How Does GPS Work?
GPS (Global Positioning System) has 24 artificial satellites that orbit the Earth at a distance of 12,600 miles/ 20,300 kilometres, transmitting radio signals. The pattern of their orbit is ‘choreographed’ so that a GPS receiver anywhere on the earth’s surface is always ‘visible’ (and therefore receiving signals) from at least 4 satellites.
From these 4 satellite readings your GPS can work out your location through ‘Trilateration’. It is essentially the same idea as triangulation, but without using angles.
Explaining trilateration in 3-D space is a little tricky, so let’s start with simple 2-D trilateration.
2-D Trilateration
Imagine you’re completely lost. You wake up in a strange hotel room one morning with no idea at all where you are. You go downstairs and ask the hotel receptionist, "Where am I?"
“I can’t say” he says, “but I will tell you you’re 593 miles/ 955 kilometres from Copenhagen.”
You now know you’re somewhere on a circle round Copenhagen with a radius of 593 miles/ 955 kilometres.
You stroll into town and, stopping off for a coffee, ask the waitress where you are. “375 miles/ 604 kilometres from Paris" she says and walks away.
You then notice the table napkins. As luck would have it, they are perfect detailed maps of Europe! You take one, pull out your handy compass-and-ruler accessory set and draw two circles. So:
You now know you must be at one of the two points where the circles intersect. The only two points both 593 miles/ 955 kilometres from Copenhagen and 375 miles/ 604 kilometres from Paris.
Back on the street an old man calls you over. He tells you that you are 317 miles/ 510 kilometres from Prague. You whip out your napkin and compass and draw another circle.
You now know exactly where you are: Frankfurt!
3-D Trilateration
3-D trilateration is basically the same idea as 2-D trilateration. You just need to imagine the 2-D example above, but with 3 spheres instead of 3 circles.
Let’s say you know you’re 10 miles/ kilometres from satellite A. This means you could be anywhere on the surface of a huge, imaginary sphere with a 10 mile/ kilometre radius.
But if you also know you’re 15 miles/ kilometres from satellite B, you can overlap the first sphere with this second sphere with a 15 mile/ kilometre radius.
The two spheres will intersect in a perfect 2-D circle.
And if you also know you’re 8 miles/ kilometres from a third satellite, when you make this third sphere, you will find it intersects with the circle at two points (just like the two-circle diagram in the 2-D example).
But you also have a 4th sphere handy: the Earth itself. Only one of the two intersecting points you’ve just identified will actually be on the Earth’s surface. So, assuming that you’re not floating around somewhere in space, you now know exactly where you are.
However, GPS receivers normally use at least 4 satellites to improve accuracy.
How to choose the right GPS for your automobile?
1.Think about the reasons why you need a GPS. Is it for personal use or for business use? Will you beusing it occasionally or everyday? If the GPS is for personal use or will be used occasionally, I would suggest purchasing a less expensive model with less bells and whistles. If the GPS is going to be used for business purposes or on a daily basis, I would suggest purchasing a higher end model that has more to offer.
2.Decide how much money you want to spend. GPS prices range from $150 for low end systems to $1000 for high end systems.
3.Decide if you want Turn-by-Turn or Text-to-Speech audible directions. Turn-by-Turn audible directions are more commonly found on lower end systems and tell the driver directions in basic terms, for example, "make a right turn in 500 feet". Where as Text-to-Speech audible directions are more descriptive and tell the driver to "make a right turn in 500 feet on Main Street". I have experienced both and prefer the Text-to-Speech directions. Text-to-Speech also allows the driver to keep his or her eyes on the road making for a safer ride.
4.Decide how important POI or Points of Interest is to you. The more POI's a GPS has, the better chance of finding a gas station, shopping center, food store, a movie theater, restaurants, or state park. The list can go on and on. If you travel often for recreation or work and are unfamiliar with where you are going, having a lot of POI's would be important. If you are somewhat familiar with where you are going and not traveling far enough to need gas, then POI's should not be a high priority.
5.Decide if you need traffic routing. The traffic routing feature alerts your GPS to traffic congestion on your route and routes you around it. In most cases, traffic routing is an additional feature that you need to subscribe to. Most GPS manufacturers make you pay a one time fee for the feature.
6.Decide on the quality of your "routing engine" software. Your routing engine is what makes your GPS pick a particular route from point A to point B. Depending on the quality of the routing engine software you choose, your route could be longer then expected. Your goal is to find a GPS with very good routing engine software.
7.Decide if you want your GPS to have Bluetooth technology or an FM transmitter. Some GPS model offer internet connected applications that require the use of a Bluetooth enabled cell phone. An FM transmitter will allow you to play your own recorded music through your GPS to your FM radio via a 3rd party device like an IPOD.
8.Decide on what size screen you want. Screen sizes range from 3.5 inches to 4.5 inches
