ACR X UNAUTH RWY ENTRY. RWY INCURSION.

Narrative

UPON ARR AT SAT; THE ENTIRE FIELD AND APCH CTL WAS BEING OPERATED BY INDIVIDUAL IN TWR TAXIING IN ON THE SAME FREQ AS YOU ARRIVE ON. AFTER BLOCKOUT; THE OP HAD CHANGED DUE TO SHIFT CHANGE AND WE WERE BLOCKED OUT BY TWR AND THEN GIVEN TAXI INSTRUCTIONS WHICH WERE NOT THE NORMAL WE USUALLY RECEIVE AT OUR NORMAL DEP TIME. THE RWY WAS DIFFERENT AND WE MISUNDERSTOOD THE CTLR'S TAXI INSTRUCTIONS. WE LEFT OUR GATE AREA AND PROCEEDED TO THE TO THE RWY IN FRONT OF US (RWY 3/21); AND CROSSED THE RWY TO PROCEED TO THE RWY 12/30. AFTER XING; THE TWR CTLR ASKED US TO SWITCH TO GND CTL. GND CTL TOLD US WE HAD CROSSED A RWY 21 WITHOUT CLRNC. THE TWR CTLR WHO HAD US ON HIS RADIO MADE NO ATTEMPT TO CALL US AND STOP US APCHING THE RWY 21 FOR XING. WE THEN RECEIVED GND CTL'S INSTRUCTIONS TO PROCEED TO RWY 21 FOR DEP. NOTHING WAS INDICATED AS TO A VIOLATION. DEP WAS NORMAL. FIELD OPS ARE CONFUSING WHEN ONLY 1 INDIVIDUAL IN TWR IS THE SOLE CTLR. IT LEAVES AREAS TO QUESTION AS TO CLRNCS; AND WHO IS IN CHARGE. THIS SHOULD BE CHANGED AT LARGE ARPTS DURING LATE HRS; BECAUSE IT DOES NOT TRANSITION PROPERLY WHEN THE CHANGE COMES DURING THE MORNING SHIFT CHANGE.