The SSWG’s analyses referenced in the rule are found in-- 

RTCA DO-318 Safety, Performance and Interoperability 

Requirements Document for Enhanced Air Traffic Services in 

Radar-Controlled Areas Using ADS-B Surveillance (ADS-B RAD) 

Annex B—Operational Performance Assessment (OPA) (See below) 

Annex G—Additional Considerations for Implementing ADS-B RAD in the US National 

Airspace System (NAS) (See below) 

(This material is copyrighted by RTCA, Inc., and 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; telephone (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

© 2009, RTCA, Inc. 

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 using 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 of 

two ADS-B-equipped aircraft, this application addresses separation of one ADS-Bequipped aircraft from one non-ADS-B-equipped aircraft whose surveillance information 

is 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 Comparison - 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 

boundaries, 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 position 
128

© 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: Per 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 is 
129

© 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 separation 

minima (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-density 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 

data 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. 
130

© 2009, RTCA, Inc. 

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. Local 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 60 
131

© 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 demanding 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

© 2009, RTCA, Inc. 

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 component 
133

© 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. 

Range Errors 

Equal 

Cross-Range 

Errors

Range 

Errors 

Greater 

than 

Azimuth 

Errors

Radar ADS-B

Case 1

Reference Range from Radar

Cross-Range Error

ADS-B

Case 2

Azimuth 

Errors 

Greater 

than 

Range 

Errors

RoA1

RoA2

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-B 
135

© 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 aircraft 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 at the Input of 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 (callsign 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, azimuth Latitude, longitude

Pressure Altitude Mode C from SSR Pressure altitude from ADS-B

Time stamp5

Time stamp corresponds to 

time of measurement of target 

position

Time of applicability of ADS-B 

information in the report

Quality indicator6

affecting individual 

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 position quality 

is suitable for surveillance 

separation 

Emergency indicators From Mode A Codes

Emergency indicators from ADSB 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 system7

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

© 2009, RTCA, Inc. 

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. Hence, 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 ADS-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 requirements 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 Route)

Value 

(TMA)

Update interval

Reference radar: 

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

ADS-B systems: 

Update interval for ADS-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

Probability

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)

Emergency and SPI code validation (same as for Mode A Code

validation)

(Values taken from Eurocontrol 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 Route)

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 rate 

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, 
142

© 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 second) 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 

implementation 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 aircraft 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 (ε95) 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 positions 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 (ra) 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 dimension radar 95% accuracy (ε95). The required ADS-B 

95% accuracy 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 (ra) is given by: 

ra = (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 provides 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 NACP quality indicators convey information on reported ADS-B position 

integrity and accuracy, respectively. Ideally, NIC and NACP should be sent in every 

ADS-B message that contains position information. However, some ADS-B link formats 

do not send NACP in the message containing position, but instead send NACP in a 

separate message, and at a different rate. Reports assembled by the ground system may 

therefore contain the latest position information with a NACP value received some time 

earlier. Consequently, if the NACP value changes from a good value (i.e., NACP ≥ 

minimum) to a poor value (i.e., NACP < minimum), the ATC system will not 

immediately be aware of the degradation in position accuracy and will use a previous 

NACP, that does not accurately reflect the quality of the reported accuracy, for some time 

until a change in NACP value is received by the ground system. 

Requirements must therefore be levied to limit the age of the NACP information, 

considering the following factors: 

1. The likelihood of a NACP 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 NACP change. 

First, the likelihood of a NACP change from a good to poor value will be very low 

because of service continuity requirements (i.e., both NIC and NACP 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 NACP 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 Pm

3

, where 

Pm 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 Pm is small, the probability of getting thee plots anywhere in an hour is 

approximately (Pm

3

) * 720. 

The comparison is made with a probability of 0.001 per hour of undetected ADS-B 

accuracy degradation [(Pm

3

) * 720 = 0.001, resulting in Pm=0.0111572]. Given the 

Gaussian Cumulative Distribution function, a Pm 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, Pm=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 ~ 0.33*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 (NACP) change. Therefore, if NACP is unknown or 

old, the minimum possible NACP can be determined from NIC as above, or alternately 

using RTCA DO-303, or EUROCAE ED-126, Table 64. The scope for an undetected 

NACP change from good to poor without a change of NIC will depend upon the 

minimum NIC and NACP 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 NACP=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 NACP 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 Route)

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 accuracy

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 Route)

Value 

(TMA)

Horizontal Position Accuracy

ADS-B system: 

Radial accuracy (ra)

Note: The radial accuracy (ra ) is scaled 

from the radar single dimension 

(ε95) (see §B.4.2.1).

