This device is a cooled multi-blade system, in horizontal and vertical disposition, placed within the fringes of the beam in order to detect the position of the photon beam. There are two BPMs; the second is placed about 3.6 m downstream of the first. Together they yield precise beam position information.

Within the Storage Ring there are similar devices monitoring the electron, or particle, beam. They are mechanically different, but perform the same function of identifying the location of the beam within the vacuum chamber. To distinquish the two, sometimes they are refered to as the xBPMs and the rfBPMs, or pBPMs. The "x" indicating an x-ray beam while "rf"or "p" indicate radio frequency or particle beams.

Beam mis-steering is to be confined by limitations of both the beam position and its angular divergence. When excessive deviations are detected, the BPM output is fed back into software which controls ring correction (steering) magnets. These magnets adjust the electron beam’s position and angle at the ID center. Changes to the electron beam result in a corresponding change in the photon beam. In the extreme case, the pBPM can trigger a beam dump. (A beam dump is a deliberate action intended to disperse stored beam to a zero current level). The xBPMs are often used by the FC; current readings are available on the BPM EPICS display.