Appendix H – Guidance on 1090-MHz extended squitter ADS-B Out

Part 1 – ADS-B Out Data Parameters (AMC ACNS.D.ADSB.020(a))

Part 1 of this Appendix provides guidance to the aircraft integrator on the minimum ADS-B Out surveillance data requirements (Table 5 and associated Definitions).

In addition, guidance is given for the overall understanding of the ADS-B Out system, in support of equipment configuration and ADS-B Out data parameter testing, as appropriate. This includes the presentation of data encodings related to the so-called BDS registers (Table 4), as extracted from ED‑102A. The content of the various BDS registers are loaded into the 56-bit ADS-B message (ME) field of the Mode S Downlink Format 17 (DF17, bits 33-88), in line with their respective transmission rates.

Table 5 below makes reference to the BDS registers that contain the various ADS-B Out data parameters. When Table 5 states Same source as for Mode S replies, reference is made to the requirement that the content of ADS-B broadcasts and Mode S replies that carry the same information need to come from the same source (CS ACNS.D.ADSB.025(b)).

The reference to the BDS registers is provided in order to facilitate a detailed understanding and traceability of ADS-B Out requirements at ADS-B transmit unit level, also in support of integration testing, as appropriate.

The relationship between the BDS registers and the ADS-B message Type Codes (first 5 bits in the 56‑bit ADS-B message field) is thereby as shown in Table 4. The Type Code is used to differentiate between ADS-B message types (i.e. BDS registers). In addition, for Airborne and Surface Position Messages, the Type Code is used to encode the horizontal position integrity containment bounds (NIC). The Subtype Code is used to further differentiate between ADS-B messages of a certain type (e.g. Operational Status Message).

A number of service bulletins have been issued to rectify some observed deficiencies and have already been addressed by the equipment manufacturers. Therefore, the installed transponders should have all published corrective transponder equipment service bulletins (SB) relating to the correct operation of the ADS-B functionality embodied.

Table 4: BDS Register Overview

Note: Although BDS registers 0716 and 0A16 are not conveying ADS-B data items their implementation is needed to complement the ADS-B protocol.

Table 5: Minimum ADS-B Out Surveillance Data Transmission Requirements

Definition 1: Aircraft Identification Data Sources

Aircraft Identification is provided to the ADS-B transmit unit so that the information is identical to the filed ICAO flight plan. This information may be provided from, amongst others:

A flight management system; or

A pilot control panel; or

For aircraft, which always operate with the same aircraft identification (e.g. using registration as the aircraft identification), it may be programmed into equipment at installation.

In case no ICAO flight plan is filed, the Aircraft Registration is provided to the ADS-B transmit unit.

Definition 2: Mode A Code

Refer to AMC1 ACNS.D.ELS.015 for general guidance.

When the ADS-B transmit unit receives a Mode A Code containing the Mode S conspicuity code (1000), the broadcast of Mode A code information is stopped.

Note: The broadcast of the Mode A Code is provided as a transitional feature, e.g. to aid operation of legacy ATC automation systems that use Mode A Code for Flight Plan correlation. Entry of the Mode A Code of 1000 will disable the transmission of the Mode A Code, and, hence, reduce the overall 1090 ES transmission rate.

Definition 3: Horizontal Position Information

The Mode S Extended Squitter position format uses the Compact Position Reporting (CPR) algorithm to encode latitude and longitude efficiently into messages. The resulting messages are compact in the sense that several higher order bits which are normally constant for long periods of time, are not transmitted in every message.

The CPR technique enables a receiving system to unambiguously determine the location of the aircraft, and, hence, reconstruct the original information provided by the source. If required for integration testing purposes, detailed guidance on the CPR algorithm is provided in ED‑102A/DO‑260B.

A horizontal position data source provides position information for both the airborne and surface horizontal position data formats (i.e. registers 05₁₆ or 06₁₆, respectively), accordingly encoded by the ADS-B transmit unit depending on the aircraft airborne/surface state.

