Time quality of each sample is evaluated whether GPS is received or not :

/* convert GPS Time-Quality code to text  */
void tq(int code, char *txt)
{
  switch(code) {
   case '*' : strcpy(txt,"No GPS"); break;
   case '1' : strcpy(txt,"err < 1 nsec"); break;
   case '2' : strcpy(txt,"err < 10 nsec"); break;
   case '3' : strcpy(txt,"err < 100 nsec"); break;
   case '4' : strcpy(txt,"err < 1 usec"); break;
   case '5' : strcpy(txt,"err < 10 usec"); break;
   case '6' : strcpy(txt,"err < 100 usec"); break;
   case '7' : strcpy(txt,"err < 1 millisec"); break;
   case '8' : strcpy(txt,"err < 10 millisec"); break;
   case '9' : strcpy(txt,"err < 100 millisec"); break;
   case 'F' :
   case 'f' : strcpy(txt,"(F)"); break;
   default  : sprintf(txt,"(%X)",code);
   }
}