Approach Scenarios

Radial accuracy (ra)

<308 meters (based 

on radar crossrange)

(NACp of 7 is 185 

meters)

<171 meters (based on radar 

cross-range)

(NACp of 8 is 93 meters )

Approach Scenarios

< 171 meters (based on 

radar along-range)

(NACP of 8 is 93 meters )
149

© 2009, RTCA, Inc. 

Accuracy Value 

(En Route)

Value 

(TMA)

Vertical Position Accuracy

Reference radar system:

Altimeter accuracy

Note: These are minimum accuracy 

requirements for altimeters and are 

dependent on the type of airspace. 

Many 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 

Avionics 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 (NACP) 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 NACP

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 NACP=8 ensures global applicability, as NACP=8 has been 

determined to indicate equivalent or better performance in comparison to 

radar based upon the comparative RFG analysis using either the US or 
150

© 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 NACP 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 NACP=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 

(NACP=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 

(NACP=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 

(NACP=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 

Accuracy 

Requirement

Value 

(En

Route)

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.

NACP reporting

Determine a 

minimum NACP

value from the 

NIC value of the 

position when 

NACP 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 for 

loss of accuracy

NACP = 7

(En Route)

NACP = 8

(TMA)

NACP = 8

(Approach Scenarios)

Vertical Position Accuracy

Altimeter accuracy 38.1 meters (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 

NACP=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 the 

distance 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 NACP=8. A NACP=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 NACP=8. An NACP=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 ED108A) 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 NACP 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 NACP=7 and 8 against aircraft speed 

are shown in Figure B-4. It can be seen that the degradation in accuracy from 

uncompensated latency uncertainty, for any likely aircraft speeds, does not cause the 

NACP=7 curve to exceed the en-route radar reference accuracy, or the NACP=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 Comparison 
154

© 2009, RTCA, Inc. 

0

50

100

150

200

250

0 100 200 300 400 500 600

Speed (knot)

Along track accuracy 95% (m) 

En-route radar reference

NACp=7 with latency uncertainty

TMA radar reference

NACp=8 with latency uncertainty

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 preservation 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-alert 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 Route)

Value 

(TMA)

Integrity of Horizontal Position 

Reference radar system: 

Surveillance 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 azimuth or 700 meter 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

(OPAASSUMP.17)

Avionic standard

(OPAASSUMP.17)

Integrity of Identity, Emergency and SPI

Reference radar: 

Mode A source integrity Avionic standard Avionic standard

ADS-B system: 

Identity, Emergency and SPI Integrity

Assumed to be at 

least as good as 

SSR Mode A 

(OPAASSUMP.17)

Assumed to be at 

least as good as 

SSR Mode A 

(OPAASSUMP.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-7Table 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 Integrity 

Requirement

Value 

(En Route)

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 Scenarios)

Integrity Change <2 s <2 s N/A <2 s

Vertical Position

Altimeter Source 

Integrity

Avionic 

standard

Avionic 

standard N/A No worse than for SSR case

Identity and Emergency and SPI

Identity, Emergency and 

SPI Integrity

At least as 

good as SSR

Mode A

[TGL 13]

At least as 

good as SSR

Mode A

[TGL 13]

N/A No 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-8Table 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 Route)

Value 

(TMA)

Identity, Emergency and SPI

Reference radar: 

Probability of incorrect Mode A validation

Probability of incorrect emergency and SPI code8

< 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, based 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 reporting 
160

© 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-9Table B. 

Table B-9: Summary of Data Preservation Requirements 

Data Preservation 

Requirement

Value 

(En Route)

Value 

(TMA)

GROUND

(“D”  “E2”)

AIRBORNE

(“A1”  “D”)

Identity, Emergency 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 SSR

(OPAASSUMP.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

(OPAASSUMP.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) Failure 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-10Table 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 Route)

Value

(TMA)

Surveillance Provision Continuity for multiple

(or all) aircraft

Reference radar system:

Continuity probability per 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. .

>0.99966 per 

hour 

(≤ 3 per year)

>0.99966 per 

hour

(≤ 3 per year)

ADS-B system (including ground receive function + 

GNSS Signal-in-Space):

Multiple aircraft surveillance provision continuity 

probability per hour (failures per year)

Note: Because this performance comparison 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 continuity and data continuity. This 

includes respective allocations to the Ground 

Domain and external functional segments.

