**THE TWENTY-SIXTH MEETING OF THE**

**INFORMAL PACIFIC ATC CO-ORDINATING GROUP**

**(IPACG/26)**

(Anchorage, Alaska, USA, May 14-18, 2007)

Agenda Item 6: Air Traffic Management (ATM) Issues

**Brief Description of the Safety Assessment Process and Data
Requirements to Support Operational-Trial Use of ADS-based Separation
Standards In the Anchorage Flight Information Region**

(Presented by the Federal Aviation Administration)

[Summary]{.underline}

This paper provides a brief description of the safety assessment process
and data requirements for Automatic Dependent Surveillance (ADS)-based
separation standards in the Anchorage FIR. It also provides a
preliminary summary of ADS operations observed to date in Anchorage
oceanic airspace.

**1. Introduction**

1.  On 1 March 2007, the Ocean21 oceanic automation system was
    introduced into full-time operation at the Anchorage Air Route
    Traffic Control Center (ARTCC) in oceanic sectors 10 and 11. This
    event followed the introduction of the Ocean21 system to the Oakland
    ARTCC in October 2005. The FAA initiated the use of the 50-nm
    longitudinal separation standard in the Oakland FIR with the
    introduction of the Ocean21 system in October 2005.

2.  After demonstration that necessary performance is adequate, the FAA
    intends to extend use of the 50-nm longitudinal separation standard
    to portions of the Anchorage oceanic airspace within which the
    agency provides air traffic services under delegation from the
    International Civil Aviation Organization (ICAO). ICAO Document 9689
    (reference 1) contains detailed information pertaining to the
    required analyses needed to justify the application of the 50-nm
    longitudinal separation standard in Anchorage oceanic airspace.

3.  This paper will present a brief description of the data needed and
    the safety assessment methodology required to implement Automatic
    Dependent Surveillance (ADS)-based separation standards into
    Anchorage oceanic airspace.

4.  This paper will also present a preliminary summary of ADS operations
    observed to date in Anchorage oceanic airspace.

```{=html}
<!-- -->
```
1.  **Discussion**

    1.  *Requirements for the 50-nm Longitudinal Separation Standard*

        1.  The requirements for implementation of a 50-nm longitudinal
            separation standard using ADS are listed in Section 5.4.2.6
            of ICAO Document 4444 (reference 2). Among other items, this
            Section requires that aircraft must exhibit Required
            Navigation Performance Type 10 (RNP-10), and specifies the
            need for ADS with a maximum periodic reporting interval of
            30 minutes. Additional ADS requirements are specified in
            Chapter 13 of reference 2.

    2.  *30-nm Lateral and Longitudinal Separation Standards*

        1.  On 22 December 2005, the FAA implemented 30-nm lateral and
            longitudinal separation standards on an operational trial
            basis in a portion of the Oakland Oceanic FIR. The FAA
            expects to extend the use of the 30-nm standards to other
            portions of Oakland oceanic airspace in the near future.
            Implementation of the 30-nm ADS-based separation standards
            in Anchorage oceanic airspace will take place after the
            necessary performance has been demonstrated and a safety
            assessment has been conducted.

        2.  The requirements for implementation of a 30-nm lateral
            separation standard are listed in Attachment B to the ICAO
            Annex 11 (reference 3). This documentation specifies that
            aircraft to be separated laterally by 30-nm must exhibit
            RNP-4 navigation performance as approved by the relevant
            State authority. In addition, there must be a communication
            capability which is either direct pilot-controller voice
            communications or controller-pilot data link communications
            (CPDLC). The requirements of this Part also call for ADS
            with an event contract established such that an ADS message
            containing appropriate information is provided to air
            traffic control (ATC) whenever an aircraft deviates by 5 nm
            or more laterally from approved routing.

        3.  The requirements for implementation of a 30-nm longitudinal
            separation standard using ADS are listed in Section 5.4.2.6
            of reference 2. Among other items, this Section requires
            that aircraft be approved to be RNP-4, and specifies the
            need for ADS with a maximum periodic reporting interval of
            14 minutes. Additional ADS requirements are specified in
            Chapter 13 of reference 2.

