Aircraft-based airport surface traffic indications and alerting systems
This is an edited version of the presentation made at the recent ESAVS 2010 conference by Doug Arbuckle of the FAA. Coauthors of the paper were David E. Gray of FAA, Peter Moertl of Mitre Corporation and Jim Duke of SAIC. You can download the original text of their paper here and the slides here.
Introduction
As discussed before, runway incursions and collisions is a major area of concern world-wide. There are on average more than two runway incursion events per day in Europe alone and the situation in the United States is similarly serious. The US National Transportation Safety Board (NTSB) has on its most wanted list a system to “give immediate warnings of probable collisions/incursions directly to cockpit flight crews.”
In our previous two articles we covered the visual tools for preventing runway incursions (RWSL and FAROS) and the communications related causes of runway incursions. In this third article we will look into aircraft based airport surface traffic indications and alerting systems being developed in the US as a further line of defense against runway incursions.
The background
As you may be aware, in the US two different data links have been adopted for ADS-B: 1090 MHz Extended Squitter (1090 ES) and the 978 MHz Universal Access Transceiver (UAT). Given that the international community has agreed to the use of the 1090 ES link, most air transport and international business aircraft are expected to equip with this link; the UAT link is expected to be primarily used by general aviation aircraft whose operations are confined to the US. The US is implementing uplink services on both links. One such uplink broadcast service is Traffic Information Service-Broadcast (TIS-B). TIS-B derives traffic information from one or more ground-based surveillance sources and uplinks this traffic information to ADS-B-equipped aircraft, enabling them to receive position reports about non-ADS-B-equipped aircraft; this service supports the transition period to full ADS-B equipage in the NAS. ADS-R is another uplink broadcast service which rebroadcasts ADS-B messages received from aircraft on one link to nearby aircraft broadcasting on the other link, making it possible for all ADS-B-equipped aircraft to receive the information being transmitted on the other link.
The FAA established a program office to deploy the ground infrastructure needed to support the use of ADS-B in air traffic control and to provide the broadcast services listed above (as well as Flight Information Service-Broadcast, which is not discussed here). The ground infrastructure is planned to provide ADS-B information to controllers for all aircraft currently under radar surveillance by 2013. Additionally, the FAA has proposed a rule mandating “ADS-B Out” avionics to fly in certain designated airspace—generally the same busy airspace where transponders are required today. In the Notice of Proposed Rulemaking issued by the FAA in 2007, the proposed rule effective date was 2020. The final rule is under review by the US Office of Management and Budget (OMB), with a target publication date in late-April to mid-May 2010.
RTCA Special Committee 186 (SC-186) has defined two applications for Cockpit Display of Traffic Information (CDTI) that use ADS-B information to enhance situational awareness for pilots by presenting ownship and traffic on a moving map of the airport surface. These applications are Airport Surface Situational Awareness (ASSA) and Final Approach & Runway Occupancy Awareness (FAROA). Additionally, EUROCAE and RTCA are currently working jointly in the Requirements Focus Group (RFG) to define Enhanced Traffic Situational Awareness on the Surface (ATSA-SURF). These applications allow pilots to correlate information about the airport, ownship, and other airport traffic on a CDTI, providing numerous safety and efficiency benefits.
However, these applications present a significant amount of information to pilots, and though the CDTI may improve situational awareness under many conditions, it is not always practical for pilots to use it under certain conditions (e.g., during takeoff). A recent US simulation study found that only 25% of pilots who had a CDTI available during various airport surface operations were able to detect runway incursion situations while performing takeoffs or when a conflict aircraft was approaching from behind. Because pilots must scan the CDTI to get useful information, situational awareness benefits only accrue in situations/scenarios where a pilot has time to use the CDTI.
SURF-IA to the rescue
The FAA is working with industry to develop standards for an ADS-B application called Enhanced Traffic Situational Awareness on the Surface with Indications and Alerts (SURF-IA) to help decrease the likelihood and severity of runway incursions and collisions on or near the airport surface by the following means:
• Increase pilot awareness of runway-safety related traffic information;
• Increase the likelihood that this information is correctly understood by the flight crew in a timely manner; and
• Facilitate an appropriate compensatory response once a traffic conflict has been detected.
