An analysis of ads c surveillance services and implications by tyler lane of inmarsat aviation

An Analysis of ADS-C Surveillance Services and Implications for Separation Standards and Other Efficiency Improvements Tyler Lane, Inmarsat Aviation

Abstract Surveillance-enhanced procedural separation provides the most cost-effective approach, for both airspace users and the FAA, to maximize NAS oceanic airspace safety and efficiency. Automatic Dependent Surveillance Contract (ADS-C) Surveillance Services1 enable more efficient air traffic control services through the integration of ADS-C surveillance data and Controller-Pilot Data Link Communications (CPDLC) capabilities into the FAA’s Advanced Technologies & Oceanic Procedures (ATOP) system. Since the introduction of the Future Air Navigation System (FANS) and ADS-C surveillance enabled oceanic procedures, reductions in separation minima have made operations increasingly more efficient. In November 2016, ICAO published ADS-C enabled separation minima of 23 NM lateral and 30 NM longitudinal as well as a 15 NM ADS-C Climb Descend Procedure (CDP). Additional reductions in separation minima, enabled by ADS-C surveillance services, are planned for publication in 2020, thereby leveraging the existing FANS 1/A equipage base for safety and efficiency improvements with no additional capital expenditure required of airlines. Additionally, these improvements can be achieved while maintaining procedural airspace in which a single controller can manage far greater numbers of aircraft (60-80 are not unusual) in much larger sectors of airspace than can a controller using tactical radar procedures. Finally, improvements in the configuration of CPDLC and satellite voice as well as the introduction of an IP-based satellite communications infrastructure will provide the speed, capacity and two-way communications capabilities between pilots and controllers and between flight deck and air traffic ground systems to achieve optimal flight profiles and thus maximize en route fuel savings. For National Airspace System (NAS) oceanic operations, further optimization of the highly successful FANS architecture along with separation minima and operational concepts that fully leverage FANS capabilities, therefore, offer the most flexible and most economical way forward.

Introduction With the integration of FANS ADS-C Surveillance Services (satellite-based ADS-C aircraft surveillance and CPDLC), into the FAA’s oceanic air traffic automation system, operations in FAA’s oceanic airspace have become increasingly more efficient. In fact, FANS-enabled reductions in procedural separation minima form the most significant contribution to an estimated $1.1 billion in savings that airlines have realized between 2001 and 2016 due to satellite communications enabled air traffic efficiency improvements, which included user preferred routes and dynamic airborne rerouting procedures. During the same time period, the same satellite communications network and avionics enabled an additional $1.9 billion in savings in airline efficiency improvements through Aircraft Communications Addressing and Reporting System (ACARS) Airline Operational Control (AOC) applications, such as improved delay management and scheduling, improved maintenance capability, better fleet management, and reduced turnaround times.

Still, the potential of the current Classic Aero FANS network is yet to be fully leveraged. For example, in 2016 ICAO published FANS enabled separation minima of 23 NM lateral and 30 NM longitudinal, as well as a 15 NM ADS-C climb/descend procedure. The ICAO Separation and Airspace Safety Panel (SASP) is currently developing separation minima of 20 NM longitudinal, and potentially lower, for planned publication in 2020.

Additionally, a 2017 report by the ICAO North Atlantic Economic, Financial and Forecast Group (NAT EFFG)2

1

ICAO Document 4444, PANS-ATM, Chapter 5, provides separation minima and methods supported by surveillance services described in Chapter 13, Automatic Dependent Surveillance Contract (ADS-C) Services.

2 ICAO NAT EFFG, Phase 2 Space-based ADS-B Business Case Analysis for the NAT Region, May 2017

calculated potential benefits associated with reduced separation standards and operational changes enabled by high update interval aircraft surveillance information transmitted from aircraft to ground stations via satellite.

While the report only addressed one specific surveillance technology, in fact, almost all of the described benefits can be achieved through ADS-C Surveillance Services, at significantly less cost, enabled by the FANS 1/A equipage (ADS-C surveillance and CPDLC) onboard over 90% of today’s oceanic commercial air transport fleet. Additionally, the estimated costs for an Air Navigation Service Provider (ANSP) to configure their ground systems and for airlines and ANSPs to operate with ADS-C surveillance at higher reporting intervals are a fraction of the estimated costs described in the NAT EFFG report.

