[00:00.000 --> 00:05.800]  Now that we've seen how precession takes place, let's try to predict its direction in this case.
[00:14.910 --> 00:20.690]  So, how do satellites know where they are? There are actually two completely separate dimensions
[00:20.690 --> 00:25.910]  that we need to consider with satellites. The first being orbital parameters. This describes
[00:25.910 --> 00:35.570]  the movement of the satellite over time, kind of like referencing a flight plan to tell where a
[00:36.080 --> 00:42.010]  location for satellites, because there's no natural state that it's expected to be in.
[00:42.010 --> 00:48.370]  On Earth, that natural state is aligned with gravity, basically upright, and satellites have
[00:48.370 --> 00:55.390]  no force or no natural state to adhere to. Figuring out the orbital parameters of a satellite requires
[00:55.390 --> 01:00.410]  taking several observations and deriving them. One way to obtain these observations is to use
[01:00.410 --> 01:06.190]  the same technology we use here on Earth, an onboard GPS receiver. Additionally, we can look
[01:06.190 --> 01:11.310]  for them through external sensors. NORAD tracks all objects in space through the United States
[01:11.310 --> 01:16.890]  Space Surveillance Network, primarily using radar. Each object in space gets a unique ID
[01:16.890 --> 01:21.770]  assigned to it called a NORAD ID, which is universally used by satellite operators to
[01:21.770 --> 01:27.730]  reference it. NORAD uses these observations to derive the orbital parameters and uses two-line
[01:27.730 --> 01:34.570]  elements, or TLEs, as a standard for providing all the orbital information. Here's an example of one
[01:34.570 --> 01:39.950]  for the International Space Station. You can see the ISS's NORAD ID at the beginning of each line,
[01:40.550 --> 01:46.530]  25544. The rest is used to describe the orbital motion. This TLE is simply an estimation of the
[01:46.530 --> 01:50.970]  orbital trajectory, and there are inherent inaccuracies. The way we propagate orbital
[01:50.970 --> 01:56.610]  trajectories introduces an error that becomes significant over time. The TLE format accounts
[01:56.610 --> 02:01.510]  for this by having a timestamp, or epoch, built into it to indicate when it was taken. It is then
[02:01.510 --> 02:06.570]  valid for only about a week or two around that epoch, depending on how important accuracy is
[02:06.570 --> 02:12.210]  for that specific satellite. As a result, they must be constantly updated to keep from going stale.
[02:12.370 --> 02:17.930]  Now we know our orbital parameters, what about the second category, attitude? Space is full of a lot
[02:17.930 --> 02:22.670]  of nothing, but there are still a few somethings that you can sense. The most common sensors are
[02:22.670 --> 02:28.010]  magnetometers, sun sensors, and star cameras. Magnetometers are used by satellites when they
[02:28.010 --> 02:32.890]  are within Earth's magnetic field. This is a very low-fidelity measurement though, so it's typically
[02:32.890 --> 02:38.450]  not used by itself. Sun sensors are photodiodes that measure light intensity. By putting these
[02:38.450 --> 02:44.330]  sensors in several places surrounding the satellite, we can measure sunlight on each sensor,
[02:44.330 --> 02:48.890]  giving us the direction of the sun. That, combined with location data, can give us a pointing
[02:48.890 --> 02:55.270]  accuracy of 10 degrees or less. And finally, star cameras are the primary means of precise
[02:55.270 --> 03:00.370]  attitude measurement. Stars are far enough away from us and moving slowly enough that we can treat
[03:00.370 --> 03:06.110]  them as constant for all but the highest precision tasks. This means we can create a map of the star
[03:06.110 --> 03:11.270]  field that surrounds Earth and we can reference it to calculate exactly where a camera mounted
[03:11.270 --> 03:16.330]  on the satellite is pointing. Now that we have determined our orbital parameters and orientation,
[03:16.330 --> 03:21.210]  we have a natural follow-on question. How do I change them? Getting around in space, meaning both
[03:21.210 --> 03:25.370]  changing your direction and changing your orientation, are difficult primarily because
[03:25.370 --> 03:30.410]  there is simply very, very little to work with. All the modes of transportation here on Earth
