This is another interesting topic that will require quite a few pages to deal with it properly. Maybe someone who sells calibration impedance standard substrates (ISSs) will pick it up and sponsor it?
Calibrations can be done with coax standards, or planar structures in the case of RF probing. Calibrating refers to establiching a reference plane, and determining correction factors that will allow precise measurements of reflection and transmission coefficients, including magnitude and phase. Standards are are required which must bemeasured just for this purpose.
More to come!
LRM
Line/reflect/match
Requires a two-port "thru" standard, a good 50 ohm standard, and a reflection standard which is typically a short circuit.
This is often done as a last resort when probing MMICs where true de-embedding standards were omitted fromt the mask set. In this case it is known as a "probe tip" cal.
Often when you measure a MMIC it is not possible to have a perfect 50 ohm termination on board the wafer, so many MMIC foundries rely on other types of calibrations that don't require an accurate load.
SOLT
short/open/load/thru
In this case you will need to know the electrical length of the thru line.