How do spacecraft navigate in space?
In: Planetary Science
In order to navigate, you need to know your position and orientation (and velocity and angular velocity, but those can often be derived from position and orientation data taken over time). You then need to do some math to figure out how to get from your current position and orientation to your desired position and orientation. Then you need to actually do it. I’m going to gloss over step two, but here are a number of devices that are used to determine and control position and orientation.
Depending on where you are in space, you may need to use a variety of different methods to determine your position and orientation.
If you are close enough to Earth, you can use:
* GPS (position) (works just like it does on Earth)
* Ground tracking (position) (shoot radar off of the satellite and see how far it is from the radar station)
* Earth sensor (rotation, rough) (If that direction is bright, Earth is probably in that direction)
* Terrain relative navigation (rotation and position) (look at Earth to see where you are) (Frequently used when landing on the Moon, not as much on Earth)
* Magnetometer (rotational velocity, rotation if you are edgy, maybe position if you are very edgy) (Earth has a magnetic field and you can measure it to figure out how fast you are spinning and what part of it you are in) (usable near other planets with a magnetic field)
* Radar/Lidar altimetry (position, possibly rotation but not usually) (see how far from Earth you are by bouncing radar or light off of it) (not normally used on Earth, common when landing on the Moon)
If you are not close to Earth, or even if you are close to Earth, you can use:
* Gyroscopes (rotation) (spinny thing tends to stay spinning without changing spin direction, if your spacecraft spins you can check the spinny thing to see how much you’ve spun by)
* Signal timing (position) (determine how long a signal takes to reach a spacecraft, gives distance)
* Star tracking (rotation) (look at the stars and do a bunch of fancy math to see how you are rotated, the stars are so far away that they don’t vary in position too much, so you can use them as a fixed background that doesn’t rotate, a bit like a gyroscope)
* Sun and/or planet sensing (rotation and position if you know the time) (Star tracking but for planets. Because planets move, if you know the time, you can do the math on their orbits and get your position within the solar system)
* Inertial guidance (rotation and position, not typically accurate over long timescales) (really accurate accelerometers (linear and angular) that can determine how much you have moved relative to the orientation you started at, so you can know where you are relative to your starting position)
* Sun sensors (rotation) (small light sensors around the spacecraft, can determine rotation relative to the sun by comparing how bright each one is)
Most spacecraft use a combination of these methods rather than relying on a single method as they all have their advantages and disadvantages. There are doubtless a few I have forgotten and/or don’t know about, and a number of theoretical systems that could work but have never been tried for one reason or another.
Ran out of characters, see part 2.
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