An A320 flight crew experienced large attitude changes on a ORD Runway 28 ILS coupled approach when the glide slope signal moved rapidly from 1.5 dot high to 1.5 dot low. ATC reported that an aircraft parked in front of the antenna with weather greater than 800 ft and 2 sm was acceptable.

Narrative

On approach into ORD; weather 15BKN 22OVC 27027G32 ILS Runway 28 inside WILLT outer marker. The aircraft was on the approach stable with the autopilot 2 engaged. Around 1100 feet AGL in VFR conditions the aircraft suddenly aggressively pitched down to chase the glide slope which seemed to drop too fast. The glide slope immediately reversed direction and shot up. The glide slope moved from center to 1.5 dot high to a 1.5 dot low indication within seconds. I disengaged the autopilot immediately and the Captain reported a unreliable glide slope to the tower. The tower response was: 'Yeah there is an aircraft parked in front of the glide slope.....this happens all the time.' The quick pitch changes could have hurt a flight attendant and caused for a confusing situation in the aircraft. In light of the recent accident I found it important to bring this situation up so that it hopefully will be avoided in the future. I called the Tower Supervisor after landing and he informed me that since the weather was better than 800 ft and 2 sm the glide slope did not have to be protected. I advised him that a 'heads up' from the tower when we checked in over the marker could have avoided this potentially dangerous situation. If the weather would have been 900 ft overcast we could have been in an even more serious situation.