It's also only valid at a certain radius (probably the surface). So the sensor wouldn't be calibrated correctly after you went inside (it would be off by differ

It's also only valid at a certain radius (probably the surface). So the sensor wouldn't be calibrated correctly after you went inside (it would be off by differing amounts depending how far from the center you were). The same is true of the constants for the planets, but at least they are (mostly) solid and not designed to be lived inside of.