The application being defined by RTCA SC-186 adds enhancements to ASSA/FAROA and ATSA-SURF. SURF-IA calls the pilot’s attention to safety-relevant traffic and circumstances by highlighting information on the CDTI. Indications are intended for normal traffic situations and alerts are intended for non-normal traffic situations. Alerts include attention-getting cues to attract the pilot’s attention during non-normal situations.
SURF-IA is applicable to aircraft on the surface in proximity to runways, on runways, or in-flight on approach to a runway. Neither alerts nor indications are provided for taxiway conflicts. Essentially, SURF-IA provides indications or alerts for combinations of ownship and traffic states as follows:
• Taxiing toward or holding short of a runway
• Entering, exiting and crossing a runway
• Stopped or taxiing along a runway
• Traffic taking off from ownship’s runway or an intersecting runway
• Traffic on approach to ownship’s runway or an intersecting runway
• Traffic landing on ownship’s runway or intersecting runway
• Traffic on approach to or landing on a very closely spaced parallel runway
The SURF-IA application does not communicate the indications and alerts to ATC, and does not consider temporary runway closures, obstructions or conditions. Nor does SURF-IA have any requirement for explicit taxi intent.
Indications and alerts
SURF-IA indications consist of highlighting (or otherwise emphasizing) relevant traffic and runways on a CDTI when the relevant traffic is entering, exiting, taxiing, taking off, on approach, or has landed on the runway. SURF-IA automatically determines what information is relevant to the pilot depending on the relationship between ownship and traffic. Indications are supplemental to, and do not replace, the pilot’s normal out-the-window traffic scan responsibilities. There are two types of indications:
• Traffic Indications (TIs) are provided if there is no immediate collision hazard with ownship but a potential collision hazard could develop over some
time. TI’s are intended to increase the flight crews’ situational awareness about particular relevant traffic that could affect runway safety.
• Runway Status Indications (RSIs) are provided if the flight crew should verify the runway status prior to proceeding. For example, a collision hazard would result if ownship were to taxi onto a runway with an RSI, takeoff when ownship has lined up and waited on a runway with an RSI, or land when ownship is lined up on approach for a runway with an RSI.
A SURF-IA alert consists of highlighting or otherwise emphasizing both traffic and runways on a CDTI. They also include auditory and visual attention-getting cues. Alerts are annunciations meant to attract the attention of the flight crew to a non-normal operational or airplane system condition. SURF-IA alerts are nondirective and do not provide resolution commands to the flight crew about how to resolve a conflict once it has been detected. Caution alerts require immediate flight crew awareness and require subsequent flight crew response. Warnings require immediate flight crew awareness and immediate flight crew response.
Clear messages from SURF-IA simulations
Multiple human factors studies have been conducted in support of SURF-IA application definition and requirements development. Studies commissioned by the FAA investigated the benefits of baseline CDTI applications with traffic only; the benefits of CDTI with traffic and indications only; the benefits of CDTI with traffic, indications and alerts combined; the feasibility of using a Class 2 EFB for CDTI with traffic with indications only; the value to pilots of surface moving maps with taxiway information during surface operations; and, an assessment of the effectiveness of non-directive alerts relative to directive alerting. The results of these studies showed that both indications and alerts are more beneficial than a traffic only application; display of off-scale traffic is necessary for indications and alerts; Class 2 EFB limitations are too restrictive to host SURF-IA; non-directive alerts appear sufficiently effective for SURF-IA; and, pilots prefer the more complete situational awareness picture provided by a surface moving map display with both taxiways and runways to the situational awareness picture provided by runway-only moving maps (which are still acceptable for minimum functionality).
The proof of the pudding – real life demonstrations
Since adoption of SURF-IA would provide desirable aircraft-to-aircraft (i.e., ADS-B In) capabilities with significant safety benefits to operators, while addressing an NTSB “most wanted” item to provide warnings of runway incursions directly to pilots, the FAA, with concurrence of the user community, chose to invest in accelerating standards development for the SURF-IA application.
Two contracts supporting this effort were awarded in November 2008. Honeywell was awarded a $3 million contract to develop requirements for draft avionics standards, perform human factors analyses, and develop/demonstrate prototype avionics. Honeywell partnered with Alaska and JetBlue airlines for the human factors analyses. In parallel, Aviation Communication & Surveillance Systems (ACSS) was awarded $6.3 million to also develop requirements for draft avionics standards and develop/demonstrate prototype avionics. The award to ACSS also included a provision to equip ACSS’s partner, US Airways, with ADS-B In avionics for their Airbus A330 fleet.