Definition 4: Horizontal Position Quality – NIC and NACp

The encoding of the NIC and NACp horizontal position quality indicators should be directly derived from the corresponding integrity and accuracy information as being reported by the selected horizontal position source (refer also to CS ACNS.D.ADSB.025(c)).

In case a measurement integrity failure has been indicated by the selected horizontal position source (e.g. bit 11 of ARINC label 130 for ARINC 743A compliant sources), both the NIC and NACp quality indicators will be set to invalid (zero), regardless of the indicated integrity containment bound (e.g. HPL).

Definition 5: Airborne NIC Value

NIC is reported so that surveillance applications, such as by ATC or other aircraft, may determine whether the reported horizontal position has an acceptable level of measurement integrity for the intended use. (Note that the NIC parameter is closely associated with the SIL quality metric.)

The NIC (and SIL) values are associated with a possible failure condition of the position measurement function and the detection thereof. For most ADS-B applications, the NIC (and SIL) values are the key horizontal position quality metrics on which the horizontal position data is determined to be of sufficient quality for its intended use. The NIC value is encoded on the respective horizontal position integrity containment radius as provided by the source.

The NIC values, including the NIC Supplements values, are encoded for airborne position messages as follows (Rc is the horizontal position integrity containment bound, typically HPL/HIL for GNSS systems):

Table 6: Airborne NIC Encoding

Note: The minimum NIC values required for the ADS-B-RAD application can be found in Table 20, in Part 3 of Appendix H. They are met through the horizontal position source requirements defined in CS ACNS.D.ADSB.070.

Definition 6: NACp

NACp specifies the 95 % radial accuracy of the aircraft’s horizontal position information (latitude and longitude) derived from the position source’s accuracy output, typically the HFOM metric from GNSS based sources.

Whereas the NIC value is associated with a possible failure condition of the position measurement function, the NACp value describes the nominal performance of the measurement function in terms of horizontal position accuracy as provided by the source.

The NACp value is encoded as follows:

Table 7: NACp Encoding

Note: The minimum NACp values required for the ADS-B-RAD application can be found in Table 20, in Part 3 of Appendix H. This value is met through the horizontal position source requirements defined in CS ACNS.D.ADSB.070.

The NACp encoding is the same for airborne position messages and surface position messages.

Definition 7: SIL

The encoding of the horizontal position source integrity level (SIL) is based on the probability of the reported horizontal position exceeding the radius of containment defined by the NIC, without alerting, assuming no avionics faults. The SIL value is set as follows:

Table 8: SIL Encoding

Note: The minimum SIL value required for the ADS-B-RAD application can be found in Table 20, in Part 3 of Appendix H. This value is met through the horizontal position source requirements defined in CS ACNS.D.ADSB.070 (see also related AMC guidance).

Whereas SIL assumes that there are no system integrity failures, the SIL should consider the effects of a faulted signal-in-space.

For horizontal position sources compliant with CS ACNS.D.ADSB.070, the probability of exceeding a NIC radius of containment without alerting is based on a per hour rate. Hence, the SIL Supplement should be set to ‘zero’. If based on per sample, the SIL Supplement would be set to ‘one’.

The SIL encoding is the same for airborne position messages and surface position messages.

Definition 8: SDA

The encoding of the system design assurance level (SDA) is based on the failure condition that the entire ADS-B Out system, with respect to the horizontal position data and associated quality indicators, is designed to support.

The SDA value is encoded as follows:

Table 9: SDA Encoding

Note 1: Software Design Assurance per EUROCAE ED-12C (RTCA DO-178C). Airborne Electronic Hardware Design Assurance per EUROCAE ED-80 (RTCA DO-254).

Note 2: In line with the ADS-B-RAD requirements, the minimum value required for the horizontal position source is SDA=2 ().

The SDA encoding is the same for airborne position messages and surface position messages.

Definition 9: Pressure Altitude Data Sources

Refer to AMC1 ACNS.D.ELS.015 for guidance.

