Narrative:

During our initial descent into iad; we were level at 6000 ft when the captain's radio altimeter started 'coming alive.' we then received an aural 'terrain; terrain; pull up; pull up.' although we both are familiar with the area and knew we were not near terrain; being IMC and night; therefore; unable to visually confirm the terrain; we complied with the 'escape' maneuver and rapidly climbed to 11000 ft while declaring an emergency with ATC. Once we could confirm (via ATC and a radical climb) that we were absolutely not going to hit terrain; we stabilized the aircraft and determined that the likely cause of the terrain warning was the captain's radio altimeter; we continued the flight. The captain designated me 'PF;' as we could not trust her instruments. As we were intercepting the localizer at approximately 5000 ft; we again received a 'terrain; too low; terrain' warning and selected the ground proximity override switch. We continued on the approach for an uneventful landing and taxi to the gate. As a side note; during climb out of ZZZ; the captain's EFIS became intermittent and she selected alternate; and all instrumentation seemed normal through the entire flight up until that point.

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Original NASA ASRS Text

Title: B757-200 FLT CREW REACTS AGGRESSIVELY TO APPARENTLY FALSE GPWS TERRAIN WARNING WHILE MANEUVERING AT 6000 MSL NEAR IAD.

Narrative: DURING OUR INITIAL DSCNT INTO IAD; WE WERE LEVEL AT 6000 FT WHEN THE CAPT'S RADIO ALTIMETER STARTED 'COMING ALIVE.' WE THEN RECEIVED AN AURAL 'TERRAIN; TERRAIN; PULL UP; PULL UP.' ALTHOUGH WE BOTH ARE FAMILIAR WITH THE AREA AND KNEW WE WERE NOT NEAR TERRAIN; BEING IMC AND NIGHT; THEREFORE; UNABLE TO VISUALLY CONFIRM THE TERRAIN; WE COMPLIED WITH THE 'ESCAPE' MANEUVER AND RAPIDLY CLBED TO 11000 FT WHILE DECLARING AN EMER WITH ATC. ONCE WE COULD CONFIRM (VIA ATC AND A RADICAL CLB) THAT WE WERE ABSOLUTELY NOT GOING TO HIT TERRAIN; WE STABILIZED THE ACFT AND DETERMINED THAT THE LIKELY CAUSE OF THE TERRAIN WARNING WAS THE CAPT'S RADIO ALTIMETER; WE CONTINUED THE FLT. THE CAPT DESIGNATED ME 'PF;' AS WE COULD NOT TRUST HER INSTS. AS WE WERE INTERCEPTING THE LOC AT APPROX 5000 FT; WE AGAIN RECEIVED A 'TERRAIN; TOO LOW; TERRAIN' WARNING AND SELECTED THE GND PROX OVERRIDE SWITCH. WE CONTINUED ON THE APCH FOR AN UNEVENTFUL LNDG AND TAXI TO THE GATE. AS A SIDE NOTE; DURING CLB OUT OF ZZZ; THE CAPT'S EFIS BECAME INTERMITTENT AND SHE SELECTED ALTERNATE; AND ALL INSTRUMENTATION SEEMED NORMAL THROUGH THE ENTIRE FLT UP UNTIL THAT POINT.

Data retrieved from NASA's ASRS site as of May 2009 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.