B737-800
Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Flight Crew Last 90 Days 441
Troubleshooting; Confusion; Workload; Distraction; Human-Machine Interface; Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented
ATC Equipment / Nav Facility / Buildings; Airspace Structure; Aircraft
On descent on the ANTHM 3 Arrival into BWI; as we neared CURBY intersection we noticed on the flight display the RNP/ANP numbers were amber. It displayed 2.0/4.04 respectively. I begin looking at the NAV status to see if we still showed 2 GPS. We were handed off to Washington Center. When I checked in; the Controller asked if we were on a heading because we appeared to be not on course to BUBBI. I confirmed we appeared to not have proper RNAV function. We were given a heading and continued descent altitude and told expect radar vectors to BWI. As we started our new heading we saw RNP/ANP display turn back to white showing 1.00/.04. I informed Washington Center we had RNAV capability again and we were told to fly direct ROKTT and resume the ANTHM 3 Arrival. We proceeded on the arrival with no further issues.
B737-800 Captain reported while on approach ANP was greater than RNP. ATC confirmed aircraft off course.