The FAA team integrated safety and performance analyses from ACSS and Honeywell with their own independent analyses and provided the results to a subgroup of RTCA SC-186 that is developing the SURF-IA Safety, Performance, and interoperability Requirements (SPR) document. The need to unambiguously determine aircraft position relative to typical airport runway/taxiway geometries and missed/false alert tradeoffs appear to be the major drivers of avionics requirements to support the SURF-IA application. To realize immediate safety benefits, it is desired to accommodate use of “legacy” ADS-B standards and GNSS receivers to the degree possible in the SURF-IA application — this is another key challenge.
The FAA funded ACSS and Honeywell to conduct independent demonstrations at US airports. Both vendors demonstrated similar scenarios, adapted to the airports used, to exercise key functionality of SURF-IA. In November and December 2009, ACSS demonstrated their prototype system during 3 periods of low traffic at Philadelphia International Airport (KPHL). KPHL is the FAA’s “key site” for initial implementation of surface TIS-B service; this service was functioning in a “test” status during the demonstration, having not yet reached Initial Operational Capability. Two aircraft, a Beechcraft C90 King Air and an Airbus A330, were used to evaluate SURF-IA utilizing ADS-B and TIS-B source information. The A330 remained on the airport surface during the demonstration testing. Feedback about the SURF-IA display was collected from pilots and observers.
Honeywell evaluated and demonstrated their prototype system over several weeks in December 2009 and January 2010 at Seattle-Tacoma International Airport (KSEA) and Snohomish County Airport (Paine Field—KPAE) using two aircraft, a Beechcraft C90 King Air and a Cessna Citation Sovereign. ADS-B was the primary surveillance source at both airports; TIS-B source information was available at KSEA only from a test system provided by FAA. Formal Human Factors evaluations were also conducted during the demonstration activities.
What did the demos reveal?
The results of these demonstrations generally confirmed that the concept for SURF-IA is viable. Both vendors were able to develop a prototype SURF-IA system that provided indications and alerts generally as expected during each scenario. Participating pilots indicated that the SURF-IA system provided useful information to them during airport operations.
Besides these general findings, several specific findings were also identified:
• Pilots found that information presented on the CDTI was useful for gaining and maintaining situational awareness, and workload was not excessive. However, as more information is presented on a CDTI, it can become too cluttered for a pilot to make quick, accurate interpretation of an alert scenario.
• The timing of TIs and RSIs was often too late to give pilots enough time to stop prior to the hold short line. This was attributable, at least in part, to the reported position accuracy of available traffic information.
• The prototype systems used different ways of presenting information to pilots regarding intersecting runway alerts. Also, the presentation of information on the CDTI about relevant off-screen traffic can be difficult to interpret by pilots. The most effective way to present this information has not yet been standardized.
• Some airports have geometries and features that interfere with line-of-sight ADS-B communications. Masking may occur particularly in scenarios involving intersecting runways due to buildings, hangars, or even trees. Masking also may occur due to changes in airfield elevation. Demonstrations at KPHL and KPAE confirmed this issue for intersecting runway scenarios. At KPHL, an intersecting runway scenario was conducted using aircraft-to-aircraft ADS-B and then re-run utilizing the TIS-B service as a surveillance source for one of the aircraft, with favorable results. Although the TIS-B service as currently implemented by the FAA will not mitigate this ADS-B line-of-sight problem, the demonstration showed the potential need for using ground infrastructure to mitigate physical blockage of aircraft-to-aircraft ADS-B transmissions.
• Occasionally observed unexpected loss of ADS-B signal reception while the two demonstration aircraft were within line-of-sight, possibly due to multi-path interference.
• The DO-260A (and DO-260B) standards stipulate that stationary ADS-B surface targets transmit position reports at reduced rates. This may have consequences for acquiring tracks and maintaining track continuity for aircraft during taxi operations including the holding position short of the runway and for aircraft that have been issued a “line up and wait” clearance on the runway.