[03:30.410 --> 03:35.950]  require exerting a force on your external environment. Cars push their tires against the
[03:35.950 --> 03:41.010]  friction of the roadway, planes pull air through their engines, and ships push water with their
[03:41.010 --> 03:46.610]  propellers. Again, satellite orbit and satellite attitude are generally disconnected, so we have to
[03:46.610 --> 03:51.390]  control them separately. First, we'll cover how do I change my orbit. In orbital motion, we don't
[03:51.390 --> 03:56.750]  actually point to where we want to go and start moving there. Satellites are already moving at a
[03:56.750 --> 04:02.810]  tremendous speed, around 17,000 miles per hour in low-Earth orbit. Changing orbit in space actually
[04:02.810 --> 04:07.890]  consists of carefully adding and subtracting energy at the right times. Not electrical energy,
[04:07.890 --> 04:13.170]  but rather the satellite's combined kinetic and potential energy. Adding energy raises the orbit
[04:13.170 --> 04:17.870]  farther away from Earth, and subtracting brings you closer. Notice I didn't say going faster raises
[04:17.870 --> 04:22.870]  the orbit, because as you get farther away from Earth, you actually slow down due to the properties
[04:22.870 --> 04:28.950]  of orbital dynamics. So we'll stick with the term energy. Since the satellite already has so much
[04:28.950 --> 04:34.550]  energy that it acquired from launch to stay in orbit, most changes are very small by comparison.
[04:34.550 --> 04:39.790]  Raising or lowering orbits to maintain or change certain orbital characteristics a small amount.
[04:39.790 --> 04:44.530]  There are all kinds of technologies being developed and in use to add and subtract energy from
[04:44.530 --> 04:49.910]  satellites, but the most common are thrusters. Thrusters exist to eject fuel out of a spacecraft
[04:49.910 --> 04:54.710]  fast enough to change its momentum significantly. This is the same way rockets work, and it comes
[04:54.710 --> 04:59.510]  from one of the fundamental laws of physics, conservation of momentum. There are several
[04:59.510 --> 05:03.510]  different types of thrusters for spacecraft that each have different trade-offs, but all of them
[05:03.510 --> 05:09.390]  operate on the core principle. Throw stuff out the back really fast. There are cold gas thrusters,
[05:09.390 --> 05:13.950]  which are the simplest ones. They released compressed air to generate a small amount of
[05:13.950 --> 05:19.350]  thrust. There are chemical thrusters, or miniaturized rockets, that are harnessing a chemical reaction
[05:19.350 --> 05:25.250]  to energize and release fuel to generate thrust. And finally there are ion thrusters, which use
[05:25.250 --> 05:30.850]  electromagnetic force to control, accelerate, and release charged particles to generate thrust.
[05:30.850 --> 05:35.350]  Now how do I control the attitude? Satellites again have the problem that they exist in a
[05:35.350 --> 05:41.630]  literal vacuum. You can't change your direction with a steering wheel, rudder, or flaps. To tackle this
[05:41.630 --> 05:46.850]  they actually make use of the same law of physics, but with a slight twist, conservation of angular
[05:46.850 --> 05:52.210]  momentum. This one is easier to demonstrate in your daily life. If you ever spun yourself on a
[05:52.210 --> 05:57.130]  chair, you probably realized that pulling your arms in will make you go slightly faster. This is a
[05:57.130 --> 06:02.970]  demonstration of that law. Satellites make use of this through a technology called reaction wheels.
[06:02.970 --> 06:08.310]  The core concept is there are very small wheels contained within the spacecraft, and if you spin
[06:08.310 --> 06:13.370]  that wheel very very fast, you'll cause the spacecraft to react and spin in the opposite
[06:13.370 --> 06:18.670]  direction. The motors that spin that wheel need to exert a torque to make that happen, and the same
[06:18.670 --> 06:24.490]  opposing torque is then exerted back on the spacecraft, causing it to spin as well. All of
[06:24.490 --> 06:29.510]  these technologies put together that allow you to determine the orbit, determine the attitude,
[06:29.510 --> 06:34.330]  and control the orbit, and control the attitude, make up the satellite's attitude and orbital
[06:34.330 --> 06:39.550]  control system, or AOCS, which is the most complex subsystem on most satellites.