a$)$ When dealing with an SIS system, explain the meaning of PFD avg. Do not

just say what it stands for.

b$)$ What is the INVERSE of the PFD ${?}_{\text{a}\text{v}\text{g}}$ known as$?($i$.$e$.\frac{1}{?}$ PFD ${?}_{\text{a}\text{v}\text{g}}=??)$