## The SSWG’s analyses reference d in the rule are found in--

RTCA DO-318 Safety, Perf ormance and Interoperability Requirements Document for Enhanced Air Traffic Services in Radar-Controlled Areas Using AD S-B Surveillance (ADS-B RAD)

Annex B—Operational Performa nce Assessment (OPA) ( See below )

Annex G—Additional Considerations for Impl ementing ADS-B RAD in the US National Airspace System (NAS) ( See below )

(This material is copyrighted by RTCA, Inc., a nd used with permission. The complete RTCA DO-318 documents and other RTCA documents may be purchased from RTCA, Inc., 1828 L St., NW, Suite 805, Washington, DC 20036; te lephone (202) 833-9339; www.rtca.org.)125

© 2009, RTCA, Inc. ANNEX B - OPERATIONAL PERFORMANCE ASSESSMENT (OPA)

B.1 Introduction

B.1.1 Purpose

The purpose of this Annex is to summarize the Operational Performance Assessment (OPA) for Enhancing ATS in Radar Areas using ADS-B (ADS-B-RAD) application.

B.1.2 The CNS/ATM system

A functional description of the CNS/ATM system required for RAD operations is depicted in §3.1

.

B.2 ADS -B-RAD OPA

The OPA is summarized in the following sections. §B.2.1 covers the scope of the analysis, §B.2.2 lists the relevant assumptions, and §B.2.3 provides the analysis detail.

This Annex identifies a set of performance requirements for the use of ADS-B in radar airspace (RAD) based on a comparison to modeled MSSR performance. This comparative methodology by its nature imposes certain limitations that suggest further investigations may be appropriate. For example, some of the analyses in this Annex are based on an assumption that the tails of the azimuth error distribution for the MSSR can be accurately modeled using a double-Gaussian curve, based on the Eurocontrol surveillance standard and supported by a limited set of US MSSR data (see Annex B, Appendix B- 3) . This assumption differs from some previously published reports where the data collected showed that the radar azimuth error distribution could be modeled using a single Gaussian curve. As with any requirements development process, additional data collection and analyses will be performed to confirm that the assumptions made in this standard remain valid. If these data and analyses indicate that assumptions need to be modified, this document will be updated accordingly.

B.2.1 Scope of the ADS-B-RAD OPA

The ADS-B-RAD application involves the ADS-B Transmit Aircraft Domain and Ground Domain, as well as radar systems. The performance of the radar system is assumed to be known and is therefore a constraint in determining required ADS-B system performance. The OPA addresses the performance from the navigation source to the input of the ATC processing system (interfaces A1 to E2) and additionally allocates performance to each of the domains, as applicable

. The OPA analysis considers the following RAD environments introduced in Annex A:

RAD 2a: A Single Monopulse Secondary Surveillance Radar (SSR) with ADSB surveillance in high-density En Route;

RAD 2b: Single Mode S Radar with ADS-B surveillance in high-density En Route;

RAD 3: Single Primary Surveillance Radar (PSR) with a collocated single SSR.126

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Note that RAD 1: Single Primary Surveillance Radar (PSR) with ADS-B surveillance in medium-density TMA is not addressed, as it is not envisioned that services us ing primary and ADS-B position information would be supported.

In the OPA, Mode A Code is used as the reference source of aircraft identification

. The OPA focuses on the performance necessary to support the ATS separation task, as it is deemed to be the most demanding ATS task, in an environment where both ADS- B and radar data are available. Both radar systems and ADS-B systems provide data to ATC Processing Systems in this application. Therefore, in addition to the separation oftwo ADS-B-equipped aircraft, this application addresses separation of one ADS-Bequipped aircraft from one non- ADS -B-equipped aircraft whose surveillance informationis provided by secondary radar. The OPA does not consider the separation of an ADS-Bequipped aircraft and an aircraft surveilled by PSR radar. Per Annex A §4.1.3.8, the OPA specifically addresses the following separation environments:

TMA

o 3 NM separation in TMA

o 2 1⁄2 NM in-trail on approach

o 2 NM dependent parallel approach

o Independent parallel approach

En Route

o 5 NM separation

B.2.2 OPA Assumptions

OPA assumptions are grouped into the following categories:

Nominal Operations - Intended to characterize the operational environment and the implementation of ADS-B within that environment

. Approach Scenario Assumptions - Specific to the 2.5 NM In-Trail, 2 NM Dependent Parallel, and Independent Parallel Approach applications only. Reference Radar Compariso n - Established as a basis for the radar comparative
assessment

.

B.2.2.1 Nominal Operations Assumptions

1. OPA-ASSUMP.1: The most demanding ATS task involving surveillance is assumed to be assuring aircraft separation. Therefore, ADS-B requirements that can support the separation task are assumed to be adequate for all other air traffic services contained in the ADS-B-RAD application. This assumption scopes the OPA to an analysis of separation.

2. OPA-ASSUMP.2: Although 100% of aircraft are assumed to be ADS-B-equipped and are also under radar surveillance, the separation of one aircraft presented to the controller with radar data and another aircraft presented to the controller with ADS- B data is addressed for a variety of reasons, including RAIM holes, coverage bo undaries, or avionics failures

.127

© 2009, RTCA, Inc. 3. OPA-ASSUMP.3: It is assumed that all aircraft are acquired by ATC processing prior to entering the defined airspace volume, thus the OPA does not consider the acquisition of aircraft

. 4. OPA-ASSUMP.4: The transmissions by the airborne system of the messages containing the minimum as well as the optional ADS-B-RAD surveillance data items are assumed to occur at the required specific ADS-B data link rate

. It is assumed that the ADS-B ground infrastructure has sufficient coverage to assure that the required ADS -B update intervals are satisfied, including the use of error-checking capabilities (e.g., CRC check) on received ADS-B data. In addition, it is assumed that appropriate frequency channel management is in place to ensure nominal concurrent operation of ADS -B and other data links (e.g., SSR). These assumptions are necessary to support the update interval analysis and SSR availability assumptions.

5. OPA-ASSUMP.5: If GNSS is used as the horizontal positioning source, it is assumed that the GNSS constellation is sufficient to assure that ADS-B integrity monitoring confirms the integrity of the surveillance position data. This assumes GNSS receivers are approved in accordance with E/TSO -C129 or a higher integrity performance standard as necessary to provide the required position data integrity

. The applicability to other horizontal positioning sources should be examined separately

. 6. OPA-ASSUMP.6: The OPA does not consider the effects of any unexpected (e.g., spoofing, environmental) interference.

7. OPA-ASSUMP.7: ATC processing systems are assumed to time-register the asynchronously received ADS-B position updates from ADS-B-equipped aircraft.

8. OPA-ASSUMP.8: The comparison of ADS-B and radar surveillance data quality is assumed to be made at the input of the ATC Processing System. For SSE purposes, this comparison includes the time synchronization function

. ADS -B requirements are therefore expressed at the ADS-B surveillance report level. It is assumed, based on existing experience, that current ATC processing systems should achieve adequate levels of performance (availability, continuity, integrity, latency) for implementation of the ADS-B-RAD application. Functions required to process ADS-B are largely equivalent to those required to process radar. Additional functions may be needed to synchronize the radar and ADS-B report outputs, to perform radar monitoring of ADS -B reports, and possibly to implement display rates greater than the radar scan interval. The following factors should be considered by implementers when determining the impact of ATC processing system processing on ADS-B position data:

a. Coordinate Conversion Errors: Radar position information in radar coordinates are translated in ATC processing system coordinates in some systems, but used in radar coordinates directly in others. ADS -B will be translated from latitude/longitude to ATC processing system coordinates. There may be some minor differences in the errors introduced into the position data, though in both cases, it is expected that the errors will be very small.

b. Time-Registration Errors: ADS -B provides asynchronous updates in comparison to a single radar which provides near synchronous updates for aircraft. It is assumed that the ADS-B positions will have to be extrapolated (time-registered)to a common display time, whether displaying position data as reported in ADSB reports or tracked position data. This will result in some amount of a position128

© 2009, RTCA, Inc. extrapolation error. This error is considered in Surveillance Separation Error

(SSE), but not in the accuracy performance.

c. Sensor Bias Error Compensation: If an automation system is currently used to separate traffic across radar systems, a radar bias compensation function that determines and corrects for surveillance source bias errors may already be implemented. The function attempts to remove the relative sensor biases to the extent possible, resulting in a small residual bias. This residual bias should be even smaller between radar and ADS-B. If an automation system does not currently have a bias compensation function, one should be implemented and must also be considered in determining requirements for ADS-B, as it is an additional error component (residual bias).

d. Error Characteristics: The characteristics of radar errors and ADS-B errors differ. Radar errors are largely uncorrelated while ADS-B errors can be more correlated. The impact of differences in error characteristics on the ATC processing system tracker should be considered.

e. Tracker Lag: Tracker lag improvements can be realized with ADS-B as reported velocity is available with a quality indicator (See Annex H.3 ).

9. OPA-ASSUMP.9: The OPA is focused on the technical aspects of surveillance, and it is assumed that all other non-technical surveillance aspects of the service (i.e., people and procedures) will be largely unchanged. This assumption supports the use of the comparative methodology in the OPA.

10. OPA-ASSUMP.10: It is assumed that the ADS-B and radar coordinate systems are registered to each other with residual radar azimuth bias ≤ 0. 05 degrees and residual radar range bias ≤ 40 meters (see Appendix B-3 §2)

.

B.2.2.2 Approach Scenario Assumptions

1. OPA-ASSUMP.11: Non-GNSS-based navigation landing systems (i.e., ILS) are used by aircraft to support terminal approaches. Approaches/landings based on GNSS navigation (e.g., RNAV) are not considered in this analysis.

Note: Although Augmented GNSS-based landing systems (such as GBAS) have not been included in the analysis it is understood that some terminal areas have approved Category I ,II and III precision approaches using GBAS. Any authority wishing to implement ADS-B- RAD into such an environment should conduct its own analysis, particularly in relation to common mode failure cases. This analysis has considered common mode failures for the application of 5 NM and 3 NM minima and provided mitigation through the use of radar association; however, the OPA has not considered the common failure mode for the approach scenarios given.

2. OPA-ASSUMP.12: Pe r OSED §4.1.3.8, the OPA excludes parallel approach operations that require 0.06 degree azimuth accuracy and/or high update rate surveillance systems.

B.2.2.3 Reference Radar Comparison Assumptions

1. OPA-ASSUMP.13: The reference radar is assumed to be operating in a no-fault condition. This provides for a comparison with the best case of radar performance. For the purposes of comparison of radar measurement errors in this OPA it is129

© 2009, RTCA, Inc. assumed that all false plots are removed from the radar measurement error characteristics.

2. OPA-ASSUMP.14: It is assumed that the ADS-B surveillance system characteristics may be compared to those of a reference radar system to determine its suitability as a surveillance source. The reference radar for comparison is assumed to be capable of supporting safe ATC separation services, with the required separationminima (5 NM in En Route, 3 NM, 2 NM dependent parallel approach, 2.5 NM intrail approach, and independent parallel approach in terminal area).

3. OPA-ASSUMP.15: Given the high-densi ty environments, the reference radar for accuracy comparisons will be MSSR rather than the lower performing sliding window radar, as the desire is to have ADS-B performance comparable with more modern radar. Consequently, the reference radar model for comparative accuracy and integrity performance is based upon the Eurocontrol Surveillance Standard requirements with the characteristics shapes of the range and azimuth error distribution supported by measurements from US MSSR data (See Appendix B- 3)

. See §B.2.3.1.2 for further information on the reference radar.

4. OPA-ASSUMP.16: The radar measurement time of applicability is assumed to be known to 200 ms (95%) to support accurate extrapolation and time synchronization to the radar Time of Applicability (TOA).

5. OPA-ASSUMP.17: The altimeter source and sources of identity, emergency, and SPI have the same or better data integrity and data preservation performance than the sources used with SSR.

B.2.3 Analysis Approach

The RAD OPA analysis steps are as follows:

1. Identify performance parameters that will be assessed

. This is largely the same set analyzed for the NRA application.

2. Identify scenarios that are to be analyzed

. 3. Determine approach for analyzing each parameter:

 Comparative Approach - “As good as radar” ;

 Surveillance Separation Error (SSE) analysis (see Annex D).

4. Perform analysis to determine ADS-B performance required in each of the scenarios.

5. Where applicable, allocate the requirements derived from the above steps between the Transmit Aircraft and Ground Domains.

Completing these steps produces the final OPA requirements, which specify the minimum criteria needed for ADS-B surveillance data in an environment that includes radar surveillance. If ADS-B meets these criteria, it can be concluded that the ADS- B da ta can be used for ATC separation services commensurate with those services provided by radar in the medium- to -high-density TMA environments and high-density En Route environments identified in the OSED

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This OPA pertains to nominal operations only. The performance requirements derived in this assessment are reconciled with safety requirements derived from the Operational Safety Assessment contained in Annex C and a Surveillance Separation Error (SSE) assessment in Annex D to produce the final Safety and Performance Requirements (SPR) for the RAD application

. These final SPR requirements are found in §3.

B.2.3.1 Comparative Approach and Reference Radar

B.2.3.1.1 Comparative Approach Methodology

The comparative approach requires a reference transponder and a reference radar for which the relevant service attribute qualities are known (or can be defined). It is assumed that the reference radar, in conjunction with the reference transponder, is capable of supporting safe ATC separation services with the required separation minima. The required ADS-B surveillance attributes are derived in comparison with the reference transponder and radar attributes. The comparative approach methodology is used in analyzing each of the performance parameters to the extent feasible. However, given the differences in radar and ADS-B systems, a direct performance comparison is sometimes not sufficient; therefore, additional analysis is performed.

B.2.3.1.2 Reference Radar System

The radar performance values shown in the Tables contained in the following sections have been determined from a combination of the values specified in the Eurocontrol Surveillance Standard [14] and values for current USA radars

.

The Eurocontrol Surveillance Standard [14] has been used as the basis of radar measurement performance (i.e., range and azimuth accuracies). This standard specifies the minimum measurement performance required for an MSSR. The characteristic shapes of the radar error distributions (i.e., core and tail error) are not specified in the standard, therefore they have been determined from analysis of recent US MSSR radar data

. See Appendix B-3 §2 for details of radar error model.

Consequently, the reference radar performance values are believed to be “representative” of present MSSR radar for comparison purposes. It is also recognized that some performance parameters will be subject to different circumstances in local environments (e.g., capacity), but nevertheless some indicative values are given. Lo cal authorities may choose to adapt the radar requirements to the prevailing local conditions.

B.2.3.1.3 Basis for Comparison of ADS-B and Radar Accuracy

If minimum ADS-B performance requirements are to be based on a comparison to current radar performance (specifically MSSR range or azimuth, whichever is less stringent), then radar performance must first be characterized sufficiently for each operational environment of interest. One of the most critical points of comparison is surveillance accuracy (or position uncertainty). For today‟s ATC radar systems, azimuth position uncertainty can be plotted as a function of range from the radar, given a specific radar characteristic. In general, radar azimuth position uncertainty increases for a given target as the target moves away from the radar. Based on current operational procedures, for MSSR, two points have been defined relative to supported separations. In general, 3 NM separations can be supported by MSSR for targets out to a maximum range of 60131

© 2009, RTCA, Inc. NM from the radar; beyond 60 NM

, 5 NM separations must be used, to a maximum range of 200 NM (see Figure B-1 )

.

Figure B-1: Error and Radar Range

ADS -B, on the other hand, does not exhibit the same uncertainty versus range characteristic as radar. ADS -B position uncertainty is primarily a function of GPS satellite geometry relative to the aircraft, and not related to range from any ground-based system. For this reason, ADS-B position uncertainty is treated as a constant for a given environment when being compared to radar performance. Therefore, in order to determine minimum performance requirements for ADS- B, a radar “reference range” must be determined for each environment to provide the appropriate comparison points between ADS-B and radar

. It is relatively easy to specify a target set of range- of -applicability ( ROA ) values that span each operational domain, i.e., 60 NM to 200 NM radar ranges for En Route separation minima (see §2.3.1.5). However, the selection of a specific ROA value for the radar to ADS-B comparisons is much more problematic, since the most demanding environments for radar separation may not be those where radar performance is least capable, i.e., at maximum operational radar ranges. For example, in En Route airspace, climbs and descents to or from the cruise phase of flight can be more demanding in terms of both traffic density and airspace complexity as compared with En Route cruise operations, independent of radar range. Similarly, in TMA airspace, complex operations such as vectoring to transition from downwind to final approaches generally occurs well inside the 60 NM range limit of an airport-based radar system. For this reason, even a “mid

- range” ROA value may not reflect the most de manding operational environments for radar to ADS-B comparisons. The ROA values selected for the RAD document assume that ADS-B performance must be as good as radar everywhere within the operation domain considered, in order to assure that the resulting performance standards are as good as radar under the most demanding environments. This analysis is necessarily conservative. Local implementers may pursue further studies and analyses

, such as target level- of

-safety (TLS) studies, to confirm minimum performance standards.132

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If these studies indicate that a different set of requirements can be supported, the relevant standards may be updated accordingly. Local administrations may also consider relaxing some requirements in the light of their own airspace conditions and other comparative radar considerations.

B.2.3.1.4 Role of Surveillance Performance in Separation Requirements

Several key points must be well understood before a comparison of ADS-B and radar performance, based on ranges of applicability, can be performed:

1) Although surveillance performance is an important factor in determining separation standards, other factors such as aircraft speed, relative geometry, human response times, and ground processing characteristics play a critical part

. The specific contribution of each of these factors can also change as a function of many interrelated conditions, such as phase of flight, airspace density, controller and pilot workload, and environmental conditions such as Instrument Flight Rules vs. Visual Flight Rules (IFR/VFR). Although the current ATC system meets current safety objectives, the specific relationship between each of these factors and conditions - specifically as a function of radar range - is not explicitly known. In fact, a target level- of -safety analysis (as yet to be completed) would be required to quantify these factors and conditions, and to determine their explicit relationship to any separation standard in a radar environment

. 2) In general, the density of radar placement is related to the demands of the airspace to be surveilled, in order to ensure sufficient surveillance coverage and performance for the desired airspace. Specific placement criteria are dependent upon individual circumstances, and can depend heavily on (qualitative) experience and expert judgment of the service provider. Given this, it can be assumed that the surveillance performance demands of an airspace tend to increase as one approaches a radar site

– this is more consistently the case for TMAs, but also applies in many circumstances to much of the En Route airspace. Therefore, in a mono-radar environment, many of the factors and conditions that contribute to a specific separation standard can, and most likely will, change as a function of radar range

. Based on these key points, to ensure that ADS-B can support the same separation standards, safety objectives, and service provider expectations that are supported today by radar (without performing a target level- of -safety analysis), ADS-B performance must be such that it meets or exceeds the performance of radar in the operational domain considered

.

