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CP for SARPs on the Use of Pressure Sourced Altitude Data

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					                                                                     SICASP/WG-1 & 2
                                                                              WP/1/837
                                                                              WP/2/831
                                                                       26 February 2000

        SSR IMPROVEMENTS AND COLLISION AVOIDANCE SYSTEMS PANEL

                                SSR MODE S SYSTEM

                              WORKING GROUP-1 & 2

                              13 –24 March 2000, London



                              CP for SARPs
               on the Use of Pressure Sourced Altitude Data




                            (Presented by Roland Mallwitz )


                                      SUMMARY

                This working paper presents a proposed change to the
                system SARPs to clarify the encoding of altitude. Issues
                addressed include the use of 25 foot encoding and the
                requirement to report uncorrected altitude



Ref.:
/1/ WP1/684, WP2/681; ”Use of 25 Foot Increments by ACAS”, Panel Secretary, WG1/2
Toulouse
/2/ WP1/732, WP2/706; ”Requirements for Altitude Reports”, R. Mallwitz, WG1/2, Brussels
/3/ WP1/759, WP2/728; ”Draft SARPs Changes for Altitude Reporting”, TSG, WG1/2, Montreal
/4/ WP1/765, WP2/744; ”A Comment on Draft SARPs Changes for Altitude Reporting”, K.
Carpenter, WG 1/2, Montreal
1.     Background
During the 28th Working Group meeting in Toulouse the Panel Secretary asked the WGs to
consider a SARPs change reflecting 25-ft increments. This was an action arising from states
comments on the ICAO state letters reflecting SARPs changes approved at SICASP/6 and the
ICAO world-wide ACAS II mandate. During the discussions at the following meeting in
Brussels WG2 supported the recommendation to carry aircraft equipment able to report
pressure altitude in 25 ft to improve ACAS performance and the safety net. On the other hand
the WGs, and WG2 in particular, did not support a SARPs change requiring such capabilities
and appropriate equipment for all aircraft. In addition the WGs noted that the information
provided in ICAO documentation may be not unambiguous. Therefore, the TSG was tasked to
consider changes to SARPs and GM. Consequently, a draft SARPs change was presented at
the WG meeting in Montreal together with comments triggered by information on digitized
altitude sources.
This working paper presents a proposed change to the system SARPs to clarify the encoding of
altitude. Issues addressed include the use of 25 foot encoding and the requirement to report
uncorrected altitude
                  Proposed change to:         MODE S SYSTEM SARPS

     As published in Annex 10 Volume IV Amendment 73                    Dated July 98



Submit to: Rapporteur SICASP Working Group 1                                      Page 1 of 2

1.   Change No          (Assigned by Rapporteur)       Date submitted 27 February 2000


     Title: Specification of 25-foot increment pressure altitude report

2. List of all relevant SICASP WG/1 Working Papers:

This paper, WP1/684, WP2/681, WP1/732, WP2/706, WP1/759, WP2/728, WP1/765, WP2/744

3.   Background:
             th
At the 28 meeting in Toulouse, an action was put on the Panel Secretary to prepare a change
proposal to strengthen the use of 25-foot increments in Mode S transponders for improved ACAS
performance. Annex 10, Volume IV, 2.1.3.2.3 already recommends the use of the "best altitude
quantization". The proposed change strengthens this recommendation. It also clarifies the encoding of
altitude, including the requirement that 25 foot encoding is not applie d to 100 foot quantized data and
the requirement to report uncorrected altitude.




4.   Need for change:

To ensure consistency and correctness of the encoding of the reported altitude

5.   Change:

See attachment.

6. Priority: (confirmed by Rapporteur)

*       1.        Necessary      - must be made to make SARPs technically correct.
        2.        Useful         - will enhance understanding of the SARPs.
        3.        Cosmetic       - needed to correct minor editorial error.