• The timing of ASDE-X alerts presented to airport traffic controllers differed from the prototype SURF-IA system. In one scenario at KPHL, the alerts were presented to pilots several seconds before separate ground-based alerts were presented to controllers. Based on this finding, triggers for SURF-IA warnings are being redefined to allow more flexibility in the timing of alerts to pilots. Any implementation of SURF-IA will need to address the systemic effects that these timing issues could introduce.
• The SURF-IA prototype systems did not exclude ADS-B information from generating indications and alerts based upon the target’s performance requirements (accuracy, integrity, etc.). Over 95% of the available traffic during the demonstrations would not have qualified for SURF-IA indications or alerts, based upon draft SURF-IA performance requirements. While TIS-B could be used to increase availability of traffic information, targets presented by the current FAA TIS-B service do not broadcast integrity information, which will likely be required by the SURF-IA performance requirements.
• The demonstrations also revealed that some stationary ADS-B targets and all TIS-B targets moving below 12 knots did not include directionality (heading) information that improves alerting performance. Results suggest that useful alerting can be performed in situations when directionality is estimated from track history.
Where do we go from here?
Simulations and field demonstrations indicate that SURF-IA is a promising CDTI application. Many benefits can be expected from using CDTI-based surface applications alone, and runway safety benefits should be enhanced by the availability of SURF-IA indications and alerts. Alerts will be especially useful in situations where the pilot does not observe a situation on the CDTI display.
The availability of high-quality aircraft positioning information will be the primary factor in the adoption of SURF-IA. A relatively small number of aircraft are currently ADS-B Out equipped, and only a few of those equipped aircraft have the level of positioning performance that may be needed to provide the full safety benefits of SURF-IA (e.g., accuracy, integrity, length/width codes, and position offset). The FAA would like to increase availability in the near-term of traffic information for the SURF-IA application by utilizing TIS-B as a surveillance source. The use of TIS-B will likely require the service to provide integrity parameters for targets. The FAA is investigating how the TIS-B service could be modified at a reasonable cost.
The RTCA has developed a mature draft of an SPR for SURF-IA (RTCA 2010). Plans are to finalize the analysis for public comment and approval in 2010. Development of a Minimum Operational Performance Standards document (MOPS) will begin subsequently to support certification of a SURF-IA system in the near term.
As this standards development work progresses within RTCA, the FAA intends to conduct additional testing to gain insight about the extent to which the unexpected losses of ADS-B signal reception while within line-of-sight is due to multipath effects on the airport surface. Additionally, based upon the demonstrations, it appears that it will be common for airports to have geometries where obstructions block aircraft-to-aircraft ADS-B transmissions between crossing runways that might be in simultaneous use. The FAA plans to analyze the alternatives available to mitigate masking of ADS-B aircraft-to-aircraft transmissions due to obstructions at key airports. Implementation of a service utilizing ground infrastructure to broadcast same-link ADS-B information would likely also mitigate signal losses due to multipath effects. This type of solution will, however, likely adversely affect the frequency congestion associated with the 1090 MHz frequency band.
There are plans underway within FAA to initiate an Aviation Rulemaking Committee (ARC) to engage the industry in discussion of a strategy for ADS-B In. The requirements analysis for SURF-IA will prove to be an important input to these deliberations.
As the standards for SURF-IA and other applications using the CDTI are developed, there will be an ongoing need to optimize the display so that the information presented is as useful as possible to pilots.
Editor’s epilogue
One of the novel features of the signaling system of the TGV (high speed) trains is that the signals are displayed to the driver in the cab. The speed of those trains makes it impossible for humans to read signals placed at trackside reliably. Although aircraft do not taxi at 300 km/h, statistics seem to imply that relying on outside visual signals to keep them from becoming a runway incursion is almost as hopeless as asking the TGV to rely on trackside signals. It just does not work.
A system that generates a warning when an incursion is about to happen in the tower only is equally useless. By the time the controller gets through to the pilot it is probably too late.
Systems like SURF-IA finally place the information and alerts about impending danger where it belongs, the cockpit. Of course the pilot must still notice the alert and react appropriately… Even those who have taxied through lit stop-bars will hopefully be less inclined to ignore a flashing red light right on the glare-shield.
And if they still do? Well, the TGV driver’s responsibility is to obey the signals displayed in the cab… if he does not, computers will bring the train to a safe stop.
Pilots, you have been warned…
——————-
Read about the communications related causes of runway incursions here.
You can find information on outside visual cues-based runway incursion prevention tools here.