The ADS-B NICbaro quality indicator is encoded as follows:

Table 10: NICbaro Encoding

Definition 10: Emergency Status

The provision of the ‘Emergency Status’ values that do not have a corresponding Mode A value (see CS ACNS.D.ELS.015(a)(6)), denoting the other emergency conditions defined in 61₁₆, is optional. This applies to the decimal values 2, 3, 6 and 7 in Table 11.

Table 11: Emergency Status Encoding

Definition 11: Horizontal Velocity (Ground Velocity)

The horizontal velocity provides the rate at which an aircraft changes its horizontal position with a clearly stated direction.

Velocity data sources provide ground velocity vector information for both the airborne and surface velocity data transmit formats, allowing for the transmission of east/west and north/south velocity information (09₁₆), or velocity scalar (06₁₆, movement) and possibly ground track information[3] (06₁₆), respectively.

In case of a failure of the provision of ground velocity data, the ADS-B transmit unit will broadcast airspeed (and heading) information instead (using subtypes 3 or 4 of register 09₁₆.

Definition 12: Horizontal Velocity Quality Indicator NACv

The NACv is an estimate of the accuracy of the horizontal geometric velocity data.

The NACv value is encoded as follows:

Table 12: NACv Encoding

The NACv encoding is the same for airborne position messages and surface position messages.

Definition 13: Emitter Category

Emitter Category settings describe the size and performance of an aircraft, primarily expressed with respect to its maximum take-off weight.

The Emitter Category value is encoded as follows:

Table 13: Emitter Category Encoding

The ADS-B Emitter Category Sets A, B, C or D are identified by the Message Format TYPE Codes 4, 3, 2, and 1 respectively.

Note 1: A coding of ‘0’ within an Emitter Category Set is not allowed.

Note 2: The Emitter Category codes 1 to 5 in category set A are intended to advise other aircraft of the transmitting aircraft’s wake vortex characteristics, and not necessarily the transmitting aircraft’s actual maximum take-off weight. In case of doubt, the next higher aircraft category code should be used

Definition 14: Vertical Rate

Vertical Rate is either the barometric or geometric rate at which the aircraft is climbing or descending, measured in feet per minute. The vertical rate is typically generated by an air data computer or GNSS position source, or equipment which blends barometric vertical rate with inertial vertical rate and/or GNSS vertical rate.

As the geometric vertical rate can be readily derived from the ADS-B Out position source, it is classified as a minimum requirement rather than an (effectively Mode S Enhanced Surveillance) conditional requirement.

Definition 15: Surface NIC Value

The Surface NIC value, including the NIC Supplement A and C values, is encoded as follows:

Table 14: Surface NIC Encoding

Definition 16: Surface Heading/Ground Track

Aircraft Heading indicates the direction in which the nose of the aircraft is pointing. It should be used as the primary source and be expressed (in ME bit 54 in 65₁₆) as either true north (‘0’, preferred) or magnetic north (‘1’).

If an approved heading source is not available (or failed during operation), the Ground Track angle information from the selected ground velocity data source will be used instead by the ADS-B transmit unit for the determination of the direction of the horizontal velocity vector.

If the position source ground track is used and inaccurate below a certain ground speed, and the position source does not inhibit output of the ground track at these slower speeds, the installer should ensure that the ADS-B transmit unit has the capability to invalidate the ground track when the GNSS ground speed falls below a threshold specified by the position source manufacturer (e.g. 3.6 m/s (7 knots)).

Definition 17: Aircraft Length and Width

Aircraft Length and Width settings describe the aircraft dimensions by the width and length of a rectangle that is aligned parallel to the aircraft’s heading. The aircraft’s length is to be measured along its axis of symmetry (i.e. from nose to tail). The aircraft’s width is to be measured from wing-tip to wing-tip.