The demonstrated FANS performance, operational experience with FANS enabled procedures, and an understanding of current ADS-C capabilities and pilot-controller satellite communications are key to understanding how the current FANS equipage on over 90% of today’s oceanic commercial transport fleet and Advanced Technologies and Operating Procedures (ATOP) may be configured to optimally support FAA’s concepts for maximizing safety and efficiency in oceanic operations. They are also key to defining the most costeffective modernization approach for industry and FAA to ensure that future traffic growth will have the safest and most efficient air traffic services. Building upon the current Classic Aero network, improvements such as controller-pilot communications for intervention and IP-based satellite communications will bring additional speed, capacity and two-way integration of flight deck and Air Traffic Management (ATM) data systems.

Finally, airlines are beginning to equip with the next generation satcom – which provides for broadband connectivity over IP between flight deck and ATM, AOC, and Air Traffic Control (ATC). This will enable further efficiency improvements with reduced separation, improved traffic flow management, shared situational awareness, as well as enhanced capabilities for airline operations.

Separation Minima in Oceanic Airspace The ICAO SASP is responsible for developing standards, procedures and guidance material, including separation minima. Development of these standards and materials must consider concepts of operation, projected airspace demand, available Communication Navigations Surveillance (CNS) systems and target levels of safety.

Prior to the integration of FANS 1/A, separation standards for oceanic areas were based on pilot initiated position information and communications with aircraft over high frequency (HF) radio. Oceanic separation minima were established at 100 NM lateral and 120 NM longitudinal. The introduction of FANS 1/A ADS-C and CPDLC in the mid-1990s enabled significant reductions in separation minima, with commensurate savings in flight time and fuel. Based on extensive operational experience with FANS, its capabilities have been enhanced, and additional reductions in separation minima have been published. In fact, in November 2016, ICAO published separation minima of 23 NM lateral, 30 NM longitudinal, and a 15 NM climb/descend procedure enabled by ADS-C and CPDLC. These new standards are performance-based, that is, they fall under ICAO Performance Based Communications and Surveillance requirements that became effective on 29 March 2018.

The SASP is currently developing additional standards, planned for publication in 2020 that will further reduce separation in oceanic airspace. There are two sets of standards under development: 

One set of standards is enabled by ADS-C Surveillance Services (ADS-C at RCP 180 and CPDLC at RCP 240) The standards will include additional configuration requirements such as: specified ADS-C periodic reporting intervals (current models are examining rates between 5 minutes and 64 seconds); specified parameters for aircraft route conformance monitoring and alerts, and airspace performance monitoring, which will consider data such as rates and extent of aircraft deviations and system performance. Once published, ANSPs can begin using the new standards since the entire system from the aircraft to the ANSP has been approved and validated to meet standards.



A second set of standards is intended for the planned ATS Surveillance System with RCP 240 (with planned configuration of Space-based Automatic Dependent Surveillance Broadcast (ADS-B) along

with ADS-C and CPDLC with RCP 240). The standards will include additional configuration requirements such as: specified surveillance reporting intervals, specified parameters for aircraft route conformance monitoring and alerts, and airspace performance monitoring, which will consider data such as rates and extent of aircraft deviations and system performance. (Space-based ADS-B still needs to undergo approval and validation before it can be used to separate aircraft.)

While there are differences between aircraft surveillance and position reporting rates enabled by ADS-C over Classic Aero and those planned for ADS-B, the standards will be very similar due to the capability of both systems to increase the frequency of location reporting and to communication constraints between controllers and pilots.

Required Communications and Surveillance Performance ICAO published required performance criteria for ADS-C (RSP 180) and CPDLC (RCP 240) to support integration of FANS into the ATM CNS infrastructure. These performance criteria provide specific allocations for various segments within the message networks. They also provide a basis for collision risk modeling used to develop separation standards and against which operational performance may be monitored in order to ensure that ATC services meet target levels of safety.

However, these criteria do not specify the performance capability of either ADS-C or CPDLC. The Inmarsat satellite communications network, including ADS-C and CPDLC, is a scalable, managed resource.

The network has evolved over time, with investments in new satellite constellations, ground earth stations, and enhancements to the related network infrastructure ensuring capacity demands and broadband cockpit satcom needs can be met over the next decades. The system is designed to allow further upgrades in the future. It can be configured to support new or evolving operational needs and concepts.