B.2.3.1.5 Reference Ranges

Radar position errors can be thought of in terms of a 95% error composed from range and azimuth (cross-range) components. The radar cross-range error component increases with range from the radar, however, the range error component is constant (see Radar in Figure 2). The radar error distributions are Gaussian in a single dimension, but in two dimensions, the 95% radar position error area is an ellipse

. At the range where the radar cross-range error approximately equals the radar range error, the 95% radar position error area becomes circular. The 95% ADS-B position error area is also circular when it is based on GPS position performance. The surveillance performance limiting component133

© 2009, RTCA, Inc. for a target at any given point in radar airspace will depend on the range of that target from the radar: at longer ranges, the azimuth component is larger; at shorter ranges, the azimuth uncertainty decreases to a point where range uncertainty (constant) becomes larger. The radar range (distance from radar) used to establish ADS-B performance requirements will be referred to as the reference range. Figure B-2 highlights the implications of reference range selection.

For ADS-B Case 1 in Figure B-2, the reference range is the distance at which the radar range error equals the cross-range error. The ADS-B accuracy performance requirement is set such that the ADS-B position error area is approximately equal to that of radar at this point. However, the circular 95% ADS-B position error area cannot be identical to radar because the discrete ADS-B navigation accuracy categories do not exactly match the radar accuracy. The 95% ADS-B accuracy performance is constant throughout the airspace where a given navigation accuracy requirement is established. Therefore, the area bounding 95% of the radar positions is larger than the ADS-B 95% area when the distance is greater than the ROA and smaller than the ADS -B area when the distance is less than the ROA

. For ADS-B Case 2 in Figure B-2, the reference range is chosen beyond the distance at which the range and cross-range errors are equal

. At this reference range, the radar crossrange error is greater than the constant range error. The ADS-B accuracy performance requirement is set such that the ADS-B position error area is approximately equal to the radar cross-range error, the major axis of the ellipse. The 95% ADS-B position error area is slightly larger than that of radar at this distance because the radar range error is less than its cross-range error. However, as the distance from the radar increases, the radar cross-range error continues to increase and the area bounding 95% of the radar positions becomes larger than the constant circular area set for ADS-B

. If the location of a radar site relative to two aircraft is known, the elliptical error areas representing radar position accuracy performance for each aircraft can be used to define the comparative performance requirements for ADS-B. Since this orientation varies between separation scenarios and may not be constant during a given scenario, the comparative accuracy performance is based on the larger radar errors represented by the major axis of the ellipse. The resulting comparison is not to the best available radar performance, but it represents a comparison to the surveillance performance that can be assured for any orientation of aircraft with respect to the radar

. In order to ensure that ADS-B performance meets or exceeds the performance of radar throughout the airspace in question, the reference range for a given environment is that range where the performance of the limiting component is minimized. For the RAD environment, this results in the following ranges of applicability:134

© 2009, RTCA, Inc. Figure B-2: Error and Reference Range

En Route : This is defined as the radar airspace beyond 60 NM from any MSSR where 5 NM separations are supported, to a maximum range of 200 NM. The limiting component of radar position uncertainty in this region is the azimuth component, for all ranges. Since azimuth uncertainty is directly proportional to the range from the radar, minimum azimuth uncertainty is achieved at the closest range to the radar that can be achieved in the airspace. Therefore, the requirements will be based upon a reference range of 60 NM for En Route. Analysis based on other reference ranges is provided in Appendix B-6 in order to provide options for local RAD implementations which may be useful for states which do not require ADS- B accuracy better than the radar performance at the radar range authorized for delivery of the relevant separation standard

.

TMA : This is defined as the radar airspace within 60 NM from any MSSR where 3 NM separations are supported. At approximately 33 NM (for the radar model selected), the azimuth uncertainty decreases to a point where range uncertainty becomes larger, and remains so for all ranges down to the minimum range of the radar. From an accuracy perspective, any range between ZERO (0) NM and 33 NM could therefore be chosen as an appropriate reference range. However, since ADS- B135

© 2009, RTCA, Inc. integrity requirements are determined from a comparison to radar tail distributions, and not the RMS values as is the case for accuracy, a further assessment is required

. Integrity as it is defined for ADS-B does not have a direct corollary in typical radar performance parameters. For the purposes of RAD, it is assumed that the range that results in the least stringent ADS-B integrity requirement, within the constraints of the ranges determined for the accuracy requirement, can be used. Therefore, the reference range for TMA is approximately 33 NM.

Approaches : This is defined as the radar airspace within 10 NM from the runway threshold where 2 .5 NM in-trail, 2 NM dependent parallel, and/or independent parallel (>4300 ft

. centerline separation) approaches are supported. In these cases, in order to provide consistent coverage, the radar cannot be greater than 15 NM from the point at which the minimum decision height is reached (assumed to be 200 ft .)

. Additionally, the position of the radar can be at any point such that this requirement is met (i.e., the radar is not necessarily located at the airport), and so the larger component of radar position uncertainty can be either the azimuth component or the range component at various points along the approach. For the maximum range in question, the range component will always be larger. Also, as was the case for TMA, it is assumed that the range that results in the least stringent ADS-B integrity requirement, within the constraints of the ranges determined for the accuracy requirement, can be used. For independent parallel approach, the radar is typically less than 2 or 3 miles from the threshold. Given these factors, the reference range for Approaches is 15 NM

.

B.2.3.2 Surveillance Separation Error

Surveillance Separation Error (SSE) analysis provides the probability that the separation error between two aircra ft is greater than a given specific amount. SSE is used in the OPA to determine the navigation integrity quality indicator (NIC) needed to support the separation standards

. The SSE analysis is provided in Annex D

.

B.3 Required Data Items a t the Input o f the ATC Processing System

B.3.1 List of Required Surveillance Data Items

ADS -B requirements are expressed in this document at the input of the ATC system, where the comparison with radar reports is performed

. The set of data items shown in the table below is required. 4

4 The OSED ( Annex A ) contains the list of data items as required at ADS-B-RAD Application Description level (Controller‟s perspective).136

© 2009, RTCA, Inc. Table B-1: Required data Items at the input of the ATC processing system
Required Surveillance Data Items Reference Radar (SSR) ADS

- B (surveillance report level) Aircraft Identity (call

sign or registration mark as filed in field 7 of the ICAO flight plan) and/or Mode A Code Mode 3/A Aircraft Identification and /or Mode 3/A code

Special Position Indicator (SPI) SPI pulse transmitted when replying to ATCRBS Mode 3/A interrogations SPI indicator from ADS

- B
message
Horizontal Position Range
, azi muth Latitude

, longitude Pressure Altitude Mode C from SSR Pressure altitude from ADS

- B

Time stamp 5 Time stamp corresponds to time of measurement of target position Time of applicability of ADS

- B information in the report

Quality indicator 6 affecting i ndividual aircraft In general, radar range imposes limits for certain separation minima (affects surveillance quality of individual aircraft according to their range) Quality Indication on an individual aircraft basis as a means to determine whether posit ion quality is suitable for surveillance separation

Emergency indicators From Mode A Code s Emergency indicators from ADS

B messages

Quality indicators are required to inform the controller whether or not the surveillance information is good enough for providing separation services. The ADS-B system 7 provides information on the quality of the position reported for each individual aircraft.

An ADS-B surveillance report may contain one or more of three types of information: (1) State Vector, (2) Identity, and (3) Emergency and SPI. Though the data items listed in Table B-1 are required at the ATC processing system, this does not mean to imply that each data item must appear in each ADS-B report, as the information may be transmitted by the avionics in separate ADS-B messages. Some ADS-B implementations may require that data from the multiple messages be retained and output to the ATC processing system in a complete report, while others may transmit partial information in the report

.

5 Some means for the ground system to determine the time of applicability of the information is required by most ATC Processing Systems. This does not imply that absolute time has to be sent in ADS-B messages, but may be determined for an ADS-B message by the data link requirements on time of applicability of data. For example, under certain conditions 1090ES message information is assumed to be valid at the time of transmission, with an uncertainty of 200 msec.
6 This is required as it is assumed that the position quality may vary in time and from aircraft to aircraft.
7 The ADS-B system is limited to the Transmit Aircraft Domain and the Ground Domain

– see §3.1.137

© 2009, RTCA, Inc. B.4 Definition and Specification of Performance Parameters

The OPA analysis is divided into logical groups of ADS-B parameters. The parameter groups and the analysis approaches used for each are as follows:

Update interval and update probability (§B.4.1)

– Comparative Approach; Position Accuracy (§B.4.2)

– Comparative Approach; Data Integrity and Data Preservation (including time- to -alert) (§B.4.3)

– Comparative Approach and SSE; Reliability (§B.4.4)

–

Comparative Approach; Other surveillance parameters (§B.4.5) Comparative Approach

. For each parameter group, the following information is provided:

An overview of the parameters; Definitions of the parameters, relative to radar and ADS-B; Expected radar performance and required comparable ADS-B performance; As applicable, an allocation of the ADS-B requirements between Transmit Aircraft Domain and the Ground Domain.

ADS -B surveillance report is defined in § 1.3 , however, as the term is used extensively in the OPA, it should be noted that the ADS-B surveillance report may contain one or more of three types of information : (1) State Vector, (2) Identity, and (3) Emergency and SPI

.

B.4.1 Update Interval and Update Probability

Surveillance reports are the major input to ATC processing systems. These reports are assembled from acquired surveillance data. In a radar environment, this data is obtained from radar interrogations and onboard transponder replies from the surveilled aircraft. For ADS-B, this data is obtained from messages received from ADS-B-equipped aircraft. Surveillance reports are generated periodically with newly acquired data. For ADS- B there is the potential for multiple messages to be received containing updates of the same data parameters between surveillance reports. It also should be noted that many ADS- B messages are not received due to other radio frequency ( RF ) interference, transmission nulls, and other factors. ADS -B systems must be designed to assure that surveillance reports (and ADS-B reports for air- to -air communications) are updated at the required update rate

.

B.4.1.1 Definitions

Report Update (as defined in § 1.3 ) : The delivery of an updated ADS-B surveillance report, which contains newly received information from ADS-B messages, to the ATC processing system.

Note: Depending on the ADS-B link, a single report update might contain information from multiple messages received since the last report update.138

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Update Interval (as defined in § 1.3 ) is the time interval within which there is a 95% probability of receiving at least one ADS-B surveillance report update (at interface E2)

.

For the reference mono-radar (Mode A/C SSR), this is fixed. Furthermore, all the required surveillance report information (position, Mode C pressure altitude

, Mode A Code, emergency indicators and SPI) is gathered each scan, and there is no more than one report each scan.

For ADS-B the required surveillance information may be spread over more than one ADS -B surveillance report. ADS -B surveillance reports containing each type of information (position, identity, and emergency and SPI) may be produced at different times and rates or may be combined into a single report. The reports are linked to the same aircraft by the ICAO 24-bit airframe address. Hen ce, for the purposes of comparison in this OPA, the required update interval between ADS-B surveillance reports may be different for each type of information.

Update Probability is defined as the probability that a report with the required information is received within the required update interval

.

For radar , the Update Probability includes the following factors :
Probability that a radar target report is generated (i.e., detected by the radar); Probability of Mode A Code validated; Probability of Mode C code validated; Probability of emergency and SPI code validated.

Note: Probability of code validation is the probability that a radar target report contains the same information transmitted by the aircraft (the information is correct) and an indication that the code has been validated (deemed to be correct by the radar system).

For ADS-B , the probability may be different for each type of ADS-B surveillance report information (state vector, identity, emergency and SPI).

B.4.1.2 Requirements

The update interval and update probability requirements are based entirely on the comparative approach. The update interval and update probability performance is data link dependent (i.e., involving the rate of transmission of ADS-B messages from the airborne end of the link and the probability of reception of the messages by the ground end of the link)

. The airborne system will provide the required information to the link via transmitted ADS-B messages and the ground system will convey the information to the ATC system via A DS -B surveillance reports.

As discussed in §B.4.1, for a particular link, the update interval and update probability of surveillance reports will depend upon the probability of successfully receiving an ADS- B message, which is dependent upon various environment factors (e.g., locations of receiving stations, design of receiving stations, numbers of aircraft transmitting ADS-B, other co-channel interference sources, etc.)

.139

© 2009, RTCA, Inc. Table B-2 shows expected reference radar performance and the comparable ADS- B performance required

. While it appears that the update probability requirements for ADS- B may be less stringent than the expected performance of the reference radar system for certain data items other than lateral position

, it should be noted that the update probabilities for these data items are specified in Table B- 2 relative to the number of valid position reports generated by the radar, not to the number of update cycles (i.e., scans) as is the case for lateral position

. Therefore, the overall probability of receiving these data items (per scan) from the reference radar system is the probability of decoding the relevant data item multiplied by the probability of generating a valid position report (i.e., radar probability of detection). This is the corresponding requirement used for ADS-B for the relevant data item.

Note that the ground domain processing includes the formation of ADS-B surveillance reports from ADS-B messages and the transfer of the reports to the ATC system in the appropriate format (a link independent format). Since the update requirements are specified at the report level, and report assembly performance (e.g., assembly time, reliability, etc.) of the ground system is assumed to be equivalent for radar and ADS- B environments, the update r equirements are allocated between interfaces “D” and “E2 .”

For some ADS-B links, a change in Mode A Code results in more frequent transmission of the Mode A Code. As long as the Mode A Code is not changing, it is not necessary that the value be updated in the surveillance report sent to ATC processing systems

. Therefore, the Mode A update interval requirements are stated in terms of “a change in Mode A Code .” An operational need has not been established to place an update interval requirement on the aircraft identification data, which is not expected to change in flight.

For the various ADS-B surveillance reports, the following update interval and update probability requirements are derived for this OPA:

PR 1: The update interval for Surveillance Reports containing ADS-B Position data associated with any aircraft in En Route airspace shall be no longer than 8 seconds ( 97 %).

PR 2: The update interval for Surveillance Reports containing ADS-B Position data associated with any aircraft in TMA airspace shall be no longer than 5 seconds ( 97 %).

PR 3: The time interval between a change of Mode A Code provided by the ADS- B aircraft domain and an ADS-B surveillance report containing the new Mode A Code at interface E2 shall be no longer than 8 seconds (95%) En Route.140

© 2009, RTCA, Inc. Table B-2: Update Interval and Probability Comparisons

Update Interval and Probability Value (En R oute) Value

(TMA)
Update interval

Reference radar : Update interval (all items) = radar scan period 8 s 5 s

ADS

- B systems : Update interval for A DS

- B surveillance reports containing:
1. State Vector (state vector information, including change in quality indicators) (equivalent to a radar scan)
2. Identity ( aircraft identification and Mode A Code )
3. Emergency and SPI 8 s

8 s 8 s 5 s

5 s
5 s

Pro bability

Reference radar: Target report (with at least lateral position):
Mode A Code validation (per target report) Equivalent Mode A Code validation (per scan) Mode C code validation (per target report) Equivalent Mode C code validation (per scan) Emerg ency and SPI code validation (same as for Mode A Code validation) ( Values taken from Eu rocontrol Surveillance Standard) ≥ 0.97
≥ 0.98
≥ 0.95
≥ 0.96
≥ 0.93
≥ 0.98 ≥ 0.97

≥ 0.98
≥ 0.95
≥ 0.96
≥ 0.93
≥ 0.98

ADS

- B systems : Probability of update for surveillance reports containing:
1. State Vector (same as for radar target)
2. Identity ( aircraft identification / Mode A)
Note : Here, "

Identity" is the particular information (either Aircraft Identification or Mode A) that is required by the local ANSP

. The probabilities relate to a change in this Identity.
3. Emergency and SPI
Note : The probabilities relate to a change in emergency and SPI information. ≥ 0. 97
≥ 0.95

≥ 0.95 ≥ 0. 97

≥ 0.95

≥ 0.95

PR 4: The time interval between a change of Mode A Code provided by the ADS- B aircraft domain and an ADS-B surveillance report containing the new Mode A Code at interface E2 shall be no longer than 5 seconds (95%) TMA.141

© 2009, RTCA, Inc. PR 5: The time interval between a change of emergency and SPI information provided by the ADS-B aircraft domain and an ADS-B surveillance report containing the new emergency and SPI information at interface E2 shall be no longer than 8 seconds (95%) En Route.

PR 6: The time interval between a change of emergency and SPI information provided by the ADS-B aircraft domain and an ADS-B surveillance report containing the new emergency and SPI information at interface E2 shall be no longer than 5 seconds (95%) TMA.

The ADS-B performance requirements for Update Interval and Probability are summarized in Table B-3.

Table B-3: Summary of ADS-B Update Interval and Update Probability Requirements

Update Interval and Update Probability Requirement Value (En R oute) Value

(TMA) (“D”  “E2”)

Update Interval and Probability

ADS

- B surveillance report

– state vector (“D”  “E2”) 8 s ( 97 %) 5 s (95%) 8 s ( 97 %) En Route

5 s ( 97 %) TMA

ADS

- B surveillance report

– Mode A Code change (the input to the ADS

- B transmit function  “E2”) 8 s (95%) 5 s (95%) 8 s (95%) En Route

5 s (95%) TMA

ADS

- B surveillance report

– emergency and SPI change (the input to the ADS

- B transmit function  “E2”) 8 s (95%) 5 s (95%) 8 s (95%) En Route

5 s (95%) TMA

B.4.1.3 Benefits of Reduced Report Update Interval

With ADS-B surveillance, the report update period is potentially much more flexible than with traditional constant scan mono-radar systems. Even though the received ADS- B reports are typically random in time, using velocity-based extrapolation enables ADS- B reports to be synchronized on ATC traffic displays to any desired behavior and update rate, e.g., synchronized to wall clock time or alternately to the azimuth scan pattern of the backup radar. As a minimum, ADS-B reports sent to the ATC processing system and backup radar position reports should be synchronized to common times for position comparison. However, there are various benefits that can be derived by decreasing the report update interval that is displayed to the ATC operators in both TMA and En Route ATC facilities.

As a general rule, position accuracy and display update rate are correlated to the extent that a given level of position accuracy is needed to support a specified display update rat e and visa versa. One means to express this relationship is analyzed in Appendix B-3 §3 (i.e., the analysis of the displayed accuracy at the controller‟s traffic display , including the position accuracy delivered to the ATC processing system by the surveillance sensor,1 42

© 2009, RTCA, Inc.

and the degradation in position accuracy as a function of display latency). The effect of latency on the airborne broadcast of ADS-B positions is also important, and is addressed in §B.4.5.1. This is an important factor to be considered in specifying enhanced surveillance performance (i.e., reductions in separation standards or in operational buffers commonly used today may require increased update rates as well as increased position accuracy). In any event, adjustments in display rate should be considered whenever changes in position accuracy are contemplated for future operations

. In order to quantify the benefit of a reduced report update interval, more in-depth analysis of the relationship between display update rate and measured position accuracy were examined to see how changes in display update rate affect separation performance. It should be noted that the recommendations based on this analysis are preliminary, and have not been validated by human factors or safety analysis. The results of this analysis are summarized in the following recommendations:

1. For TMA operations, the use of standard (~5 sec ond ) display update periods can result in a increase in position uncertainty on the controller screen compared to that delivered by the surveillance system (ADS-B or radar). For local operations that may require more precision, a reduced update interval of 3 seconds or less may be beneficial. For the operational concepts covered in the OSED, reduced update intervals are not required.

