resistance of the whole grounding system on the main ground bus or on the main grounding electrode?
You could but Earth Impedance does not mean much. I am a Co-Author of IEEE 1100-2005 Chapter 9 aka Power & Grounding Electronic Equipment. I also design protective grounding systems for Telephone Switching Offices, Cell Towers, and Data Centers for a living.
You are in a very specialized field and not picked up quickly as it goes well beyond NEC requirement and focuses on operation in addition to safety. Today grounding equipment racks and Signal Grounds is not of much importance today from a signal transmission POV. Gone are ground referenced signal protocols like RS-232. Today everything is either Balanced or Optical so a low noise reference is not needed or required. Gone are Single Point Grounds and In with Mesh Grounds where everything is bonded to everything to form a Equipotential Ground Plane. Not for signal or noise purposes but for lightning fault protection.
There are a few free documents out there but most are antiquated. But here is a FREEBIE that might interest you.
ANSI/BICSI -002.
EDIT: If you send me a PM I can email you a couple of documents that should help.