The Grid Locator identifies where you currently are in the Lambert Azimuthal Equal Area (LAEA) coordinate system, and makes a sound whenever crossing the boundary from one grid to another. It also provides a compass that points north if your phone has a geomagnetic sensor.
Note that the Grid Locator does not navigate you to a target location, it only reports your current Grid ID.
We use the LAEA to divide the world into grids of size 1km-by-1km, which are further subdivided into 100m by 100m subgrids. We set 20°E, 5°N as the center, which is near the center of Africa.
Using your GPS sensor, the app determines your current Grid ID (GID) by taking the LAEA coordinates (x,y) in meters relative to the origin and computing E-value = ⌊abs(x)/1000⌋ and N-value=⌊abs(y)/1000⌋.
The GID string is then written as:
where 43 and 51 are example E-values and N-values, respectively. If x is negative, we replace "E" with "W", and if y is negative, we replace "N" with "S".