its position by carefully timing the signals sent by the GPS satellites high above the Earth. Each satellite continually transmits messages containing the time the message was sent, precise orbital information )the ephemeris), and the general system health and rough orbits of all GPS satellites )the almanac). The receiver measures the transit time of each message and computes the distance to each satellite. Geometric trilateration is used to combine these distances with the location of the satellites to determine the receiver's location. The position is displayed, perhaps with a moving map display or latitude and longitude; elevation information may be included. Many GPS units also show derived information such as direction and speed, calculated from position changes.

What is the difference between the two pictures )A ?) versusB

How many satellite is required for positioning GPS ?receiver? Why Three satellites are enough to solve for position, since space has three dimensions. But a very small clock error times the very large speed of light the speed at which satellite signals propagate; results in a large positional error. The receiver uses a fourth satellite to solve for x, y, z, and t which is used to correct the receiver's clock.

A visual example of the GPS constellation in motion with the Earth rotating. Notice how the number ofsatellites in view from a given point on the Earth's surface, in this example at 45°N, changes with

Sometimes GPS reading may have an error…. ?Why

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 for 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.

Sometimes GPS reading may have an error…. (.Why? (Cont 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 — Also known as ephemeris 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

Sometimes GPS reading may have an error…. (.Why? (Cont 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. Intentional degradation of the satellite signal — Selective Availability )SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defense. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off

Practical instruction Determine your classroom location using your GPS and return us to discuss. N=……°………´………˝….. E=…….°………´……… ˝ …