FLC UNFAMILIAR WITH PARAMETERS OF GPWS; FLEW AT ALT WHICH CAUSED GPWS WARNING. NO ACTION WAS TAKEN BECAUSE THEY WERE IN VISUAL CONDITIONS.

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I WAS THE CAPT/PF ON THE DEADHEAD POSITIONING FLT FROM WICHITA; KS; TO TOLUCA (MMTO) ARPT IN MEXICO CITY; MEXICO. THE ACFT WAS A CESSNA CITATION V ULTRA. THE ACFT WAS EQUIPPED WITH THE ALLIED SIGNAL MK-V1 GPWS. THE WX FOR THE TOLUCA ARPT WAS RECEIVED AND WAS GOOD (BETTER THAN 20000 FT SCATTERED AND 5 MI VISIBILITY). WE WERE BEING HANDLED BY MEXICO CITY APCH. INITIALLY; WE WERE FLYING AN AIRWAY INBOUND FROM THE N WHEN WE WERE RECLRED DIRECT TO THE MEX VOR. I DISCUSSED WITH MY COPLT (HIS FIRST TRIP INTO MEXICO) THE POSSIBILITY OF BEING CLRED DIRECT TO THE ARPT; INSTEAD OF HAVING TO FLY THE FULL ARR. WE REVIEWED THE TERMINAL CHART TO MAKE SURE THAT ANY ALT THAT APCH GAVE US WOULD COMPARE CORRECTLY TO OUR CHARTS (INCLUDING THE MINIMUM RADAR SECTOR ALTS ON THE CHART). SURE ENOUGH; APCH CLRED US DIRECT TO THE ARPT AND TO MAINTAIN 13000 FT. I DSNDED TO AND LEVELED AT 13000 FT WITH THE AUTOPLT ENGAGED. I HAVE SHOWN OUR APPROX TRACK IN RED ON THE ENCLOSED TERMINAL CHART. OUR TRACK TOOK US OVER THE MOUNTAIN RIDGE AS SHOWN. I WAS DOING 250 KIAS. AS WE APCHED THE RIDGE LINE; I COMMENTED TO THE OTHER PLT THAT THIS WOULD NOT BE PRUDENT AT NIGHT OR IN IMC AND THAT WE WOULD HAVE ADHERED TO THE ARR AND SUBSEQUENT APCH. AS WE NEARED THE RIDGE LINE; I WONDERED TO MYSELF IF THERE WOULD BE ANY ADVERSE EFFECT WITH REGARD TO THE GPWS. SURE ENOUGH; AS THE TERRAIN STARTED RISING UNDERNEATH US; THE GPWS WENT OFF. 'TERRAIN; TERRAIN' WAS SOUNDED AND THE RED 'PULL UP' LIGHT ILLUMINATED. I WAS JUST SET TO DISENGAGE THE AUTOPLT AND START A PULL UP MANEUVER WHEN I REALIZED THAT WE WERE IN THE CLR TO BEGIN WITH. A MOMENTARY GLANCE AT THE RADAR ALTIMETER SHOWED APPROX 2080 FT AND CLBING RAPIDLY. WE WERE BY THEN ON THE FAR SIDE OF THE RIDGE LINE. MY CO-CAPT CONFIRMED THAT HE HAD FELT COMFORTABLE WITH THE TERRAIN CLRNC (HE HAS A MIL FIGHTER BACKGROUND WITH EXTENSIVE LOW LEVEL FLYING EXPERIENCE); AND HE VERIFIED ALL INDICATIONS AND WARNINGS. AFTER RETURNING HOME; I READ UP AGAIN ON THE GPWS. WE OBVIOUSLY RECEIVED A WARNING DUE TO AN 'EXCESSIVE CLOSURE RATE TO TERRAIN.' I'VE ENCLOSED THE PERTINENT PAGES FROM OUR MANUAL. WE MUST HAVE HAD A MOMENTARY SPIKE ON THE RADAR ALTIMETER; AND BEING AT THE HIGHER INDICATED AIRSPD; FELL INTO THE PARAMETERS OF THE SYS. THUS; THE WARNING. WHAT A WAKE-UP CALL; EVEN IN THE BROAD DAYLIGHT AND LOOKING AT THE TERRAIN WITH THE KNOWLEDGE THAT WE WERE GOING TO CLR. THIS EXPERIENCE BRINGS ME TO ASK THIS QUESTION. HAS ANY OTHER FLC EVER EXPERIENCED A SIMILAR SIT IN THIS AREA? MY RECOMMENDATION. MEXICO CITY ATC SHOULD REVIEW THEIR MINIMUM RADAR SECTOR ALTS FOR THESE MOUNTAINOUS AREAS AND SHOULD CONSIDER RAISING THE ALTS. THIS WOULD PREVENT ANY POSSIBLE GPWS FALSE ALERTS TO AIRCREWS.