ADS-C Capabilities and Configuration The ADS-C service currently used in support of oceanic air traffic control, provides a capability for aircraft to automatically transmit, via a data link, data derived from on-board systems. At a minimum, the data must include aircraft identification, the three-dimensional position, heading and speed, the corresponding time of the position data, and a Figure of Merit (FOM) that characterizes the accuracy of the position data. Additional information may be transmitted as required and includes intent, waypoint and deviation information, emergency reporting and other information. Examination of data available for transmission in an ADS-C report, as compared to that available in an ADS-B report demonstrates that ADS-C supports all of the data content of ADS-B as well as additional route, altitude and waypoint conformance information. ADS-C also provides additional intent information, which is a key enabler of trajectory based operations. Table 1 presents a comparison of data values available through ADS-C as compared to ADS-B. The ADS-C message update rate is set by specifying the reporting interval in the contract requested by the ANSP and is constrained by the provisions in RTCA DO-212.

A complete ADS-C message is generated by the aircraft avionics and transmitted by the satcom equipment installed on the aircraft at the specified interval. In the case of Inmarsat Classic Aero, the most widely used service, the message is sent over the ACARS network through a Satellite Data Unit (SDU) to the satellite currently providing service for that aircraft and from there to the Ground Earth Station (GES), which interfaces with the ACARS network. In the case of the latest satcom technology, SB-S, a much smaller and lighter SwiftBroadband unit (SBU) facilitates communication over Inmarsat’s broadband satellite network, which sends the message through satellite access stations (SAS) to be routed to the ACARS network for further processing.

The shortest periodic reporting interval possible per each ADS-C contract is specified by DO-212 as 64 seconds.

Additionally, up to 5 individual ADS-C contracts may be established for a given aircraft. It is not uncommon to see 3 separate ADS-C contracts established for a single aircraft, one for the ANSP with current data authority (CDA), one for the following ANSP, or next data authority (NDA), and one for the AOC.

Most ANSPs currently specify an ADS-C update rate (periodic reporting interval) of once per 10 to 15 minutes, however this is a user-specified requirement and not a system limitation. Several ANSPs have satisfactory experience using intervals of 4 minutes or less. Influencing factors may include: 

Separation standards: 12 minutes is currently the fastest reporting rate required by published separation standards (30 NM longitudinal, published in November 2016);

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ICAO Global Aeronautical Distress and Safety System requirements, which specify a tracking rate of 15 minutes or faster for aircraft operating under normal conditions;

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ANSP or AOC unique needs: a number of ANSPs and AOCs establish faster ADS-C intervals to implement specific CONOPS. Surveillance Requirement for Separation Type*

Data Content Value ADS-C (DO212)

ADS-B (DO260B)

Procedural Separation

Radar Separation

Latitude

Latitude

Latitude (WGS-84)

X

X

Longitude

Longitude

Longitude (WGS-84)

X

X

Altitude (Geometric)

Altitude

Altitude, Geometric (WGS84)

X

Altitude (Baro)

Altitude, Barometric

Altitude, Barometric

Timestamp

Time

Time of Applicability

X

X

Air Ref Velocity

Air Ref Group: speed

Air speed and type

X

X

Air Ref Heading

Air Ref Group: hdg

Heading/subsonic Heading/supersonic

X

Air Ref Vertical Rate

Air Ref Group: vertical rate

Vertical Rate Baro

X

Ground Ref Velocity

Earth Ref Group: speed

North/South Velocity East/West Velocity

X

Ground Ref Heading

Earth Ref Group: track

Heading/ground track

X

Ground Ref Vertical Rate

Earth Ref Group: v rate

Vertical Rate, Geom

X

Accuracy

Figure of Merit

NAC

X

Integrity

X

X X X X X X X

NIC, SIL

System Performance

Navigation System Redundancy, TCAS Health

SDA

X

Flight ID

Flight Identification Group

Call sign

X

Aircraft ID

Airframe Ident Group

Emitter category, aircraft address, participant address, address qualifier

X

Intent: Heading

Selected Heading Predicted Route Group, Intermediate Projected Intent MCP/FCU or FMS Selected Altitude

Intent: Altitude

X

X X X

X X

X X

Surveillance Requirement for Separation Type*

Data Content Value ADS-C (DO212)

ADS-B (DO260B)

Procedural Separation

Intent: Routing

Group, Fixed Projected Intent Group

X

Deviation

Change Event Tags (Lateral, Altitude, Waypoint)

X

Emergency Reporting

Emergency Periodic Report

Meteorological Data

Meteorological Group

Emergency/Priority Status & Mode A Code

X

Radar Separation

X

Table 1: Comparison of ADS-C and ADS-B Message Content

Controller-Pilot Communications Currently the technical constraint with the most significant influence on lower thresholds in oceanic separation is controller-pilot communications. The ICAO standard for CPDLC, RCP 2403 is currently the most rigorous standard applicable to air traffic control communications in oceanic airspace. However, the requirements for interventional communications rise as separation minima are reduced. Reduced minima for oceanic areas under development by the ICAO SASP for both ADS-C and ADS-B enabled separation are constrained by RCP240 communications. While satellite voice and/or data communications could be configured to provide interventional communications, standards and concepts of operation are needed.

