Wednesday, February 10, 2010

Part 1 WGS-84 What is it? Current world GPS status

WGS-84 or World Geodetic Standard-84 is a specific standard by by which surveyors describe their work results and cartographers produce maps. Surveyors have worked to as many as 200 different standards which vary according to the time period in which they were established and which parts of the world use them and which of the surveying results are of prime importance. The many standards developed through the years are due to the continuing development of more and more accurate means to acquire data.  One of the more accurate ways to gather data today is the result of the establishment of the Global Navigation Satellite Systems (GNSS). Data acquired by the GPS can be as accurate as plus or minus a centimeter.

My old Garmin GPS-195 hand held battery powered GPS receiver has the capability to accept any of 110 different geodetic standards.

The Global Positioning System (GPS) delivers user services today based upon the WGS-84 model.  If a country has utilized other geodetic standards to locate its airports and runways, an error in what GPS is telling you and what you are looking at out the window will occur. This is not a very good situation if you happen to be using your GPS receiver to accomplish an instrument approach. The best source for you to use to determine if the country you are flying in is using the WGS-84 model for surveying its airports is found at the Jeppesen web site.  Jeppesen has a page that will deliver this information for you to consider.

The current Jeppesen WGS-84 page is dated March 2009.  If you want the most current data for a country, it will be necessary to access its Aeronautical Information Publication (AIP).  Many of the AIP's are available for free down load and viewing on the internet.  For example, click here for the AIP for the United States.   Caution: it is 786 pages in length. Here is the indication in the USA AIP that WGS-84 is the standard used in the USA. It is found in Table TBL GEN 2.1−1

"3. Geodetic Reference Datum
3.1 All published geographic coordinates indicating latitude and longitude are expressed in terms of the World Geodetic System − 1984 (WGS−84) geodetic reference datum."

For those of you flying into Russia and the Commonwealth of Independent States and planning on using your GPS receiver this may prove to be an issue.  Russia uses an different geodetic standard model for its Global Navigation Satellite System (GLONASS) satellite navigation system.  It is the PZ-90 (Earth Parameters 1990 - Parametry Zemli 1990) standard. 
(To be continued...)