Submitted by: The Technical Sub Group

Organization:      SICASP
Address:   ICAO
                       ATTACHMENT A: PROPOSED CHANGES TO 2.1.3.1


                                                   AERONAUTICAL
                                                TELECOMMUNICATIONS

                                               ANNEX 10
                           TO THE CONVENTION ON INTERNATIONAL CIVIL AVIATION

                                                         VOLUME IV

                        (SURVEILLANCE RADAR AND COLLISION AVOIDANCE SYSTEMS)



                                                  CHAPTER 2. GENERAL



                                   2.1 SECONDARY SURVEILLANCE RADAR (SSR)

                                      2.1.3 Transponder reply modes (air-to-ground)

2.1.3.1 Transponders shall respond to Mode A interrogations in accordance with the provisions of 3.1.1.7.12.1 and to
Mode C interrogations in accordance with the provisions of 3.1.1.7.12.2.

          Note. If pressure-altitude information is not available, transponders reply to Mode C interrogations with framing
pulses only.

2.1.3.1.1 The pressure altitude reports contained in Mode S replies shall be derived as specified in 3.1.1.7.12.2.2.

          Note. 3.1.1.7.12.2.2 is intended to relate to Mode C replies and specifies, inter alia, that Mode C pressure altitude
reports be referenced to a standard pressure setting of 1013.25 hectopascals. The intention of 2.1.3.1.1 is to ensure that all
transponders, not just Mode C transponders, report uncorrected pressure altitude.

2.1.3.2 Where the need for Mode C automatic pressure-altitude transmission capability within a specified airspace has been
determined, transponders, when used within the airspace concerned, shall respond to Mode C interrogations with pressure-
altitude encoding in the information pulses.

2.1.3.2.1 From 1 January 1999, all transponders, regardless of the airspace in which they will be used, shall respond to
Mode C interrogations with pressure-altitude information.

2.1.3.2.2 Recommendation. All transponders, regardless of the airspace in which they will be used, should respond to
Mode C interrogations with pressure-altitude information.

Note. Operation of the airborne collision avoidance system (ACAS) depends upon intruder aircraft reporting pressure-altitude
in Mode C replies.

2.1.3.2.3 Recommendation. For aircraft with 25-foot or better pressure altitude sources the TheThe pressure-altitude
information provided by Mode S transponders in response to selective interrogations (i.e. in the AC field) should be reported
with the best altitude quantization increment available in the aircraft in accordance with the provisions of 3.1.2.6.5.4.field,
3.1.2.6.5.4) should be reported in 25 foot increments, subject to 2.1.3.2.7 .

           Note. Performance of the airborne collision avoidance system (ACAS) is significantly enhanced when an intruder
aircraft is reporting pressure-altitude in 25-foot increments.
2.1.2.3.2.4 All Mode A/C transponders installed on or after 1 January 1992 shall report pressure-altitude encoded in the
information pulses in Mode C replies.

2.1.2.3.2.5 All Mode S transponders installed on or after 1 January 1992 shall report pressure-altitude encoded in the
information pulses in Mode C replies and in the AC field of Mode S replies.

2.1.3.2.6 All Mode S transponder equipped aircraft with 25 ft or better pressure altitude sources shall report pressure
altitude encoded in 25 ft increments in the AC field of Mode S replies from 1 January 2005.

2.1.3.2.7 When a Mode S transponder reports altitude in 25ft increments, the reported value of the altitude shall be the value
obtained by expressing the measured value of the uncorrected pressure altitude of the aircraft in 25 ft increments.

           Note:     This requirement relates to the installation and use of the Mode S transponder. The purpose is to ensure
that altitude data obtained from a 100 ft increment source are not reported using the formats intended for 25 ft data.

2.1.3.3 Transponders used within airspace where the need for Mode S airborne capability has been determined shall also
respond to intermode and Mode S interrogations in accordance with the applicable provisions of 3.1.2.

2.1.3.3.1 Requirements for mandatory carriage of SSR Mode S transponders shall be on the basis of regional air navigation
agreements which shall specify the airspace and the airborne implementation time-scales.