    3.  *Data Sources Used in the Safety Assessment for the 50-nm
        Longitudinal Separation Standard and the 30-nm Lateral and
        Longitudinal Standards*

        1.  Several data sources are available for use in conducting the
            safety assessment. These data sources provide insight into
            the operation of Anchorage oceanic airspace, as well as
            support estimation of values for several of the collision
            risk model parameters to be discussed later in this paper.

        2.  FAA Enhanced Traffic Management System

            1.  The FAA's Enhanced Traffic Management System (ETMS)
                provides the means of applying air traffic management
                principles to traffic movements in the U.S. National
                Airspace System, as well as in oceanic airspace
                delegated to the FAA. The FAA Technical Center will
                assemble all available ETMS records pertaining to
                operations in Anchorage airspace during the data sample
                period to assist in the safety assessment.

        3.  Safety Databases

            1.  A group of FAA databases, collectively called the
                National Airspace Incident Monitoring System (NAIMS),
                contains information regarding all reported instances of
                operational errors made by flight crews or air traffic
                controllers, with such reports being compulsory for FAA
                personnel witnessing such errors. In addition, the U.S.
                National Aeronautics and Space Administration (NASA)
                administers the Aviation Safety Reporting System (ASRS),
                a database of similar incident reports, with reporting
                being non-compulsory and the identity of reporters being
                kept anonymous.

            2.  The Technical Center will assemble all reports from
                NAIMS and ASRS for the data sample period for use in the
                safety assessment.

        4.  Data from the Ocean21 System

            1.  As noted in the introduction, the Ocean21 automation
                system became fully operational in portions of Anchorage
                oceanic airspace on 1 March 2007. Prior to March 2007,
                the system was in full-time use in the Oakland FIR
                beginning in October 2005. Through the support of system
                operations experts at Oakland Center, the Technical
                Center was provided with all Ocean21 operations data
                available for the Oakland FIR from October 2005 through
                the present date. Processes developed to analyze the
                Ocean21 data from the Oakland FIR are also used to
                analyze Ocean21 data from the Anchorage FIR since the
                data file structure is the same from both Ocean21
                systems.

            2.  These data consist of all CPDLC, HF, and ADS messages
                provided from the comprehensive data reduction and
                analysis capabilities of Ocean21. In addition, the
                aircraft filed flight plans and electronic ATC
                coordination messages are contained within the data from
                the Ocean21 system.

    4.  A key difference between safety assessments conducted to support
        implementation of previous changes to ocean horizontal-plane
        separation minima and the safety assessment described in this
        document is the explicit account taken of a controller
        decision-support system to reduce risk. In this document, the
        system consists of aids provided by Ocean21 -- such as conflict
        probe and a display of accurate aircraft position - and the
        information supplied to this automation system by CPDLC and ADS.

    5.  As was noted earlier, safe introduction of ADS-based separation
        minima requires, in accordance with references 2 and 3, that
        CPDLC and ADS be operational in the airspace and that the ADS
        function have certain properties. In particular, for the 30-nm
        separation standards, there is the requirement for a 14-minute
        periodic ADS reporting rate, the establishment of a contract
        calling for an event report when an aircraft deviates laterally
        by 5 nm or more from cleared routing, and a request for a
        position update if a periodic ADS position report is delayed by
        three minutes or more. The final safety assessment documents for
        both 50-nm longitudinal and 30-nm lateral and longitudinal
        separation standards will outline these requirements and provide
        evidence of compliance using data from the Ocean21 system.

    6.  *Brief Overview of the Safety Assessment Methodology*

        1.  In accordance with the requirements and guidance of
            references 1, 2 and 3, the safety assessment described in
            this document consists of estimating the risk of collision
            associated which will pertain when the 50-nm longitudinal
            separation standard and the 30-nm lateral and longitudinal
            separation standards are applied in OC3 and comparing this
            risk to the specified Target Level of Safety (TLS).

