An apparent false glide slope on an ILS approach to ORF caused a Lear 35 to descend below the actual GS.

Narrative

Highly experienced crew; both in terms of total and in type. 9th leg together in last 30 days. Non-event return flight after maintenance; no passengers. Cleared for approach; maintain 2;000 FT until established. Checklist was completed as required for phase of flight -- approach. Done; waiting final landing checks. Aircraft was; in fact; slower than normal due to slower aircraft ahead. In other words; no rush. Aircraft was on autopilot; approach briefed; all set up for autopilot coupled approach. LOC intercept called by both pilots at the same time. Captain call = GS alive -- all OK. I checked both GS needles moving out of top of box; went to finish before landing checks/make sure everything was done. Felt aircraft begin descent; had ground contact; checked GS needle close to centered. I am very familiar with area. A long bridge is on final with turns on the bridge -- all looked normal; then realized the 'bridge turn' I was looking at was the one that was +/-4 miles further out than where I thought we were/visual picture. ILS DME confirmed we were indeed too far out for our altitude. Called climb to the GS to the Captain/Pilot Flying. Received low altitude alert from the Tower as we climbed to regain GS/get to proper altitude. My guess is that we were about 500 FT low. How we both saw on LOC/GS and then deviated that much I do not know. I believe that the altitude hold/GS arm function disarmed at/shortly after LOC intercept and that the GS alive/off the peg was false or incorrect; but it occurred on both sides. At ground contact; I looked out of the windshield for the runway; then realized how far out/low we were. Without the visual over the water and IMC over the airport; I would have remained on instruments and may have seen the GS deviation in a timely fashion. The aircraft does not have GPWS -- a shortcoming; but the real shortcoming was the crew's performance. I (both crew members) know that this event has happened in this model aircraft before. Both knew to be vigilant and somehow we stepped into the same trap. Sure am glad we were over water; not mountains. Callback conversation with Reporter revealed the following information: Reporter advised the 'bridge turns' mentioned in the narrative addressed the change in direction of the long bridge that roughly parallels the approach course of the ILS; not 'procedural turns' as a part of the ILS. As the pilot not flying Reporter's attention wasn't focused on the ILS indicator but it is his understanding that the autopilot remained coupled to the GS throughout the descent and the descent was only arrested when he advised the pilot flying they were visually well below the GS. This occurred simultaneously with the MSAW from the Tower. They climbed back to approximately 1700 ft AGL and recaptured a correct GS at the appropriate point.