The current performance standard for communication alternatives to CPDLC, satellite voice or high frequency radio, RCP 400, is not as rigorous as RCP 240. Additionally, legacy security requirements for satellite voice ground-to-air connections result in extended ground-to-air connection times, and constrain the ability to leverage contemporary security protocols. While satellite voice could be configured to perform to more rigorous standards, standards for security and performance as well as concepts of operation are needed.

For present needs, CPDLC with RCP 240 provides a safe and reliable communications capability. The intervention capability necessary for closer minima (e.g., 5 NM or 3 NM) similar to those supported by groundbased ATS surveillance systems and VHF communications, however, cannot be supported by RCP 240 and a new standard would be required.

Geostationary Satellites Latency and Performance Performance assumptions are sometimes discussed regarding latencies inherent to Geostationary Earth Orbit (GEO) and Low Earth Orbit (LEO) satellite constellations due to the relative difference in orbital distance (approximately 35,800 km for GEO satellites providing ACARS and ADS-C services and 780 km for the new LEO constellation proposed to provide space-based ADS-B surveillance service). With an average ground-toGEO and return latency of approximately 300 milliseconds, and an average ground-to-LEO and return latency between 100 and 200 milliseconds, the difference of approximately 100 milliseconds is insignificant to overall network latency in the relevant applications (ADS-C, CPDLC, ADS-B) for air traffic control. In fact, for either system, most of the signal latency is a function of the number of space and ground network nodes.

Figures 1 through 3 provide information on current ACARS data transmissions over Inmarsat GEO and Iridium LEO constellations. The figures provide measurements of end-to-end network latencies as observed by the FAA. Attachment A provides a more thorough discussion of GEO and LEO architectures and satellite signal processing.

3

ICAO GOLD

Figure 1 depicts the actual surveillance performance for FANS over Inmarsat GEOs and Iridium LEOs in the Anchorage FIR. While this chart includes both ADS-C data from both Inmarsat’s GEO network and Iridium’s LEO network, there are significantly more Inmarsat processed reports. This is important when comparing Figure 1 to Figure 2 which depicts the performance of FANS over Iridium LEOs. The reader will note significantly better performance with Inmarsat GEO/Iridium LEO aggregate data than Iridium LEO data alone.

Figure 1

Figure 2 Figure 3 depicts ADS-C downlink latency over the Inmarsat I4 satellite network and SB-S. The end-to-end downlink latency averaging 4.5 seconds includes signal processing and the satellite transmission path.

Figure 3: ADS-C downlink latency over Inmarsat SB-S (September 2016 – July 2017)

Introducing IP Capability to Oceanic Operations The powerful Air Traffic Services applications that have enabled the expansion of oceanic airspace will be faster, stronger, and more reliable with IP connectivity for the cockpit. With IP powering cockpit avionics, surveillance with ADS-C and communication with CPDLC will help airlines exceed the minimum performance standards required by aviation regulators across the world and further reduce separation standards.

Additionally, IP delivers game-changing visibility for airline operations, transforming the role of satcom from a safety utility to a unique source of strategic value. Operational benefits include: 

Fuel and CO2 Savings: Flight route optimization and trajectory-based operations save time and fuel.

EFB apps, such as chart updates and real-time graphical weather, assist efficient flight paths.



Better Asset Utilization: Pre-position parts for improved turnaround time through air to ground Tech Log notifications and reduce medical diversions with the telemedicine.

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Increased Capacity: High frequency positional reporting enables reduced separation minima, whilst VoIP and ACARS over IP bypass VHF congestion to improve ATC communications.

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Secure, assured safety: Encrypted, segregated network. Better than 99.9% L-band availability gate-togate worldwide coverage to ICAO GOLD (Global Operational Data Link Document) standard.