>0.99966 per 

hour 

(≤ 3 per year)

>0.99966 per 

hour 

(≤ 3 per year)

System continuity for single aircraft

Reference transponder:

System continuity of SSR Mode A/C transponder system 

(typical values per flight hour)

Notes:

1. These values are based on typical values of 5000 

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 continuity of single aircraft overall ADS-B 

function (between A1 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 0.9998
164

© 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. 

For 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-12Table B examines latency, time-stamp accuracy, and capacity for radar and 

the 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 E2), 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

(En Route)

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 Route 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: Any latency uncertainties onboard the aircraft have the effects of a 

reduction in position accuracy in the along-track direction (See also 

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 Route 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: Any 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 be 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 is 

split 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 Route)

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 

items, 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 Within 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-12Table 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

Route)

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 performance 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 

environment as 

described in the 

OSED without 

degradation.

N/A
171

© 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-stamping 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 

For 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 can 

determine 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 receiving 
172

© 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 

and 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. 
175

© 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 scan
176

© 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 azimuth 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 B15Table 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 Louisville 
177

© 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-7Figure 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-8Figure B. 
178

© 2009, RTCA, Inc. 

Figure B-6: FAA Single Gaussian Model 

FAA

FAA 
179

© 2009, RTCA, Inc. 

Figure B-7: Normal-Normal MSSR Model With Empirical Fit to Measured Data 
180

© 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:

b 0.05 w 0.25 0.05

FAA Gaussian pnS:

nS 0.068

1 0.5 0 0.5 1

1 10 3

0.01

0.1

1

10

APL/LL data

AFS-450 Gaus fit

N-N MSSR fit

Azimuth angle, degrees

Prob density

FAA
181

© 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-9Figure 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 the 

radar 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 (NACP=7) will degrade to a NACP=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 NACP=6 equivalent over the display interval, whenever a NACP=7 or better ADSB report is received). 
182

© 2009, RTCA, Inc. 

Display Accuracy Example - (4.8 sec Radar Scan)

(Display Accuracy Degrades to ~ 0.4 nm Prior to Refresh)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0 4.8 9.6 14.4 19.2

Time in Seconds

Display Accuracy in nm

Assumptions: 

Radar Accuracy = 0.1 nm

Target Speed = 300 knots

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 cycle that is 180 degrees out of phase with the radar scan 

(see Figure B-10Figure 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 this 

Appendix, 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 scan 
183

© 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). 

Traffic Display Concept for Two Times Refresh ADS-B Surveillance (Azimuth Stabilized Display)

Range 

 360 deg 270 deg 180 deg 90 deg 0 deg

5 sec Radar Azimuth Sweep

*

*

*

*

*

*

Radar Azimuth

Sweep Position

*

Updated Radar 

/ADS-B Plot *

180o Predicted 

ADS-B Positions

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 of 

velocity 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 extrapolating 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 operations. 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 may 
184

© 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 (NACP =4 equivalent) by limiting the 

display interval to 5 seconds or less). Moreover, separation buffers are typically added to 

mitigate position uncertainty in various controller decision support tools such as CenterTRACON Automation System timed arrival management, User Request Evaluation Tool 

medium-term conflict probe, and short-term conflict alerting that could potentially be 

reduced on the basis of enhanced surveillance, coupled with a reduced traffic display 

interval. 

En-Route Radar Display Accuracy with 10 Sec Sweep Rate

(Desired MSSR Accuracy = 0.5 nm) 

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 5 10 15 20

Time in Seconds

Display Accuracy in nm

Assumptions

* Radar Acc <= 0.5 nm

* Target Speed = 600 kt

* Display Latency = 10s

Figure B-11: Example Long Range En Route MSSR Display Accuracy 
185

© 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-12Figure 

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

T0 T1 Time

0

T2 T3

HPL

Position from 

positioning source

∂

Position fault 

detected
187

© 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 (RC). 

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 (RC) 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 meters. 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). 
188

© 2009, RTCA, Inc. 

ANNEX B APPENDIX B-5 

CONSIDERATIONS FOR TIME CORRELATION OF POSITION ERROR 

B-5.1 Introduction 

B-5.1.1 Purpose 

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 position 

errors 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 to 

support 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-13Figure 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-14Figure 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-15Figure 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 these 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 integrity 

performance 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). 
191

© 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-15Figure B, the 

controller is unaware of the undetected GPS bias RC, 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 the 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-16Figure 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. 

Displayed positions shown in blue. 

Actual position shown in red

Note: Actual aircraft positions will not be displayed to the 

controller

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 NACP=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 be 

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 of 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. 