The Aircraft Length and Width values are encoded as shown in Table 15 to be less than or equal to a respective upper bound length and width as expressed in the two right-side columns. The Length and Width Codes are based on a combined encoding of the actual length and width whereby the largest respective upper bound prevails. If the Aircraft or Vehicle is longer than 85 meters, or wider than 90 meters, then decimal Aircraft/Vehicle Length/Width Code 15 is used.

Table 15: Aircraft Length/Width Encoding

Example: a powered glider with an overall length of 24 meters and wingspan of 50 meters would, normally, have a length code of ‘001’. However, since the wingspan exceeds 34 meters, it does not qualify for either Width subcategory of length category ‘001’. In line with its actual width, such an aircraft would be assigned a length code of ‘100’ and width code of ‘1’, meaning length less than 55 meters and width less than 52 meters.

Definition 18: GPS Antenna Offset (lateral and longitudinal)

GPS Antenna Offset information provides the position offset of the GNSS antenna used for the provision of horizontal position information.

Both a lateral distance of the GPS Antenna (from the longitudinal axis of the aircraft) and a longitudinal distance of the GPS Antenna (from the nose of the aircraft) are provided.

The accuracy of the information should be better than 2 meters, consistent with the data resolution.

The lateral and longitudinal GPS Antenna Offset values are encoded as follows:

Table 16: Lateral Axis GPS Antenna Offset Encoding

Supplementary Notes

Maximum distance left or right of aircraft longitudinal (roll) axis is 6 meters or 19.685 feet. If the distance is greater than 6 meters, then the encoding should be set to 6 meters.

The No Data case is indicated by encoding of 000 as above, while the ZERO offset case is represented by encoding of 100 as above.

The rounding should be performed to half of the resolution of the GPS antenna offset information, i.e. +/- 1 meter.

Table 17: Longitudinal Axis GPS Antenna Offset Encoding

Supplementary Notes:

If the distance is greater than 60 meters, the encoding should be set to 60 meters.

Position Offset Applied by the Sensor applies to future cases where the antenna offset is compensated by the horizontal position source to the centre of the rectangle describing the aircraft’s length and width (refer to Definition 17).

The encoding of the values from decimal ‘2’ (only bit 1 one set to ‘1’) to ‘31’ (all five bits set to ‘1’) is as follows: encoded binary value = offset [m]) / 2 + 1  (e.g. an offset of 4 meters leads to a binary value of (4/2 + 1 = 3), i.e. Bits 0-1 equal ‘1’ and Bits 2-4 equal ‘0’).

Definition 19: Geometric Altitude

The geometric altitude is a measure of the aircraft’s height above a geometric reference and is provided by a GNSS-based position source.

Both within 05₁₆ and 09₁₆, Geometric Altitude is provided as height above ellipsoid (HAE) in accordance with the WGS 84 coordinate system (AMC1 ACNS.D.ADSB.085(b)).

Definition 20: Geometric altitude quality indicator information (GVA)

The GVA parameter expresses the actual performance of the geometric altitude data source as valid at the time of applicability of the measurement.

The GVA value is encoded as follows:

Table 18: GVA Encoding

Part 2 – ADS-B Out Surveillance Data Parameters (AMC1 ACNS.D.ADSB.020(b))

Table 19 below makes reference to the BDS register(s) that contain the various ADS-B Out surveillance data parameters. When Table 19 states Same source as for Mode S replies, reference is made to the requirement that the content of ADS-B broadcasts and Mode S replies that carry the same information and need to come from the same source (CS ACNS.D.ADSB.025(b)).

Guidance on the content of the various BDS registers and their relationship with the ADS-B message Type Codes is provided in Table 4 in part 1 of Appendix H.

Table 19: ADS-B-ADD Surveillance Data Transmission Requirements

Definition 21: Selected Altitude/Barometric Pressure Setting

Refer to AMC1 ACNS.D.EHS.015(c)(1) and (c)(3) for detailed guidance.

Definition 22: ACAS Operational /Resolution Advisory (RA)

Refer to AMC1 ACNS.D.ELS.015(f) for detailed guidance.

