Now that we've seen how precession takes place, let's try to predict its direction in this case.
So, how do satellites know where they are? There are actually two completely separate dimensions
that we need to consider with satellites. The first being orbital parameters. This describes
the movement of the satellite over time, kind of like referencing a flight plan to tell where a
location for satellites, because there's no natural state that it's expected to be in.
On Earth, that natural state is aligned with gravity, basically upright, and satellites have
no force or no natural state to adhere to. Figuring out the orbital parameters of a satellite requires
taking several observations and deriving them. One way to obtain these observations is to use
the same technology we use here on Earth, an onboard GPS receiver. Additionally, we can look
for them through external sensors. NORAD tracks all objects in space through the United States
Space Surveillance Network, primarily using radar. Each object in space gets a unique ID
assigned to it called a NORAD ID, which is universally used by satellite operators to
reference it. NORAD uses these observations to derive the orbital parameters and uses two-line
elements, or TLEs, as a standard for providing all the orbital information. Here's an example of one
for the International Space Station. You can see the ISS's NORAD ID at the beginning of each line,
25544. The rest is used to describe the orbital motion. This TLE is simply an estimation of the
orbital trajectory, and there are inherent inaccuracies. The way we propagate orbital
trajectories introduces an error that becomes significant over time. The TLE format accounts
for this by having a timestamp, or epoch, built into it to indicate when it was taken. It is then
valid for only about a week or two around that epoch, depending on how important accuracy is
for that specific satellite. As a result, they must be constantly updated to keep from going stale.
Now we know our orbital parameters, what about the second category, attitude? Space is full of a lot
of nothing, but there are still a few somethings that you can sense. The most common sensors are
magnetometers, sun sensors, and star cameras. Magnetometers are used by satellites when they
are within Earth's magnetic field. This is a very low-fidelity measurement though, so it's typically
not used by itself. Sun sensors are photodiodes that measure light intensity. By putting these
sensors in several places surrounding the satellite, we can measure sunlight on each sensor,
giving us the direction of the sun. That, combined with location data, can give us a pointing
accuracy of 10 degrees or less. And finally, star cameras are the primary means of precise
attitude measurement. Stars are far enough away from us and moving slowly enough that we can treat
them as constant for all but the highest precision tasks. This means we can create a map of the star
field that surrounds Earth and we can reference it to calculate exactly where a camera mounted
on the satellite is pointing. Now that we have determined our orbital parameters and orientation,
we have a natural follow-on question. How do I change them? Getting around in space, meaning both
changing your direction and changing your orientation, are difficult primarily because
there is simply very, very little to work with. All the modes of transportation here on Earth
require exerting a force on your external environment. Cars push their tires against the
friction of the roadway, planes pull air through their engines, and ships push water with their
propellers. Again, satellite orbit and satellite attitude are generally disconnected, so we have to
control them separately. First, we'll cover how do I change my orbit. In orbital motion, we don't
actually point to where we want to go and start moving there. Satellites are already moving at a
tremendous speed, around 17,000 miles per hour in low-Earth orbit. Changing orbit in space actually
consists of carefully adding and subtracting energy at the right times. Not electrical energy,
but rather the satellite's combined kinetic and potential energy. Adding energy raises the orbit
farther away from Earth, and subtracting brings you closer. Notice I didn't say going faster raises
the orbit, because as you get farther away from Earth, you actually slow down due to the properties
of orbital dynamics. So we'll stick with the term energy. Since the satellite already has so much
energy that it acquired from launch to stay in orbit, most changes are very small by comparison.
Raising or lowering orbits to maintain or change certain orbital characteristics a small amount.
There are all kinds of technologies being developed and in use to add and subtract energy from
satellites, but the most common are thrusters. Thrusters exist to eject fuel out of a spacecraft
fast enough to change its momentum significantly. This is the same way rockets work, and it comes
from one of the fundamental laws of physics, conservation of momentum. There are several
different types of thrusters for spacecraft that each have different trade-offs, but all of them
operate on the core principle. Throw stuff out the back really fast. There are cold gas thrusters,
which are the simplest ones. They released compressed air to generate a small amount of
thrust. There are chemical thrusters, or miniaturized rockets, that are harnessing a chemical reaction
to energize and release fuel to generate thrust. And finally there are ion thrusters, which use
electromagnetic force to control, accelerate, and release charged particles to generate thrust.
Now how do I control the attitude? Satellites again have the problem that they exist in a
literal vacuum. You can't change your direction with a steering wheel, rudder, or flaps. To tackle this
they actually make use of the same law of physics, but with a slight twist, conservation of angular
momentum. This one is easier to demonstrate in your daily life. If you ever spun yourself on a
chair, you probably realized that pulling your arms in will make you go slightly faster. This is a
demonstration of that law. Satellites make use of this through a technology called reaction wheels.
The core concept is there are very small wheels contained within the spacecraft, and if you spin
that wheel very very fast, you'll cause the spacecraft to react and spin in the opposite
direction. The motors that spin that wheel need to exert a torque to make that happen, and the same
opposing torque is then exerted back on the spacecraft, causing it to spin as well. All of
these technologies put together that allow you to determine the orbit, determine the attitude,
and control the orbit, and control the attitude, make up the satellite's attitude and orbital
control system, or AOCS, which is the most complex subsystem on most satellites.