Title: CPDLC - Modification UnitName definition

PDR Reference:                          	97080010
Originator reference:                   	unit_nme
SARPs document reference:         	 	CPDLC SARPs, Section 2.3.4.2.1
Status:                                 	ADOPTED
PDR revision date:                      	22/09/97 (SUBMITTED -> ACCEPTED)
			    		22/09/97 (ACCEPTED -> PROPOSED)
					20/10/97 (PROPOSED -> RESOLVED)
					29/05/98 (alignment to ICAO 2.2 SARPs - see PDR 98050019)
PDR submission date:                    	18/08/97
Submitting state / organization        	ICAO ATNP WG3 SG2
Submitting author name              	R.G.W.J. Esser
                                        	J. Hamelink
Submitting author e-mail address   		esser@nlr.nl
                                        	Jhamelin@adsystech.com
Submitting author supplemental 
contact information:
SARPs date                              	IV1.1, March 1997
SARPs language                          	English

Summary of defect

At the ATNP WG3 meeting in Langen, it was identified that when considering the European operational requirements, during a transfer of communications / control of an aircraft between two data link equipped ATC centres, an operational message can go to the unintended ground ATC system (for details refer to WG3-WP9-18). WG3 recognised that the issue was safety critical. The WG3 decided to forward the subject to the subgroup.

The operational sequence of messages during a transfer of commmunications is, according to the European requirements:
* Uplink of a Voice Channel Instruction (being either a CONTACT or MONITOR instruction, UM117 to UM 122);
* responded by a downink Logical Acknowledgement (LACK) or ERROR message
* if responded by a LACK, then followed by a Pilot Acknowledgement Message (i.e. WILCO or UNABLE)
* which has to be responded by an uplink LACK or ERROR message
* if a WILCO had been submitted which was responded by a LACK, a Moitoring R/T (MRT) message should be followed;
* which will be responded by an uplink LACK or ERROR message.

In order to allow the aircraft to send the MRT message to the right data authority (NDA instead of CDA for a transfer between two centres, CDA for a transfer between controller working positions within that centre), it must be made possible for the aircraft to relate the VCI & MRT message with the CDA and NDA, if any.
	In addition, the VCI message should also remain the ability to inform the air-crew of the next Unit name, In other words, the VCI message should still allow human (controller - pilot) oriented language.

Assigned SME                            Sub-volume II SME

Proposed SARPs amendment
In order to enable an aircraft to determine whether the VCI sent is actually the last VCI for that ATC centre (i.e. CDA), and subsequently to be able to withhold the MRT message meant for the NDA centre (if required to be sent), until the NDA link has become the CDA link, the FacilityDesination always has to be provided in a CONTACT or MONITOR instruction (UM117 to UM122). Moreover, as the VCI is a real controller-to-pilot message, it has to be possible to provide a name of the next controlling working position.
Therefore, it is recommended to change the defintion of unit name in the CPDLC SARPs ASN.1 notation, from:
UnitName ::= SEQUENCE
	{
	facilityId	FacilityIdentification,
	facilityFunction	FacilityFunction
	}
(which only allowed a CHOICE between FacilityDesignation and FacilityName)
to:
UnitName ::= SEQUENCE
{
facilityDesignation		[0] FacilityDesignation,
facilityName			[1] FacilityName	OPTIONAL,
facilityFunction			[2] FacilityFunction
}
(which requires the FacillityDesignation to be sent, optionally accompanied by a FacilityName).

SME recommendation to CCB:              -

CCB decision:                           atnp_ccb_chair: SUBMITTED (18/08/97)
						    atnp_ccb_chair: ACCEPTED (22/09/97)
						    atnp_ccb_chair: RESOLVED (20/10/97)