DCA CTLR ISSUES LOW ALT ALERT TO C206 ON GPS RNAV RWY 15 APCH TO CGS.

Narrative

I WAS SHOOTING THE GPS 15 APCH TO CGS (COLLEGE PARK; MD). I WAS USING THE REAGAN ALTIMETER SETTING; PER THE PLATE INSTRUCTIONS. AT 2 TIMES DURING THE STEPDOWN PLAN; I RECEIVED 'LOW ALT' WARNINGS FROM ATC. ONE WAS JUST OUTSIDE OF CAJJA AND THE OTHER AT ABOUT 3 MI FROM RWY 15. BOTH TIMES I CHKED MY 2 ALTIMETERS AND BOTH TIMES I WAS WELL ABOVE THE STEPDOWN MINIMUM FOR THAT PHASE OF THE APCH. THE ATC ALT ALARMS MUST BE SET TOO CONSERVATIVELY OR WERE OUT OF CALIBRATION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR'S GREATEST CONCERN WAS THAT IF MSAW ALERT WAS FALSE; THEN THE ATC EQUIP NEEDED ADJUSTMENT. RPTR EQUIP WAS A GARMAN 530; WITH A 10 MI RADIUS SELECTED. RPTR HAS NOT BEEN BACK TO CGS SINCE THIS INCIDENT TO VALIDATE HIS EQUIP OR CONCERNS. HIS EQUIP HAS NOT PROVIDED AN ALERT ON ANY OTHER ARPT APCH. DCA SPOKESPERSONS ADVISED THAT THIS WAS A RARE ISSUE. THE CGS MSAW ALERT BOXES ARE BUILT OFF THE DCA VOR DME; 2 MI SQUARE. ADJOINING THE CGS ARPT AND RWY 15 FINAL APCH COURSE; MSAW BOXES VARY FROM 500 FT TO OVER 1400 FT. AS THE MSAW PROJECTS THE PROPOSED AND ACTUAL RTE OF FLT; MSAW ESTABLISHES AN 'ORDER OF MAGNITUDE;' WHICH FAULTS ON THE CONSERVATIVE SIDE; ESPECIALLY IF THE ACFT IS TURNED ON FROM THE N; WHICH HAS THE HIGHER TERRAIN. SO IF ATC IS ISSUING MSAW ALERTS; IT'S MORE THAN LIKELY THAT THE ACFT WAS INBOUND FROM THE N. A SPOKESMAN ADVISED THAT TO COMPLETELY DETERMINE THE ALERT VALIDITY; A 'DATA REDUCTION' MUST BE ACCOMPLISHED. I ADVISED THAT DUE TO THE NATURE AND STRUCTURE OF ASRS; I WOULD BE UNABLE TO PROVIDE THE DATA REQUIRES TO VALIDATE THE RTE OF FLT.