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GPS Tutorial





  • What is GPS?
    The Global Positioning System (GPS) is a space age navigational system that can pinpoint your position anywhere on the globe, usually within a few meters. This amazing technology is available to everyone, everywhere, day and night, and best of all, at no cost for use of the navigational data. GPS uses a constellation of 24 satellites in precise orbits approximately 11,000 miles above the earth. The satellites transmit data via high frequency radio waves back to Earth and, by locking onto these signals, a GPS receiver can process this data to triangulate its precise location on the globe. GPS operates 24 hours a day, in all weather conditions, and can be used worldwide for precise navigation on land, on water and even in the air. Some of its many current applications include: boating, fishing, hunting, scouting on land or from the air, hiking, camping, biking, rafting, pack trips by horseback, hot air ballooning, general aviation, snowmobiling and skiing, search and rescue, emergency vehicle tracking, 4 wheeling, highway driving and a host of other outdoor activities where accurate positioning is required.
  • How GPS Determines Your Position?
    GPS uses satellite ranging to triangulate your position. In other words, the GPS unit simply measures the travel time of the signals transmitted from the satellites, then multiplies them by the speed of light to determine exactly how far the unit is from every satellite it's sampling.
    By locking onto the signals from a minimum of 3 different satellites, a GPS receiver can calculate a 2D positional fix, consisting of your latitude and longitude. By locking onto a fourth satellite, the GPS can compute a 3D fix, calculating your altitude as well as your latitude/longitude position.
    In order to do this Eagle uses a 12 parallel channel receiver in all of it's current products. Three of the channels lock on to satellites for triangulation. Another channel locks on to a fourth satellite for 3D navigation, which lets the unit calculate altitude in addition to latitude and longitude. These four channels continuously and simultaneously track the four satellites in the best geometrical positions relative to you. The additional eight channels track all other visible satellites, then add this data to the data from the original four satellites. The unit then over-resolves a solution, creating an accuracy-enhanced reading. The additional channels also ensure reliable, continuous, and uninterrupted navigation, even in adverse conditions such as valleys or dense woods.
  • What is the accuracy of GPS? GPS was conceived in the 1970s, and is controlled by the United States Department of Defense. Although GPS was initially envisioned for military use, the Government realized early on that there would be numerous civilian applications as well. Subsequently, the Department of Defense created two transmission codes; the P code (Precision code) for military use, and the C/A code (Civilian Access code) for civilian use.
    The highest accuracy levels were to be reserved for the military so as to prevent hostile enemy attacks against the U.S. using our own navigational system. However, once in operation, the civilian GPS receivers using the C/A code proved to be more accurate than the D.O.D. had intended. Consequently, the military developed a system for randomly degrading the accuracy of the signals being transmitted to civilian GPS receivers. This intentional degradation in accuracy is called Selective Availability or S/A. This reduced the civilian GPS accuracy levels to being within 100 meters or less, 95% of the time. However, typical accuracy for most users averaged between 20 and 50 meters the majority of the time. You could easily see the effects of S/A on a GPS receiver when you were not moving. Typically there would be random movements in speed, altitude and position readings, along with slow position "wandering" on the plotter trail, easily seen when you are on a 0.1 or 0.2 mile Plotter scale, and not moving. For example, while parked at the dock in your boat, you would see unexplainable changes in your digital speed readings up to a few miles per hour, even though you were not moving.
  • What is a map datum?
    A datum is a mathematical model of the Earth which approximates the shape of the Earth, and enables calculations such as position and area to be carried out in a consistent and accurate manner. The datum is physically represented by a framework of ground monuments (i.e., trig stations) whose positions have been accurately measured and calculated on this reference surface. Lines of latitude and longitude on a map or chart are referenced to a specific map datum. Every chart has a map datum reference. The map datum is usually listed in the title block of the chart.
    If you are comparing the GPS coordinates to a chart or other reference, the map datum in the GPS unit should be selected to the same map datum that is listed as the datum used to generate the chart or other reference.
  • What is the MOB function?
    The Man Over Board (MOB) function is a common feature for marine GPS units. If a person falls overboard, the skipper uses this function to immediately mark the current location. The GPS goes into a "Goto" mode and directs the skipper back to the point where the button was pressed.
  • What is a route?
    A route is a series of waypoints entered in the order that you want to navigate them.
  • What is WAAS?
    WAAS stands for the "Wide Area Augmentation System".
    The Wide Area Augmentation System (WAAS) is an FAA funded project to improve the overall accuracy and integrity of the GPS signal for flying in instrument meteorological conditions, primarily during the approach to landing phase of flight. It is a space-based system that broadcasts integrity information and correction data as determined by ground reference stations. At this time the system is still in the development stage with a goal of providing reliable signals with an accuracy of 7 meters (21-22ft) both horizontally and vertically 95+% of the time. Current tests have shown the actual accuracy to be on the order of 2-3 meters.

    Sources of GPS signal errors

    Factors that can degrade the GPS signal and thus affect accuracy include the following:

    • 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.
    • 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 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.
    • 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 SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.






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