2. If the ADS-B reports are synchronized to the backup radar azimuth scan, then impl ementation of a two-times radar update rate (forward and backward scan pattern synchronization) is recommended as one means of achieving higher display update rates, e.g., a 2.5 second display interval given a 5 second radar

scan . For ADS-B targets this means extrapolating position reports twice per radar report. For radar targets, some means of extrapolating position reports by

one -half the radar update interval should be considered for continuity of surveillance if one or more aircraft are not being tracked by the ADS-B system

.

3. The 8 second update interval assumed for En Route operations is partly driven by legacy radar limitations such as dwell time needed for PSR radars at maximum line- of -sight ranges, and also by other operational factors. A reduction in the update interval (i.e., use of display update intervals on the order of 5 seconds) should be considered for traffic display of MSSR and ADS-B targets. The increased update rates may enable the application of tighter separation standards with possible increases in sector productivity and/or flight efficiency, more fully utilizing the enhanced position accuracy available with ADS-B and MSSR radar systems.

The analysis of increased ADS-B update rates is further detailed in Appendix B-3 §3.

B.4.1.4 Effects of ADS-B Asynchronous Updates

Closely separated aircraft are measured at similar times when they are detected by the same radar. ADS -B, in contrast, will generally provide measurements at different times and the latest measurements for two aircraft could vary in time by up to the maximum specified update interval. Therefore, in keeping with the time registration of position assumption, the position of one of the aircraft may be extrapolated to a common time.143

© 2009, RTCA, Inc. Successive position updates are necessary to estimate target velocity using radar data

. Each ADS-B surveillance report containing position and other state vector information is expected to include an accurate air-derived instantaneous velocity as well as a position estimate. Initial implementations of ADS-B may be unable to capitalize on the use of reported velocity, as a change in current ATC processing systems is required to process the velocity data. A key assumption of the OPA analysis, however, is that the ATC processing system will use the instantaneous velocity to time-register the asynchronously received ADS-B position updates from adjacent aircraft.

Reference [34] examines how uncompensated position reports (no time registration using velocity) can be accommodated in the comparative analysis, and what implications this has on acceptable update intervals. Generally, more frequent updates are required in the

non -time-registered case. In both the time-registered and non-time-registered cases, the possibility exists that an aircraft starts a maneuver after the last update, thus introducing an additional error in the displayed position

.

B.4.2 Position Accuracy

Accuracy is a critical attribute of surveillance position data. The accuracy conveys the uncertainty of the position data, which must be considered when separating aircraft

.

For radar , accuracy is a known quantity with expected performance under normal, fault-free conditions. The nominal accuracy and error distribution of the position data is usually predetermined and static

.

For ADS-B , accuracy is more of a variable, as it is dependent upon the characteristics of the position source from which it was obtained onboard the transmitting ai rcraft and how well that source is currently performing (e.g., GNSS satellites, the visible satellite constellation, etc.). Therefore, ADS-B includes an accuracy quality indicator.

B.4.2.1 Definitions

Accuracy is defined as the uncertainty in the position measurement. It characterizes the error distribution

. Horizontal Position Accuracy characterizes the horizontal position measurement error distribution. Position Accuracy comparisons are made at the 95% error bounds. The 95% errors provide a more stringent comparison than one sigma errors, as they account for noise component of the radar error (tail errors) and latency and random errors for ADS

- B.144

© 2009, RTCA, Inc.

For radar , the horizontal position accuracy is normally expressed in terms of range and azimuth. The overall errors are considered to have the following component

errors:
Systematic biases; Stochastic errors (usually expressed as a standard deviation ); Another important term for radar is core accuracy , which is a starting point for examining radar accuracy as compared to that of ADS-B data. Core accuracy ( ε 95) is defined as the 95% limit of the error distribution in a certain (single direction) dimension with respect to the radar

. In the crossrange direction, the error is calculated from the azimuth error multiplied by the reference range

. In the range direction, errors are assumed to be the same for any reference range (See Appendix B- 3)

. Systematic radar biases do not significantly affect the separation of targets seen by the same radar, as the aircraft pos itions will have the same biases. However, when separating two aircraft detected by different radar systems, any residual radar systematic biases will affect the apparent separation. The magnitudes of the residual radar biases will depend upon the inherent biases from the radars and the capability of the sensor registration processing in the ATC processing system to detect and compensate for the biases. Similarly, when separating one aircraft with ADS-B surveillance from another with radar surveillance, the apparent separation will be affected by residual biases (see Appendix B-3 §2)

. In ATC processing systems currently providing separation services across different radar systems, the residual biases that are present after registering radar systems are expected to be no smaller than the residual biases remaining when registering ADS-B surveillance and radar, thus the bias need not be considered in determining the comparative ADS-B accuracy. However, in ATC processing systems that do not currently provide for separation of two targets across radar systems, these residual biases may need to be considered in determining comparative ADS-B requirements.

For ADS-B , horizontal position accuracy is defined as the radius ( r a ) of a circle centered on the target ‟ s reported position such that the probability of the target‟s actual position being inside the circle is 95%. Note that this radial error is the resultant of errors in two dimensions (x and y). Assuming that the error distributions in each dimension are independent Gaussian distributions with the same standard deviation (σ), the resultant radial error distribution will be Rayleigh and the 95% point of this distribution will be 2.45σ . The required accuracy is derived from comparison with a single dimen sion radar 95% accuracy (ε95) . The required ADS- B 95% accur acy is compared against the radar ε95 accuracy without systematic biases (i.e., stochastic errors only). This ensures that the normal 95% accuracy of ADS-B is always as good as, or better than, radar accuracy at the chosen range of applicability and it also ensures that the ADS-B to ADS-B separation accuracy is always as good or better than the best-case radar to radar separation (i.e., both aircraft separated by the same radar).145

© 2009, RTCA, Inc. Knowing that the 95% point of a Gaussian distribution is 1.96σ, the required ADS- B 95% accuracy radius ( r a ) is given by:

r a = (2.45/1.96)*ε95 = 1.25*ε95 (B

- 1)

Note that any uncompensated latency (see §B.4.5.1) will impact the along-track accuracy. The uncompensated latency therefore must be limited so as to meet the specified accuracy requirements. The impacts of uncompensated latency on ADS- B accuracy are considered in §B.4.2.3.

Vertical Position Accuracy characterizes the vertical position measurement error distribution

.
For both radar and ADS-B , an encoder on the aircraft pr ovides the altitude, and the encoded measurement is transmitted to the radar or ADS-B ground station

. Therefore, vertical accuracy at ATC processing should be identical for both radar and ADS -B, depending only on :
altimeter accuracy;
transmission errors;

altitude resolution.

B.4.2.1.1 Timing of Position Quality Indicators

NIC and NAC P quality indicators convey information on reported ADS-B position integrity and accuracy, respectively. Ideally, NIC and NAC P should be sent in every ADS -B message that contains position information. However, some ADS-B link formats do not send NAC P in the message containing position, but instead send NAC P in a separate message, and at a different rate. Reports assembled by the ground system may therefore contain the latest position information with a NAC P value received some time earlier. Consequently, if the NAC P value changes from a good value (i.e., NAC P ≥ minimum) to a poor value (i.e., NAC P < minimum), the ATC system will not immediately be aware of the degradation in position accuracy and will use a previous NAC P , that does not accurately reflect the quality of the reported accuracy, for some time until a change in NAC P value is received by the ground system.

Requirements must therefore be levied to limit the age of the NAC P information, considering the following factors:

1. The likelihood of a NAC P change from a good to a poor value;

2. A comparison with radar position error probabilities;

3. The availability of NIC to limit the possible NAC P change.

First, the likelihood of a NAC P change from a good to poor value will be very low because of service continuity requirements (i.e., both NIC and NAC P have to be at or above their minimum values with high probability of > 0.999 (see Table 10)). Although in practice, the minimum NIC requirement may be the more dominant factor for continuity, a worst case probability of a NAC P change from good to poor of 0.001 per hour is assumed

.146

© 2009, RTCA, Inc.

Second, a simple comparison with radar is made to determine the likelihood of a sequence of radar plots appearing to have degraded accuracy, solely from random plot error probabilities. Assuming random Gaussian plot errors with standard deviation σ, the probability of an error greater than a certain magnitude (m) can be calculated

. Furthermore, the probability of three consecutive scans of radar plots with magnitude greater than (m) occurring anywhere in an hour of flight can be calculated

. The random probability of three consecutive plots each having an error > m is P m 3 , where P m is the probability of an error > m, and can be calculated from the Gaussian distribution. Assume that there are about 720 radar plots per flight hour (5 second scan)

. A sequence of three plots with errors > m can occur anywhere in the sequence of plots. Hence, if P m

is small, the probability of getting thee plots anywhere in an hour is approximately (P m 3 ) * 720.

The comparison is made with a probability of 0.001 per hour of undetected ADS- B accuracy degradation [(P m 3 ) * 720 = 0.001, resulting in P m =0.0111572]. Given the Gaussian Cumulative Distribution function, a P m of 0.0111572 occurs at 2.54σ . This result is not very sensitive to the number of plots per hour or the scan rate, for example, given 360 plots and a 10 second scan rate, P m =2.47 σ

. For RAIM-based GPS systems, a conservative means of estimating HFOM 95% accuracy is obtained from the GPS RAIM model used for SSE analysis in ANNEX D. This model uses the formula σ = HPL/ 7.5 to estimate the relationship between σ and HPL . The 95% radial accuracy is then bounded by

HFOM = 2.45* σ ≤ 2.45*HPL / 7.5 ~ 0.3 3*HPL

The above derived formula is a conservative overbound of HFOM for HPL ≥ 0.2 NM
, which has been validated by the results reported in [1] . Specifically, for both 2 degree and 5 degree masking angles, and for both Selective Availability ( SA )-Aware and SAUnaware (SA assumed On) RAIM-based GPS receivers, 0.33*HPL bounds the locus of all simulated HFOM values for HPL ≥ 0.2 NM (NIC ≤ 7) , based on long-term simulations of 20 to 24 GPS active satellite constellation geometries (0 to 4 satellite outages) [42]

. This relationship can be used to derive a conservative NACp limit from a received NIC, assuming that the ADS-B data source is a RAIM-based GPS system

. Finally, the integrity containment radius (NIC) received with each position will limit the possible magnitude of an accuracy (NAC P ) change. Therefore, if NAC P is unknown or old, the minimum possible NAC P can be determined from NIC as above, or alternately using RTCA DO-303, or EUROCAE ED -126, Table 64 . The scope for an undetected NAC P change from good to poor without a change of NIC will depend upon the minimum NIC and NAC P values for the application (e.g., HPL ≤ 0.5 NM as adequate substitute for the 308 meter accuracy requirement for En Route, and NIC=7 as substitution for the NAC P =8 requirement for TMA).

B.4.2.2 Requirements

Accuracy requirements were derived from the reference radar model described in Appendix B-3. The core position accuracy 95% limits (ε95) were determined at the reference range of the radar.147

© 2009, RTCA, Inc. The analysis is based on the performance of a single MSSR reference radar, per Appendix B-3 §2. Implementers using a different single surveillance source should do a corresponding analysis with the appropriate radar reference model.

Table B- 4 shows expected reference radar performance and the comparable ADS- B performance required.

The following requirements result from the timing of position quality indicator analysis:

PR 7: The position integrity quality indicator (NIC) shall be broadcast in every ADS -B position message.

PR 8: For TMA, if the position accuracy quality indicator (NACp) is not received within 15 seconds of

a position message, then the ADS-B ground domain shall determine the position accuracy requirement has been met using a NIC encoding that corresponds to 513 meters (or less) as a substitute for the NACp requirement

. Note: GNSS avionic systems reporting position integrity can be assumed to have a corresponding 95% position accuracy measure no greater than 33% of the reported position integrity. The reported integrity may be used to meet position accuracy requirements normally satisfied through NAC P when this quality measure is not reported. For TMA, a reported integrity less than or equal to 513 meters assures the 171 meter accuracy requirement is met. For En Route, a reported integrity less than or equal to 926 meters assures the 308 meter accuracy requirement is met.148

© 2009, RTCA, Inc. Table B-4: Accuracy Performance Comparison

Accuracy Value (En R oute) Value (TMA)
Horizontal Position Accuracy
Reference radar system : 95% core azimuth accuracy MSSR reference range Horizontal position accuracy (ε95):
95% cross

- range accuracy 95% along

- range accuracy Approach Scenarios MSSR reference range Horizontal position accuracy (ε95):
95% cross

- range accu racy 95% along

- range accuracy Note : Above values are single dimension accuracies 0.127 degrees
60 NM

246 meters 137 meters 0.127 degrees
33 NM

137 meters 137 meters Approach Scenarios
15 NM

62 meters 137 meters

Accuracy Value (En R oute) Value (TMA)
Horizontal Position Accuracy

ADS

- B system : Radial accuracy ( r a )
Note : The rad ial accuracy (r a ) is scaled from the radar single dimension (ε95) (see §B.4.2.1). Approach Scenarios Radial accuracy ( r a ) <308 meters (based on radar cross

range ) (NAC p of 7 is 18 5
meters ) <171 meters (based on radar

cross

- range ) (NAC p of 8 is 9 3 meters )

Approach Sc enarios
< 171 meters (based on radar along

- range ) (NAC P of 8 is 9 3 meters )149

© 2009, RTCA, Inc. Accuracy Value (En R oute) Value (TMA)
Vertical Position Accuracy
Reference radar system : Altimeter accuracy
Not e: These are minimum accuracy requirements for altimeters and are dependent on the type of airspace

. Man y airspace regions, such as RVSM, will require better altimeter performance than specified

. Per Annex 10, Vol

. IV (§4.3.9.3.1) it is recommended to use a source providing a resolution less than or equal to 7.62 meters (25 ft. ).
Resolution in Mode C Avi onics standard

100 ft. Avionics standard

100 ft.

ADS

- B system Same as above Same as above

PR 9: For En Route aircraft, if the position accuracy quality indicator (NAC P ) is not received within 24 seconds of a position message, then the ADS-B ground domain shall satisfy the position accuracy requirement using a NIC encoding that corresponds to 926 meters (or less) as a substitute for the NAC P requirement

. The entire allocation for accuracy is made to the Transmit Airborne Domain. This means that the associated accuracy for data obtained from the position source (e.g., GNSS, FMS, etc.), combined with any uncertainty or deterioration of precision incurred prior to transmission due to randomly varying uncompensated latency or other factors must meet the overall ADS-B accuracy requirements. This also means that the Ground Domain must not introduce any further deterioration of accuracy into the received data through loss of resolution or randomly varying uncompensated latency

. Vertical position is obtained in much the same way in radar and ADS-B environments

. Therefore the vertical position accuracy requirements are specified with respect to current Mode C performance.

The following are the requirements for ADS-B accuracy as allocated between the Transmit Aircraft and Ground Domain:

PR 10: For 5 NM separation, the 95% accuracy of the horizontal position measured at interface D shall be less than 308 meters (NAC p =7 ).

PR 11: For 3 NM separation, the 95% accuracy of the horizontal position measured at interface D shall be less than 171 meters (NAC p =8).

Note: The use of a NAC P =8 ensures global applicability, as NAC P =8 has been determined to indicate equivalent or better performance in comparison to radar based upon the comparative RFG analysis using either the US or150

© 2009, RTCA, Inc. European radar models, taking into account the set of ADS-B-RAD ground domain functions applicable to the assessment. Local implementations may permit a lower NAC P value, such as 7, based on additional ADS-B-RAD ground domain functions as a means of achieving the end- to -end performance necessary for the application. Moreover, an implementing authority may also permit a NAC P =7 encoding in line with the arguments produced in Appendix B-7

. PR 12: For 2.5 NM separation in-trail on approach, the 95% accuracy of the horizontal position measured at interface D shall be less than 171 meters (NAC P =8).

PR 13: For 2 NM dependent parallel approach separation, the 95% accuracy of the horizontal position measured at interface D shall be less than 171 meters (NAC P =8).

PR 14: For independent parallel approach separation, the 95% accuracy of the horizontal position measured at interface D shall be less than 121 meters (NAC P =8) (see Annex D)

.

Note: Although the analysis above establishes a minimum requirement of 171 meters, the analysis in Annex D.6 shows that a tighter requirement of 121 meters is required to achieve a one sigma value of 49.4 meters for adequate fault detection capability.

PR 15: The ADS-B Ground Domain shall not introduce any additional horizontal position error greater than that which might otherwise be introduced by a linear extrapolation using the instantaneous velocity for the target.

Note: The purpose of this requirement is for the Ground Domain to preserve the accuracy of the position information provided by the aircraft, introducing only an error associated with the extrapolation of the position information, as time-registration of the position information has been assumed

. PR 16: Altimeter accuracy - including accuracy of measurement and accuracy of reported value through use of encoding - shall be at least as good as Mode C provisions in ICAO Annex 10, which specifies 38.1 meters (125 feet)

. PR 17: The Ground Domain (including data link) shall not degrade altitude resolution to worse than 100 feet.

PR 18: The resolution of the Pressure altitude shall be 100 feet.151

© 2009, RTCA, Inc. The ADS-B performance requirements for Position Accuracy are summarized in Table B- 5.

Table B-5: Summary of ADS-B Position Accuracy Requirements

Accura cy Requirement Value (En R oute) Value

(TMA) GROUND

(“D”  “E2”) AIRBORNE (at “B1”)

Timing of Position Quality Indicators

NIC broadcast Broadcast NIC in every ADS

- B message containing position information.

NAC P reporting Determine a minimum NAC P
va lue from the NIC value of the position when NAC P is not received within Tnac seconds of a position message
Horizontal Position Accuracy

95% accuracy (ra) < 308
meters (MSSR at 60 NM) < 171

meters (TMA)

< 171
meters (Approach Scenarios) No allocation f or loss of accuracy NAC P = 7 (En Route)

NAC P = 8 (TMA)

NAC P = 8 (Approach Scenarios)

Vertical Position Accuracy
Altimeter accuracy 38.1 m eters (125 ft.)

Mode C Resolution 100 ft. 100 ft

. 100 ft

. 100 ft.

Figure B- 3 summarizes the implications of the above results. For En Route 5 NM separation, the selected reference range is 60 NM. The 95% radar position error is elliptical at this point, because the cross-range errors are larger than the range errors. The larger radar cross-range error is the basis for the ADS-B performance comparison. The NAC P =7 results in an ADS-B error that is less than the radar cross-range error at that point.152

© 2009, RTCA, Inc.

