Sunday, February 14, 2010

Part 4 WGS-84 What is it? Current World GPS Status.

Which is most  important where GPS navigation systems is concerned, accuracy or integrity? Well first of all, how accurate are the current GPS signals? The US Coast Guard says the current system has an accuracy of plus or minus 7 meters horizontally and plus or minus 12 meters vertically. Prior to Selective Availability being turned off on May 2, 2000 the accuracy of the signal was  plus or minus 100 meters horizontally and plus or minus 300 meters vertically.

The accuracy of the signals was much improved when the Wide Area Augmentation System became fully operational over North America in 2003.  When this happened, the accuracy became plus or minus one meter both horizontally and vertically and never worse than plus or minus 2 meters both horizontally and vertically.

The cost of WAAS capability is not significant today.  Even the most basic portable GPS units such as those used by joggers and physical fitness advocates  cost less than $100 and offer WAAS accuracy.

A Local Area Augmentation System (LAAS) is in development which delivers high accuracy signals to a local area.  Usually about 20 to 30 miles in radius, it has the advantage of only requiring one ground station which will provide precision curved path approaches to all runways at airports within its coverage area. The accuracy of the Local Area Augmentation System is plus or minus one meter both horizontally and vertically.

GPS is capable of delivering millimeter accuracies when differential techniques are used by surveyors.  These require a highly accurate reference point and a transmitter. Whether we require this level of accuracy when operating our aircraft has not been answered yet.

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