For more adverse geometries and opposing biases (as shown in Figure B-15Figure B and 

Figure B-16Figure B, these smaller integrity biases may still have a significant impact on 

a controller‟s capability to provide the desired 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/NACP) 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 NACP is directly determined based upon 

the reference range and the radar error model. The NACP is then used in the analysis that 

is performed to determine NIC (see Annex D). 

B-6.1.2 Purpose 

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 at 

the 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 

NACP 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 

NACP 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 NACP values. 

B-6.2.2 NIC

The SSE analysis detailed in Annex D was performed and the resulting NIC values per 

required containment radius (RC) are as shown in Table B-17. Corresponding graphs in 
195

© 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 NACP

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

(RC = 0.6 NM)

60 NM 5

(RC = 1 NM)

En Route

120 NM 4

(RC = 2 NM)

200 NM 4

(RC = 2 NM)
196

© 2009, RTCA, Inc. 

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

10-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

E (NM)

Prob(Separation Error > E)

HPE=HPL

HPE=HPL

HPE=.27 HPL

HPE=.27 HPL

HPE=.59 HPL

HPE=.59 HPL

ADSB-to-ADSB, NIC=6,NAC=7

ADSB-to-ReferenceRadar

Reference Radar-to-Radar, R=40NM

Radar-to-Radar Range, R=70m

Figure B-17: SSE Probability, R=40 NM 

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

10-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

E (NM)

Prob(Separation Error > E)

HPE=HPL

HPE=HPL

HPE=.27 HPL

HPE=.27 HPL

HPE=.59 HPL

HPE=.59 HPL

ADSB-to-ADSB, NIC=5,NAC=7

ADSB-to-ReferenceRadar

Reference Radar-to-Radar, R=60NM

Radar-to-Radar Range, R=70m

Figure B-18: SSE Probability, R=60 NM Terminal 
197

© 2009, RTCA, Inc. 

Figure B-19: SSE Probability, R=120 NM 

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

10-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

E (NM)

Prob(Separation Error > E)

HPE=HPL

HPE=.27 HPL

HPE=.59 HPL

ADSB-to-ADSB, NIC=4,NAC=5

ADSB-to-ReferenceRadar

Reference Radar-to-Radar, R=200NM

Radar-to-Radar Range, R=70m

Figure B-20: SSE Probability, R=200 NM 
198

© 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 Terminal 
199

© 2009, RTCA, Inc. 

Figure B-23: Partial SSE Probability, Separation Reduction > HPL, R=120 NM 

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

10-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

E (NM)

Prob(Separation Error > E)

ADSB-to-ADSB, NIC=4, NAC=5, ERRORFAULT=2.0xHPL

ADSB-to-ADSB, NIC=4, NAC=5, ERRORFAULT=1.5xHPL

Ref Radar-to-Radar, R=200NM

Radar-to-Radar Range, R=70m

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

© 2009, RTCA, Inc. 

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

E (NM)

Prob(Separation Error > E)

HPE=HPL

HPE=.32 HPL

HPE=.62 HPL

Figure B-25: Conditional ADS-B SSE Probability, NIC=4 

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

E (NM)

Prob(Separation Error > E)

NIC=4, ERRORFAULT=2.0xHPL

NIC=4, ERRORFAULT=1.5xHPL

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

Reduction > HPL 
201

© 2009, RTCA, Inc. 

ANNEX B APPENDIX B-7 

CONSIDERATION OF NACP=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 NACP=8 (0.05 NM, 93 meters) 

is needed. This value also satisfies more stringent USA radar comparison models. 

Hence, NACP=8 is considered as being the required minimum value to ensure “global 

interoperability.”

B-7.2 Consideration for NACP=7 Accuracy 

Local implementers may consider a lower accuracy of NACP=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 NACP=7 being 

adequate accuracy for TMA may be based upon: 

1. The NACP=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 NACP=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 NACP=7 is likely to be very pessimistic. 

Firstly, the 14 meter difference between the upper limit (i.e., worst case) NACP=7 

accuracy of 185 meters and the required 171 meters may be considered to be 

operationally 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 from 

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 NACP=7 is better than the required accuracy of 171 

meters. Assuming that NACP=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 NACP=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 NACP=7 are shown in the figure below. It 
202

© 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. 

60

80

100

120

140

160

180

200

220

0 100 200 300 400

Speed (knot)

 95% accuracy (m) 

TMA radar reference

NACp7(upper) with latency uncertainty

NACp7(average) with latency uncertainty

NACp7(lower) with latency uncertainty

Figure B-27: Effects of 0.2 second uncompensated latency uncertainty 

Finally, for legacy SA-unaware GPS receivers, the availability of reported NACP=8 is a 

problem because the receivers always assume the degraded ranging accuracy of the 

satellite signals. (Typically the availability of reported NACP=8 is in the order of 99% or 

less and this would be insufficient for ATC operations). In comparison, a reported 

NACP=7 or better has very high availability. Furthermore, the actual accuracy of a 

reported NACP=7 from an SA-unaware receiver is very likely to be much better. 

