ADS-B SITF/6-IP/3 International Civil Aviation Organization AUTOMATIC DEPENDENT SURVEILLANCE BROADCAST (ADS-B) SEMINAR AND THE SIXTH MEETING OF ADS-B STUDY AND IMPLEMENTATION TASK FORCE (ADS-B SITF/6) Seoul, Republic of Korea, 23 27 April 2007 Agenda Item 10: Discuss issues observed during the trial and implementation of ADS-B including review items from ADS-B Problem report database ADS-B ISSUES (Presented by Australia) SUMMARY This paper reports on avionics issues related to the use of ADS-B 1. Background 1. There is currently no internationally agreed scheme for certification of extended squitter ADS-B installations in existing aircraft. As a result aircraft operators who want to join the Australian ADS-B UAP program must apply for their aircraft to be approved by the Civil Aviation Safety Authority (CASA). Once aircraft have been approved, and flight crew have received the necessary training, the ADS-B ground stations are updated to allow data from these aircraft to be processed and displayed to ATC. This allows ATC to provide services based on ADS-B. 1.2 In addition to certification for ATC use, Australia now has a number of years of operational experience with ADS-B. During that time a number of issues have been discovered. 1.3 This paper reports on issues related to the ATC use of ADS-B. 2. Issues preventing certification of ADS-B for ATC use in Australia 2.1 ACSS XS-950 transponders not upgraded to software Mod A status: - This transponder is mainly used on air transport aircraft. Prior to the Mod A software upgrade the indication of ADS-B position quality (Navigational Uncertainty Category for position - NUCp) was based on GPS accuracy (Horizontal Figure of Merit - HFOM). HFOM does not reflect unannounced faults in a GPS satellite. To determine whether ADS-B data may be used to provide ATC separation services, a measure of the position integrity is required. GPS position integrity is conveyed via the
ADS-B SITF/6-IP/3-2- Horizontal Protection Limit (HPL). The Mod A software upgrade ensures NUC is based on HPL. 2.2 Early Rockwell Collins TDR-94(D) transponders: - This transponder is mainly used on regional aircraft. Rockwell Collins certified a version of the TDR-94 (-108) to the RTCA DO-260A standard in 2005. Earlier versions of the transponder with ADS-B capability were used to demonstrate feasibility in proving trials and are not compliant with current standards. It was intended that the ADS-B function on these transponders would be disabled, by grounding an interface pin, prior to the aircraft leaving the factory. A service bulletin is available for this purpose. Unfortunately this did not occur in all cases, and ADS-B data has been received from aircraft with uncertified versions of the TDR-94. The data exhibits anomalies making it unsuitable for operational use. 2.3 Random changes in Flight Identification B747: The call-sign or flight identification on some Boeing 747s has been observed to change randomly during flight. The problem has been isolated to an ACSS transponder. A service bulletin is expected by the end of March 2007 to address the problem. The flight identification is the primary means of coupling the ADS-B track to the ATC flight plan within the air traffic management system. A change in flight identification would cause a decoupling, resulting in problems for the controller. This problem also has implications for the European Mode S radar program and will need to be resolved for airlines to gain compliance. 2.4 Northrop Grumman Litton LTN-2001 Mk I GPSSU: - This unit is not compliant with the current version of the ARINC 743A standard. It uses bit 11 of ARINC Label 130 (HPL) as a data bit. Bit 11 is currently defined as the RAIM flag in the standard, and is used to indicate a satellite failure has been detected but the satellite has not been excluded from position determination. Monitoring in Australia has found that Aircraft with this unit transmit a high proportion of ADS-B position messages with NUC=0. 2.5 ICAO Aircraft Address: The ICAO aircraft address is a 24-bit number that uniquely identifies an airframe. The 24-bit address is transmitted in each reply to a Mode S radar interrogation, in each ADS-B and TCAS message, and is the basis for tracking Mode S equipped aircraft. It is important that the 24-bit address assigned by the appropriate regulatory authority (CASA in the case of Australia) is programmed into the transponder. The address is allocated by CASA when the aircraft is registered. An ADS-B monitoring station in Brisbane identified an aircraft recently registered in Australia that was still using the 24-bit address previously allocated by Germany.
-3- ADS-B SITF/6-IP/3 3. Other ADS-B issues 3.1 Longitude jumps: This problem has been reported previously (see the diagram following). It affects the encoding of an aircraft s longitude when it is very close to a transition latitude (i.e. within 5m). The transponder reports a jump in longitude as the aircraft crosses the transition latitude. The problem is due to the use of an early CPR encoding algorithm. The manufacturer is aware of the problem and has indicated it will be addressed. ADS-B standards are being updated to deal, more generally, with unrealistic shifts in reported position. A Reasonableness Test has been defined to detect sudden jumps in position to deal with this problem. This feature has been implemented in Australian ADS-B ground stations. 3.2 Backward position jumps: Backward jumps in reported position have been noted with some aircraft. The jump is of the order of 0.6 NM in the direction of the aircraft s track. In one particular case it occurred 22 times within a 15 minute period (video). It is suspected that the fault lies with the extrapolation of position that occurs between updates from the position data source. The problem has been reported to manufacturers, but the current status is not known. The jump is not visible on the range scales that controllers typically use in Australia. It is also easily detected and the faulty position report may be discarded. This problem, while not significant operationally, needs to be addressed.
