Accelerometers

Features of a GPS system
A list of what a GPS system can do.

GPS Tutorial
A great site that deals with the basics of GPS

GPS History, Chronology, and Budgets
Adobe File that does shows just what it says.

Accelerometers
Technical data on Accelerometers (Adobe file)

Hummingbird 1.0
A website that uses accelerometers in their products

Using Micromachined Motion Sensors to Improve Human/Machine Interface
Adobe file that goes into detail about the uses of Accelerometers and other motion sensors in human/machine interaction

In the 1920s a technique called radionavigation (radios that allowed navigators to locate the direction of the shore-based transmitters) became the start of a military tracking system.  Once artificial satellites were designed to make possible tranmissions of more precise, line-of-sight radionavigation they led way for a Navy system called Transit (a position-finding system that was two deminsional).  This was the first GPS (Global Positioning System).

Here's how GPS works in five logical steps:

1. The basis of GPS is "triangulation" from satellites.
2. To "triangulate," a GPS receiver measures distance using the travel time of radio signals.
3. To measure travel time, GPS needs very accurate timing which it achieves with some tricks.
4. Along with distance, you need to know exactly where the satellites are in space. High orbits and careful monitoring are the secret.
5. Finally you must correct for any delays the signal experiences as it travels through the atmosphere.

Trilateration is a basic geometric principle that allows you to find one location if you know its distance from other, already known locations. The geometry behind this is very easy to understand in two dimensional space.

This same concept works in three dimensional space as well, but you're dealing with spheres instead of circles. You also need four spheres instead of three circles to find your exact location. The heart of a GPS receiver is the ability to find the receiver's distance from four (or more) GPS satellites. Once it determines its distance from the four satellites, the receiver can calculate its exact location and altitude on Earth! If the receiver can only find three satellites, then it can use an imaginary sphere to represent the Earth and can give you location information but no altitude information.

GPS satellites send out radio signals that your GPS receiver can detect. But how does the signal let the receiver know how far away the satellite is? The simple answer is: A GPS receiver measures the amount of time it takes for the signal to travel from the satellite to the receiver. Since we know how fast radio signals travel -- they are electromagnetic waves and so (in a vacuum) travel at the speed of light, about 186,000 miles per second -- we can figure out how far they've traveled by figuring out how long it took for them to arrive.

A satellite begins transmitting a long digital pattern, called a pseudo-random code, as part of its signal at a certain time, let's say midnight. The receiver begins running the same digital pattern, also exactly at midnight. When the satellite's signal reaches the receiver, its transmission of the pattern will lag a bit behind the receiver's playing of the pattern. The length of the delay is equal to the time of the signal's travel. The receiver multiplies this time by the speed of light to determine how far the signal traveled. If the signal traveled in a straight line, this distance would be the distance to the satellite.

The only way to implement a system like this would require a level of accuracy only found in atomic clocks. This is because the time measured in these calculations amounts to nanoseconds. To make a GPS using only synchronized clocks, you would need to have atomic clocks not only on all the satellites, but also in the receiver itself. Atomic clocks usually cost somewhere between $50,000 and $100,000, which makes them a little too expensive for everyday consumer use.

Details from How stuff Works.com

The Global Positioning System has a very effective solution to this problem -- a GPS receiver contains no atomic clock at all. It has a normal quartz clock. The receiver looks at all the signals it is receiving and uses calculations to find both the exact time and the exact location simultaneously. When you measure the distance to four located satellites, you can draw four spheres that all intersect at one point, as illustrated above. Four spheres of this sort will not intersect at one point if you've measured incorrectly. Since the receiver makes all of its time measurements, and therefore its distance measurements, using the clock it is equipped with, the distances will all be proportionally incorrect. The receiver can therefore easily calculate exactly what distance adjustment will cause the four spheres to intersect at one point. This allows it to adjust its clock to adjust its measure of distance. For this reason, a GPS receiver actually keeps extremely accurate time, on the order of the actual atomic clocks in the satellites!

Accelerometers
These devices measure the acceleration in a particular direction.

This data can be used to determine the probability of injury. Acceleration is the rate at which speed changes. For example, if you bang your head into a brick wall, the speed of your head changes very quickly. But, if you bang your head into a pillow, the speed of your head changes more slowly as the pillow crushes.

These devices have been used in crash test dummies, satellites, solar sails, and some printable computers.  

They sense and respond to translational accelarations.  Acceloramters are made of mechanical proof masses, hinges, and servos; gyroscopes have been built with multiple mechanical proof masses.

Gyroscopes and Accelerometers

If you mount two gyroscopes with their axles at right angles to one another on a platform, and place the platform inside a set of gimbals, the platform will remain completely rigid as the gimbals rotate in any way they please. This is this basis of inertial navigation systems (INS).

In an INS, sensors on the gimbals' axles detect when the platform rotates. The INS uses those signals to understand the vehicle's rotations relative to the platform. If you add to the platform a set of three sensitive accelerometers, you can tell exactly where the vehicle is heading and how its motion is changing in all three directions. With this information, an airplane's autopilot can keep the plane on course, and a rocket's guidance system can insert the rocket into a desired orbit!

Satellites and Accelerometers

A rocket must be controlled very precisely to insert a satellite into the desired orbit. An inertial guidance system (IGS) inside the rocket makes this control possible. The IGS determines a rocket's exact location and orientation by precisely measuring all of the accelerations the rocket experiences, using gyroscopes and accelerometers. Mounted in gimbals, the gyroscopes' axes stay pointing in the same direction. This gyroscopically-stable platform contains accelerometers that measure changes in acceleration on three different axes. If it knows exactly where the rocket was at launch and it knows the accelerations the rocket experiences during flight, the IGS can calculate the rocket's position and orientation in space.

From "How Stuff works" and technical data website link.