        2.  As stated in reference 3, Paragraph 3.2.1, the value of the
            TLS which applies to both the lateral and longitudinal
            dimensions is 5 x 10^-9^ fatal accidents per flight hour.

        3.  Estimation of collision risk in this safety assessment is
            carried out using the general collision risk model, which
            has different forms for the lateral and longitudinal
            dimensions. No explicit derivations of these two model forms
            will be provided in this safety assessment. The interested
            reader is referred to the portions of reference 1 cited
            below for the technical details of the assumptions and
            mathematical details of the models.

        4.  The form of the lateral collision risk model applicable to
            assessing the risk, N~ay~, of a 30-nm lateral separation
            standard from Appendix 15 of reference 1 is:

$$N_{\text{ay}} = P_{y}(S_{y})P_{z}(0)\frac{\lambda_{x}}{S_{x}}\left\{ E_{y}(\text{same})\left\lbrack \frac{\overline{\left| \overset{˙}{x} \right|}}{2\lambda_{x}} + \frac{\overline{\left| \overset{˙}{y} \right|}}{2\lambda_{y}} + \frac{\overline{\left| \overset{˙}{z} \right|}}{2\lambda_{z}} \right\rbrack \right.\ \left. \  + E_{y}(\text{opp})\left\lbrack \frac{\overline{V}}{\lambda_{x}} + \frac{\overline{\left| \overset{˙}{y} \right|}}{2\lambda_{y}} + \frac{\overline{\left| \overset{˙}{z} \right|}}{2\lambda_{z}} \right\rbrack \right\}$$

where the individual parameters of the lateral collision risk model and
their definitions are given in Table 1.

  ----------------------------------------------------- -------------------------------------------------------
  **Term**                                              **Definition**

  *S~x~*                                                Nominal distance defining proximity of aircraft on
                                                        adjacent parallel track to a typical aircraft

  *S~y~*                                                Lateral separation minimum

  *P~z~(0)*                                             Probability of vertical overlap (with planned vertical
                                                        separation equal to zero)

  *P~y~(S~y~)*                                          Probability of lateral overlap (with planned lateral
                                                        separation equal to *S~y~*)

  λ*x*                                                  Average aircraft length

  λ*y*                                                  Average aircraft wingspan (or width)

  λ*z*                                                  Average aircraft height with undercarriage retracted

  *E~y~(same)*                                          Same-direction lateral occupancy for a pair of aircraft
                                                        on adjacent routes separated by distance *S~y~* on the
                                                        same flight level

  *E~y~(opp)*                                           Opposite-direction lateral occupancy for a pair of
                                                        aircraft on adjacent routes separated by distance
                                                        *S~y~* on the same flight level.

  *N~x~(same)*                                          Same direction passing longitudinal frequency

  *N~x~(opp)*                                           Opposite direction longitudinal passing frequency

  $$\overline{V}$$                                      Average aircraft ground speed

  $$\overline{\left| \overset{˙}{x} \right|}$$          Average absolute relative along-track speed between
                                                        aircraft pairs

  ![](media/image1.wmf){width="0.18055555555555555in"   Average absolute relative cross-track speed between
  height="0.3055555555555556in"}                        aircraft pairs

  ![](media/image2.wmf){width="0.16666666666666666in"   Average absolute relative vertical speed between
  height="0.3055555555555556in"}                        aircraft pairs
  ----------------------------------------------------- -------------------------------------------------------

**Table 1.** Lateral Collision Risk Model Parameters

5.  The generalized form of the longitudinal collision risk model
    applicable to assessing the risk, N~ax~, associated with a 50-nm and
    30-nm longitudinal separation standard is given in Appendix 1 of
    reference 1. Assuming that flight is on the same ground track, this
    generalized form becomes:

$$N_{\text{ax}} = P_{y}(0)P_{z}(0)\frac{2\lambda_{x}}{\overline{\left| \overset{˙}{x} \right|}}\left\lbrack \frac{\overline{\left| \overset{˙}{x} \right|}}{2\lambda_{x}} + \frac{\overline{\left| \overset{˙}{y} \right|}}{2\lambda_{y}} + \frac{\overline{\left| \overset{˙}{z} \right|}}{2\lambda_{z}} \right\rbrack \times \int_{}^{}\left( \int_{}^{}{f(s,l)\text{dl}} \right)\text{ds}$$