ADS-B SITF/6-IP/3-4- 3.3 Aircraft transmitting Surface Position Messages while airborne: This situation was experienced due a short circuit on one of the air/ground sensor inputs. While this is specifically the result of an equipment fault it emphasises the requirement for global CPR decoding of both Airborne and Surface Position Messages to initiate the aircraft s position. Local decoding of Surface Position Messages will only guarantee the correct initial position if the aircraft is within 45 NM of the reference position (ground station). The other issue is that the aircraft did not have on-ground validation as required by Annex 10 Vol. IV (3.1.2.6.10.3). As a result the on-ground indication was not over-ridden on the basis of speed and radio altitude. A survey indicates none of the major manufacturers have implemented on-ground validation. Other cases on this problem have been reported. Manufacturers need to consider this issue more carefully in association with the next revisions of avionics. 3.4 Excessive fluctuation in reported Vertical Rate: It appears the instantaneous value of Vertical Rate is transmitted in the Velocity Message. It exhibits too much fluctuation to be of much value in its raw form (the diagram below shows one example). A smoothing algorithm is applied to the received Vertical Rate data by the Australian ATM system. The source of data on board the aircraft needs to be determined, and it may need to be averaged over a shot period to be more useful. Vertical Rate based on GNSS tends to be a little better than that based on barometric pressure altitude. This issue requires further investigation by avionics manufacturers.
-5- ADS-B SITF/6-IP/3 3.5 Incorrect entry of Aircraft Identification (Flight Identification): Flight ID is used to correlate the ADS-B track to the Flight Plan. There is a requirement for the Flight ID entered into the transponder by the flight crew to be exactly the same as that filed in the Flight Plan. The main problem with Flight ID is that it is sometimes not entered into the transponder correctly. Problems include the use of leading zeroes, the use of IATA rather than ICAO airline prefixes, missing airline prefixes, and the odd incorrect digit in the flight number. This is more an issue about flight crew training and human factors than avionics. Avionics should be designed such that it is relatively easy for flight crew to change the flight identification during flight if an error is discovered. It is not possible in some aircraft to change the Flight ID during flight. This problem may be addressed within ground systems by correlating the ADS-B track to the flight plan on the basis of the aircraft s 24-bit address. The entry of 24-bit address could be done manually by the controller, or a learning database could be implemented within the ATM system to maintain a table of 24-bit addresses against registration. In the latter case the ATM system would automatically fill the 24-bit address in the flight plan. This is however subject to problems involving late substitution of aircraft if the flight plan is not updated. 3.6 Incorrect Ground Track during push back: During push back from the terminal gate the ground track of an aircraft was observed to be incorrect by 180 degrees. The ground track was reporting the aircraft nose heading rather than the direction of travel of the aircraft (see diagram below). It is not known how widespread this problem is. This problem may have an impact in surface surveillance applications.
ADS-B SITF/6-IP/3-6- 3.7 NUC in Surface Position messages: The issue with Surface Position Messages is that many are transmitted with NUCp=6. NUCp=6 has no upper bound on integrity (HPL >= 0.1 NM) or accuracy (HFOM>=0.05 NM). This means that a Surface Squitter with NUCp=6 has to be considered as unknown integrity/accuracy, resulting in a lot of surface messages being discarded. Analysis of over 1 million surface position messages in Australia in 2006 found that over 60% were broadcast with NUCp=6. It appears that in most cases the NUC is based on integrity which tends to be conservative due to the 1 in 105 requirement to be within the reported containment radius. Transition to DO-260A may help, since it reports accuracy and integrity separately. This would allow users to be more selective in deciding whether to accept or reject surface messages. applications. This issue will become more significant as ADS-B is used for surface surveillance 3.8 NUC drops to 0 for a single message and then returns to its correct value: This seems to be an avionics issue. It does not pose any problems operationally but should be investigated further by manufacturers. UK NATS has also reported the same issue. 4. Conclusion 4.1 Issues have been identified that would prevent some aircraft from being approved to receive ATC services based on ADS-B in Australia. The identification of these issues is a result of the operational implementation of ADS-B. It should also be noted that most of the issues identified are the result of pre-operational ADS-B implementations and may be addressed by equipment upgrades or manufacturer s service bulletins. 4.2 Other issues reported in this paper should be investigated further by manufacturers and all States considering the use of ADS-B, and should be addressed as a process of continual improvement. 4.3 Finally it is important that these issues be reported to increase awareness within the aviation community, and to allow further discussion leading to internationally agreed solutions. _ (Prepared by Kojo Owusu, Airservices Australia) (Presented by Greg Dunstone, Airservices Australia)