Inertial Guidance Systems

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 the accelerations the rocket experiences during flight, the IGS can calculate the rocket's position and orientation in space.

How Is a Satellite Launched Into an Orbit?

All satellites today get into orbit by riding on a rocket. Many used to hitch a ride in the cargo bay of the space shuttle. Several countries and businesses have rocket launch capabilities, and satellites as large as several tons make it into orbit regularly and safely.

For most satellite launches, the scheduled launch rocket is aimed straight up at first. This gets the rocket through the thickest part of the atmosphere most quickly and best minimizes fuel consumption.

After a rocket launches straight up, the rocket control mechanism uses the inertial guidance system (see sidebar) to calculate necessary adjustments to the rocket's nozzles to tilt the rocket to the course described in the flight plan. In most cases, the flight plan calls for the rocket to head east because Earth rotates to the east, giving the launch vehicle a free boost. The strength of this boost depends on the rotational velocity of Earth at the launch location. The boost is greatest at the equator, where the distance around Earth is greatest and so rotation is fastest.

How big is the boost from an equatorial launch? To make a rough estimate, we can determine Earth's circumference by multiplying its diameter by pi (3.1416). The diameter of Earth is approximately 7,926 miles (12,753 kilometers). Multiplying by pi yields a circumference of something like 24,900 miles (40,065 kilometers). To travel around that circumference in 24 hours, a point on Earth's surface has to move at 1,038 mph (1,669 kph). A launch from Florida's Cape Canaveral doesn't get as big a boost from Earth's rotational speed. The Kennedy Space Center's Launch Complex 39-A is located at 28 degrees 36 minutes 29.7014 seconds north latitude. The Earth's rotational speed there is about 894 mph (1,440 kph). The difference in Earth's surface speed between the equator and Kennedy Space Center, then, is about 144 mph (229 kph). (Note: The Earth is actually oblate -- fatter around the middle -- not a perfect sphere. For that reason, our estimate of Earth's circumference is a little small.)

Considering that rockets can go thousands of miles per hour, you may wonder why a difference of only 144 mph would even matter. The answer is that rockets, together with their fuel and their payloads, are very heavy. For example, the Feb. 11, 2000, liftoff of the space shuttle Endeavour required launching a total weight of 4,520,415 pounds (2,050,447 kilograms) [source: NASA]. It takes a huge amount of energy to accelerate such a mass to 144 mph, and therefore a significant amount of fuel. Launching from the equator makes a real difference.

Once the rocket reaches extremely thin air, at about 120 miles (193 kilometers) up, the rocket's navigational system fires small rockets, just enough to turn the launch vehicle into a horizontal position. The satellite is then released. At that point, rockets are fired again to ensure some separation between the launch vehicle and the satellite itself.