http://compnetworking.about.com/cs/wireless/g/bldef_ap.htm
Small businesses and home networks can use devices that are specifically pre-configured and dedicated to serve as wireless access points (also known as WAN'S or AP's). These access points have nodes that are configured and designed to act as a central receiver and transmitter of the wireless local area networks (LAN's) radio signals. The access points support communication via the wireless WI-FI standards.
It is true that some very small WLAN's can function via "ad-hoc" or peer-to-peer mode as access points to support the mode of infrastructure. It should be noted that the "ad-hoc' supporting infrastructure mode bridges wireless local area networks (that use no physical wires) with a wired local area network, therefore more clients are able to be served. Many newer access points can now support up to 255 clients whereas the first "base model" access points could only serve up to a 20 client maximum.
http://www8.garmin.com/aboutGPS/
GPS is a Global Positioning Satellite-based navigational system. The system ids free of charge, works worldwide and in any weather conditions.
A network of 24 satellites that have been placed in orbit by the US Department of Defense make up the GPS system. Originally GPS was intended only for military applications but it became available to civilians in the 1980's.
Two times per day the satellites preciselycircle the earth and constantly and consistently transmit information back to the receiver's units on the earth. The satellites travel at speeds of 7000 miles per hour at 12000 miles above our planet. It is the GPS recievers that calculate exact positions. The receivers compare what exact time a location transmision was received with the time the transmission was sent to configure a users exact positional location and display it almost instantly on the users unit electronic map. Once the GPS has calculated a user position it also has the capabilities of computing information such as speed, trip distance, the sun's location, distance to destination and more.
Global Positioning Satellite systems are extremely quick and accurate in determining location within an error for margin of 15 meters. The GPS system can pin-down a location even in tall buildings in suburban setting or in the thick of the woods in seemingly un-navigational locations. There are now newer GPS receivers with Wide Area Augmentation Systems (WAAS) with improved capability to determine accurate coordinates within 3 meters.
Interesting facts about the GPS System
- In 1978, the first GPS satellite was launched.
- In 1994, the first full constellation of 24 satellites was acheived.
- 10 years is the estimated lifespan of each satellite. They are continually being rebuilt and launched.
- The weight of a Global Positioning Satellite is about 2000 pounds and with the solar panels extended, the satellite measures approximately 17 feet across.
- The transmitter power is 50 watts or less.
Sources of error signals include, Ionosphere and troposphere delays- the satellite slows as it passes through the atmosphere. Signal multipath occurs when signals are reflected or deflected off of objects. Sometimes there are slight timing errors in different clocks such as atomic clocks meshing with the GPS satellite clocks. Also, intentional degradation of the GPS signals by the U.S. Department of Defense, called Selective Availability (S.A.) was used to protect the United States from military adversaries. S.A. was once a source of signal error to civilians. The U.S. government turned S.A. off in may 2000, which significantly improved signal accuracy.
