Assuming that any obstructions of the signal line of sight will be constant
Right off the bat, you make an invalid assumption.
The RFM22B appears to have RSSI capabilities. The signal strength is not an accurate measure of distance. It is, in fact, influenced by many things, with distance being a less important factor than things like power supply voltage, signal interference caused by noise, and solar radiation.
If you have COMPLETE control over the environment in which the generated signal and the received signal exist, then RSSI will give you a rough measure of distance (rough being near, not too near, a long ways away, etc.; not 1 inch, 4 inches, 24 inches).