Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 144; Flight Crew Total 17000; Flight Crew Type 11000
Communication Breakdown
Aircraft Equipment Problem Less Severe; Ground Event / Encounter Ground Equipment Issue
Air Traffic Control Provided Assistance; Flight Crew Overcame Equipment Problem
Aircraft; Airspace Structure
During RNAV GPS approach Runway 27R to Chicago; aircraft had erroneous path indication showing almost full scale below. Acquired airport visually and flew below VNAV path to require proper glide path.  And we flown aircraft using auto pilot in the VNAV; I missed approach would've been required.
Captain reported loss of GPS signal and VNAV on GPS approach resulted in change to a visual approach.