GPS lecture - Global Positioning Systems(GPS Aamir Shakeel...

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Aamir Shakeel Global Positioning Systems (GPS)
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What is GPS? The Global Positioning System (GPS) is a constellation of 24 satellites that orbit the Earth. This constellation, when used with a GPS receiver, makes it possible for a person to pinpoint their geographic position. The NAVSTAR (GPS) satellite system is set up and maintained by the US Dept. of Defense Providing 3 Dimensional Positioning for 24 hrs a day to military and civilian users. Provide unequal accuracy, flexibility of positioning for navigation, surveying and GIS data capturing.
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The GPS satellite system The 24 satellites that make up the GPS space segment are orbiting the earth about 12,000 miles above us. They are constantly moving, making two complete orbits in less than 24 hours. These satellites are traveling at speeds of roughly 7,000 miles an hour. The first GPS satellite was launched in 1978. A full constellation of 24 satellites was achieved in 1994. Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit. A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.
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What's the signal? 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. The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects such as buildings and mountains.
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A GPS signal contains three different bits of information — Pseudorandom code , ephemeris data and almanac data. The pseudorandom code is simply an I.D. code that identifies which satellite is transmitting information. You can view this number on your Garmin GPS unit's satellite page, as it identifies which satellites it's receiving.
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Ephemeris data tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits ephemeris data showing the orbital information for that satellite and for every other satellite in the system.
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Almanac data , which is constantly transmitted by each satellite, contains important information about the status of the satellite (healthy or unhealthy), current date and time. This part of the signal is essential for determining a position.
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Why does the GPS say I am 40 to 100 feet from the turn when I am already there? A GPS computes a position within 50 feet (usually within 30 feet) and will move around within those accuracy numbers due to the fact that the satellites are moving and being replaced by other ones that are coming upon the horizon. The GPS is constantly updating its position (once per second). It is amazing technology but don’t expect it to be accurate to a few inches.
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Sources of GPS 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 for this type of error.
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Signal multipath
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GPS lecture - Global Positioning Systems(GPS Aamir Shakeel...

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