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0	5800	Now that we've seen how precession takes place, let's try to predict its direction in this case.
14910	20690	So, how do satellites know where they are? There are actually two completely separate dimensions
20690	25910	that we need to consider with satellites. The first being orbital parameters. This describes
25910	35570	the movement of the satellite over time, kind of like referencing a flight plan to tell where a
36080	42010	location for satellites, because there's no natural state that it's expected to be in.
42010	48370	On Earth, that natural state is aligned with gravity, basically upright, and satellites have
48370	55390	no force or no natural state to adhere to. Figuring out the orbital parameters of a satellite requires
55390	60410	taking several observations and deriving them. One way to obtain these observations is to use
60410	66190	the same technology we use here on Earth, an onboard GPS receiver. Additionally, we can look
66190	71310	for them through external sensors. NORAD tracks all objects in space through the United States
71310	76890	Space Surveillance Network, primarily using radar. Each object in space gets a unique ID
76890	81770	assigned to it called a NORAD ID, which is universally used by satellite operators to
81770	87730	reference it. NORAD uses these observations to derive the orbital parameters and uses two-line
87730	94570	elements, or TLEs, as a standard for providing all the orbital information. Here's an example of one
94570	99950	for the International Space Station. You can see the ISS's NORAD ID at the beginning of each line,
100550	106530	25544. The rest is used to describe the orbital motion. This TLE is simply an estimation of the
106530	110970	orbital trajectory, and there are inherent inaccuracies. The way we propagate orbital
110970	116610	trajectories introduces an error that becomes significant over time. The TLE format accounts
116610	121510	for this by having a timestamp, or epoch, built into it to indicate when it was taken. It is then
121510	126570	valid for only about a week or two around that epoch, depending on how important accuracy is
126570	132210	for that specific satellite. As a result, they must be constantly updated to keep from going stale.
132370	137930	Now we know our orbital parameters, what about the second category, attitude? Space is full of a lot
137930	142670	of nothing, but there are still a few somethings that you can sense. The most common sensors are
142670	148010	magnetometers, sun sensors, and star cameras. Magnetometers are used by satellites when they
148010	152890	are within Earth's magnetic field. This is a very low-fidelity measurement though, so it's typically
152890	158450	not used by itself. Sun sensors are photodiodes that measure light intensity. By putting these
158450	164330	sensors in several places surrounding the satellite, we can measure sunlight on each sensor,
164330	168890	giving us the direction of the sun. That, combined with location data, can give us a pointing
168890	175270	accuracy of 10 degrees or less. And finally, star cameras are the primary means of precise
175270	180370	attitude measurement. Stars are far enough away from us and moving slowly enough that we can treat
180370	186110	them as constant for all but the highest precision tasks. This means we can create a map of the star
186110	191270	field that surrounds Earth and we can reference it to calculate exactly where a camera mounted
191270	196330	on the satellite is pointing. Now that we have determined our orbital parameters and orientation,
196330	201210	we have a natural follow-on question. How do I change them? Getting around in space, meaning both
201210	205370	changing your direction and changing your orientation, are difficult primarily because
205370	210410	there is simply very, very little to work with. All the modes of transportation here on Earth
210410	215950	require exerting a force on your external environment. Cars push their tires against the
215950	221010	friction of the roadway, planes pull air through their engines, and ships push water with their
221010	226610	propellers. Again, satellite orbit and satellite attitude are generally disconnected, so we have to
226610	231390	control them separately. First, we'll cover how do I change my orbit. In orbital motion, we don't
231390	236750	actually point to where we want to go and start moving there. Satellites are already moving at a
236750	242810	tremendous speed, around 17,000 miles per hour in low-Earth orbit. Changing orbit in space actually
242810	247890	consists of carefully adding and subtracting energy at the right times. Not electrical energy,
247890	253170	but rather the satellite's combined kinetic and potential energy. Adding energy raises the orbit
253170	257870	farther away from Earth, and subtracting brings you closer. Notice I didn't say going faster raises
257870	262870	the orbit, because as you get farther away from Earth, you actually slow down due to the properties
262870	268950	of orbital dynamics. So we'll stick with the term energy. Since the satellite already has so much
268950	274550	energy that it acquired from launch to stay in orbit, most changes are very small by comparison.
274550	279790	Raising or lowering orbits to maintain or change certain orbital characteristics a small amount.
279790	284530	There are all kinds of technologies being developed and in use to add and subtract energy from
284530	289910	satellites, but the most common are thrusters. Thrusters exist to eject fuel out of a spacecraft
289910	294710	fast enough to change its momentum significantly. This is the same way rockets work, and it comes
294710	299510	from one of the fundamental laws of physics, conservation of momentum. There are several
299510	303510	different types of thrusters for spacecraft that each have different trade-offs, but all of them
303510	309390	operate on the core principle. Throw stuff out the back really fast. There are cold gas thrusters,
309390	313950	which are the simplest ones. They released compressed air to generate a small amount of
313950	319350	thrust. There are chemical thrusters, or miniaturized rockets, that are harnessing a chemical reaction
319350	325250	to energize and release fuel to generate thrust. And finally there are ion thrusters, which use
325250	330850	electromagnetic force to control, accelerate, and release charged particles to generate thrust.
330850	335350	Now how do I control the attitude? Satellites again have the problem that they exist in a
335350	341630	literal vacuum. You can't change your direction with a steering wheel, rudder, or flaps. To tackle this
341630	346850	they actually make use of the same law of physics, but with a slight twist, conservation of angular
346850	352210	momentum. This one is easier to demonstrate in your daily life. If you ever spun yourself on a
352210	357130	chair, you probably realized that pulling your arms in will make you go slightly faster. This is a
357130	362970	demonstration of that law. Satellites make use of this through a technology called reaction wheels.
362970	368310	The core concept is there are very small wheels contained within the spacecraft, and if you spin
368310	373370	that wheel very very fast, you'll cause the spacecraft to react and spin in the opposite
373370	378670	direction. The motors that spin that wheel need to exert a torque to make that happen, and the same
378670	384490	opposing torque is then exerted back on the spacecraft, causing it to spin as well. All of
384490	389510	these technologies put together that allow you to determine the orbit, determine the attitude,
389510	394330	and control the orbit, and control the attitude, make up the satellite's attitude and orbital
394330	399550	control system, or AOCS, which is the most complex subsystem on most satellites.