For TMA 3 NM separation, the reference range is selected per Case 1 above as thedistance from the radar at which the radar range errors approximately equal the crossrange errors. For the radar error model being used, this range is approximately 33 NM. The ADS-B performance requirement is set such that the 95% ADS-B position error is approximately equal to that of radar at the reference range, smaller at greater ranges and larger at lesser ranges (see Table B-5). The resulting requirement for TMA 3 NM separation is a NAC P =8

. A NAC P =7 is shown for comparison

. For approach separation, the constant radar range error is used to establish the ADS- B accuracy requirement, as it is the major axis of the radar error. The resulting requirement for the approach scenarios is a NAC P =8

. An NAC P =7 is shown for comparison

.

B.4.2.3 Impact of Uncompensated Latency on ADS-B Accuracy

The requirement in §B.4.5.2 allows for 0.6 seconds of uncompensated latency (95%) and this is supported by measurement data. In particular, the measurement data indicates a mean bias of 0.3 second and a standard deviation of less than 0.2 second across a population of currently fitted aircraft. The mean bias is assumed to be compensated by the ground, while the standard deviation is the resultant of both the individual aircraft (message to message) variability and aircraft to aircraft variability of the mean value

.

Note: The present link MOPS (RTCA DO-260A, RTCA DO -282A, and EUROCAE ED

108A) do not currently address reporting characteristics of uncompensated latency from the aircraft. For aircraft that perform better than the assumed 0.6 seconds uncompensated latency distribution, the link MOPS should allow for the reporting of this information

. Assuming an uncompensated latency standard deviation of 0.2 seconds and combining this in a random manner with the measurement source NAC P position uncertainty (i.e., the resultant sum of squares of the standard deviations of each are computed), the resulting along-track 95% position accuracies for NAC P =7 and 8 against aircraft speed are shown in Figure B-4. It can be seen that the degradation in accuracy fro m uncompensated latency uncertainty, for any likely aircraft speeds, does not cause the NAC P =7 curve to exceed the en-route radar reference accuracy, or the NAC P =8 curve to exceed the TMA radar reference accuracy (which is the same for in-trail and dependent parallel cases). For the independent parallel case, uncompensated latency has no significant effect on the more demanding across track accuracy requirement

.153

© 2009, RTCA, Inc. Figure B-3: Radar and ADS-B Error Comparison154

© 2009, RTCA, Inc.

Figure B-4: Changes in accuracy for uncompensated latency uncertainty of 0.2 seconds

B.4.3 Data Integrity and Data Preservation

Integrity concerns the level of trust the end user can have for information. Surveillance data integrity is determined by two primary factors :

1. The data source and confidence that the measurement (for position data) or pronouncement (for identity, emergency and SPI and other discrete parameters) is correct, and

2. The confidence that the data, as reported by the source, was not corrupted in any way by the system.

For the purposes of this document, the integrity of the position measurement, or pronouncement of other source information, is referred to as data integrity . The confidence that the data is correctly conveyed from interface to interface is referred to as data preservation . Data integrity has an associated tolerance (e.g., containment radius) for which integrity is expressed as the probability that the reported value is accurate within that tolerance. Data p reservation relates to the system‟s ability to deliver the measured data without modification.

This OPA examines the integrity of position data as well as discrete parameters such as identity, SPI, and emergency indication information. Data integrity and data preservation are addressed in §B.4.3.1 and §B.4.3.2, respectively.155

© 2009, RTCA, Inc. B.4.3.1 Data Integrity

B.4.3.1.1 Definitions

Horizontal Position Data Integrity is defined as the probability that the surveillance horizontal position error is outside a given distance (containment radius) for a given duration without alert

.

For radar , data integrity is not quantified in this way. It is assumed that combinations of various internal self-tests and external monitoring methods will ensure that the radar equipment is working properly (or otherwise a fault is detected quickly). When the radar is declared to be in a fault-free operation, there is no error detection (or integrity values) in the radar plot measurements provided to the input of the ATC system. Hence, in fault-free operation, radar plot position errors of any magnitude are all passed as undetected errors and the likelihood of an error being beyond a certain magnitude is entirely determined from the radar error distribution function. The nominal radar accuracy error distribution (see Appendix B-4 ) can be compared to undetected position error distribution from ADS-B (see Annex D).

For ADS-B , the above definition can be used directly. ADS -B is expected to use integrity monitoring of the position sources, so that the reported aircraft horizontal position will be characterized by a level of integrity (i.e., the ADS-B quality indicators for individual aircraft ).

Vertical Position Data Integrity is defined as the probability of the surveillance vertical position being incorrect.

For both radar and ADS-B , vertical position data integrity is assumed to be the same, as the same altimeter standards apply (see OPA-ASSUMP.17)

. Identity, Emergency and SPI Data Integrity is defined as the probability of the aircraft identification, Mode A Code

, 24 -bit Aircraft address, SPI, or emergency indicator information being incorrect

.

For both radar and ADS-B , data integrity is assumed to be the same, as the same information should be made available to both through the same or similar sources (see OPA-ASSUMP.17)

.

B.4.3.1.2 Requirements

Horizontal position data integrity requirements are determined using SSE (See Annex D)

.Other data integrity requirements are determined using the comparative approach. Timeto

- al ert considerations are documented in Appendix B-4. Table B- 6 summarizes the Data Integrity requirements and assumptions.156

© 2009, RTCA, Inc. Table B-6: Data Integrity Comparisons

Data Integrity Value (En R oute) Value

(TMA)
Integrity of Horizontal Position

Reference rad ar system : S urveillance position errors being outside certain limits are indicated from tails of nominal radar accuracy (see Appendix B

- 3 )

. Note : The Eurocontrol Surveillance Standard simply specifies such tail errors as “jumps” of more than 1 degree az imuth or 700 m eter range , and the probability of a jump shall be less than 0.05%

. ADS

- B System : Integrity of position information (see Annex D)

Approach Scenarios Integrity of position information (see Annex D) Time between change of position integrity and reflection of change at interface B1 (see Appendix B

- 4) NIC= 5

< 2 s NIC= 6

Approach Scenarios
NIC= 7

< 2 s

Integrity of Vertical Position

Reference radar : Altimeter source integrity Avionic standard Avionic standard

ADS

- B system : Altimeter source integrity Avionic standard (OPA

ASSUMP.17) Avionic standard

(OPA

ASSUMP.17)

Integrity of Identity, Emergency and SPI

Reference radar : Mode A source integrity Avionic standard Avionic standard

ADS

- B system : Identity, Emergency and SPI Int egrity Assumed to be at least as good as SSR Mode A (OPA

ASSUMP.17) Assumed to be at

least as good as SSR Mode A (OPA

ASSUMP.17)

The minimum operational performance requirements derived from Table B-6 for data integrity are as follows :
PR 19: The time between a change in satellite reception status in the GNSS receiver and the delivery of the resulting integrity quality indicator change to B1 shall be no more than 2 seconds

.157

© 2009, RTCA, Inc. Note: This applies to a nominal ( no -fault) change of the integrity quality indicator value between A1 and B1.

PR 20: The probability that a horizontal position error exceeds the integrity containment bound for more than 10 seconds without reflecting the fault at B1 shall be no greater than 0.001

.

From Annex D :

PR 21 : For 5 NM separation, the value of the ADS-B NIC parameter shall be greater than or equal to 5 (ER 1).

PR 22 : For 3 NM separation, the value of the ADS-B NIC parameter shall be greater than or equal to 6 (ER 2).

PR 23 : For 2.5 NM in-trail separation on approach, the value of the ADS-B NIC parameter shall be greater than or equal to 7 (ER 3).

PR 24 : For 2 NM dependent parallel approach separation, the value of the ADS- B NIC parameter shall be greater than or equal to 7 (ER 4)

.

PR 25 : For independent parallel approach separation, the value of the ADS-B NIC parameter shall be greater than or equal to 7 (ER 5).

Note: For the TMA and approach environments, local implementers may pursue a reduction in the required NIC value if radar integrity monitoring is implemented on approach (see Annex H).

These performance requirements are summarized in Table B-7 Table B

.

B.4.3.2 Data Preservation

Data preservation is measured from the time of measurement (A1) to the input of the ATC processing function (E2). To measure the data preservation the following factors need to be considered:

a. The onboard avionics (excluding the data sources);

b. The data link to the ground sensor system;

c. The processing in the ground sensor (such as radar signal processing);

d. The data link from the ground sensor to the ATC processing system

. The contributions from (a), (b), and (c) will differ between radar and ADS-B, while the contribution from (d) is assumed to be the same for both

.158

© 2009, RTCA, Inc. Table B-7: Summary of Data Integrity Requirements

Data Int egrity Requirement Value (En R oute) Value

(TMA) GROUND

(“D”  “E2”) AIRBORNE

(at “D”)

Horizontal Position

Position Data Integrity (see Annex D) N/A NIC= 5 (1852 meters ) (En Route)

NIC= 6 (1111.2 meters ) ( TMA)

NIC= 7 (370.4 meters ) (Approach Scenario s)
Integrity Change <2 s <2 s N/A <2 s

Vertical Position

Altimeter Source Integrity Avionic standard Avionic

standard N/A N o worse than for SSR

case Identity and Emergency and SPI

Identity, Emergency and SPI Integrity At least as good as SSR Mode A [T GL 13] At least as

good as SSR Mode A [TGL 13] N/A N o worse than for SSR

case

B.4.3.2.1 Definitions

Vertical Position Data Preservation relates to the probability that an error is introduced into the vertical position data.

For radar , the vertical position data preservation can be described by the probability of a validated Mode C error in a SSR plot report.

For ADS-B , the vertical position data preservation the comparison can be described by the probability of the vertical position being transmitted or decoded incorrectly, or otherwise corrupted.

Identity and Emergency and SPI Data Preservation relates to the probability that an error is introduced into the identity, SPI, and emergency indicator information

.

For radar , the identity and emergency and SPI data preservation can be described by the probability of a validated Mode A Code error in a SSR plot report.159

© 2009, RTCA, Inc. For ADS-B

, the comparison can be described by the probability of the aircraft identification, Mode A Code

, ICAO 24 -bit Aircraft address, SPI, or emergency indicator information being transmitted or decoded incorrectly, or otherwise corrupted.

B.4.3.2.2 Requirements

The data preservation requirements are analyzed using the comparative approach. Table B-8 Table B provides expected performance for the reference radar, which is used to derive the comparable ADS-B requirements shown.

Table B-8: Data Preservation Comparisons

Data Preservation Value (En R oute) Value

(TMA)
Identity, Emergency and SPI

Reference radar : Probability of incorrect Mode A val idation Probability of incorrect emergency and SPI code 8 < 0.001
as for Mode A < 0.001

as for Mode A

ADS

- B system : Probability of introducing an error in aircraft identification, Mode A Code

, 24

- bit Aircraft address, SPI, or emergency indicators, base d on radar Mode A validation comparison
Note : For ADS

- B, the requirement probabilities are on a per report basis

. < 0.001 < 0.001

Vertical Position

Reference radar : Probability of incorrect Mode C validation (when Mode C is provided) < 0.001 < 0. 001

ADS

- B system : Probability of incorrect height information < 0.001 < 0.001

The following are the resulting minimum operational requirements for ADS-B Data Preservation :

Note: Refer to § 3.1 for a depiction of the interfaces and points of measurement included in the requirements below.

PR 26 : The probability that the ground domain (between D and E2) introduces an error in the aircraft identification, Mode A Code

, ICAO 24 -bit Aircraft address, SPI, or emergency indicator information shall be less than 0.001 per ADS -B surveillance report.

8 For aircraft capable of Emergency and SPI reporting160

© 2009, RTCA, Inc.

PR 27 : The probability that the ground domain (between D and E2) introduces an error in the aircraft height information shall be less than 0.001 per ADS- B surveillance report.

The ADS-B data preservation requirements are summarized in Table B-9 Table B

.

Table B-9: Summary of Data Preservation Requirements

Data Preservation Requirement Value (En R oute) Value

(TMA) GROUND

(“D”  “E2”) AIRBORNE

(“A1”  “D”)
Identity, Em ergency and SPI
Probability of introducing an error in aircraft identification, Mode A Code

, 24

- bit Aircraft address, SPI, or emergency indicator information (different than source) < 0.001 < 0.001 < 0.001 per report Assumed to be no

worse than current S SR (OPA

ASSUMP.17)

Vertical Position
Probability of introducing an error in Height Information (different than altimeter source) < 0.001 < 0.001 < 0.001 per report Assumed to be no

worse than current SSR (OPA

ASSUMP.17)

B.4.4 Reliability

The reliability of surveillance data relates to the probability that surveillance data will be provided for all aircraft in the service volume at any given time and is considered in terms of availability and continuity (see definitions in §1.3). For the context of this comparative assessment, reliability of surveillance data must be considered for both the case of a single aircraft (e.g., the SSR transponder or ADS-B transmitter provides the necessary data), and the case of an area-wide loss (e.g., a radar fails in a single SSR environment or a GNSS outage prevents ADS-B from providing position information).

B.4.4.1 Availability

Overall availability is composed of:

a) the availability of functions affecting all aircraft (e.g., external positioning function, ground data acquisition function); and b) the availability of functions affecting only one aircraft (e.g., transponder function) : expressed per flight hour.

For radar , the availability of ground radar and data transmission equipment will affect the service for all aircraft. The availability of an individual aircraft SSR transponder function will affect the service for that one aircraft.161

© 2009, RTCA, Inc. For ADS-B , the availability of positioning sources (including satellite configurations) of sufficient quality in the region will affect many aircraft and must be considered in addition to the availability of ground receiving and data transmission systems.

Note: While defined here for completeness, the OPA does not specifiy availability requirements for the ADS-B-RAD application. See §B.4.4.2 for more details. However, the OSA establishes assumptions on the inherent SSR and PSR availability.

B.4.4.2 Continuity

Within the scope of the OPA, continuity performance of radar and ADS-B is compared and specified in terms of:

a) the surveillance provision continuity affecting all (or at least multiple) aircraft, provided by the entire surveillance source chain up to the input to the ATC Processing System (E2 in Figure 5) expressed as a number of disruptions per year or probability per hour; and b) the system continuity of functions affecting a single aircraft expressed per flight hour.

For radar , the system continuity of ground radar and data transmission equipment will affect the service for all aircraft. The system continuity of an individual aircraft SSR transponder function will affect the service for that one aircraft.

For ADS-B

, in addition to the system continuity of the ground receiving and data transmission equipment, the data continuity of position information with sufficient quality in the region will affect the continuity of service for many aircraft. For a single aircraft, the system continuity of the aircraft ADS-B avionics function is specified while the data continuity of position with sufficient quality affecting only a single aircraft (e.g., temporary loss of satellite signal due to aircraft banking) was considered in the position update requirements.

B.4.4.3 Requirements

To the extent that reliability requirements are specified, they are based upon the comparative approach

.

Ground Domain

Reliability (i.e., Availability and Continuity) of ground systems will depend upon various equipment and environment factors such as the amount of equipment redundancy, local site conditions, accessibility, etc. Reliability of ground systems can be improved by increased expenditure on equipment and maintenance infrastructure, thus there are cost effectiveness considerations to be made by the local ATS administrations. In particular, the consequences of loss of service with respect to the traffic levels in the region will be important considerations. Hence, the required reliability of ground systems is really a local issue

.162

© 2009, RTCA, Inc.

Airborne Domain

Two main aspects need to be considered in the airborne domain, namely:

a) Failure of the actual avionics. The reliability of the onboard avionics of the transmitting aircraft (i.e., the “ ADS -B- OUT ” system) is expected to be similar to present SSR transponders and the sources of Mode A/C on the same type of aircraft

. Avionics failures are independent failure events between aircraft.

b) Fa ilure of the GNSS positional source to deliver adequate quality data, both from a single aircraft perspective (non-correlated between aircraft) and multiple aircraft perspective (correlated between aircraft).

With ADS-B, the reliability of the external position sources, in particular the GNSS satellite sources, is also to be considered. The reliability not only concerns complete failure (however unlikely) but also the occasions where the quality of the information may fall below that required for a certain separation (e.g., when numbers of satellites and their geometry is unfavorable). These conditions may affect all or a substantial proportion of aircraft in the locality and the likelihood of occurrence has to be considered with respect to the local ATC environment. In addition, unfavorable satellite geometry situations might occur on a single aircraft basis as a result of e.g., aircraft maneuvers (e.g.

, banking).

It is noted that only a sustained loss of position data is considered to lead to a reliability failure. The sustained loss must thereby be long enough for the surveillance track to be dropped from the controller‟s display (refer also to Section C.3.4).

It is further noted that susceptibility of the GNSS on-board receive function to unfavorable satellite geometry is a function of the performance specification of the airborne avionics.

It may be possible to predict when satellite conditions will be unfavorable and this might be used as mitigation against the loss of availability (i.e., the outages become planned)

. Within the context of the ADS-B-RAD application it has been decided to consider comparative continuity requirements for the ground system

. This is in line with the scope of the OSED which assumes that aircraft have already entered radar airspace. This means that the operation has already commenced, and therefore was “available” at the start of the operation

.

Table B- 10 Table B provides expected reliability performance from the reference radar system and compares that to required comparable ADS-B performance

.163

© 2009, RTCA, Inc. Table B- 10 : Reliability Performance Comparisons

Reliability Value (En R oute) Value

(TMA)

Surveillance Pr ovision Continuity for multiple (or all) aircraft
Reference radar system : Continuity probability p er hour (failures per year)
Note : This is a single radar layer continuity , based on the Eurocontrol Surveillance Standard

, which specifies an availability of 0.999 (i.e.

, overall 10 hours of outages per year) and the maximum duration of a single outage of 4 hours.

. >0.999 66 per hour ( ≤ 3 per year) >0.999 66 per

hour ( ≤ 3 per year)

ADS

- B system (including ground receive function + GNSS Signal

- in

- Space ): Multiple aircraft surveillance provision c ontinuity probability per hour (failures per year)
Note: Because this performance compariso n spans functional segments that are outside the scope of ADS

- B

- RAD ( e.g., GNSS Signal

- in

- Space), the OSA further details the requirements and environmental assumptions, as appropriate, for the various ADS

- B service continuity components, i.e.

, system con tinuity and data continuity. This includes respective allocations to the Ground Domain and external functional segments. >0.999 66 per hour ( ≤ 3 per year) >0.999 66 per

hour ( ≤ 3 per year)

System c ontinuity for single aircraft
Reference transponder : System c ontinuity of SSR Mode A/C transponder system (typical values per flight hour)
Not e s : 1. These values a re based on typical values of 5 000 hours between failures for single

- thread transponders.
2. The specification of continuity requirements for General Aviation is a local issue. 0.9998 0.9998

ADS

- B system : System c ontinuity of single aircraft overall ADS

- B function (between A 1 and D) Note: This does not include satellite coverage and constellation, i.e. data continuity aspects. Data continuity for a single aircraft is encompassed by the OPA requirements for position update. 0.9998 0.9998164

© 2009, RTCA, Inc.

