GPS Frequently Asked Questions (FAQs)
From
its beginnings as space-age technology more than three decades ago, GPS technology
has progressed from electronic novelty to the point where, for many, it's an
indispensable requirement of daily life. Indeed, GPS technology may one day be
built into almost every electronic unit we operate.
Don't throw out that GPS receiver you bought back in 1997 just yet though. Far from being obsolete, that unit's basic function—a compass that works day or night every day of the year-still works just fine (provided the batteries are fresh). But newer generation GPS units can do some amazing things. In fact, the future of GPS technology and its uses is limited only by man's imagination.
If you're new to the world of Global Positioning, these five often-asked questions will help acquaint you with this now-ubiquitous technology:
Q. What does the acronym GPS stand for?
A. Global Positioning System.
Q. How does GPS work?
A. GPS technology is a global navigation system based on two dozen satellites that orbit the earth at an altitude of 12,000 statute miles and provide very precise, worldwide positioning and navigation information 24 hours a day, in any weather. GPS technology captures a combination of three or more satellite-transmitted waves to triangulate, or trilaterate, your GPS receiver's position on earth.
Q. Who maintains the satellites?
A. Built by the United States Department of Defense (DOD) at a cost of $12 billion and intended primarily for military use, the DOD manages and controls the two dozen satellites which make up the Global Positioning System. The system includes three backup satellites. The United States Government has officially named this satellite system NAVSTAR (NAVigation Satellite Timing And Ranging).
Q. How accurate is GPS technology?
A. Initially, the signals of the GPS satellites contained random errors, known as Selective Availability (SA). The United States government disabled SA in May 2000, releasing the proper transmission of these satellite signals for public use. This made GPS-secured locations ten-times more accurate than they were prior to 2000 and aids not only outdoor adventurers, but public-service agencies such as fire and police departments as well.
In 2003, the United States government flipped the switch on the latest advancement in ensuring the accuracy of GPS technology—Wide Area Augmentation System (WAAS). From earth, data that has been corrected is collected from stations across the United States and uploaded to a geostationary satellite, which in turns transmits the data back to a WAAS-enabled GPS receiver. This data can give your WAAS-enabled GPS unit a position accuracy of better than three meters 95% of the time, and can achieve sub-meter results—a far cry from the football field-sized positioning provided by the first GPS units of the 1980s.
Differential GPS (DGPS) is an extension of GPS that uses land-based radio beacons to transmit satellite position corrections to your DGPS receiver. Approximately 60 broadcast stations, including two control centers, have been established by the United States Coast Guard. DGPS stations recalculate the effect of random error, propagation delay, ionosphere and troposphere irregularities, etc. and can improve position accuracy to between 1 and 3 meters. Most recreation-grade GPS units require the purchase an additional receiver antenna to access DGPS signals.
Q. Do I have to subscribe to a service in order to use my GPS unit?
A. The DOD Navstar GPS is free. However, there are some augmented systems, such as DGPS, that require receiver parts which may not be part of your GPS unit and will have to be purchased separately. The Coast Guard maritime DGPS service, for example, became fully operational in March 1999 and as of 2004 supports approximately 1.5 millions users. There are other competing positioning systems, both ground-based and satellite-based, that provide correction signals for a subscription fee. These are accessed primarily by commercial concerns such as surveyors and oil-rigging companies. Additional business-related advancements in GPS technology on the horizon include Local Area Augmentation System (LAAS), which will provide GPS support for automatic landings of airplanes at commercial airports.
More information:
How to Choose a GPS Receiver