How your drone knows where it is — transcript

00:00:00.500 --> 00:00:04.340 How does your drone know where it is? Sensors, and a flight controller.

00:00:06.900 --> 00:00:12.436 The flight controller is the brains of the operation. Sensor data flows in,

00:00:12.436 --> 00:00:15.130 and commands flow out to the motors.

00:00:15.130 --> 00:00:19.282 Its tiny gyros and accelerometers together form the IMU.

00:00:19.632 --> 00:00:24.842 A gyro measures rotation rate about each axis. When a gust tips the drone,

00:00:24.842 --> 00:00:29.010 the controller senses it and corrects, many times a second.

00:00:31.632 --> 00:00:35.764 The accelerometer measures acceleration, including gravity.

00:00:35.764 --> 00:00:40.792 Gravity always acts straight down, so the drone knows which way is down.

00:00:40.792 --> 00:00:43.753 With the gyro, that gives its orientation.

00:00:44.103 --> 00:00:47.342 Quick check. What does the accelerometer measure?

00:00:48.803 --> 00:00:52.800 Linear acceleration. Rotation rate is the gyro's job.

00:00:53.150 --> 00:00:56.549 Heading comes from the magnetometer, or compass.

00:00:56.549 --> 00:01:00.919 A hovering multirotor relies on it most: when it isn't moving,

00:01:00.919 --> 00:01:03.416 GPS gives it no track to work with.

00:01:04.650 --> 00:01:09.358 Current makes magnetic fields, so batteries and motors can upset it.

00:01:09.358 --> 00:01:13.725 So can power lines, iron ore stockpiles, ships and other metal.

00:01:13.725 --> 00:01:18.024 Many drones check the compass and warn you, or won't take off.

00:01:19.150 --> 00:01:23.695 Next to a steel shed or the ute? Move to open ground before you calibrate.

00:01:26.650 --> 00:01:31.307 Position comes from GNSS, like GPS. Signals travel at the speed of light,

00:01:31.307 --> 00:01:36.028 so the receiver turns each tiny delay into a distance from that satellite.

00:01:38.150 --> 00:01:43.719 Two satellites leave a big area of doubt. A third narrows it to two points.

00:01:43.719 --> 00:01:48.630 A fourth fixes your position and altitude, and corrects the clock.

00:01:48.630 --> 00:01:53.980 Four is the minimum for a 3D fix. Many drones want more before they arm.

00:01:55.650 --> 00:02:01.374 Errors creep in: the ionosphere slows signals, clocks drift, and signals bounce.

00:02:01.374 --> 00:02:06.534 That bounce is multipath. The same signal arrives late by a longer path,

00:02:06.534 --> 00:02:08.371 so your position wanders.

00:02:08.721 --> 00:02:12.761 Among tall buildings, expect it. If GPS fails, take manual control,

00:02:12.761 --> 00:02:15.435 land as soon as you can, and tell your crew.

00:02:18.221 --> 00:02:23.468 So: gyro for rotation, accelerometer for which way is down, compass for heading,

00:02:23.468 --> 00:02:26.189 and four or more satellites for position.

00:02:27.221 --> 00:02:31.753 Last one. Flying between tall buildings, your position starts to wander.

00:02:31.753 --> 00:02:33.491 Which error is most likely?

00:02:34.621 --> 00:02:37.442 Multipath. Signals bouncing off the buildings.