Based on Table B- 10 , and the above discussion, the following operational performance requirements are derived:

PR 28 : The System Continuity of the ADS -B Aircraft Domain shall be 0.9998 per flight hour.

PR 29 : Reserved

.

B.4.5 Other Surveillance Parameters

This section concludes the comparative assessment between radar and ADS-B as surveillance data sources by examining a final few attributes of surveillance systems. While most of the comparative assessment used for this OPA examines the content, quality, and other attributes of the actual surveillance data, this section examines performance capabilities of the overall surveillance system.

B.4.5.1 Definitions

Latency is defined in § 1.3 as the amount of time it takes to deliver data from one interface point to another interface point. Latency requirements may be quantified in terms of the maximum age of the information contained in a report being no longer than a certain time since measurement or extrapolation. Latency is particularly significant to position and pressure altitude information. It is also of significance for the emergency and SPI indicators. Note that any delays in the processing and display of the surveillance information to the controller that may occur within the ATC system are outside the scope of this OPA.

Fo r radar , the latency is the time between the measurement of aircraft position (including pressure altitude) when it is in the radar beam and the output of the report by the radar system. The latency is produced by delays in the radar processing at the radar site and the communication of the data to the ATC processing system. The onboard latency of the pressure altitude input into the airborne transponder is assumed to be insignificant.

Emergency and SPI indicators are included with the Mode A reply to the radar. It is assumed that the onboard latency between the setting of this information and its input to the transponder is insignificant.

For ADS-B , it is recognized that the airborne position may be determined from the positioning sensors (e.g., GPS) at some time prior to the ADS-B output. Any onboard latency that is not known by the airborne ADS-B equipment (and hence by the ground receiving system) will have the effect of degrading the along-track accuracy of the ADS-B position (depending upon the speed of the aircraft. See Appendix B-1 ). The time of applicability is provided in ADS-B reports so that the known latencies can be compensated for on the ground.

For SPI and emergency indicators, the onboard latency is expected to be better than the radar case because of the higher update rate.165

© 2009, RTCA, Inc. Unlike radar, ADS -B may sample and broadcast aircraft identification and Mode A Code separately from the SPI and emergency indicators. The onboard sampling of aircraft identification, Mode A Code

, SPI, and emergency indicators must ensure that a change of these parameters is recognized within the corresponding required update interval (see §B.4.1).

Delays in the ground communications of ADS-B reports from the ADS-B ground system to the input of the ATC processing system are expected to be similar to the radar case.

Time Stamp Accuracy is defined as the accuracy to which the time of applicability is determined from information in the message and by timing in the ground detection system. 9

For radar , this will depend upon the time-stamping accuracy of the received radar returns and the method of time synchronization with the user system.

For ADS-B , any onboard timing inaccuracies will have the effect of degrading the along-track position accuracy (depending upon the speed of the aircraft)

. The time-stamping of ADS-B messages at the reception by the ground system and the time information sent in ADS- B surveillance reports will have similar time stamp accuracy requirements to radar.

Capacity relates to the maximum number of aircraft in the system for which all the service surveillance performance parameters have to be provided. Capacity will depend upon the particular environment characteristics (e.g., traffic densities, area of coverage required)

. Two key terms associated with capacity are:

Total load : Maximum number of aircraft in coverage; and

Density : Maximum number of targets within a confined area or azimuth sector.

For radar

, the capacity of an individual radar is usually designed for high-density environments. The number of aircraft in an azimuth sector is important for radar processing in high-density environments.

For ADS-B , the capacity figures do not necessarily imply the processing capability of a single sensor, since the required capacity may be achieved by using a number of ground sensors distributed geographically. The numbers of sensors and their receiving range will depend upon the particular environment.

9 Time-stamping may not be required for some systems that simply display the ADS-B information as it is received. However, time-stamping accuracy is important for more sophisticated and multi-sensor tracking systems where a means of determining the time of applicability is required.166

© 2009, RTCA, Inc.

Coverage is defined as the geographical volume in which all the service surveillance attributes can be provided.

For both radar and ADS-B , coverage will depend upon the detection range and locations of ground sensors. In practice, the required coverage will be determined by the local operational environment and will require confirmation by flight trials.

B.4.5.2 Requirements

The comparative approach was used in the analysis of the other surveillance parameters

.

Table B- 12 Table B examines latency, time-stamp accuracy, and capacity for radar and th e comparable required ADS-B performance.

The performance requirements are as follows:

PR 30 : The ground domain shall provide in each ADS-B surveillance report a time of applicability (Interface E2) of the position information.

Note: The time of applicability must be provided in ADS-B reports so that the known latencies can be compensated for on the ground

.

PR 31 : If the time of applicability within each ADS-B surveillance report is not applicable for all state vector data items of that report (Interface E 2) , separate times of applicability shall be provided for the specific state vector data items that differ.167

© 2009, RTCA, Inc. Table B- 12 : Other Surveillance Parameters Performance Comparisons

Surveillance Performance Parameter (at the surveillance report level) Value (E n R oute) Value

(TMA)
Latency
Reference radar system : Maximum latency between the moment a radar target for a given aircraft is detected and the moment when the corresponding report starts to be transmitted (applies to position, pressure altitude, Mode A Code

, emergency and SPI in radar reports).
Note : The 2 second value is per the Eurocontrol Standard Document for Radar Surveillance in En R oute Airspace and Major Terminal Areas

. 2 s 2 s

ADS

- B system : Maximum total latency from position measurement at B1 or generation of aircraft identification, Mode A and emergency/SPI data at interface B1 to interface E2

. Note : A ny latency uncertainties onboard the aircraft have the effects of a reduction in position accuracy in the along

- track direction (See al so Appendix B

- 1 )

. 2 s 2 s

Time Stamp
Reference radar system : Maximum time stamp inaccuracy of radar reports, as determined by the ground system.
Note : The 0.1 second value is per the Eurocontrol Standard Document for Radar Surveillance in En R oute Airspace and Major Terminal Areas. 0.1 s 0.1 s

ADS

- B system : Maximum time stamp inaccuracy of ADS

- B surveillance reports as determined by the ground system

. Note: A ny time uncertainties onboard the aircraft have the effects of a reduction in position accuracy

. 0.1 s 0.1 s

Capacity
Annex A §A.1 includes typical maximum figures for ADS

- B

- RAD environment

. These (generic) figures will be used as conditions under which the performance requirements must b e met

. In addition

, local capacity figures will have to be taken into account when implementing the system.
Reference radar system See A.1 See A.1

ADS

- B system See A.1 See A.1

Note: If the time of applicability varies for a particular state vector data item, a separate time of applicability must be provided for that data. For example, if the TOA for position differs from that of velocity.168

© 2009, RTCA, Inc.

PR 32 : The time of applicability conveyed in the ADS-B surveillance report shall have an absolute accuracy relative to UTC of ±0.1 seconds or less.

Note: This is the time stamp accuracy from Table B- 13

.

PR 33 : The 95% latency for ADS-B surveillance reports (measured between points D and E2

–

output of the ADS-B ground system) shall be no greater than 0.5 seconds

, excluding communication latency to the ATC processing system

.

Notes : 1. To meet the overall 2 second latency performance of radar, 2 seconds issplit between the airborne (1.5 seconds) and the ADS-B Receive subsystem (0.5 seconds). 2. Assume that the communication delay to ATC processing systems is comparable to that for radar. 3. This requirement does not include the latency associated with the communication of the ADS-B reports to the ATC processing system. Local implementers may add the communication latency and instead specify the total latency to include the communication delay to the ATC processing system

.

PR 34 : The Aircraft Transmit Domain (B1 to D) shall have a 95% total latency of 1.5 seconds or less for horizontal position and quality indicators.

Notes :
1. Any uncompensated latency will have the effect of degrading position accuracy. See Appendix B-1. Therefore, uncompensated latency must be limited in order to meet accuracy requirements.

2. Total latency = uncompensated latency plus compensated latency.

3. Compensated latency: An interval of time over which position information is extrapolated

.

PR 35 : The Aircraft Transmit Domain (B1 to D) shall have a 95% uncompensated latency of less than 0.6 seconds for horizontal position.

PR 36 : The Aircraft Transmit Domain (B1 to D) shall have a maximum uncompensated latency of less than 1 second for horizontal position (99.9%).

The requirements for time-stamping and latency are summarized in Table B- 13

.169

© 2009, RTCA, Inc. Table B- 13 : Summary of Latency and Time Stamp Requirements

Timing Requirement Value

(En R oute) Value

(TMA) GROUND

(“D”  “E2”) AIRBORNE (at “D”)
Time of applicability

Per surveillance report Within each surveillance report Note : If the time of applicability is not applicable for all data item s, separate times of applicability shall be provided for the specific data items that differ

. 95% Latency

Surveillance Reports 2 s 2 s ≤ 0.5 s out of ADS

- B Ground System ≤ 1.5 s (95%)

Uncompensated latency ≤ 0.6 s (95%)

Maximum uncompensated latency ≤ 1 s (99.9%) Time Stamp

Time accuracy of ADS

- B report information (as determined by ground system) ≤ 0.1 s ≤ 0.1 s W ithin 0.1 s Any unknown latency onboard the aircraft will have the effect of degrading the accuracy of the position information seen by the ground system (See Appendix B

- 1 )

.

Also obtained from Table B- 12 Table B is the capacity requirement for the Ground Domain as specified below

. PR 37: The ground system shall have the capacity to handle the reports from the maximum load of aircraft in the environment as described in the OSED without degradation

.170

© 2009, RTCA, Inc.

The requirement for system capacity is summarized in Table B- 14.

Table B- 14 : Summary of System Capacity Requirements

Capacity Requirement Value (En R oute) Value

(TMA) GROUND

(“D”  “E2”) AIRBORNE

(at “D”)

Capacity/Total Load

Capacity (See Annex A, § A.1 )

. These (generic) figures will be used as conditions under which the performanc e requirements should be met

. In addition

, local capacity figures will have to be taken into account when implementing the system. See OSED See OSED The ground system shall have the capacity to handle reports from the maximum load of aircraft in the

env ironment as described in the OSED without degradation. N/A171

© 2009, RTCA, Inc. ANNEX B APPENDIX B-1

TIME-STAMPING AND LATENCY ISSUES B-1.1 Introduction

The ground system needs to know the time of applicability (TOA) of the surveillance information

. The TOA is the time the information is valid.

Knowledge of the time characteristics of surveillance information is very important for a position that is changing rapidly with time. In the OPA, the required time characteristics are specified by two parameters : time- st amping accuracy and latency.

The time-stamping accuracy is the accuracy in time to which the TOA of the information can be determined by the ground system and indicated in surveillance reports sent to the ATC processing system.

Latency is the age of the information at the point where it becomes available. In this case, the end point of concern is at the input of the ATC processing system

. Overall latency = the time that information is available at the input of ATC system minus TOA (See definition of latency in § 1.3 ).

For radar surveillance, the time characteristics of position information are entirely dependent upon the ground system, whereas for ADS-B, they also depend upon the airborne system

.

B-1.2 Radar Time Characteristics

Fo r radar, the TOA of position information is the time of measurement by the radar. This is generally assumed to be the time when the aircraft is at the center of the radar antenna beam.

For some older radar and display systems, the TOA is simply assumed to be the time of receipt of the radar report data, and there is no qualification of time-stamping or latency requirements. However, for more modern systems, particularly those using multi-radar data processing, time-stamping and latency are very important requirements.

The TOA of radar information may be determined in various ways, for example:

An absolute time stamp is put on each radar report by the radar site equipment. Absolute time stamps are put on radar rotation markers (azimuth sector messages) at the radar site. The receiving system determines the radar report time from its azimuth position with respect to the sector messages. Radar sector messages (minimally North/South marker) are sent with known (or minimal) delay from the radar site. Consequently, the receiving system candetermine the radar antenna azimuth of the radar with time and therefore timestamp the radar reports according to azimuth position. The radar sends “time in store” information on each plot . This is the time difference between the time of measurement and the time of output of the plot from the radar (i.e., the age of the information). Consequently, the receiving172

© 2009, RTCA, Inc. system can determine the time stamp from the time it receives the plot (assuming

negligible or known transmission delay from the radar)

. The latency of radar plot information is generally due to two factors:

Inherent delay in the radar equipment processing. Most notably, many modern radars include scan to scan processing (i.e., tracking) to enhance plot detection

an d this may introduce a significant delay in confirming a plot. Queuing delay in the communication of plots from radar to ATC system. The delay will depend upon the bunching of plots in azimuth and the bandwidth of the communications link

. Generally, in most radar data processing systems used in ATC, a plot may be discarded if its latency is beyond a certain threshold age.

B-1.3 ADS -B Time Characteristics

Just as for radar, the ground system needs to determine the time of applicability (TOA) of the ADS-B information. For ADS-B, the time that the position measurement was obtained at airborne domain level is not necessarily the same as the time the information is transmitted.

The TOA of the information contained in an ADS-B message is determined by the processing of the data onboard the aircraft that is required by the data link standard. For example, for the 1090ES MOPS specify that the TOA may be either the time of transmission (in the non-coupled case) or the most recent 200-msec time mark (in the coupled case). The ground system has no knowledge of onboard latency between the time of position measurement (e.g., positioning source) and the TOA indicated by the data link. To account for the position change during this time difference, the avionics may extrapolate its navigational position to the time of transmission, using the latest velocity vector

– this is referred to as “position latency compensation .” The loss of position accuracy due to uncertainty in the velocity is likely to be very small for short extrapolation times (e.g., 1 second).

Any uncompensated onboard latency or timing uncertainties that are not known to the ground system will have the effect of degrading the position accuracy (predominantly in the along-track direction) of the ADS-B information received by the ground system.173

© 2009, RTCA, Inc. ANNEX B APPENDIX B-2

CLASSES OF RADAR ERRORS

For the purposes of the OPA, radar errors are subdivided into the following three classes :

1

. Radar Equipment Failures

2

. False targets

3

. Radar measurement errors

Radar equipment failures (hardware or software) may have many effects on the resulting radar plot outputs. Although such failures may be regarded as a loss of radar equipment integrity, in practice such occurrences are not specified in integrity terms. Usually, various internal monitoring (e.g., self tests with injected test signals) and external monitoring (e.g., use of site monitor transponders) are required to ensure that the radar equipment is being used in a fault-free condition. In the OPA, the only aspect of radar equipment failures concerns the overall reliability. For the purposes of comparison of radar measurement errors in this OPA it is assumed that errors from radar equipment failures are not present, (i.e., the radar equipment is working in a fault-free state when it is providing radar reports)

. False targets are due to various imperfections in the environment where the radar is located (e.g., reflecting surfaces, sidelobe responses, FRUIT replies from other interrogators). Such false targets may be regarded as a radar integrity failure in the detection of an object of interest. Years of experience of radar processing has enabled such false detections to be minimized to an acceptable level. For the purposes of comparison of radar measurement errors in this OPA it is assumed that all false plots are removed from the radar measurement error characteristics.

Radar measurement errors are present on the target positions reported by the radar in its normal mode of operation (i.e., no radar equipment faults). There is no indication from the radar about the magnitude of the error (i.e., all these errors are undetected by the radar). The important characteristics of radar measurement errors are described in more detail in Appendix B-3.174

© 2009, RTCA, Inc. ANNEX B APPENDIX B-3

RADAR MEASUREMENT ERRORS

B-3.1 Introduction

Radar measurement errors are usually quantified in the radar range and azimuth dimensions. Azimuth errors are sometime referred to as „ cross-range ‟ errors , and when projected in a plan, the position error will be proportional to the range from the radar

. Radar error distribution in range and azimuth are considered to have the following components:

Systematic biases.

Stochastic errors.

Systematic biases will cause a common offset in range and azimuth for all targets seen by the same radar. For the comparative assessment, the best-case radar separation situation of two targets seen by the same radar is assumed, so radar systematic biases have no significant effect on apparent radar separation.

Stochastic errors are random-like errors that vary from measurement to measurement or from aircraft to aircraft (see Errors within core accuracy in Figure B-3- 1) . These errors are typically due to the inherent accuracy limitations in the particular radar sensor in its environment, such as receiver signal to noise, quantization, individual aircraft transponder performance, etc. The radar stochastic measurement performance is usually expressed as a standard deviation ( ) determined from the distribution of a large population of measurements.

In practice, radar azimuth errors are found to have larger “tail” errors than expected from a single Gaussian distribution with the measured . Such errors are typically caused by various local radar site factors, such as local multi-path, diffraction, multiple target resolution, and the effects of other interrogators. Furthermore, the exceptionally large tail errors are often highly correlated for the same target from scan to scan (see Errors highly correlated from scan to scan and Exceptional errors in tail distribution in Figure B-3-1)

. On the other hand, range errors are considered to be accurately represented by a single Gaussian distribution

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© 2009, RTCA, Inc. Figure B-3-1: Radar Measurement Errors

B-3.2 Radar Error Model

A representative radar performance model is needed for OPA comparative analysis and SSE . ATC administrations now require at least an MSSR to support ATC radar services in medium and high-density airspace

.

The Eurocontrol Surveillance Standard [14] specifies the minimum measurement performance required to be achieved by an MSSR radar in European Civil Aviation Conference states. Although this standard is not as stringent as US MSSR standards, it is the standard that has also been used widely by other administrations outside Europe and provides an acceptable basis for the required range and azimuth measurement accuracies in the OPA radar error model.

For the stochastic (random-like) measurement errors, the standard specifies the following range and azimuth accuracy standard deviations (σ):

Azimuth σa ≤ 0.08 degrees Range σr ≤ 70 meters

These σ values were determined from the overall error distributions of data collected over a suitably large sample of aircraft measurements in the actual radar operating environment. Note that in the case of range, the errors include the effects of aircraft transponder turn-around delays that typically vary from aircraft to aircraft

. (ICAO Annex 10 SSR transponder standards allow ± 0.5μs tolerance in the time between receipt of a Mode A/C interrogation and the associated transponder reply).

The standard does not explicitly specify the expected shapes of the error probability distribution, except to say that large errors (jumps) in azimuth greater than 1 degree and range errors greater than 700 meters shall be less than 0.05% of measurements

. However, for the OPA SSE analysis, it is necessary to model the characteristic shapes of the error distributions in some detail. Therefore, in order to characterize the shapes of range and azimuth error distributions, an analysis of recent measurements from the US MSSR has been used. Errors within core accuracy Exceptional errors in tail distribution
Errors highly correlated from
scan to scan176

© 2009, RTCA, Inc.

