A B737-300 flight crew flying an HGS low minimum approach is distracted by the automated AGL altitude call outs initiated between their mandated PF/PM callouts as they approach DH.

Narrative

Landing Cat III utilizing the HGS (Heads up Guidance System) with an RVR of 1000. The aircraft was a B737-300 with automated altitude callouts feature. We thoroughly briefed all aspects of the approach IAW the FOM. Approach and landing were safe and smooth. The only item that interrupted the normal flow of cockpit communication cadence was the automated 100 foot altitude callout that came between the approaching minimums and minimums calls from the PM. Obviously; as Pilots we don't execute Cat III Approaches every day and even less in a -300 with the automated altitude feature enabled. Most of the practice we have with Cat III Approach procedures comes in the simulator without this feature. The result was an unexpected interruption and brief distraction during a time critical phase of flight. As PF; I am acutely aware that the PM's job during these low vis approaches is to miss the approach if I don't promptly answer his callouts in close proximity to the ground. We are programmed by simulator training to expect the minimums call after the approaching minimums call out by the PM. In this case; we were not mentally prepared for the additional automated 100 ft call between the two normal HGS PM callouts. The result was that as PF I reflexively answered the automated 100 ft call with 'landing!' I had the touchdown zone in sight so it was an honest answer; but it lead to some brief confusion; non-standardization and professional embarrassment. The PM made the proper minimums call and I answered again 'landing.' 30; 10 followed and landing was uneventful I suggest the system be modified to activate the automated altitude callouts staring at 500 AGL vice 100 ft so as to allow crews to settle into the proper communication cadence with the PM and automation. This will help until the entire fleet is standardized with the automation features.