PLT OF SR22 DEPARTING RWY 13R AT ALPHA NINE INTXN AT BFI USES WRONG LCL TWR FREQ AND ACCEPTS TKOF CLRNC DIRECTED TO CITATION IN POSITION FULL LENGTH.

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I THOUGHT I RECEIVED TKOF CLRNC AND WAS ADVISED; AS I ROTATED THAT I WAS NOT CLRED TO TAKE OFF. I GOT THE ATIS; CALLED GND AND WAS CLRED TO TAXI TO RWY 13R AT ALPHA 9. I BELIEVE THAT THE TWR XMITS ON BOTH OF THE TWR FREQS 120.6 AND 118.3 AT THE SAME TIME. THERE WAS A CESSNA CITATION AT THE FAR END OF RWY 13R ALSO WITH THE SAME LAST ID LETTER. I THOUGHT THE TAXI INTO POSITION WAS FOR ME AND THEN THE TKOF CLRNCE WAS FOR ME. I BELIEVE THAT THE CITATION WAS ON 120.6 AND I WAS ON 118.3 AND MY REPLY WAS STEPPED ON. I WAS ALSO DISTRACTED BY A CELL PHONE CALL IN THE RUN UP AREA. SINCE LIGHT ACFT LAND ON RWY 13L AND DEP 1/2 WAY DOWN RWY 13R; THERE SHOULD BE A SIGN THAT TELLS THE TWR FREQ IS DIFFERENT OR THE GND CTLR ADVISES THE CORRECT FREQ WHEN TAXI INSTRUCTIONS ARE GIVEN. I SHOULD ALSO NOT TAKE INCOMING CELL CALLS DURING THIS STAGE OF FLT (JUST PRIOR TO TKOF CLRNC). MANY TWRS XMIT ON MORE THAN ONE FREQ AT A TIME AND THIS CONTRIBUTED TO THE CONFUSION AS I HEARD CLRNCS GOING TO BOTH OF THE ACFT THAT DEPARTED RWY 13R AT A9; SO I THOUGHT I WAS ON THE CORRECT FREQ. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR ADVISED HE WAS UNFAMILIAR WITH THE AREA; THIS BEING HIS FIRST EXPOSURE. HE STATED THAT HE USES THE DATABASE ON HIS PANEL MOUNTED GPS UNIT AS A RESOURCE FOR ARPT DATA INCLUDING TWR FREQS. HIS DATABASE DISPLAYED BOTH FREQS; BUT HAD NO EXPLANATORY INFO REGARDING THE APPROPRIATE USE; SUCH AS PRINTED IN THE AFD. HE USED THE 118.3 FREQ SOLELY BECAUSE HE HAD BEEN ADVISED TO SWITCH TO IT ON ARR BY SEATTLE APCH. IT SEEMED LOGICAL TO USE IT FOR DEP SINCE HE WAS ABLE TO TALK TO THE LCL CTLR ON IT AND HE WAS HEARING XMISSIONS TO THE OTHER ACFT. ONLY AFTER THE EVENT DID IT OCCUR TO HIM THAT HE WAS ONLY HEARING ONE SIDE OF THOSE XMISSIONS. HE SUGGESTED THAT THE ATIS SHOULD PROVIDE DIRECTION; THAT SIGNS AT COMMON DEP INTXNS COULD SO INDICATE AND THAT GND CTL COULD PROVIDE THE CORRECT FREQ WHEN GIVING THE TAXI CLRNC.