The data is populated from ACAS II systems if installed on the aircraft. Both parameters should be preset to ‘zero’ if an ACAS II system is not installed (refer to ADS-B transmit unit manufacturer instructions).

Part 3 – ADS-B Out Minimum Horizontal Position and Velocity Data Requirements

Table 20 provides a summary of the minimum horizontal position data requirements as specified in the defining ADS-B-RAD Safety and Performance Requirements/Interoperability document (ED-161).

Table 20: Minimum Horizontal Position and Velocity Data Quality Requirements

Note 1: The requirement of NACp<=0.1NM in support of 3NM separation is based on the arguments produced in Annex B to ED-161 (ADS-B-RAD Safety and Performance Requirements/Interoperability Requirements Document).

Note 2: The SDA encoding of ‘2’ (10-5/fight-hour) applies to individual components of the ADS-B Out system, i.e. 10-5/fight-hour for the ADS-B transmit unit and 10-5/flight-hour for the horizontal position and velocity source.

Note 3: ADS-B transmit units interfaced with a GNSS position source that is compliant with CS ACNS.D.ADSB.070 (and the related AMC guidance) should preset the SIL Supplement to ‘zero’.

Note 4: If set as fixed value, NACv should be always ‘one’. For quality indications that are dynamically provided by the velocity source, NACv should be ‘one’ or ‘two’. There is currently no established guidance on establishing a NACv performance of ‘three’ or better.

This should be verified through appropriate tests, as follows. With respect to NIC and NACp testing, the ADS-B Out system installer should check for satellite shielding and masking effects if the stated performance is not achieved.

(a)     Airborne & Surface NIC:

During testing under nominal GNSS satellite constellation and visibility conditions, the transmitted NIC value should be a minimum of ‘six’.

(b)     NACp:

During testing under nominal GNSS satellite constellation and visibility conditions, the transmitted NACp value should be a minimum of ‘eight’

In order to validate the correctness of the transmitted horizontal position, the aircraft should be positioned on a known location.

(c)      SIL:

SIL is typically a static (unchanging) value and may be set at the time of installation if a single type of position source is integrated with the ADS-B transmit unit. SIL should be set based on design data from the position source equipment manufacturer. Installations which derive SIL from GNSS position sources compliant with CS ACNS.D.ADSB.070 should set the SIL to ‘three’.

ADS-B transmit units interfaced with a GNSS position source that is compliant with CS ACNS.D.ADSB.070 (and the related AMC guidance) should pre-set the SIL Supplement to ‘zero’.

(d)     NACv:

If set as fixed value, NACv should be always ‘one’. For quality indications that are dynamically provided by the velocity source, NACv should be ‘one’ or ‘two’.

It is noted that there is currently no established guidance on establishing a NACv performance of ‘three’ or better.

Part 4 – ADS-B Out Integrity and Continuity Requirements

CS ACNS.D.ADSB.100 and CS ACNS.D.ADSB.105 summarise, per data parameter, the integrity and continuity probability levels applicable to the ADS-B Out system.

In the first place, the ADS-B Out System installed in the aircraft needs to deliver data that satisfy the ADS-B-RAD airborne domain system safety and performance requirements in line with Section 3.4 of the ADS-B-RAD Safety and Performance Requirements/Interoperability standard ED-161.

As, for the purpose of framing the ADS-B-RAD operational safety assessment, the ADS-B-RAD airborne domain only comprises the horizontal position data source and the ADS-B transmit unit, including the interconnecting avionics, the data sources providing surveillance information other than horizontal position and velocity are assumed to operate as within today’s SSR environment. Hence, in line with CS ACNS.D.ADSB.080, the related Mode S Elementary and Enhanced Surveillance requirements apply.

It is noted that the respective Mode S Elementary and Enhanced Surveillance requirements have to be understood within their given context, in particular taking into account applicable procedural mitigation means (e.g. as currently performed by means of the ICAO required controller-pilot verification procedure for pressure altitude reporting).