Radar range errors closely follow a single Gaussian error distribution, but azimuth errors from US MSSR measured data (see the Radar Error Model Curve Fit section below) have much wider “tails .” Consequently, the azimuth error distribution Pd(a) is best described by a double Gaussian distribution as follows:

Pd(a) = (1- )Pb(a) + Pw(a) (B

- 2) Where: „a‟ is the azimut h error Pb(a) is the probability density function distribution of the „core‟ errors Pw(a) is the probability density function distribution of the „tail‟ errors is a weighting factor (determined empirically, see below)

Pb(a) and Pw(a) are assumed to be Gaussian distributions with different standard deviations ( b and w) . The values of b, w and the weighting factor were determined to fit radar errors experienced for US MSSR radars

.

The characteristic shape of the azimuth error distribution is determined from the ratio σw / σb and the weighting factor found from the above measurements as :

σw/σb = 5, = 0.05

The model radar azimuth error distribution characteristics are summarized in Table B15 Table B

.

Table B- 15 : Radar Model Azimuth Error Distribution Parameters

Overall

a

d

e g

r

e

e

s Core b degrees Tails

w degrees Weighting f

a

c t

o

r 95% error de

gr

ee

s

0.08 0.054 0.27 0.05 0.127

The 95% error point in the final column of the table is determined from the error distribution

. This is used for comparison with ADS-B accuracy values

.

B-3.2.1 Radar Error Model Curve Fit

MSSR azimuth angle estimates relative to precise time-registered Ashtech® reference position reports were collected in several flight tests by Johns Hopkins University Applied Physics Laboratory (APL) and Massachusetts Institute of Technology Lincoln Laboratories (LL). The flight tests were conducted using Philadelphia and Louisville177

© 2009, RTCA, Inc. terminal radar sites, which employ ASR-9/Mode S radar systems. The total number of usable samples collected ( nS ) was 4137; since this is only a limited data set, additional data (millions of samples) will be collected to more completely validate the azimuth error distribution model. For this analysis, it is assumed that the limited data set used is a valid representation of the assumed azimuth radar error distribution.

This combined azimuth angle error data set was normalized to provide a measured MSSR probability density function ( pdf )

, called MS . Two analytic azimuth angle error models were compared with these measured errors for goodness of fit using the MathCad correlation function

. The first model is an FAA single Gaussian model, pnS , with a standard deviation, σnS = 0.068 degrees. This provided a correlation coefficient, corr=0.979. Comparison of this model with the measured distribution is shown in Figure B-6

. The second model is a linear combination of Gaussian functions with empirical fits to the measured data set. Details of this model, pwd, termed the Normal-Normal MSSR model, are summarized in Figure B-7 Figure B with the resulting goodness of fit correlation, corr = 0.999.

A comparison of each model with the measured data is shown in a semi-log plot in Figure B-8 Figure B

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© 2009, RTCA, Inc. Figure B-6: FAA Single Gaussian Model

FAA

FAA179

© 2009, RTCA, Inc. Figure B-7: Normal-Normal MSSR Model With Empirical Fit to Measured Data180

© 2009, RTCA, Inc. Figure B-8: Comparison of Each Model

B-3.2.2 Residual Systematic Radar Biases

While systematic radar range and azimuth biases do not significantly affect the apparent radar to radar separation of targets seen by the same radar, they will affect ADS-B to radar separations. Consequently, some residual systematic radar biases need to be assumed for ADS-B to radar separation considerations.

In practice, systematic range and azimuth biases will be present at the radar site, due to limitations in the radar site equipment, such as mechanical alignment of the radar antenna. Limits of such systematic biases at the radar site are specified in the Eurocontrol Surveillance Standard. However, modern multi-sensor surveillance processing systems are also able to perform automatic systematic bias calculation, sometimes called sensor registration. Hence, in the ADS-B-RAD environment, it is assumed that sensor registration between radar and ADS-B will be performed by the surveillance data processing system to reduce any inherent radar systematic biases. For OPA model comparisons, the performance of this system in eliminating radar biases is assumed to be as follows:

Residual radar azimuth bias ≤ 0. 05 degrees. Residual radar range bias ≤ 40 meters

. Norm

- Norm model, pwd:

FAA Gaussian pnS:



### FAA181

© 2009, RTCA, Inc. B-3.3 Reduced Report Update Interval Benefits

In order to quantify the benefit of a reduced report update interval, more in-depth analysis of the relationship between display update rate and measured position accuracy were examined to see how changes in display update rate affect separation performance. The results of this analysis are summarized in §B.4.1.3. The following sections provide detailed analysis, justifying the §B.4.1.3 recommendations.

B-3.3.1 Reduced Display Interval for More Precise TMA Control

In classic track-while-scan radar automation and display systems, the measured radar position or „plot‟ data is processed and sent to the display system as soon as possible to avoid time lags between the radar measurement and the time that aircraft positions are refreshed on the controller traffic display. The position uncertainty during the display period prior to refresh increases due to aircraft motion and can dominate the position uncertainty due to the radar sensor, as the displayed measurement data ages over the refresh cycle. The same phenomenon will occur to some extent with ADS-B data, but can be mitigated by use of a higher display rate than that of the backup radar system.

Figure B-9 Figure B shows an example of the decrease in position accuracy with classic radar „plot‟ displays, assuming a radar update interval of 4.8 seconds, and a 95% position accuracy of 0.1 NM, typical of MSSR systems in terminal airspace. It is assumed in this figure that the initial error on display refresh is entirely due to the radar sensor, and that the display uncertainty or „display accuracy‟ is root -sum-squared with the linearly increasing error due to the display lag over the refresh period, ignoring other ATC processing system errors. In this case, display uncertainty from measurement display lag quickly dominates the position uncertainty and results in a four-fold increase in position uncertainty prior to display refresh. If the ADS-B data is simply synchronized to theradar scan with one report per radar scan, then the same pattern can be expected for ADSB data degradation over the radar display interval, i.e., a sensor accuracy of 0.1 NM or less (NAC P =7) will degrade to a NAC P =6 equivalent or worse over the radar scan interval. This degradation can be mitigated by reducing the display interval, using velocity extrapolation to synchronize the ADS-B and radar reports to common display times. In this case, reducing the display interval to 2.4 seconds, i.e., twice the radar scan interval, will reduce the display uncertainty by almost a factor of two so that the effective position uncertainty on the display is reduced on average by a factor of two, and does not exceed 0.25 NM at any time prior to display refresh.

In terminal applications where precise control is desired, such as vectoring onto final approach and merging of traffic streams, a reduced display interval of 3 seconds or less may be considered in order to give the controller most of the benefit of the increased sensor accuracy available with MSSR and ADS-B surveillance. The value of 3 seconds was chosen since it assures that the effective degradation of position uncertainty over the display interval does not exceed 0.3 NM (95%) (i.e., the accuracy will not be any worse than a NAC P =6 equivalent over the display interval, whenever a NAC P =7 or better ADSB report is received)

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© 2009, RTCA, Inc. Figure B-9: Example Radar Display Accuracy Over Several Display Cycles

One specific method of synchronizing ADS-B and the backup radar positions is to extrapolate ADS-B position data and radar position data to a common time point. This enables direct comparison of ADS-B data and radar position data, and synchronizes the ADS -B data to the latest received radar report. However, this does not fully utilize the data received from the ADS-B system and typically results in discarding most of the ADS -B reports received. This can be mitigated by using a two-times radar scan update cycle where ADS- B data is refreshed on both a „forward‟ scan comparable to the radar updates and a „backward‟ scan cycl e that is 180 degrees out of phase with the radar scan (see Figure B- 10 Figure B ). In this figure, the azimuth scan is represented as a vertical bar with the horizontal axis representing the current radar scan azimuth and the vertical axis representing the range dimension. As the radar scan sweeps past an aircraft, a specific azimuth, range point is generated representing the radar plot data. The ADS- B data is synchronized to the time of the measured radar plot and then displayed on the controller display, or the backup radar plot data is used if no ADS-B data is available for the target aircraft. At the same time, a second „back scan‟ shown as a dotted line vertical bar in Figure B.3.2-2 sweeps through targets which are 180 degrees out of phase with the radar scan, assuring that ADS-B targets are refreshed at a higher, more efficient update
rate

. In principle, additional synthetic scan beams could be generated if needed for high precision applications (i.e., three-times or four-times radar display rate synchronization). However, for the terminal applications and separation standards considered in thisAppendix, there does not seem to be a need for display refresh rates greater than twotimes the radar scan. If the ADS-B reports are synchronized to the backup radar azimuth scan, then implementation of a two-times radar update rate (forward and backward scan183

© 2009, RTCA, Inc. pattern synchronization) may be considered as one means of achieving higher display update rates (e.g., a 2.5 second display interval given a 5 second radar scan)

.

Figure B- 10 : Example ADS-B Forward / Backward Azimuth Synchronized Display

One issue with the above synchronization scheme may be to provide a radar backup for the backward scan as well as for the usual radar scan, for continuity of surveillance in the event that one or more aircraft are not providing ADS-B positions for surveillance. Hardware solutions such as adding additional back-scan SSRs are either not feasible or likely to be very expensive. A better transition solution may be to use whatever source ofvelocity is readily available to extrapolate the radar track or plot data to the next backscan display time. In many cases an ADS-B velocity vector will be available for this purpose, even if the ADS-B position data from GPS is not available, or is not of sufficient quality for surveillance purposes. For example, many air transport category aircraft will provide an Instrument Navigation System (INS)-derived position and velocity in the event that GPS data is not available. A second source of velocity data is the Doppler range rate that is generated with the backup PSR radar. Use of this information in a multi-sensor tracker, or use of much simpler adaptive track corrections have been proposed for extrapolatin g aircraft positions over short time periods, even in the presence of significant turn maneuvers. One solution is the development of a multi-sensor tracker using all available sensor inputs, with display of the fused radar-ADS-B track data, rather than specifying an ADS-B primary versus radar backup display scheme. Whatever data is available is used to update a fused track, assuring continuity when one or the other data source is unavailable or unusable for critical surveillance functions.

B-3.3.2 Reduced Display Interval for En Route Surveillance

The effect of display lags in radar En Route surveillance is much greater than in the TMA area for two reasons: (1) significantly higher aircraft speeds, and (2) a 10 -second scan update versus 5-second for TMA operatio ns . This means that position display uncertainty can be significantly larger than sensor accuracy, for either ADS-B or MSSR systems. At long ranges on the order of 200 NM, a MSSR sensor can deliver 95% position accuracy on the order of 0.5 NM or better (i.e., NAC=5 equivalent for ADS- B systems). Figure B- 11 shows an example of the display accuracy degradation for a MSSR system at long range with 0.5 NM position accuracy, and the effect of display lag during two refresh cycles, assuming a 10-second radar scan interval and a synchronized 10 -second display refresh. In this case, the position uncertainty can degrade by a factor of almost four before a data refresh occurs. However, with ADS-B it is feasible and may184

© 2009, RTCA, Inc.

be desirable to increase the data display rate. The figure shows that the degradation can be greatly mitigated by reducing the data display interval by a factor of two (i.e., the display uncertainty can be limited to 1.0 NM (NAC P =4 equivalent) by limiting the display interval to 5 seconds or less). Moreover, separation buffers are typically added tomitigate position uncertainty in various controller decision support tools such as CenterTRACON Automation System timed arrival management, User Request Evaluation Tool medium-term conflict probe, an d short-term conflict alerting that could potentially be reduced on the basis of enhanced surveillance, coupled with a reduced traffic display interval

.

Figure B- 11 : Example Long Range En Route MSSR Display Accuracy185

© 2009, RTCA, Inc. ANNEX B APPENDIX B-4

TIME- TO -ALERT CONSIDERATIONS

B-4.1 Background

The time- to -alert requirement concerns the onboard detection time of a signal- in -space fault by the position integrity monitoring system (e.g., RAIM). The time- to -alert is defined here as the time between the position error first exceeding the horizontal protection limit (HPL) and the eventual indication or annunciation of the condition by the position integrity monitoring system. The position error is subsequently communicated by ADS -B- OUT as a change of Quality Indicator (e.g., a change of NIC to an unacceptable value).

It is assumed that the effects of a fault condition in the horizontal position source could be either:

(a) A sudden jump in position (e.g., a clock jump from a faulty GPS satellite); or (b) A slow drift in position (e.g., timing drift from a faulty GPS satellite).

Justification of this assumption is provided by analysis of GPS faults throughout the period of its operation [ 30 , Appendix R], where it is observed that about half the faults were jumps and the remainder were drifts in the satellite ranging signal of up to 5 meters per second.

It is assumed that a significant jump (a) would be detected quickly and will not mislead the controller

. The basis of this assumption is for the following reasons:

In the ground ATC system, a significant jump in plot position may be detected by the controller as being inconsistent with the previous plot history (on plot display systems) or the bad plot position may be detected as an „outlier‟ by a tracking system in comparison to previous track history (for track display systems). In modern avionic GPS systems ( E/TSO -C129 or higher), there is a specific function to detect jumps (steps) in the satellite ranging signals and then exclude the faulty satellite. Furthermore, the position error induced before the step fault is detected is required to be constrained to a relatively small value (< 200 meters)

.

On the other hand, a slow drift (b) will not be detected by either of the above features

. Consequently, detection of this fault condition will depend upon the integrity monitoring function. An example time sequence of related events is illustrated in Figure B- 12 Figure B for a fault-induced position error drift. In this figure, the HPL is depicted as a constant value over time for ease of illustration but, in practice, HPL may change with time.186

© 2009, RTCA, Inc.

Figure B- 12 : Example Time Sequence for a Fault-Induced Position Error Drift

In the figure, the following time events are depicted:

“T0”: Horizontal position source (signal- in -space) is working normally.

“T1”: A fault starts to develop in the position source (e.g., satellite clock drift).

“T2”: The position error exceeds the HPL of the integrity monitoring system.

“T3”: The integrity monitoring system detects the position fault condition and changes HPL (and consequently the ADS-B Quality Indicator) to an unacceptable value.

The time- to -alert requirement is described by the difference T3- T2 . During this time period, the position error continues to drift beyond HPL by a distance ∂.

B-4.2 Value of Time to Alert and Drift Rate

In practice, for the implementation of onboard integrity monitoring systems, requirements for both the timely detection of a fault condition and the false alert rate from the normal measurement noise must be met. Hence, the time- to -alert delay is largely due to the integration time needed to reduce false alerts to an acceptably low rate. GNSS receivers ( E/TSO -C129 or higher) require time-to-alert <10 seconds and so this value is used as the basis of the time- to -alert requirement and associated analysis.

Note that the time- to -alert delay specified here (i.e., 10 seconds) is the maximum relative time difference between a position and its associated integrity quality indicator. All subsequent delays (e.g., onboard and ground processing) will equally affect both the position and integrity information.

Observations of GPS faults [ 30 ] and test specifications for avionic GPS and Wide-Area Augmentation Systems (WAAS) assume a worst-case satellite ranging ramp fault of 5 Position

error

Time T0 T1 0 T2 T3 HPL Position from

positioning source ∂ Position fault detected187

© 2009, RTCA, Inc. meters per second. Although the effects of such a ramp fault on the position solution will depend upon the satellite geometry at the time, it is anticipated that, for realistic satellite geometries, the associated position drift caused by the ranging error will not be substantially larger than 7.5 meters per second. Hence, a position drift of 7.5 meters per second is assumed to be a realistic worse case value in the calculations below

.

B-4.3 Taking Account of Time to Alert in the Surveillance Risk Calculations

In the Surveillance Separation Error analysis described in Annex D, it is assumed that an undetected position integrity fault causes the position to be offset in the direction of an adjacently separated aircraft, by an amount equal to the integrity containment radius (R C ). It is relatively simple to consider the effects of time- to -alert delays as follows:

Firstly, it is assumed that the position drift is in the direction of an adjacently separated aircraft. Hence, the effect of the position drift during the time- to -alert delay period is equivalent to extending the containment radius (R C ) by an amount equal to:

= position drift rate * time- to -alert (B

- 3)

Consequently, the SSE comparison with the reference radar can ensure that there is a sufficient margin of to cover the effects of time- to -alert. Using the values of position drift rate = 7.5 meters per second and time- to -alert = 10 seconds , ∂ = 75 m eters. See Annex D for further details of how ADS-B position errors, including the effects of undetected fault conditions, are compared to radar.

B-4.4 Normal Changes of Integrity Containment Radius

In the normal, no-fault position source situation, the quality indicators (i.e., NIC and NAC) may change because of changes in the geometry and visibility of available satellites. The time required for such normal quality indicator changes to be represented in the associated position information is defined elsewhere (in Annex B) and it is not the same thing as this time- to -alert requirement (that is specific to the detection of faults in the position source)

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© 2009, RTCA, Inc. ANNEX B APPENDIX B-5

CONSIDERATIONS FOR TIME CORRELATION OF POSITION ERROR

B-5.1 Introduction

B-5.1.1 Pur pose

The purpose of this section is to address some similarities and differences in the nature of position error between radar and ADS-B surveillance sources. It explains how positionerrors from ADS-B and radar may be correlated in time and the impact these timecorrelated position errors have with respect to the timing and planning of controller actions

.

B-5.1.2 Overview

The position source accuracy and integrity requirements established in this document tosupport terminal and En Route separation standards limit the potential magnitude of timecorrelated position error to values that are relatively small compared to the separation standards. The effect of these errors with respect to the timing and planning of controller actions for the performance requirements in the OPA is not considered to be of a significance warranting additional analysis

. However, if ADS-B position accuracy and integrity requirements are reduced, or if the current performance requirements are applied to reduce existing separation standards, the impacts may be significant. In either situation it is important to understand and appreciate how time-correlated position error can affect controller actions

.

B-5.2 Description of Position Errors

“Misleading” position information may be defined as one or more erroneous position reports that affect a controller‟s capability to plan and achieve the desired separation between a pair of aircraft

. For any surveillance source, a single erroneous position report will typically not be misleading because a large error (e.g., sudden jump in position) is often rejected by the surveillance data processing system or easily noticed by the controller. An example of a single erroneous position report for radar- to -radar separation of two aircraft is shown in Figure B- 13 Figure B . In this case, if the controller judged the separation at t4 to be minimally acceptable, then he will issue instructions to the aircraft to provide increased separation at or before t5 and would not judge the separation to have significantly increased at t5 based on the sudden apparent increase in separation. In a similar manner, a single jump in the position of one aircraft would not represent an equivalent decrease in the separation between that aircraft and another aircraft of interest

.189

© 2009, RTCA, Inc. Figure B- 13 : Uncorrelated Radar Position Error

B-5.2.1 Radar Comparison Case

For a radar-based surveillance system, position errors are assumed to be independent(uncorrelated). However, there can be environmental circumstances (e.g., ground multipath, obstructions, etc.) that may cause a sequence of large and time-correlated position errors. Radars are designed and installed to limit the occurrence or impact of such errors, and in many cases the controller will be aware of them. An example of such correlated error for radar in shown in Figure B- 14 Figure B . This figure also shows the difference between an uncorrelated („exceptional error‟) and errors that are correlated from scan to

scan

.

