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Knowledge Is Key
For Intelligent Decisions
Satellite Logic is a leading,
authoritative source of information in
the Satellite Industry. Located in the
heart of the Silicon Valley, Satellite
Logic provides one of the most
valuable and comprehensive
knowledge bases on the Satellite
market! This is a primary Worldwide
information center which enables our
clients to analyze, evaluate, inquire
and select their best tailored
solutions. Our company sets the
industry standards for targeted
buying leads, reflecting a dramatic
advance over traditional marketing
solutions.
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Once upon a time... we had to use pretty extreme measures to keep
from getting lost. Things are much, much easier today. For less than
$100, you can get a pocket-sized gadget that will tell you exactly
where you are on Earth at any moment. As long as you have a GPS
receiver and a clear view of the sky, you'll never be lost again. Global
Positioning System is vast, expensive and involves a lot of technical
ingenuity, but the fundamental concepts at work are quite simple and
intuitive.
GPS Navigation
Older GPS navigation units relied upon information from only one or
two satellites to track satellites, find a satellite, lock onto it, read
information and then find a second satellite. In order to be effective,
GPS navigation needs a minimum of three satellites to provide an
accurate two dimensional position fix, and at least four satellites to
provide an accurate three dimensional fix. The downfall of the older
GPS units was that they pieced together and then averaged out the
navigational information they received, thus providing lacklustre results
when it came to reliability and precision. While they performed very
well for tracking use, the older GPS units left a lot to be desired when
it came to use as a navigation tool. The newer GPS navigation units
on available today come equipped with 12-channel receivers, where
each channel is used to lock on and hold onto an individual satellite.
The best 4 signals are identified and are used to provide reliable and
precise information almost instantaneously.
The GPS receiver's job is to locate four or more satellites, figure out the
distance to each, and use this information to deduce its own location.
This operation is based on a simple mathematical principle called
trilateration.
Fundamentally, three-dimensional trilateration isn't much different from
two-dimensional trilateration, but it's a little trickier to visualize. If you
know you are 10 miles from satellite A in the sky, you could be
anywhere on the surface of a huge, imaginary sphere with a 10-mile
radius. If you also know you are 15 miles from satellite B, you can
overlap the first sphere with another, larger sphere. The spheres
intersect in a perfect circle. If you know the distance to a third satellite,
you get a third sphere, which intersects with this circle at two points.
In order to make calculations the GPS receiver has to know 2 things:
The location of at least three satellites above you
The distance between you and each of those satellites
The GPS receiver figures both of these things out by analyzing
high-frequency, low-power radio signals from the GPS satellites.
Radio waves are electromagnetic energy, which means they travel at
the speed of light (about 186,000 miles per second, 300,000 km per
second in a vacuum). The receiver can figure out how far the signal
has traveled by timing how long it took the signal to arrive. In the next
section, we'll see how the receiver and satellite work together to make
this measurement.
Measuring Distance
When you measure the distance to four located satellites, you can
draw four spheres that all intersect at one point. Three spheres will
intersect even if your numbers are way off, but four spheres will not
intersect at one point if you've measured incorrectly. Since the
receiver makes all its distance measurements using its own built-in
clock, the distances will all be proportionally incorrect.
The receiver can easily calculate the necessary adjustment that will
cause the four spheres to intersect at one point. Based on this, it
resets its clock to be in sync with the satellite's atomic clock. The
receiver does this constantly whenever it's on, which means it is
nearly as accurate as the expensive atomic clocks in the satellites.
In order for the distance information to be of any use, the receiver also
has to know where the satellites actually are. This isn't particularly
difficult because the satellites travel in very high and predictable orbits.
The GPS receiver simply stores an almanac that tells it where every
satellite should be at any given time.
So in fact, the most essential function of a GPS receiver is to pick up
the transmissions of at least four satellites and combine the
information in those transmissions with information in an electronic
almanac, all in order to figure out the receiver's position on Earth.
Once the receiver makes this calculation, it can tell you the latitude,
longitude and altitude (or some similar measurement) of its current
position. To make the navigation more user-friendly, most receivers
plug this raw data into map files stored in memory.
You can use maps stored in the receiver's memory, connect the
receiver to a computer that can hold more detailed maps in its
memory, or simply buy a detailed map of your area and find your way
using the receiver's latitude and longitude readouts. Some receivers
let you download detailed maps into memory or supply detailed maps
with plug-in map cartridges.
A standard GPS receiver will not only place you on a map at any
particular location, but will also trace your path across a map as you
move. If you leave your receiver on, it can stay in constant
communication with GPS satellites to see how your location is
changing. With this information and its built-in clock, the receiver can
give you several pieces of valuable information:
How far you've traveled (odometer)
How long you've been traveling
Your current speed (speedometer)
Your average speed
A "bread crumb" trail showing you exactly where you have traveled
The estimated time of arrival at your destination
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