For GPS clients (devices using gps.locate) to function, GPS hosts are needed. A GPS host is a computer with a modem (or several modems, in some cases) running a GPS host program, such as the built-in
gps host (others include the Opus OS GPS server and TrilateratorGPS by gollark). The role of a GPS host is just to respond with its position to pings on the GPS channel, so its position must be set either manually or using existing GPS servers. This allows clients to find their positions using true range multilateration.
GPS host setup
Generally GPS service for a dimension is provided by 4 GPS hosts positioned close to each other. With normal wireless modems these are generally placed high up to maximize communication range, but with ender modems this is no longer necessary. With the built-in
gps host program, 4 separate computers with a single modem each are used for this. To set up a computer as a GPS host, place a modem on it, then edit its
startup.lua files to contain the following (substituting in the actual coordinates for each computer):
shell.run "gps host <x coordinate> <y coordinate> <z coordinate>". This will make
gps host run on startup, with your specified coordinates provided to it. There are programs available to automate constructing and configuring these, as well as alternative GPS host programs which only use one computer. For information on use of these, please consult their documentation.