I would prefer something less than .5 ohm.
I worked in power production in the Air Force ~5 decades ago and I cannot remember getting that low of an impedance to earth. I also was detailed to Fire Alarm Telegraph System maintenance and those clockwork driven numerical telegraph transmitters had to be able to transmit via ground return if there was an open anywhere on it's single wire looped fire alarm box circuit.
I was given about the niftiest little drill rig to get the rods in and I've never seen anything like it since. It pulled out from a compartment on the side of the add on body of a pickup. I felt like a dunce, and still do when I tell about it, but I put in 3 or 4 Rods with stacked 10 foot sections until I got some of the boxes to signal via ground return. Then it donned on me that the open in the ground might be at the fire alarm office. I talked the Sergeant into letting me test the office grounding and it was over 100 Ohms to earth. That alarm office was built during world war 2 and the grounding for it was all galvanized steel. In the ensuing 30 years it had corroded open. It was under constant assault by AV Gas followed by JP-4 and it's predecessors because were they parked fuel trucks was just on the other side of the perimeter fence. The office still had it's required 100 foot clearance from all other structures and vehicles but the ground was saturated with fuel from decades of spills. They let me install the new grounding system because I really wanted to get those dammed street fire alarm stations fully functional again. A heavy equipment operator exposed the top of the footer all the way around the building. The spoil pile stank terribly of spilled fuel. There was very little left of the original Ground rods at each corner of the building. They looked to be about the size of a #10 wire run vertically in the soil. Whatever was originally used to bond the rods together was gone completely. Once I got the use of a utility pole hole borer from the communications squadron it was really easy to get holes down to 30 feet from the top of the footer which was ~10 feet below ground level. I'd have gone to 40 but they didn't have that many extensions for the augur shaft. I personally filled each of the bores with Bentonite which had to be mixed to the consistency of putty before pouring it into the holes. I did get all 40 foot of rod down in 2 or 3 of the bored holes so that the top of the coupled rod was 10 feet below ground at the top of the footer. That made the drilled end of the rod 40 or 50 foot deep. I bonded the rods to the ground ring by exothermic welding. I put in a Ground Ring because the bonding conductor for the rods was going to be 10 foot below ground level so I got permission to use the large conductor. It ended up being 2/0 because the supply folks had enough left over from some other job. The Ground Ring had a foot of Bentonite in all directions around it. I know because I tamped it. The last touch I think was a good idea and entirely mine. I got the guys from base engineering to show me how to prepare the basement walls and once I had wire brushed it all down they sprayed it with HOT tar. They said that they used the stuff to repair roads and roofs. I had seen them patching roads with it which is were I got the idea. The wire guys that had to work in that basement at times told me they really appreciated my "drying it in" for them.
When we were finished we did a 4 pole fall of potential ground impedance test on the system. We did get it down to ~3 Oms but I've never heard of getting a Grounding Electrode System below a single ohm.
OK I'll admit it. I was proud of the way it turned out. But I still feel dumb about not suspecting the grounding at the fire alarm office until after I put in my 4th 30 foot long ground rod to get the ground return to work from an alarm station.
Tom Horne