Narrative:

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 navigation 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.

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Original NASA ASRS Text

Title: B737-800 Captain reported while on approach ANP was greater than RNP. ATC confirmed aircraft off course.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.