The Global Positioning System (GPS) is a global navigation satellite system (GNSS) developed by the United States Department of Defense and managed by the United States Air Force 50th Space Wing ,  It made up of a network of 24 satellites It is the only fully functional GNSS in the world, can be used freely by anyone, anywhere, and is often used by civilians for navigation purposes.

GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user’s exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user’s position and display it on the unit’s electronic map.

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user’s 3D position (latitude, longitude and altitude). Once the user’s position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.

Putting GPS to work

GPS technology has matured into a resource that goes far beyond its original design goals. These days scientists, sportsmen, farmers, soldiers, pilots, surveyors, hikers, delivery drivers, sailors, dispatchers, lumberjacks, fire-fighters, and people from many other walks of life are using GPS in ways that make their work more productive, safer, and sometimes even easier.

Examples of real-world applications of GPS fall into five broad categories :

• Location – determining a basic position
• Navigation – getting from one location to another
• Tracking – monitoring the movement of people and things
• Mapping – creating maps of the world
• Timing - bringing precise timing to the world

A router is a networking device whose software and hardware are usually tailored to the tasks of routing “ selecting paths in a network along which to send network traffic ” and forwarding information “relaying of packets from one network segment to another by nodes in a computer network“.

The router is the only device that sees every message sent by any computer on either of the networks .

Router is extremely useful in dealing with two separate computer networks. It joins the two networks, passing information from one to the other and, in some cases, performing translations of various protocols between the two networks. It also protects the networks from one another, preventing the traffic on one from unnecessarily spilling over to the other.

One of the tools a router uses to decide where a packet should go is a configuration table. A configuration table is a collection of information, including:

• Information on which connections lead to particular groups of addresses
• Priorities for connections to be used
• Rules for handling both routine and special cases of traffic

Configuration table is very useful that make sure :

• The router ensures that information doesn’t go where it’s not needed. This is crucial for keeping large volumes of data from clogging the connections of “innocent bystanders.”
• The router makes sure that information does make it to the intended destination.

A personal digital assistant (PDA) is a handheld computer, also known as a palmtop computer,have both color screens and audio capabilities, enabling them to be used as mobile phones, (smartphones), web browsers, or portable media players. Many PDAs can access the Internet, intranets or extranets via Wi-Fi, or Wireless Wide-Area Networks (WWANs). Many PDAs employ touch screen technology.

The main purpose of  PDA  is to act as an electronic organizer or day planner that is portable, easy to use and­ capable of sharing information with your PC. It’s supposed to be an extension of the PC, not a replacement.

Software typically required to be a PDA includes an appointment calendar, a to-do list, an address book for contacts and some sort of note program. Connected PDAs also typically include E-mail and Web support.

Types of PDAs :

Palm PDAs
Most Palm devices are made by palmOne, which offers the Zire and Tungsten product lines. The company formed in 2003 when Palm Computing acquired Handspring, Inc. Sony, which produced the Palm-based CLIE, stopped producing PDAs in 2005.

Pocket PC the generic name for Windows Mobile PDAs

All PDAs come with some kind of personal information management (PIM) software that typically handles the following tasks to keep you organized:

• Store contact information (names, addresses, phone numbers, e-mail addresses)
• Make to-do lists
• Take notes
• Track appointments (date book, calendar)
• Remind you of appointments (clock, alarm functions)
• Perform calculations

PDAs can run specialized software applications:

• Windows Mobile devices come with Pocket versions of Word, Excel, Internet Explorer and Outlook (includes e-mail and PIM functions), along with Windows Media Player and voice memo recording.
• Most Palm OS devices include applications such as DataViz Documents to Go (compatible with Microsoft Word, Excel, and PowerPoint), palmOne Media (for photos and video), VersaMail e-mail software and web-browsing software.
• All types of devices can run other kinds of software including games, multimedia, expense, diet and exercise, travel, medical, time and billing, and reference.