What was found
AIRCRAFT WAS GROUNDED. DURING DESCENT AT 21,000 IN ICING CONDITIONS, HAD AIRSPEED UNRELIABLE ON CA'S SIDE, STANDBY AND F/O'S INSTRUMENTS NORMAL. CA'S AIRSPEED READING 80 KTS, BUT ACTUAL SPEED 300 KTS. FOLLOWED QRH AND EMERGENCY CHECKLIST, EMERGENCY DECLARED. CA'S AIRSPEED NORMAL AT 4 MILE FINAL. NO AIRSPEED LIMITS EXCEEDED, ANTI-ICE WAS ON. STICK SHAKER CAME ON AS EXPECTED. REPLACED LT ADIRU, IAW AMM 34-21-01. OPERATIONS CHECK GOOD. ALSO REPLACED LT AIR DATA MODULE (AALA202011199016), IAW 34-21-04 AND LT PITOT TUBE (AALA202011199010) IAW 34-11-01. OPERATIONS CHECK GOOD.