Document setting up a gps constellation (#1070)

doc/guides/gps_setup.md

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+---
+module: [kind=guide] gps_setup
+---
+
+# Setting up GPS
+The @{gps} API allows computers and turtles to find their current position using wireless modems.
+
+In order to use GPS, you'll need to set up multiple *GPS hosts*. These are computers running the special `gps host`
+program, which tell other computers the host's position. Several hosts running together are known as a *GPS
+constellation*.
+
+In order to give the best results, a GPS constellation needs at least four computers. More than four GPS hosts per
+constellation is redundant, but it does not cause problems.
+
+## Building a GPS constellation
+![An example GPS constellation.](/images/gps-constellation-example.png){.big-image}
+
+We are going to build our GPS constellation as shown in the image above. You will need 4 computers and either 4 wireless
+modems or 4 ender modems. Try not to mix ender and wireless modems together as you might get some odd behavior when your
+requesting computers are out of range.
+
+:::tip Ender modems vs wireless modems
+Ender modems have a very large range, which makes them very useful for setting up GPS hosts. If you do this then you
+will likely only need one GPS constellation for the whole dimension (such as the Overworld or Nether).
+
+If you do use wireless modems then you may find that you need multiple GPS constellations to cover your needs.
+
+A computer needs a wireless or ender modem and to be in range of a GPS constellation that is in the same dimension as it
+to use the GPS API. The reason for this is that ComputerCraft mimics real-life GPS by making use of the distance
+parameter of @{modem_message|modem messages} and some maths.
+:::
+
+Locate where you want to place your GPS constellation. You will need an area at least 6 blocks high, 6 blocks wide, and
+6 blocks deep (6x6x6). If you are using wireless modems then you may want to build your constellation as high as you can
+because high altitude boosts modem message range and thus the radius that your constellation covers.
+
+The GPS constellation will only work when it is in a loaded chunk. If you want your constellation to always be
+accessible, you may want to permanently load the chunk using a vanilla or modded chunk loader. Make sure that your 6x6x6
+area fits in a single chunk to reduce the number of chunks that need to be kept loaded.
+
+Let's get started building the constellation! Place your first computer in one of the corners of your 6x6x6. Remember
+which computer this is as other computers need to be placed relative to it. Place the second computer 4 blocks above the
+first. Go back to your first computer and place your third computer 5 blocks in front of your first computer, leaving 4
+blocks of air between them. Finally for the fourth computer, go back to your first computer and place it 5 blocks right
+of your first computer, leaving 4 blocks of air between them.
+
+With all four computers placed within the 6x6x6, place one modem on top of each computer. You should have 4 modems and 4
+computers all within your 6x6x6 where each modem is attached to a computer and each computer has a modem.
+
+Currently your GPS constellation will not work, that's because each host is not aware that it's a GPS host. We will fix
+this in the next section.
+
+## Configuring the constellation
+Now that the structure of your constellation is built, we need to configure each host in it.
+
+Go back to the first computer that you placed and create a startup file, by running `edit startup`.
+
+Type the following code into the file:
+```lua
+shell.run("gps", "host", x, y, z)
+```
+
+Escape from the computer GUI and then press <kbd>F3</kbd> to open Minecraft's debug screen and then look at the computer
+(without opening the GUI). On the right of the screen about halfway down you should see an entry labeled `Targeted
+Block`, the numbers correspond to the position of the block that you are looking at. Replace `x` with the first number,
+`y` with the second number, and `z` with the third number.
+
+For example, if I had a computer at x = 59, y = 5, z = -150, then my <kbd>F3</kbd> debug screen entry would be `Target
+Block: 59, 5, -150` and I would change my startup file to this `shell.run("gps", "host", 59, 5, -150)`.
+
+To hide Minecraft's debug screen, press <kbd>F3</kbd> again.
+
+Create similar startup files for the other computers in your constellation, making sure to input the each computer's own
+coordinates.
+
+:::caution Modem messages come from the computer's position, not the modem's
+Wireless modems transmit from the block that they are attached to *not* the block space that they occupy, the
+coordinates that you input into your GPS host should be the position of the computer and not the position of the modem.
+:::
+
+Congratulations, your constellation is now fully set up! You can test it by placing another computer close by, placing a
+wireless modem on it, and running the `gps locate` program (or calling the @{gps.locate} function).
+
+:::info Why use Minecraft's coordinates?
+CC doesn't care if you use Minecraft's coordinate system, so long as all of the GPS hosts with overlapping ranges use
+the same reference point (requesting computers will get confused if hosts have different reference points). However,
+using MC's coordinate system does provide a nice standard to adopt server-wide. It also is consistent with how command
+computers get their location, they use MC's command system to get their block which returns that in MC's coordinate
+system.
+:::