#### Additional parameters of the longitudinal risk model and their definitions are given in Table 2.

  --------------- -------------------------------------------------------
  **Term**        **Definition**

  *s, l*          Initial separation distance (*s*) and the maximum loss
                  of separation distance (*l*) for a pair of aircraft

  *M*             Minimum initial separation distance (30-nm) over all
                  observed pairs of aircraft

  *M*             Maximum loss of separation distance over all observed
                  pairs of aircraft
  --------------- -------------------------------------------------------

### **Table 2.** Additional Parameters Needed for the Longitudinal CRM

7.  *Preliminary Summary of Observed ADS Operations in Anchorage Oceanic
    Airspace*

    1.  The position reports from ADS aircraft are available in the
        Ocean21 data described in paragraph 2.3.4. Data collected from 1
        March through 23 April 2007 show the average number of ADS
        flights per day in Anchorage oceanic airspace to be 98 ADS
        flights per day. Figure 1 presents the top 15 ADS operators in
        rank order by number of observed operations.

![](media/image3.wmf){width="5.638888888888889in"
height="3.213888888888889in"}

**Figure 1.** Top 15 ADS Operators Observed in Anchorage Oceanic
Airspace

2.  The ADS operations are further analyzed by the RNP-type indicated in
    the filed flight plan. Aircraft filing RNP-4 in the flight plan are
    automatically assigned a periodic reporting rate equal to 14 minutes
    by the Ocean21 system. All other ADS flights are assumed to be
    RNP-10 and are assigned a periodic reporting rate equal to 27
    minutes by the Ocean21 system. The average number of observed RNP-4
    flights utilizing ADS is 13 flights per day. The average number of
    observed RNP-10 flights utilizing ADS is 85 flights per day. Figure
    2 presents the daily counts of observed ADS flights by the filed
    RNP-type observed in Anchorage oceanic airspace from 1 March through
    23 April 2007.

![](media/image4.wmf){width="5.497916666666667in"
height="3.5034722222222223in"}

**Figure 2.** ADS Operations by RNP-Type Observed in Anchorage Oceanic
Airspace

8.  Conclusions

    1.  This paper has presented a summary of the data sources that will
        be used to complete a safety assessment for the implementation
        of the ADS-based separation standards in Anchorage oceanic
        airspace. It has also summarized the safety assessment
        methodology.

    2.  Preliminary estimates of ADS operations show that the average
        number of aircraft utilizing ADS in Anchorage oceanic airspace
        is 98 flights per day. The average of number of ADS flights
        filing RNP-4 in their flight plans and using ADS is 13 flights
        per day.

    3.  The anticipated minimum data collection period needed to
        complete the safety assessment for the implementation of the
        50-nm longitudinal separation standard in Anchorage oceanic
        airspace is 3 months. However, a sufficient number of observed
        ADS aircraft pairs separated by the longitudinal separation
        standard are needed to compute critical collision risk
        parameters. Therefore, this data collection time period estimate
        is subject to change depending on a satisfactory number of
        observed longitudinally separated ADS aircraft pairs in data.

```{=html}
<!-- -->
```
2.  **Recommendation**

    1.  The meeting is invited to note the information provided in this
        paper.

**[References]{.underline}**

1.  *Manual on Airspace Planning Methodology For the Determination of
    Separation Minima*, First Edition, Doc 9689-AN/953, International
    Civil Aviation Organization, Montreal, 1998.

2.  *Procedures For Air Navigation Services Air Traffic Management*,
    Fourteenth Edition, Doc 4444 ATM/501, International Civil Aviation
    Organization, Montreal, 2001.

3.  *Annex 11 of the Convention on International Civil Aviation*, Air
    Traffic Services, Thirteenth Edition, International Civil Aviation
    Organization, Montreal, July 2001

\- END -