[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 --> 00:20.800]  First, let's take a quick look at our current scoreboard,
[00:20.800 --> 00:23.780]  which at this point should be pretty familiar to everyone.
[00:23.920 --> 00:25.960]  Since our last update an hour ago,
[00:25.960 --> 00:28.840]  we've actually seen no movement on the scoreboard yet.
[00:28.840 --> 00:31.280]  Challenge 4 is proving to be difficult for the teams,
[00:31.280 --> 00:32.920]  as we predicted it would be.
[00:32.920 --> 00:35.360]  But, from the sound of it on the team chats,
[00:35.360 --> 00:37.220]  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 --> 00:42.740]  While we wait a little bit more game action,
[00:42.740 --> 00:45.900]  let's learn about the complexities of navigation in space
[00:45.900 --> 00:48.360]  from our resident expert, Jess Coffey,
[00:48.360 --> 00:50.740]  in another episode of Space Talk.
[00:50.940 --> 00:53.820]  Now that we've seen how precession takes place,
[00:53.820 --> 00:56.620]  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 --> 01:11.560]  There are actually two completely separate dimensions
[01:11.560 --> 01:13.520]  that we need to consider with satellites.
[01:13.520 --> 01:15.920]  The first being orbital parameters.
[01:15.920 --> 01:18.660]  This describes the movement of the satellite over time,
[01:18.660 --> 01:22.260]  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 --> 01:28.980]  This is entirely independent of location for satellites,
[01:28.980 --> 01:32.860]  because there's no natural state that it's expected to be in.
[01:32.860 --> 01:36.140]  On Earth, that natural state is aligned with gravity,
[01:36.140 --> 01:37.740]  basically upright,
[01:37.740 --> 01:43.140]  and satellites have no force or no natural state to adhere to.
[01:43.140 --> 01:45.700]  Figuring out the orbital parameters of a satellite
[01:45.700 --> 01:49.040]  requires taking several observations and deriving them.
[01:49.040 --> 01:50.580]  One way to obtain these observations
[01:50.580 --> 01:53.400]  is to use the same technology we use here on Earth,
[01:53.400 --> 01:55.420]  an onboard GPS receiver.
[01:55.480 --> 01:59.060]  Additionally, we can look for them through external sensors.
[01:59.060 --> 02:00.940]  NORAD tracks all objects in space
[02:00.940 --> 02:04.120]  through the United States Space Surveillance Network,
[02:04.120 --> 02:05.680]  primarily using radar.
[02:05.680 --> 02:08.580]  Each object in space gets a unique ID assigned to it,
[02:08.580 --> 02:09.740]  called a NORAD ID,
[02:09.740 --> 02:13.840]  which is universally used by satellite operators to reference it.
[02:13.840 --> 02:17.400]  NORAD uses these observations to derive the orbital parameters
[02:17.400 --> 02:20.420]  and uses two-line elements, or TLEs,
[02:20.420 --> 02:23.740]  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 --> 02:38.460]  This TLE is simply an estimation of the orbital trajectory,
[02:38.460 --> 02:40.500]  and there are inherent inaccuracies.
[02:40.500 --> 02:42.620]  The way we propagate orbital trajectories
[02:42.620 --> 02:46.020]  introduces an error that becomes significant over time.
[02:46.020 --> 02:49.860]  The TLE format accounts for this by having a timestamp, or epoch,
[02:49.860 --> 02:51.940]  built into it to indicate when it was taken.
[02:51.940 --> 02:55.460]  It is then valid for only about a week or two around that epoch,
[02:55.460 --> 02:58.980]  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 --> 03:04.960]  Now we know our orbital parameters,
[03:04.960 --> 03:07.440]  what about the second category, attitude?
[03:07.440 --> 03:09.620]  Space is full of a lot of nothing,
[03:09.620 --> 03:12.340]  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 --> 03:32.440]  By putting these sensors in several places surrounding the satellite,
[03:32.440 --> 03:37.320]  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 --> 03:50.480]  Stars are far enough away from us and moving slowly enough
[03:50.480 --> 03:54.740]  that we can treat them as constant for all but the highest precision tasks.
[03:54.740 --> 03:58.740]  This means we can create a map of the star field that surrounds Earth,
[03:58.740 --> 04:03.960]  and we can reference it to calculate exactly where a camera mounted on the satellite is pointing.
[04:03.960 --> 04:07.260]  Now that we have determined our orbital parameters and orientation,
[04:07.260 --> 04:10.520]  we have a natural follow-on question. How do I change them?
[04:10.520 --> 04:14.760]  Getting around in space, meaning both changing your direction and changing your orientation,
[04:14.760 --> 04:19.360]  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 --> 05:11.200]  Notice I didn't say going faster raises the orbit, because as you get farther away from Earth,
[05:11.200 --> 05:15.540]  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 --> 05:22.780]  Since the satellite already has so much energy that it acquired from launch to stay in orbit,
[05:22.780 --> 05:25.440]  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 --> 05:36.140]  There are all kinds of technologies being developed and in use to add and subtract energy from satellites,
[05:36.140 --> 05:38.220]  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 --> 05:53.720]  There are several different types of thrusters for spacecraft that each have different trade-offs,
[05:53.720 --> 05:58.500]  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 --> 06:08.620]  There are chemical thrusters, or miniaturized rockets,
[06:08.620 --> 06:13.920]  that are harnessing a chemical reaction to energize and release fuel to generate thrust.
[06:13.920 --> 06:18.300]  And finally, there are ion thrusters, which use electromagnetic force to control,
[06:18.300 --> 06:21.720]  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 --> 06:57.770]  The core concept is there are very small wheels contained within the spacecraft,
[06:58.160 --> 07:04.980]  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 --> 07:08.920]  The motors that spin that wheel need to exert a torque to make that happen,
[07:08.920 --> 07:14.980]  and the same opposing torque is then exerted back on the spacecraft, causing it to spin as well.
[07:14.980 --> 07:19.120]  All of these technologies put together that allow you to determine the orbit,
[07:19.120 --> 07:23.040]  determine the attitude, and control the orbit and control the attitude,
[07:23.040 --> 07:27.420]  make up the satellite's Attitude and Orbital Control System, or AOCS,
[07:27.420 --> 07:30.640]  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 --> 07:57.450]  The ground control team is telling me that we have those commands scheduled to be sent
[07:57.450 --> 08:02.590]  between 1 p.m. and 4.30 p.m. Pacific, and we are just now starting that comms window.
[08:02.910 --> 08:05.210]  Given the orbit predictions of our satellite and the moon,
[08:05.210 --> 08:09.490]  we expect the actual photo to be taken around 6.30 Pacific this evening.
[08:09.550 --> 08:14.670]  With any luck, we expect to receive that moonshot back here at Earth at 1 a.m. Pacific tonight,
[08:14.670 --> 08:17.230]  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 --> 08:25.230]  We'll see you back here at 4.45 Eastern, 1 p.m. Pacific,
[08:25.230 --> 08:28.530]  for a final hourly update before our end-of-day recap tonight.
[08:28.650 --> 08:29.810]  Thanks, and have a good one.