The ADS-B Out data parameters other than the ones addressed in the preceding paragraphs, need to satisfy comparable ADS-B-RAD requirements.

The specified integrity levels are required to adequately protect against the corruption of ADS-B Out surveillance data causing false or misleading information to be transmitted.

Although the direct effects to an aircraft of an ADS-B Out failure may be minor, the ADS-B Out information will be used by ATC and other ADS-B equipped aircraft, thus provisions that would allow for a reduction in failure probabilities and design assurance level, do not apply to the ADS-B Out system.

Part 5 – GNSS Position and Velocity Source Qualification

This part 5 of Appendix H provides guidance to GNSS equipment manufacturers on how to establish a qualification for these ADS-B specific requirements, i.e. beyond the demonstration of compliance to ETSO requirements. In the following, as appropriate, reference is made to the respective:

—         ETSO material: ETSO-C129a (JTSO-C129a), ETSO-C196a, ETSO-C145()/146()

—         EUROCAE/RTCA MOPS material: ED-72A, DO-208, DO-229D, DO-316 as well as DO-235B; and

—         FAA AC material (AC 20-138C).

Note: ETSO-C145 refers to RTCA DO-229A, ETSO-C146 refers to RTCA DO-229B, ETSO-C145c/146c refers to RTCA DO-229D, and ETSO-C145()/146() refers to any of those revisions.

In addition to the ETSO minimum requirements, the requirements of this part need to be demonstrated unless this has been demonstrated as a declared non-ETSO function. It is expected that the required compliance demonstration is supplied by the position and velocity source manufacturer through a Declaration of Design and Performance (DDP), or an equivalent document.

(a)     Horizontal Position Integrity (HPL)

Horizontal Position Integrity – AMC1 ACNS.D.ADSB.070(a)(1) and (a)(2)(ii).

Applicability: ETSO-C129a (JTSO-C129a)

GNSS equipment manufacturers should provide substantiation data showing that the equipment outputs latitude and longitude information that is referenced to the WGS-84 coordinate system.

GNSS equipment manufacturers should provide substantiation data showing that the equipment outputs a 10^-7/hr Horizontal Protection Limit (HPL, or equivalent) based on the RAIM algorithm meeting the ETSO-C129a (JTSO-C129a) Class A1, A2, B1, B2, C1, or C2 RAIM requirements.

Applicability: ETSO-C145()/146()

SBAS equipment certified under any revision of ETSO-C145 or ETSO-C146 is required to have several modes of operation depending on the availability of augmentation. For example, when operating in an augmented mode intended for LPV approach guidance, the position source may determine HPL based on a lateral error versus a horizontal error and an exposure time based on the duration of the approach versus flight hour (refer to Appendix J to RTCA DO229D for details).

If the position source outputs the HPL on lateral error and approach exposure time, it is possible that the ADS-B transmit function would need to inflate the HPL by 3% in approach modes to ensure the integrity is appropriately bounded.

GNSS equipment manufacturers should provide information data to determine if the integrity output needs to be scaled (i.e., by applying an inflation factor). The same considerations apply to GBAS differentially-corrected position sources when in approach mode.

Integrity Fault – Time to Alert – AMC1 ACNS.D.ADSB.070(a)(1) and (a)(2)(iii).

Applicability: ETSO-C129a (JTSO-C129a)

For the horizontal position sources compliant with AMC ACNS.D.ADSB.070, it should to be demonstrated, that a non-isolated GNSS satellite fault detected by the position source is properly passed to the ADS-B transmit unit within the allowable time to alert of 10 seconds, at any time.

With reference to the mode-dependent time to alert in Table 3-5 of EUROCAE ED-72A, Section 3.2.1 (Table 2-1 of RTCA DO-208 Section 2.2.1.13.1), GNSS equipment manufacturers should provide information describing the equipment integrity fault output latency, along with interface instructions and/or any limitations for meeting the 10-second latency requirement of AMC1 ACNS.D.ADSB.070(a)(1) and (a)(2)(iii).