2.1.3.3.   Recommendation. The agreements indicated in 2.1.3.3.1 should provide at least seven five years' notice.

           -             ------
             ATTACHMENT B: 2.1.3.1 WITH CHANGES INCORPORATED


                                                    AERONAUTICAL
                                                 TELECOMMUNICATIONS

                                               ANNEX 10
                           TO THE CONVENTION ON INTERNATIONAL CIVIL AVIATION

                                                         VOLUME IV

                        (SURVEILLANCE RADAR AND COLLISION AVOIDANCE SYSTEMS)


                                                  CHAPTER 2. GENERAL


                                   2.1 SECONDARY SURVEILLANCE RADAR (SSR)

                                               2.1.3 Transponder reply modes

2.1.3.1 Transponders shall respond to Mode A interrogations in accordance with the provisions of 3.1.1.7.12.1 and to
Mode C interrogations in accordance with the provisions of 3.1.1.7.12.2.

         Note. If pressure-altitude information is not available, transponders reply to Mode C interrogations with
framing pulses only.

2.1.3.1.1 The pressure altitude reports contained in Mode S replies shall be derived as specified in 3.1.1.7.12.2.2.

           Note. 3.1.1.7.12.2.2 is intended to relate to Mode C replies and specifies, inter alia, that Mode C pressure
altitude reports be referenced to a standard pressure setting of 1013.25 hectopascals. The intention of 2.1.3.1.1 is to
ensure that all transponders, not just Mode C transponders, report uncorrected pressure altitude.

2.1.3.2 Where the need for Mode C automatic pressure-altitude transmission capability within a specified airspace
has been determined, transponders, when used within the airspace concerned, shall respond to Mode C interrogations
with pressure-altitude encoding in the information pulses.

2.1.3.2.1 From 1 January 1999, all transponders, regardless of the airspace in which they will be used, shall respond
to Mode C interrogations with pressure-altitude information.

2.1.3.2.2 Note. Operation of the airborne collision avoidance system (ACAS) depends upon intruder aircraft reporting
pressure-altitude in Mode C replies.

2.1.3.2.3 Recommendation. The pressure-altitude information provided by Mode S transponders in response to
selective interrogations (i.e. in the AC field, 3.1.2.6.5.4) should be reported in 25 foot increments.

          Note. P  erformance of the airborne collision avoidance system (ACAS) is significantly enhanced when an
intruder aircraft is reporting pressure-altitude in 25-foot increments.

2.1.2.3.2.4 All Mode A/C transponders installed on or after 1 January 1992 shall rep ort pressure-altitude encoded in
the information pulses in Mode C replies.
2.1.2.3.2.5 All Mode S transponders installed on or after 1 January 1992 shall report pressure-altitude encoded in the
information pulses in Mode C replies and in the AC field of Mode S replies.

2.1.3.2.6 All Mode S transponder equipped aircraft with 25 ft or better pressure altitude sources shall report pressure
altitude encoded in 25 ft increments in the AC field of Mode S replies from 1 January 2005.

2.1.3.2.7 When a Mode S transponder reports altitude in 25ft increments, the reported value of the altitude shall be the
value obtained by expressing the measured value of the uncorrected pressure altitude of the aircraft in 25 ft increments.
          Note:      This requirement relates to the installation and use of the Mode S transponder. The purpose is to
ensure that altitude data obtained from a 100 ft increment source are not reported using the formats intended for 25 ft
data.

2.1.3.3 Transponders used within airspace where the need for Mode S airborne capability has been determined shall
also respond to intermode and Mode S interrogations in accordance with the applicable provisions of 3.1.2.

2.1.3.3.1 Requirements for mandatory carriage of SSR Mode S transponders shall be on the basis of regional air
navigation agreements which shall specify the airspace and the airborne implementation time-scales.

2.1.3.3.   Recommendation. The agreements indicated in 2.1.3.3.1 should provide at least five years' notice .

				
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