GPS Mod / Stalker mod
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Remember the original GPS/Stalker mod from Minecraft 1.2.5 to 1.4.7?
This is exactly this mod updated to current Minecraft Versions.
What this does:
This mod adds a new Item called the GPS.
When you have a GPS in your inventory and it's turned on, you can see, how far away you are from other people, that also have a GPS on them on at least idle mode.
Then there's also supercharged mode, which allows you to see the distance between you and other people, if they don't have a GPS on them, but remember, supercharged mode is quite pricy.