Figure B- 14 Correlated Radar Position Error

Radar will also have correlated errors in the form of systematic range and azimuth biases. However, if both aircraft are being observed by the same radar, these biases affect both aircraft in the same direction, and therefore the separation error remains unaffected by this bias. Furthermore, for a multi-radar environment, radar systematic biases can be detected and corrected in various ways so that their effects on separation are kept relatively small (e.g., azimuth bias typically corrected to within 0.05 degrees)

.

B-5.2.2 GNSS Error Case

When ADS-B position data is derived from GNSS, the reported position is an estimate based on the center of the region where the aircraft is calculated to be. The actual position of the aircraft will vary from the reported position according to the available accuracy and, in the case of a GNSS satellite failure, the integrity of the GNSS. The magnitude and direction of the correlated position error depends on the GNSS avionics, number of satellites in view, satellite geometry and the integrity of the GNSS signal- in

-190

© 2009, RTCA, Inc.

space. This time-correlated error may appear as a short-term bias between the track inferred from the reported GPS-based positions and the actual aircraft position. This bias will not be apparent to the controller, but may be negligible compared to the scale used on the controller display and the separation standard. An example of this correlated error is shown in Figure B- 15 Figure B where both aircraft report a biased position due to an integrity fault and the bias is in the opposite direction. In this case, the apparent separation (So) the controller sees is greater than the actual separation and separation planning/action, if it were necessary for the given separation standard, would not be implemented.

Figure B- 15 : ADS -B Correlated Position Error

GNSS position errors are much smaller under normal GNSS operations than for operations with a loss of integrity since they are bounded by the required accuracy

. GNSS integrity-related biases may affect only one or possibly both aircraft. While it is unlikely to have two closely-separated aircraft report positions biased away from each other, it can occur in less than 10% of the cases where the se aircraft both use a GPS satellite in a faulted condition, but also use different GPS satellites to estimate their positions. When a significant bias affecting the position of one or more aircraft exists, the controller will not be able to detect or take early action to prevent less-than-desired separation using ADS-B position reports alone

.

B-5.3 Limitations of Mathematical Modeling

The Surveillance Separation Error (SSE) mathematical model used to derive the integrityperformance requirements in the OPA does not account for the effect that the timedependent characteristics of position errors have on controller actions. SSE calculations represent the probability of surveillance separation errors of various magnitudes based on the variability and spacing of the probability density functions used in the model

. Position errors with the same density functions will have the same modeled SSE probabilities even if the position sources have different bias due to time-correlation. This is also true for radar-based SSE calculations where the values will be the same whether the radar is operating nominally or with a position error bias due to correlation (e.g., multipath)

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© 2009, RTCA, Inc. B-5.4 Potential Operational Impact of Position Source Failures

The potential impact of the time-correlated position error may occur in two ways. First, this short-term bias will delay controller action to intervene when aircraft separation is decreasing. In the ADS-B-based surveillance system shown in Figure B- 15 Figure B , the controller is unaware of the undetected GPS bias R C , and will assume the aircraft are separated as the positions are displayed. The figure shows the potential difference between the apparent (displayed) separation and the actual separation.

A second impact of th e short-term bias is in controller planning. In this case, the bias may display aircraft in an orientation that the controller judges to be adequate to maintain the desired separation when there actual separation is less. This may result in no controller action to direct an increase in separation when such an action might otherwise be taken. In Figure B- 16 Figure B , the controller plans the separation so that the displayed aircraft positions (shown in blue) at the point of closest approach meet or exceed the desired separation. However, the actual aircraft positions (shown in red) reveal an unknown position bias that results in less-than-desired actual separation between aircraft

.

Figure B- 16 : Position Report Errors (Timing/Planning of Actions)

B-5.5 Limitations on Position Error Bias

B-5.5.1 Fault-Free GNSS Operations

There are a number of operational factors that limit the magnitude and duration of the position error bias that might be displayed from ADS-B. Under fault-free GNSS satellite operations, the magnitude of the bias will be relatively small compared to the separation minima. For example, at NAC P =7, the probability of a single aircraft bias exceeding 6% of the 3 NM separation minima is less than 10 -4 . A bias of this magnitude would b e unlikely to persist for longer than a few minutes since the de -correlation interval is typically assumed to be less than 10 minutes

.192

© 2009, RTCA, Inc. B-5.5.2 Faulted GNSS Operations

A GNSS integrity fault may also have an impact on controller actions with a potentially larger magnitude and duration, but a lower likelihood of occurrence. These faults may occur on the order of 10 -4 and create a short-term bias for a single aircraft less than or equal to 6% of the 3 NM separation minima (based on a NIC=7). Although the avionics are likely to detect and report these biases (i.e., 99.9% detection before the bias might exceed the NIC and reporting within 10 seconds), biases less than the NIC are less likely to be detected and may persist for an indefinite time period until the ATCO becomes aware that a GNSS satellite fault condition exists. The US Department o f Defense GPS Positioning Service Performance Standard allows a nominal 2 hours for the ground system to address satellite faults, with a maximum response time of 6 hours

.

Fo r more adverse geometries and opposing biases (as shown in Figure B- 15 Figure B and Figure B- 16 Figure B , these smaller integrity biases may still have a significant impact on a controller‟s capability to provide the desir ed separation. Long-term biases exceeding the NIC value are rare by design; however, they may exist without detection at a maximum likelihood of 10 -7 for a single aircraft based on the combined probability of a GNSS satellite fault (10 -4 ) and a GNSS avionics missed detection (10 -3 ). However, it is important to note that the impact of all the likelihoods described above are further reduced by the likelihood that the affected aircraft are located in close proximity at an adverse geometry with respect to their relative position biases. A combination of the bias, geometry and proximity likelihoods are required to produce the most significant impact with respect to controller actions in effecting the desired separation

.

B-5.6 Summary

The mathematical comparative analysis addresses the potential differences between surveillance separation error for radar and ADS-B. It addresses the probability that the surveillance positions of aircraft have varying magnitudes of position error. Although this analysis includes some latency compensation, the calculated probabilities are independent of the conflict trajectory before and after the point of closest approach between any two aircraft. Mathematical modeling alone may be insufficient to address how the timing of controller actions might be affected by an unknown bias in aircraft positions if the potential bias is significant relative to the separation standard. It does not fully address how the controller can account for this short-term position bias when planning separation for two or more potentially conflicting aircraft tracks

. The position source accuracy and integrity requirements established in the OPA to support terminal and En Route separation standards (e.g., NIC=7 for 3 NM) limit the potential magnitude of time-correlated position error to values that are relatively small compared to the separation standards. The impact of this position error characteristic may be more significant if ADS-B position accuracy and integrity requirements are reduced, or if current performance requirements are applied to reduce existing separation standards

.193

© 2009, RTCA, Inc. B-5.7 Recommendations

Recommendation 1 : If ADS-B surveillance is applied under different conditions, additional study to address the impact of time-correlated position errors on the planning and timing of controller actions would be useful

.

Recommendation 2 : This additional analysis could compare position errors after a tracking process to compare the error distributions resulting from tracking of ADS- B positions to those from radar

.194

© 2009, RTCA, Inc. ANNEX B APPENDIX B-6

ALTERNATE REFERENCE RANGE IMPLICATIONS ON QUALITY INDICATORS

B-6.1 Introduction

B-6.1.1 Background

The quality indicator (NIC/NAC P ) requirements, as determined in the OPA, are a function of the range at which the comparison of ADS-B performance with radar performance is made, the reference range. The NAC P is directly determined based upon the reference range and the radar error model. The NAC P is then used in the analysis that is performed to determine NIC (see Annex D).

B-6.1.2 Pur pose

The purpose of this Appendix is to provide guidance on quality indicator values that would result if the radar reference ranges were selected at ranges other than those selected in Annex B. While those ranges were selected to provide comparable ADS-B performance throughout the maximum practicable airspace in a high-density environment, local implementers may decide that they need only be as good as radar atthe maximum range at which the separation standard is supported in low- to mediumdensity environments. Local implementers choosing this approach need to perform the necessary safety assessments. This Appendix does not confirm the operational support of the application of 5 NM/3 NM minima using ADS-B data with the quality indicator values specified herein.

B-6.1.3 Reference Ranges Considered

The reference range selected for the OPA is 33 NM in terminal area, and 60 NM in En Route for the reasons documented in Annex B. In this Appendix, additional reference ranges are considered: 40 NM and 60 NM in terminal area, and 120 NM and 200 NM in En Route. These ranges were selected as they roughly represent the ranges at which the NAC P values change for En Route, given the radar error model being used. Also, 60 NM is the extent of terminal area radar and 200 NM is the extent of En Route radar

.

B-6.2 Quality Indicators

B-6.2.1 NAC

Measured radar performance at each of the reference ranges (based on the radar error model described in Annex B) and ADS-B accuracy are desired to be comparable. The NAC P value that supports the accuracy is then selected (see Annex B for additional details). The resulting NACps and associated Estimated Position Uncertainties (EPUs) are summarized in Table B- 16 . Note that the impact of latency on accuracy has not been considered when determining these NAC P values

.

B-6.2.2 NIC

The SSE analysis detailed in Annex D was performed and the resulting NIC values per required containment radius (R C ) are as shown in Table B- 17

. Corresponding graphs in195

© 2009, RTCA, Inc. Figure B-17 through Figure B-26 delineate the relationship of the Horizontal Protection Errors (HPE) and the Horizontal Protection Limits (HPL) in these environments.

Table B- 16 : Required ADS-B Accuracy

Reference Range Radar

Accuracy NAC P

Terminal Area 40 NM 205 meters 7 ( EPU < 185.2
meters )

60 NM 308 meters 7 ( EPU < 185.2
meters )

En Route 120 NM 616 meters 6 ( EPU < 555.6
meters )

200 NM 1026 meters 5 ( EPU < 926
meters )

Table B- 17 : Required ADS-B Integrity

Reference Range NIC

Terminal Area 40 NM 6 ( R C = 0.6 NM)

60 NM 5

( R C = 1 NM)

En Route 120 NM 4 ( R C = 2 NM )

200 NM 4

( R C = 2 NM)196

© 2009, RTCA, Inc. Figure B- 17 : SSE Probability, R=40 NM

Figure B- 18 : SSE Probability, R=60 NM Terminal197

© 2009, RTCA, Inc. Figure B- 19 : SSE Probability, R=120 NM

Figure B- 20 : SSE Probability, R=200 NM198

© 2009, RTCA, Inc.

Figure B- 21 : Partial SSE Probability, Separation Reduction > HPL, R=40 NM

Figure B- 22 : Partial SSE Probability, Separation Reduction > HPL, R=60 NM Terminal199

© 2009, RTCA, Inc. Figure B- 23 : Partial SSE Probability, Separation Reduction > HPL, R=120 NM

Figure B- 24 : Partial SSE Probability, Separation Reduction > HPL, R=200 NM200

© 2009, RTCA, Inc. Figure B- 25 : Conditional ADS-B SSE Probability, NIC=4

Figure B- 26 : Conditional ADS-B SSE Probability, NIC=4 and Separation Reduction > HPL201

© 2009, RTCA, Inc. ANNEX B APPENDIX B-7

CONSIDERATION OF NAC P =7 ACCURACY FOR TMA

B-7.1 TMA Accuracy Requirement

The TMA accuracy requirement was determined in Appendix B (§B.4.2.2) by comparison with the reference radar model at a range of applicability where azimuth (cross-range) accuracy equaled range accuracy. Consequently, using a range accuracy of 70 meters standard deviation, as specified by the Eurocontrol Surveillance Standard, the required 95% error radius was determined to be ≤ 171 meters. To ensure that ADS- B accuracy is always better than this requirement, a value of NAC P =8 (0.05 NM, 93 meters ) is needed. This value also satisfies more stringent USA radar comparison models. Hence, NAC P =8 is considered as being the required minimum value to ensure “global interoperability .”

B-7.2 Consideration for NAC P =7 Accuracy

Local implementers may consider a lower accuracy of NAC P =7 to be adequate for TMA purposes, in order to gain better system availability, in particular for aircraft equipped with legacy ( SA -unaware) GPS receivers. The arguments to justify NAC P =7 being adequate accuracy for TMA may be based upon:

1. The NAC P =7 upper limit (of 185 meters) is not substantially worse than the required 171 meter reference radar accuracy at the chosen range of applicability.

2. The quantization of NAC P =7 encoding (93 to 185 meters) will most probably provide an actual accuracy much better than the required accuracy.

3. With SA -unaware receivers, a reported NAC P =7 is likely to be very pessimistic.

Firstly, the 14 meter difference between the upper limit (i.e., worst case) NAC P =7 accuracy of 185 meters and the required 171 meters may be considered to be
ope rationally insignificant, for example in comparison with other features in the system such as display resolution and aircraft size. Furthermore, the reference accuracy value of 171 meters from radar may be considered to have some amount of variability fro m environmental factors. In particular, radar accuracy performance figures are long-time averages over the population of aircraft detected by the radar and there may be some variations in performance between individual aircraft or changes in radar aspect (e.g., elevation angle).

Secondly, the average accuracy of NAC P =7 is better than the required accuracy of 171 meters. Assuming that NAC P =7 can be any accuracy between the bounds of 93 and 185 meters with equal probability, then the resultant long-term average 95% accuracy will be 142 meters. (This is a very conservative result, because the probable distribution of satellite geometry heavily favors the smaller bound of NAC P =7).

There is some degradation in accuracy due to uncompensated latency that has to be taken into account, as was done in Annex B.4.2.3. Assuming an uncompensated latency standard deviation of 0.2 seconds, and combining this in a random manner with the measurement source position uncertainty, the resulting along-track 95% position accuracies for the limits and the average of NAC P =7 are shown in the figure below. It202

© 2009, RTCA, Inc.

can be seen that the degradation in accuracy from uncompensated latency uncertainty does not cause a substantial degradation in accuracy for typical TMA speeds.

Figure B- 27 : Effects of 0.2 second uncompensated latency uncertainty Finally, for legacy SA -unaware GPS receivers, the availability of reported NAC P =8 is a problem because the receivers always assume the degraded ranging accuracy of the satellite signals. (Typically the availability of reported NAC P =8 is in the order of 99% or less and this would be insufficient for ATC operations). In comparison, a reported NAC P =7 or better has very high availability. Furthermore, the actual accuracy of a reported NAC P =7 from an SA-unaware receiver is very likely to be much better

. (Typically, the actual accuracy of a reported NAC P =7 has >> 0.99 probability of being better).

Consequently, for arguments such as those indicated above, local implementers may consider a reported NAC P =7 to provide sufficient accuracy for their TMA purposes.507

© 2009, RTCA, Inc. ANNEX G - ADDITIONAL CONSIDERATIONS FOR IMPLEMENTING ADS-B-RAD IN THE US NATIONAL AIRSPACE SYSTEM (NAS)

G.1 Overview

ADS -B-RAD safety, performance and interoperability requirements defined in the body of this document are based on a generic set of operational environments, as described in Annex A (OSED), and on a generic system architecture, as described in Annex B (OPA). In order for a specific ANSP to implement ADS-B-RAD, the unique aspects of that provider‟s operational environment and system architecture(s) must also be tak en into account. This Annex provides an example of how an ANSP might apply ADS-B-RAD to a specific, unique environment.

Specifically, this Annex addresses the unique aspects of the United States (US) National Airspace System (NAS) operational and system environments, and their impact on implementing ADS-B-RAD in the USA. Unique aspects of NAS operational environments are addressed in §G.2, and unique aspects of NAS system performance in §G.3. Additional safety assessment considerations are addressed in §G.4. The impacts on safety, performance and interoperability requirements are addressed in §G.5.

This Annex identifies a set of performance requirements for the use of ADS-B in radar airspace based on a comparison to modeled NAS MSSR performance. This comparative methodology by its nature imposes certain limitations that suggest further investigations may be appropriate. For example, some of the analyses in this Annex are based on an assumption that the tails of the azimuth error distribution for a NAS MSSR can be accurately modeled using a double-Gaussian curve, supported by a limited set of US MSSR data (see Annex B, Appendix B- 3) . This assumption differs from some previously published reports where the data collected showed that the radar azimuth erro r distribution could be modeled using a single Gaussian curve. As with any requirements development process, additional data collection and analyses will be performed to confirm that the assumptions made in this standard remain valid. If these data and analyses indicate that assumptions need to be modified, this document will be updated accordingly.

G.2 Operational Environment Considerations

For the purposes of this application, NAS operational environments are either similar or identical to the environments described in Annex A

. Specifically, the following scenarios identified in §A.4.1.2 apply for the NAS, as modified below:

RAD 1 : Target Environment: Single primary surveillance radar (PSR) with ADS -B surveillance in medium-density terminal airspace; Reference Environment: PSR with collocated monopulse secondary surveillance radar (MSSR) in medium-density terminal airspace.

RAD 2a : Target Environment: Single MSSR with ADS-B surveillance in highdensity En Route airspace. Reference Environment: Single MSSR in highdensity En Route airspace.

RAD 3 : Target Environment: Single PSR with collocated single MSSR together with ADS-B surveillance in high-density terminal airspace; Reference508

© 2009, RTCA, Inc. Environment: PSR with single collocated MSSR in high-density termina l

airspace.

Scenario RAD 2b (Single Mode S radar with ADS-B surveillance in high-density En Route) identified in §A.4.1.2 does not apply for the NAS, since the US has no plans to implement Mode S elementary or enhanced surveillance, as is currently required in ECAC airspace.

The separation minima described in §A.4.2.1.2 apply for the given scenarios in the NAS, except for approaches with successive aircraft on an adjacent ILS, as described below:

Scenario Minima RAD 1 3 .0 NM RAD 2a 5 .0 NM RAD 3 Sector- wi de minima 3 .0 NM Succeeding aircraft on same final 2. 5 NM Successive aircraft on adjacent ILS 1. 5 NM

It should be noted that the minimum spacing allowed between ILS runway centerlines for adjacent ILS approach operations in the NAS is 2500 feet . In addition to these minima, independent parallel approach operations are allowed in the NAS on ILS runways spaced no less than 4300 feet

between the runway centerlines

. All environmental assumptions and operational requirements identified in Annex A, as modified by the above environmental considerations, apply for the NAS. These assumptions and requirements, as modified, provide the basis for the development of performance and safety requirements specific to the implementation of ADS-B-RAD in the NAS

.

