system
Given the following post, respond as if you're commenting on Reddit:
human
How are satellite orientated in space?

If down is relative to the ground for us how is positioning treated in space?
gpt
It depends whether the satellite is interested in pointing "down" (GPS, weather satellites) or at another object in space (space telescopes etc.)

If it wants to point "down", all it needs to do is spin slowly so it makes a full revolution once per orbit, so the same side is always facing the earth. 
For most satellites this is achieved by just getting it's rotational velocity close enough to correct using thrusters, and then fine tuning it with rotating weights (reaction wheels)

If it's looking somewhere else, usually it will be oriented relative to the stars, since on a human timescale they are pretty much perfectly stationary. For this purpose reaction wheels can last a long time let you point it at whatever you want.

There are some interesting consequences of using reaction wheels to orient satellites - if there's an unbalanced force on the satellite over a long period of time certain wheels will need to gradually speed up to counteract this, until eventually they reach a maximum speed and are no longer useful. An example of this is Hubble, it has (from memory, please correct me) 8 sets of reaction wheels, about half of which are no longer functional due to being at max speed all the time. This means Hubble can't rotate directly to look at something, and has to do a sort of wiggly roundabout way to point in the right direction.