WEBVTT 00:12.140 --> 00:13.640 Hello, and welcome back. 00:13.640 --> 00:16.240 Live with another competition update for HackASAP. 00:16.240 --> 00:17.980 I'm your host, Jordan Wines. 00:17.980 --> 00:23.780 First, let's take a quick look at our current scoreboard, which at this point should be pretty familiar to everyone. 00:23.920 --> 00:28.840 Since our last update an hour ago, we've actually seen no movement on the scoreboard yet. 00:28.840 --> 00:32.920 Challenge 4 is proving to be difficult for the teams, as we predicted it would be. 00:32.920 --> 00:37.220 But, from the sound of it on the team chats, they might be getting a little closer. 00:37.220 --> 00:40.140 So, hopefully we'll see some movement soon there. 00:40.500 --> 00:50.740 While we wait a little bit more game action, let's learn about the complexities of navigation in space from our resident expert, Jess Coffey, in another episode of Space Talk. 00:50.940 --> 00:56.620 Now that we've seen how precession takes place, let's try to predict its direction in this case. 01:06.120 --> 01:09.060 So, how do satellites know where they are? 01:09.180 --> 01:13.520 There are actually two completely separate dimensions that we need to consider with satellites. 01:13.520 --> 01:15.920 The first being orbital parameters. 01:15.920 --> 01:22.260 This describes the movement of the satellite over time, kind of like referencing a flight plan to tell where a plane is. 01:22.260 --> 01:24.040 The second is the attitude. 01:24.040 --> 01:26.040 Basically, what direction am I facing? 01:26.040 --> 01:32.860 This is entirely independent of location for satellites, because there's no natural state that it's expected to be in. 01:32.860 --> 01:43.140 On Earth, that natural state is aligned with gravity, basically upright, and satellites have no force or no natural state to adhere to. 01:43.140 --> 01:49.040 Figuring out the orbital parameters of a satellite requires taking several observations and deriving them. 01:49.040 --> 01:55.420 One way to obtain these observations is to use the same technology we use here on Earth, an onboard GPS receiver. 01:55.480 --> 01:59.060 Additionally, we can look for them through external sensors. 01:59.060 --> 02:05.680 NORAD tracks all objects in space through the United States Space Surveillance Network, primarily using radar. 02:05.680 --> 02:13.840 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. 02:13.840 --> 02:23.740 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. 02:23.740 --> 02:27.120 Here's an example of one for the International Space Station. 02:27.120 --> 02:32.360 You can see the ISS's NORAD ID at the beginning of each line, 25544. 02:32.360 --> 02:35.180 The rest is used to describe the orbital motion. 02:35.180 --> 02:40.500 This TLE is simply an estimation of the orbital trajectory, and there are inherent inaccuracies. 02:40.500 --> 02:46.020 The way we propagate orbital trajectories introduces an error that becomes significant over time. 02:46.020 --> 02:51.940 The TLE format accounts for this by having a timestamp, or epoch, built into it to indicate when it was taken. 02:51.940 --> 02:58.980 It is then valid for only about a week or two around that epoch, depending on how important accuracy is for that specific satellite. 02:58.980 --> 03:03.120 As a result, they must be constantly updated to keep from going stale. 03:03.240 --> 03:07.440 Now we know our orbital parameters, what about the second category, attitude? 03:07.440 --> 03:12.340 Space is full of a lot of nothing, but there are still a few somethings that you can sense. 03:12.340 --> 03:16.660 The most common sensors are magnetometers, sun sensors, and star cameras. 03:16.700 --> 03:20.800 Magnetometers are used by satellites when they are within Earth's magnetic field. 03:20.800 --> 03:25.340 This is a very low-fidelity measurement, though, so it's typically not used by itself. 03:25.340 --> 03:28.720 Sun sensors are photodiodes that measure light intensity. 03:28.720 --> 03:37.320 By putting these sensors in several places surrounding the satellite, we can measure sunlight on each sensor, giving us the direction of the sun. 03:37.320 --> 03:42.340 That, combined with location data, can give us a pointing accuracy of 10 degrees or less. 03:42.440 --> 03:47.440 And finally, star cameras are the primary means of precise attitude measurement. 03:47.440 --> 03:54.740 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. 03:54.740 --> 04:03.960 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. 04:03.960 --> 04:09.320 Now that we have determined our orbital parameters and orientation, we have a natural follow-on question. 04:09.460 --> 04:10.520 How do I change them? 04:10.520 --> 04:19.360 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. 