G.3 System Performance Considerations

G.3.1 MSSR Performance Model for the NAS

For the RAD application, minimum ADS-B performance requirements are based on a comparison to MSSR performance requirements. In Annex B, a generic MSSR performance model was developed to support the comparison. In the US, specific MSSR performance parameters are used based on existing MSSR specifications, summarized in Table G- 1.509

© 2009, RTCA, Inc. Table G-1: NAS MSSR Performance Parameters

Parameter Short Range Long Range

Azimuth Error Jitter σ = 0.068o

Bias ± 0.088o

Range Error Jitter σ = 25 ft.

Radar Bias ± 30 ft.
Location Bias ± 200 ft.

Interface Quantization Range 1/64 NM 1/8 NM

Azimuth 0.088o (1 ACP)
Assumed Residual Bias es (after correction in automation) Range

(including locat ion) 130 ft.

Azimuth 0.0 5 o
Update Rate 4.8s 12s

G.3.2 Horizontal Position Accuracy

In order to provide a basis for comparison of ADS-B and MSSR positional accuracy performance equivalent to that provided in Annex B, the range and azimuth errors shown above need to be restated in terms consistent with those used in Annex B (e.g., range error stated in terms of overall range error at the input to the ATC processing system, which includes contributions from the aircraft and the ground system interface, in addition to what is contributed by the MSSR alone). Only those errors that impact the relative separation of radar targets are used for the comparison of ADS-B and MSSR positional accuracy performance in the domain of interest

.

G.3.2.1 Terminal and Approach Domains

In the US NAS, terminal operations (3 NM and approach operations as described above) are supported today by no more than one MSSR for a given terminal sector, and in most cases by a single MSSR for the entire terminal airspace (i.e., multi-sensor fusion is not used). For the single MSSR case, azimuth biases and location biases do not affect the relative separation of MSSR targets, and are therefore not included in the comparison of ADS -B and MSSR accuracy performance in terminal airspace.

Overall Range Err or
The overall range error at the input to the ATC processing system consists of error contributions from the aircraft, the MSSR, and the ground system interface

. In the aircraft, range errors are introduced by variations in transponder response delay ti mes . ATCRBS transponders have the widest specified variations, with a bias of 3 ± 0.5 μsec and a jitter standard deviation (R TJ ) of 0.167 μsec (83.5 ft.). For this analysis, it is assumed that the fixed bias of 3 μsec is compensated, which results in a bias limit of ± 0.5 μsec. However, based on data recorded by the FAA over a population of aircraft equipped with ATCRBS transponders, the distribution of these biases is Gaussian, with a510

© 2009, RTCA, Inc.

standard deviation of approximately 0.2 μsec, which translates into a range bias error standard deviation (R TB ) over the population of aircraft of 100 ft. The ratio of this standard deviation to the bias limit of ±0.5 μsec gives a ratio of 0.4, which is assumed to conservatively represent typical performance results one would expect to find for similarly designed electronic equipment where performance is required to fall within a specific (uniform) range of values

. This assumption will be used later in this analysis.

As shown in Table G-1, the MSSR exhibits a range jitter standard deviation (R MJ ) of 25 ft. and a bias limit of ±30 ft. The location bias specified in Table G-1 does not affect the relative separation of MSSR targets for the single MSSR case, and so is not included in the comparison for the terminal and approach domains. For this analysis, it is assumed that, over the population of MSSRs in the NAS, the distribution of the range biases is Gaussian. It is also conservatively assumed that this distribution exhibits the same standard deviation to bias limit ratio of 0.4 as was found with the ATCRBS transponder delay time biases above. This results in a range bias standard deviation (R MB ) over the population of MSSRs of 12 ft.

As shown in Table G-1, the range output from the MSSR is quantized at the interface to the ATC processing system to 1/64 NM for terminal systems. The equivalent standard deviation of the range errors introduced by this quantization (R Q ) can be estimated as follows:

(G

- 1)

Therefore, for terminal systems R Q = 27.4 ft.

The standard deviation of the overall range error for the terminal and approach domains (R σT ) is described in terms of the individual error components described above, which are assumed to be independent, and can be approximated as follows:

= 136 ft. (41.4 meters) (G

- 2)

Overall Azimuth Error

The overall azimuth error at the input to the ATC processing system consists of error contributions from the MSSR and the ground system interface

. As shown in Table G-1, the MSSR exhibits an azimuth jitter standard deviation (A J ) of 0.068°. The residual azimuth bias specified in Table G-1 does not affect the relative separation of MSSR targets for the single MSSR case, and so is not included in the comparison for the terminal and approach domains. Similar to the discussion in AnnexB, the distribution of azimuth jitter errors for this analysis is also assumed to be doubleGaussian. This is an assumed distribution, based on a limited set of available data from selected operational systems in the NAS; should this assumed distribution change as a result of on-going data collection and analysis activities, then the results of the analysis contained in this Annex will need to be revised accordingly.

The double-Gaussian distribution can be described as a linear combination of a core and a tail distribution, with a tail weight factor (α) and a core

- to -tail standard deviation ratio

.511

© 2009, RTCA, Inc. The standard deviation of the double-Gaussian distribution can then be approximated from its core and tail components:

(G

- 3)

A core standard deviation (A JC ) of 0.046° and a tail standard deviation (A JT ) of 0.23° (core- to -tail ratio of 1:5) with a tail weight factor α = 0. 05 gives the overall azimuth jitter standard deviation (A J ) of 0.068° specified in Table G- 1.

As shown in Table G-1, the azimuth output from the MSSR is quantized at the interface to the ATC processing system to 1 ACP, or 0.088°. The equivalent standard deviation of the azimuth errors introduced by this quantization (A Q ) can be estimated as follows:

(G

- 4)

Therefore, the resulting A Q = 0.025°.

The standard deviation of the overall azimuth error for terminal and approach domains (A σT ) is described in terms of the individual error components described above, which are assumed to be independent, and can be approximated as follows:

= 0.072° (G

- 5)

G.3.2.2 En Route Domain

In the US NAS, En Route operations (5 NM operations as described above) are supported today by a mosaic of MSSRs that cover the majority of En Route airspace above 18 000 ft. mean sea level ( MSL ) (i.e., multi-sensor fusion is not used). Unlike the single MSSR case, for this mosaic case, azimuth biases and location biases will affect the relative separation of MSSR targets across mosaic sort boxes using different MSSR sources, and are therefore included in the comparison of ADS-B and MSSR accuracy performance in En Route airspace.

Overall Range Error

As in the terminal and approach domains, the overall range error at the input to the ATC processing system consists of error contributions from the aircraft, the MSSR, and the ground system interface

. In the En Route domain, range errors introduced by the aircraft are identical to those for the terminal and approach domains. Therefore, for the En Route domain, the transponder range jitter standard deviation R TJ = 83.5 ft., and the transponder range bias standard deviation R TB = 100 ft. over the population of aircraft.

For the En Route domain, the MSSR exhibits a range jitter standard deviation (R MJ ) of 25 ft. and a bias limit of ± 30 ft. as shown in Table G-1. Unlike the terminal and approach domains, however, the location bias of ± 200 ft. specified in Table G-1 will affect the relative separation of MSSR targets for the mosaic MSSR case, and so is included in the512

© 2009, RTCA, Inc.

comparison for the En Route domain. For this analysis, it is assumed that the residual range bias limit (including location bias) for a given MSSR is ±130 ft. as shown in Table G-1, and that over the population of MSSRs in the NAS, distribution of the biases is Gaussian. It is also conservatively assumed that this distribution exhibits the same standard deviation to bias limit ratio of 0.4 as was found with the ATCRBS transponder delay time biases above. This results in a range bias standard deviation (R MB ) over the population of MSSRs of 52 ft.

As shown in Table G-1, the range output from the MSSR is quantized at the interface to the ATC processing system to 1/8 NM for En Route systems. The equivalent standard deviation of the range errors introduced by this quantization (R Q ) can be estimated as follows:

(G

- 7)

Therefore, for En Route systems R Q = 219 ft.

The standard deviation of the overall range error for the En Route domain (R σE ) is described in terms of the individual error components described above, which are assumed to be independent, and can be approximated as follows:

= 261 .2 ft. ( 79

. 6 meters) (G

- 8)

Overall Azimuth Error

The overall azimuth error at the input to the ATC processing system consists of error contributions from the MSSR and the ground system interface

. The MSSR azimuth jitter standard deviation (A J ) and distribution is identical for both the En Route domain and the terminal and approach domains, and so A J = 0.068° (doubleGaussian), with a core standard deviation (A JC ) of 0.046° and a tail standard deviation (A JT ) of 0.23° (core- to -tail ratio of 1:5), with a tail weight factor α = 0. 05. Unlike the terminal and approach domains, however, the residual azimuth bias of ± 0.05o specified in Table G-1 will affect the relative separation of MSSR targets for the mosaic MSSR case, and so is included in the comparison for the En Route domain. For this analysis, it is assumed that, over the population of MSSRs in the NAS, the distribution of MSSR azimuth biases is Gaussian. It is also conservatively assumed that this distribution exhibits the same standard deviation to bias limit ratio of 0.4 as was found with the ATCRBS transponder delay time biases above. The resulting standard deviation of azimuth biases (A B ) over the population of MSSRs can then be approximated as 0. 02 °.513

© 2009, RTCA, Inc. As with terminal systems, the azimuth output from the MSSR is quantized at the interface to the ATC processing system to 1 ACP, or 0.088°. The equivalent standard deviation of the azimuth errors introduced by this quantization (A Q ) is therefore approximately 0.025°

. The standard deviation of the overall azimuth error for the En Route domain (A σE ) is described in terms of the individual error components described above, which are assumed to be independent, and can be approximated as follows:

= 0. 075 ° (G

- 9)

G.3.3 Comparative Performance Analysis

The required ADS-B performance for the US NAS can be determined in the same manner as described in Annex B, namely by comparison to MSSR performance, as shown below.

G.3.3.1 Update Rate

As shown in Table G-1, the update rate for MSSR in the US NAS is 4.8 sec onds for the terminal domain, and 12 s ec onds for the En Route domain. The minimum update rate for surveillance reports, changes in Mode A Codes, and changes in emergency and SPI information broadcast using ADS-B is therefore the same as for MSSR, namely 4.8

sec onds for terminal and 12 sec onds for En Route.

G.3.3.2 Horizontal Position Accuracy

The required horizontal position accuracy for ADS-B is determined in the same manner as described in Annex B: First, a range of applicability is selected for each domain of interest (En Route, terminal, approach); Next, the 95% point of the relevant error distribution (ε95) at that range is determined by multiplying the standard deviation of the overall error distribution by 1.96; The required ADS-B horizontal position accuracy radius ( r

a ) can then be determined as follows:

r a = 1.25*ε95 (G

- 10)

The En Route range of applicability is 60 NM, as shown in Annex B. At this range, the elliptical MSSR position error distribution is dominated by the azimuth error (see Figure 2 in Annex B), and so the MSSR azimuth error distribution is used for the comparison. Using A σE = 0.075° from §G.3.2.2 above, the 95% point of the azimuth error distribution is 0. 147 °. The magnitude (in feet) at the range of applicability is therefore:

= 935 .3 ft. (285 meters) (G

- 11)

Therefore, the En Route r a = 1247 ft. (380 meters), which results in a required NAC P = 7 (185 meters) to ensure ADS-B accuracy performance that is equivalent to or better than that of MSSR throughout the domain airspace.

For the terminal domain, the range of applicability selected is the range at which the 95% point of the azimuth and range error distributions are equal. The 95% point of the range error distribution is 266.6 ft. (81.2 meters) for terminal airspace; therefore, the range at which the 95% point of the azimuth distribution equals this value is 21 NM (this range514

© 2009, RTCA, Inc.

will also be used in the determination of the required NIC value, shown later in this Annex). Therefore, the terminal r a = 333 ft. (102 meters), which results in a required NAC P =8 (93 meters ).

For the approach domain, the range of applicability is 15 NM, as shown in Annex B. In this domain, the elliptical MSSR position error distribution is dominated by the range error (see Figure 2 in Annex B), and so the MSSR range error distribution is used for the comparison. The 95% point of the range error distribution is 266.6 ft. (81.2 meters). Therefore, the approach r a = 333 ft. (102 meters), which results in a required NAC P = 8 (93 meters ).

G.3.3.3 Data Integrity

As described in Annex B, ADS-B data integrity requirements are based on the SSE analysis presented in Annex D; Data integrity requirements for the US NAS are determined in the same fashion.

Requirements on ADS-B data integrity (NIC) are levied to ensure that the ADS- B surveillance separation error is no greater than that of radar in the event that the ADS- B position source is in a faulted condition. The ADS-B horizontal position surveillance component of risk is compared (SSE) with that of the reference MSSR, the assumption being that the reference radar already achieves safe separation in a similar air traffic environment

. The range and azimuth errors described in §G.3.2 apply for the SSE analysis; however, bias errors are treated in a different manner as explained in Annex D for determining the integrity requirements (worst-case analysis). That is, where cross-range errors are considered, as in the En Route and terminal domains, the SSE analysis assumes that there is a fixed residual 0.05° radar bias (instead of the distribution assumed in §G.3.2), and the two aircraft follow the worst-case geometry along the azimuthal bias (for example, in anorbit around the radar). Furthermore, it is assumed the radar bias is away from the ADSB-equipped aircraft. Where range errors are additionally considered, as in the approach domain, the SSE analysis assumes there is a fixed residual 1 30 ft. radar bias (instead of the distribution assumed in §G.3.2) away from the ADS-B-equipped aircraft. These radar biases apply to ADS-B to Radar mixed surveillance and to cases where different radars provide surveillance of nearby aircraft.

Figure G-1 shows the overall (nominal and faulted) SSE analysis results for ADS-B- to

ADS -B and ADS-B- to -radar surveillance separation error probabilities in comparison with the reference radar- to -radar separation, for the En Route domain, at the reference range of 60 NM and NAC P = 7. Based on this result, it is concluded that ADS-B with NIC=5 or better provides equal or less surveillance separation error for ADS-B- to

- ADS

B and radar-to- ADS -B than the reference radar- to -radar 5 NM separation.515

© 2009, RTCA, Inc. Figure G-1: Comparative SSE probability for En Route 5 NM application in the US NAS

Figures G- 2 and G-2a show the overall (nominal and faulted) SSE analysis results for ADS -B- to -ADS-B and ADS-B- to -radar surveillance separation error probabilities in comparison with the reference radar- to -radar separation, for the terminal domain, at the reference range of 21 NM and NAC P = 8, showing the results for both NIC=7 and Rc=0. 3 NM . Based on this result, it is concluded that ADS-B with Rc ≤ 0. 3 NM provides equal or less surveillance separation error for ADS-B- to -ADS-B and radar- to

- ADS -B than the reference radar- to -radar 3 NM separation. 80m516

© 2009, RTCA, Inc.

Figure G-2: Comparative SSE Probability for Terminal 3 NM Application in the US NAS517

© 2009, RTCA, Inc. Figure G- 2a : Comparative SSE probability for NAS terminal 3 NM application, Rc = 0.3 NM Figure G-3 shows the overall (nominal and faulted) SSE analysis results for ADS-B- to

ADS -B and ADS-B- to -radar surveillance separation error probabilities in comparison with the reference radar- to -radar separation, for the approach domain, at the reference range of 15 NM and NAC P = 8. Based on this result, it is concluded that ADS-B with NIC=7 or better provides equal or less surveillance separation error for ADS-B- to

- ADS

B and rad ar

- to

- ADS -B than the reference radar- to -radar separation for in-trail and dependent parallel approach scenarios.

For independent parallel approach scenarios, however, in addition to the 2.5 NM separation between the two aircraft that are in-trail, each aircraft must stay outside the Non Transgression Zone (NTZ). Following a similar approach as described in Annex D, fault condition biases are expected to be detected prior to the aircraft reaching the NTZ when a NIC of at least 7 is used for ADS-B. Additionally, with a NIC=7, the ADS- B apparent NTZ penetration probability without alerting per approach is better than that of radar. Based on this, it is concluded that a minimum value of NIC=7 is required for independent parallel approach operations. 

### Rc=0. 3 NM518

© 2009, RTCA, Inc.

Figure G-3: Comparative SSE probability for approach application in the US NAS

G.3.3.4 Surveillance Report Latency

The latency of ADS-B surveillance reports is defined such that communication latency to the automation system is included in the requirement, in contrast to how this requirement is stated in Table 4a (see SPR 35). For the US NAS, the 95% latency for ADS- B surveillance reports (measured between interface points D and E2

, including communication latency to the ATC processing system) shall be no greater than 0. 7 seconds.

G.3.3.5 Other Performance Requirements

All other ADS-B performance requirements for the US NAS are the same as described in Annex B

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© 2009, RTCA, Inc. G.4 Safety Assessment Considerations

Each hazard identified in Annex C was reassessed in light of the environmental

co nsiderations described in §G.2 above

. After careful review by NAS operational subject matter experts (SMEs), the introduction of the environmental considerations described above present no changes to the severity classifications of the hazards in Annex C for the NAS . All environmental conditions, failure modes, system failure likelihoods, and internal and external mitigation means identified in Annex C remain unchanged as well, and apply for the NAS

. As a result, all safety requirements based on the SMS process (using US safety objectives) identified in Annex C remain unchanged given the environmental considerations described above, and so these requirements apply as stated in Annex C for the NAS. Safety requirements based on the ESARR-4 process identified in Annex C, however, apply only for ECAC airspace, and do not apply for the NAS.

G.5 Impacts on Safety, Performance and Interoperability Requirements for the NAS

Unique aspects of US NAS operational and system environments resulted in several changes to A DS -B performance requirements as they apply to the NAS, as compared to the generic case presented in the body of the ADS-B-RAD document, but resulted in no changes to NAS ADS-B safety or interoperability requirements. Changes to ADS- B performance requirements for the NAS are summarized below, denoted by the prefix NAS PR :

NAS PR 1: The update interval for ADS-B surveillance reports containing position or

other state vector information measured at point E2 shall be no longer than 12s (95%) in NAS En Rou te radar airspace.

NAS PR 2: The update interval for ADS-B surveillance reports containing position or

other state vector information measured at point E2 shall be no longer than 4.8 seconds (95%) in NAS terminal radar airspace.

NAS PR 3: The time interval between a change of Mode A Code or emergency and SPI

information at point B1 and an ADS-B surveillance report containing the new Mode A Code or emergency and SPI information at point E2 shall be no longer than 12 seconds (95%) in NAS En Route radar airspa ce.
NAS PR 4: The time interval between a change of Mode A Code or emergency and SPI

information at point B1 and an ADS-B surveillance report containing the new Mode A Code or emergency and SPI information at point E2 shall be no longer than 4.8 seconds (9 5%) in NAS terminal radar airspace.

NAS PR 5: For 3 NM separation, the required containment radius Rc shall be ≤ 0. 3 NM

( NIC ≥ 7 ).

NAS PR 6: The 95% latency for ADS-B surveillance reports (measured between

interface points D and E2

, including communication latency to the ATC processing system) shall be no greater than 0. 7 seconds.520

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