code: public boolean hasValidInterrogatorID() {
  		assert(interrogator.length == 3);
  
  		// 3.1.2.3.3.2
  		// the first 17 bits have to be zero
  		if (interrogator[0] != 0 ||
  				interrogator[1] != 0 ||
  				(interrogator[2]&0x80) != 0)
  			return false;
  
  		int cl = (interrogator[2]>>4)&0x7;
  
  		// 3.1.2.5.2.1.3
  		// code label is only defined for 0-4
  		if (cl>4) return false;
  
  		// Note: seems to be used by ACAS
  //		int ii = interrogator[2]&0xF;
  //		// 3.1.2.5.2.1.2.4
  //		// surveillance identifier of 0 shall never be used
  //		if (cl>0 && ii==0) return false;
  
  		return true;
  	}
docs: Note: this can be used as an accurate check whether the all call reply
  has been received correctly without knowing the interrogator in advance.
  @return true if the interrogator ID is conformant with Annex 10 V4