(Typically, the actual accuracy of a reported NACP=7 has >> 0.99 probability of being 

better). 

Consequently, for arguments such as those indicated above, local implementers may 

consider a reported NACP=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 taken 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 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. 

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; Reference 
508

© 2009, RTCA, Inc. 

Environment: PSR with single collocated MSSR in high-density terminal 

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-wide 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 

Biases (after 

correction in 

automation)

Range

(including 

location)

130 ft.

Azimuth 0.05o

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 Error

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 

times. ATCRBS transponders have the widest specified variations, with a bias of 3 ± 0.5 

μsec and a jitter standard deviation (RTJ) 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 a 
510

© 2009, RTCA, Inc. 

standard deviation of approximately 0.2 μsec, which translates into a range bias error 

standard deviation (RTB) 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 (RMJ) 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 (RMB) 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 (RQ) can be estimated as 

follows: 

12

Quantization RQ

(G-1)

Therefore, for terminal systems RQ = 27.4 ft. 

The standard deviation of the overall range error for the terminal and approach domains 

(RσT) is described in terms of the individual error components described above, which are 

assumed to be independent, and can be approximated as follows: 

2 2 2 2 2 R T RTJ RTB RMJ RMB RQ

 = 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 (AJ) 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 Annex 

B, 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: 

(1 )( ) ( )( )

2 2 AJ AJC AJT

(G-3)

A core standard deviation (AJC) of 0.046° and a tail standard deviation (AJT) 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 (AJ) 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 (AQ) can be estimated as follows: 

12

Quantization AQ

(G-4)

Therefore, the resulting AQ = 0.025°. 

The standard deviation of the overall azimuth error for terminal and approach domains 

(AσT) is described in terms of the individual error components described above, which are 

assumed to be independent, and can be approximated as follows: 

2 2 2

(1 )( ) ( )( ) A T AJC AJT AQ

 = 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 18000

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 RTJ = 83.5 ft., and the transponder range bias standard 

deviation RTB = 100 ft. over the population of aircraft. 

For the En Route domain, the MSSR exhibits a range jitter standard deviation (RMJ) 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 the 
512

© 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 (RMB) 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 (RQ) can be estimated as 

follows: 

12

Quantization RQ

(G-7)

Therefore, for En Route systems RQ = 219 ft. 

The standard deviation of the overall range error for the En Route domain (RσE) is 

described in terms of the individual error components described above, which are 

assumed to be independent, and can be approximated as follows: 

2 2 2 2 2 R E RTJ RTB RMJ RMB RQ

 = 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 (AJ) and distribution is identical for both the 

En Route domain and the terminal and approach domains, and so AJ = 0.068° (doubleGaussian), with a core standard deviation (AJC) of 0.046° and a tail standard deviation 

(AJT) 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 (AB) 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 (AQ) is therefore approximately 0.025°. 

The standard deviation of the overall azimuth error for the En Route domain (AσE) is 

described in terms of the individual error components described above, which are 

assumed to be independent, and can be approximated as follows: 

2 2 2 2

(1 )( ) ( )( ) A E AJC AJT AB AQ

 = 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 seconds for the 

terminal domain, and 12 seconds 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 

seconds for terminal and 12 seconds 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 (ra) can then be determined as follows: 

ra = 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 AσE = 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: 

0.147 (60 6076)

180

= 935.3 ft. (285 meters) (G-11)

Therefore, the En Route ra = 1247 ft. (380 meters), which results in a required NACP=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 range 
514

© 2009, RTCA, Inc. 

will also be used in the determination of the required NIC value, shown later in this 

Annex). Therefore, the terminal ra = 333 ft. (102 meters), which results in a required 

NACP=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 ra = 333 ft. (102 meters), which results in a required NACP = 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 an 

orbit 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 130 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-toADS-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 NACP = 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-ADSB 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 NACP = 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. 

80m
516

© 2009, RTCA, Inc. 

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

US NAS
517

© 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-toADS-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 NACP = 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-ADSB and radar-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 

NM
518

© 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. 
519

© 2009, RTCA, Inc. 

G.4 Safety Assessment Considerations 

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

considerations 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 ADS-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 Route 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 airspace.

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 (95%) 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

© 2009, RTCA, Inc. 

This Page Intentionally Left Blank.