GPS stands for Global Positioning System, and was initially developed by the United States for military purposes. It is still operated by the US Defense Department, but is now free for civilian use. The GPS navigation system is made up of 24 satellites, that are situated at a distance of 12000 miles above the Earth. They circle the Earth twice a day in precisely determined orbits, each satellite providing a continuous coverage of its position. The satellites are solar-powered, with a battery back-up in the event of a solar eclipse, are equipped with atomic clocks for accurate time measurement, and a rocket booster to help stay in exact orbit.
The continuous signals received from these satellites are used by GPS units in the operation of their navigation and tracking systems. The devices note the transmission time of the signal by the satellite, along with the time when it was received by the receiver and compare their difference to calculate how far the satellite is. Based on how long it took to receive the signal after the transmission, you can determine the distance of the receiver from the satellite.
Using signals from three satellites, a GPS unit can calculate latitude and longitude. To determine altitude, you need signals from four satellites. By knowing latitude, longitude, and altitude, you can get precise position and directions - you can get an accurate position in the range of up to fifteen feet or nearer - and this knowledge is certainly vital in various personal and professional spheres.
The use of portable tracking and navigation systems has certainly made life a lot easier and convenient for professionals from fields like, geology, surveying, archeology, mapping, forestry, transport, avionics, search and rescue, to give just a few examples. Portable GPS units are also used widely for regular civilian activities, like navigating your car through traffic, for finding road directions in a new area, for finding your route when hiking, biking, or walking.
Is it for personal, or professional use, or both? Are you going to use it to plot the quickest route to your destination, or to get traffic flow information, or both? Do you plan to use it exclusively in your car, or do you want a hand-held one that you can walk around with, or both? Do you want a sturdy one that will withstand rugged use? Once you have a general idea of what you need, you can check out the various portable units available in the market and see which one best fits your requirements. Some of the leading makers for these devices are:
- Rand McNally
Here are some features to look for when buying a portable unit:
The GPS unit should contain a road map database. You should be able to update and add to the database, as and when needed, while selecting different guidance options. The device should have plenty of storage space to store the maps database, as well as all other information that you need to store. Get a portable one with a built-in and rechargeable battery, that can last at least eight hours on a single charge. Get one with a text-to-speech capability. If you have a device that announces aloud all street names and directions, you don't have to take your eyes off the street to check the screen.
The GPS should have a traffic reporting capability, so you can avoid rush and jams, or get around slow-moving traffic. Get a device with a 12 channel parallel receiver system for quicker transmissions. It won't hurt to have a unit with additional special features, like a video player, MP3 player, an FM transmitter, an iPod connection, and hands-free calling. It should be small in size and not too weighty to make it easy to lug around. Just make sure to get one with a screen that is at least 3.5 inches. Anything smaller than that will be harder to decipher. If you plan to mount it in your car, get one that is equipped with a suction cup or rigid arms.
If you are going to put it to some rough use on bumpy roads, get a portable device that comes with a shock-proof case. You can also get water-proof and floatable units, which are a great boon if you want to use them on a boat.
Just keep in mind that while you can get an amazing amount of information and directional help from it, it isn't completely fool-proof. You may encounter a few errors here and there. These days we have something called Differential Global Positioning System (DGPS), which uses a stationary unit to help calculate and convey position information to other non-stationary receivers.