Why I Like garmin gps
Sources of GPS signal errors
By by Dagfinn Rognerud
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.
* 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.
* 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.
About The Author
16 years as an officer in the Norwegian army. See my pages: http://www.gps-info.org/ and http://www.lincoln-navigator.org/
Lots of interesting articles from today's PC World. First up, Microsoft Takes on MapQuest: "MSN launches MapPoint online mapping service, offering maps, directions, and more."
Apparently this is going to be yet another component of MS' push for .Net services with hooks into other MSN services. MapPoint is XML-based, which makes it interesting in other ways, and there it launches with a phone-based direction service. The articles notes that this is another step towards location-based services. "For example, you might someday be able to click on a Windows Messenger buddy's name and retrieve a map showing the location of his or her home."
Good or bad? You make the call. I like parts of this, but I'm incredibly wary of .Net.
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posted by The GPS man @ 4:27 PM
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