Conclusion Surveillance-enhanced procedural separation enabled by FANS 1/A and ADS-C Surveillance Services provides the most cost-effective approach, for both airspace users and the FAA, to maximize NAS oceanic airspace safety and efficiency.

ADS-C Surveillance Services operating in oceanic airspace since the 1990s, have resulted in air traffic control related efficiency savings for airlines of over $1.1 billion between 2001 and 2016, by enabling reduced separation (currently 23 NM lateral and 30 NM longitudinal), user preferred routes, and dynamic airborne reroute procedures. The same avionics have saved airlines an additional $1.9 billion over the same time period through efficiency improvements related to airline operations management. The current FANS 1/A equipage can be configured to provide even more savings through additional reductions in separation minima, with ICAO publication of 20 NM or less longitudinal separation minima planned for 2020. ADS-C Surveillance Services over the certified and operational IP-based SB-S network will provide the high capacity, speed, and capabilities for even greater efficiency in the future.

Attachment A GEO vs. LEO… Geostationary earth orbiting satellites (GEO) have advantages over space-based ADS-B on low earth orbiting satellites (LEO) in most factors that affect radio link performance, and for the remaining factors, GEOs and LEOs are roughly equivalent. Figure 1 depicts the factors that affect radio link performance.

Figure 5. Factors affecting radio link performance ✓ Radiated Power – More radiated power allows more energy per bit, yielding a clearer and more reliable signal. The measure is Eb/N0, or energy per bit divided by noise floor. Total radiated power is greater in Inmarsat satellite communication links (the signal is stronger and better focused) than it is in Aireon’s LEO ADS-B solution, which has a weaker transmission and dispersed, omnidirectional broadcast. Because all ADS-B transponders broadcast on the exact same frequency, the noise floor is much higher (“louder”) and position reports are harder to distinguish. In contrast, Inmarsat uses dedicated communication channels, which create a far more reliable communication link. ✓ Antenna Gain – Inmarsat satcom antennas are specifically designed for aircraft/satellite/earth station communication and are optimized for this type of communication. 1090 MHz omnidirectional antennas, on the other hand, are designed for aircraft-to-ground omnidirectional broadcasts. ✓ Transmission feed line loss occurs when energy is lost on the wire that connects the transmitter to antenna, and is affected by quality of installation. Feed line loss on a satellite is typically very low, but can vary significantly on aircraft depending on installed equipment. The quality of Inmarsat satcom installation is typically very high, while substandard ADS-B installations are more common. While ground-based radar antennas can compensate for these shortcomings through large antenna gains, a space-based ADS-B receiver operates with much less margin. Weaker (ADS-B) signals thus run the risk of disappearing in the noise. ✓ Signal slant range/free space impacts received signal strength, but is more than offset by antenna gain achieved by focusing the antenna beam directly on the satellite. Our focused antenna beam (antenna gain) also results in a more reliable signal, because the signal through a dedicated channel does not compete with other transmissions. ~ Propagation Loss – energy lost to the medium through which the RF signal propagates, e.g. the atmosphere.

For GEOs and LEOs, this is roughly equivalent. GEOs typically experience less loss over the equator while LEOs experience less loss over the poles. ~ Receiver feed line loss results from production quality. GEOs and LEOs are roughly equivalent.

✓ Inmarsat performance advantage ~ Inmarsat GEO and Aireon/Iridium LEO performance equivalent Satellite Latency is Irrelevant Ground-to-GEO and return latency is approximately 300 milliseconds, while the average ground-to-LEO and return latency is between 100 and 200 milliseconds. This difference of ~100 milliseconds renders insignificant the contribution of space propagation delays to overall signal latency in the relevant applications (ADS-C, CPDLC, ADS-B). There is no significant advantage of one system over another. Rather, for either system, most of the signal latency is a function of the ground network.

Figure 6.

In a LEO architecture, the distance from aircraft-to-satellite is shorter, but the signal must transmit through multiple satellites (network nodes) before it reaches the ground receiving station. While the approximate distance between a GEO satellite and an aircraft ranges between 36,000 and 40,000 km, on a LEO system using a single ground station, the signal can travel a similar distance or longer between the aircraft and the ground network. Because LEO satellites move quickly over the ground, networks are more difficult to setup and signal routing is less predictable, depending heavily on the instantaneous constellation of nearby satellites. The signal delay is, therefore, less predictable: in general it is equivalent, but it may be longer.

Figure 7. Factors affecting radio link performance