Aug
14
2006

GPS and geocaching

Geocache: Typical geocache and contents.  Photo courtesy Skinny Mike, Flickr Creative Commons.
Geocache: Typical geocache and contents. Photo courtesy Skinny Mike, Flickr Creative Commons.

I thoroughly enjoy geocaching. Geocaching is basically a treasure hunt using the Global Positioning System (GPS) to locate the treasure (often simply trinkets – the “treasure” for me is the find) which can be hidden anywhere in the world. I’ve been a geocacher for over a year now, and have successfully found over 200 geocaches. I love the game because it gets me outside, gets me active, and brings me to places that I have never been before, such as city parks, wildlife refuges, scenic overlooks, and historical monuments. While I have enjoyed my geocaching experience, I have been pretty ignorant as to how the Global Positioning System that makes geocaching possible, works. Until now!

The Global Positioning System was developed by the U.S. Military as a navigation system for their use and became operational in 1993. In 1996 President Bill Clinton issued a policy directive declaring GPS to be a dual-use (civilian and military) system.


GPS satellite: GPS satellite. Image courtesy NASA.

The GPS system consists of 29 satellites (as of August 2006). 24 are all that is needed for global coverage, and three of the 29 are held back as spares in case of any malfunctions. Each satellite orbits the Earth twice each day at an altitude of around 12,000 miles. The orbits are set up so that at least four satellites are always within line of sight from any place on Earth.

The handheld GPS receivers that are used for geocaching (and other uses) locate four or more of these satellite signals, figure out the distance to each, and use this information to determine its own location. The receivers used to have a built in degree of error called selective availability that caused intentional errors in the publicly available navigation signals, making it difficult to use a privately owned receiver for malicious purposes. This feature was disabled in 2000, but can be reactivated should it be found to be needed. The best accuracy I have ever obtained was +/- 8 feet.

Besides for treasure hunting, the Global Positioning System has many other uses. Since it was developed by the military, they use it for a wide variety of purposes – including guiding missiles. GPS receivers are becoming more commonplace in cars as navigation tools.

Discovery landing: Space shuttle Discovery makes the first GPS assisted space shuttle landing on July 17, 2006.  Image courtesy NASA.
Discovery landing: Space shuttle Discovery makes the first GPS assisted space shuttle landing on July 17, 2006. Image courtesy NASA.

The system is used by an increasing number of airplanes to assist with landings, and the most recent landing of space shuttle Discovery was the first to use the GPS satellites to give navigational guidance for landing. GPS was previously a back-up capability to the microwave system currently used. We’re seeing GPS become a larger part of our lives as we look to utilize this incredible system.

I’d encourage anyone who has not gone geocaching to do so – it’s fun, and it gets you outside, and active. Good luck!

Your Comments, Thoughts, Questions, Ideas

Jestjuggle's picture
Jestjuggle says:

Great post and a nice looking blog.

Mike and Barb
At Jestcaching

posted on Mon, 08/14/2006 - 7:29pm

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