04:19.360 --> 04:25.220 All the modes of transportation here on Earth require exerting a force on your external environment. 04:25.220 --> 04:27.940 Cars push their tires against the friction of the roadway. 04:27.940 --> 04:32.740 Planes pull air through their engines, and ships push water with their propellers. 04:32.740 --> 04:38.700 Again, satellite orbit and satellite attitude are generally disconnected, so we have to control them separately. 04:38.700 --> 04:41.040 First, we'll cover how do I change my orbit. 04:41.040 --> 04:45.600 In orbital motion, we don't actually point to where we want to go and start moving there. 04:45.600 --> 04:51.520 Satellites are already moving at a tremendous speed, around 17,000 miles per hour in low-Earth orbit. 04:51.520 --> 04:57.480 Changing orbit in space actually consists of carefully adding and subtracting energy at the right times. 04:57.480 --> 05:02.360 Not electrical energy, but rather the satellites combined kinetic and potential energy. 05:02.360 --> 05:07.100 Adding energy raises the orbit farther away from Earth, and subtracting brings you closer. 05:07.100 --> 05:15.540 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. 05:15.540 --> 05:17.680 So we'll stick with the term energy. 05:17.840 --> 05:25.440 Since the satellite already has so much energy that it acquired from launch to stay in orbit, most changes are very small by comparison. 05:25.440 --> 05:30.680 We're raising or lowering orbits to maintain or change certain orbital characteristics a small amount. 05:30.680 --> 05:38.220 There are all kinds of technologies being developed and in use to add and subtract energy from satellites, but the most common are thrusters. 05:38.360 --> 05:43.620 Thrusters exist to eject fuel out of a spacecraft fast enough to change its momentum significantly. 05:43.620 --> 05:49.760 This is the same way rockets work, and it comes from one of the fundamental laws of physics, conservation of momentum. 05:49.760 --> 05:58.500 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. 05:58.540 --> 06:01.900 There are cold gas thrusters, which are the simplest ones. 06:01.900 --> 06:05.360 They release compressed air to generate a small amount of thrust. 06:05.360 --> 06:13.920 There are chemical thrusters, or miniaturized rockets, that are harnessing a chemical reaction to energize and release fuel to generate thrust. 06:13.920 --> 06:21.720 And finally, there are ion thrusters, which use electromagnetic force to control, accelerate, and release charged particles to generate thrust. 06:21.720 --> 06:23.600 Now, how do I control the attitude? 06:23.700 --> 06:26.980 Satellites again have the problem that they exist in a literal vacuum. 06:26.980 --> 06:31.620 You can't change your direction with a steering wheel, rudder, or flaps. 06:31.620 --> 06:38.220 To tackle this, they actually make use of the same law of physics, but with a slight twist, conservation of angular momentum. 06:38.280 --> 06:41.260 This one is easier to demonstrate in your daily life. 06:41.260 --> 06:47.540 If you ever spun yourself on a chair, you probably realized that pulling your arms in will make you go slightly faster. 06:47.540 --> 06:49.600 This is a demonstration of that law. 06:49.600 --> 06:53.920 Satellites make use of this through a technology called reaction wheels. 06:53.960 --> 07:04.980 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. 07:04.980 --> 07:14.980 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. 07:14.980 --> 07:30.640 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. 07:42.930 --> 07:44.790 Thanks, Jess, for that update. 07:44.850 --> 07:49.090 Before we sign off here, I want to give another update on the progress of the on-orbit mission. 07:49.090 --> 07:53.990 Remember, we chose a single-team solution to send to our orbiting satellite. 07:53.990 --> 07:58.650 The ground control team is telling me that we have those commands scheduled to be sent between 1 p.m. 07:58.650 --> 07:59.650 and 4.30 p.m. 07:59.650 --> 08:02.590 Pacific, and we are just now starting that comms window. 08:02.910 --> 08:09.490 Given the orbit predictions of our satellite and the moon, we expect the actual photo to be taken around 6.30 Pacific this evening. 08:09.550 --> 08:13.430 With any luck, we expect to receive that moonshot back here at Earth at 1 a.m. 08:13.610 --> 08:17.230 Pacific tonight, and we'll have it for you tomorrow at the closing awards. 08:17.350 --> 08:18.990 That's it for this hour's update. 08:18.990 --> 08:23.770 We'll see you back here at 4.45 Eastern, 1 p.m. 08:23.950 --> 08:28.530 Pacific, for a final hourly update before our end-of-day recap tonight. 08:28.650 --> 08:29.810 Thanks, and have a good one.