  This note is motivated by preparations of a new $ep$ elastic scattering
experiment in the low transfer momentum region to be carried out in the 720 MeV
electron beam of the Mainz Microtron MAMI. This experiment will use an
innovative method allowing for detection of recoil protons in coincidence with
the scattered electrons. The goal is to measure the $ep$ differential cross
sections in the $Q^2$ range from 0.001 GeV$^2$ to 0.04 GeV$^2$ and to determine
the proton charge radius with sub-percent precision.