Note 1: The latency of reporting nominal ADS-B ‘Quality Indicator’ changes, such as in response to changing GNSS satellite constellations or due to switching between position sources, is bounded by AMC1 ACNS.D.ADSB.070(a)(2)(iii) as well.

Note 2: ED-72A allows a provision to extend the Time to Alarm up to 30 seconds during en route phases of flight while for terminal and Non-Precision Approach the 10-second limit is applicable. For ADS-B Out, a time to alert of 10 seconds applies to any phases of flight.

Mode Output – AMC1 ACNS.D.ADSB.070(a)(1) and (a)(3)

Applicability: ETSO-C129a (JTSO-C129a), ETSO-C196a, ETSO-C145()/146()

GNSS equipment manufacturers should provide instructions describing any equipment modes affecting the interpretation of horizontal position integrity output and how the position source outputs the mode indication.

As the minimum horizontal position integrity containment bound provided by non-augmented, as well as some specific augmented GNSS source, equipment is limited to 0.1 NM by design, the GNSS equipment manufacturer should present substantiation data whether the HPL output is limited or not, and provide proper instructions for the ADS-B Out system integration. If the GNSS source equipment does not limit the HPL, although it should do so by design, the ADS-B transmit unit limits the encoded NIC value to be equal to or less than ‘eight’.

(b)     Horizontal Position Accuracy (HFOM) – AMC1 ACNS.D.ADSB.070(a)(1) and (a)(2)(v)

Applicability: ETSO-C129a, ETSO-C145, and ETSO-C146

Note 1: Compliance with RTCA/DO-229D is required by ETSO-C145c-C146c. ETSO-C145/-C146 may be acceptable by applications of a positive deviation.

Note 2: If in the following, reference is made in the qualification tests described in DO-229D, the equivalent material in DO-316 applies as well.

GNSS equipment manufacturers should provide substantiation data showing the equipment computes and outputs HFOM. The following criteria for an acceptable horizontal position output and its associated HFOM accuracy metric are recommended to be applied:

(1)     The horizontal position output should be calculated using the general least squares position solution of DO-229D Appendix J.1 (or any mathematically equivalent linear combination of range measurements). There is no restriction on the choice of the weight matrix W including non-weighted solutions; the use of the LNAV/VNAV, LP, LPV approach weight (w_i = 1/σ_i²) is optional.

(2)     The horizontal position accuracy should be tested using the procedure of DO-229D Section 2.5.8.3. The σ_i² used to compute the variance d²_major should be greater or equal to the ones listed in DO-229D Appendix J when the equipment uses SBAS-provided integrity and greater or equal to the ones listed as an acceptable means for FDE-provided integrity in section DO-229D 2.1.2.2.2.2 when the equipment does not use SBAS-provided integrity. A fixed sigma of 33.3 m is considered a sufficient over-bound when using FDE‑provided integrity. For equipment that uses SBAS-provided integrity, testing only in the highest mode attainable for its declared Operational Class as specified in the test itself is acceptable.

(3)     The accuracy metric should be greater or equal to 1.96 sqrt(d²_east + d²_north) or 2.45 d_major where d_major, d_east, and d_north are computed using the same σ_i employed during the horizontal accuracy test procedure. General certification substantiation data that the equipment meets this requirement is sufficient; no specific test is required.

Note 1: The scaling factors for the horizontal position accuracy metrics were rounded to 2 decimal places; there is no intention to prohibit the use of a more accurate number.

Note 2: The horizontal position accuracy metrics listed above are the standard metrics used to provide a minimum of 95 % containment (varying from 95 % to approximately 98.5 % for the horizontal metrics) under the assumption that a Gaussian distribution with a sigma of σi over- bounds the error of the range measurements. The use of a general least squares position solution (or mathematically equivalent) results in a joint Gaussian distribution for the components (North, East, Up) of the position error. Any accuracy metric that can be mathematically demonstrated to provide a minimum 95 % containment in the position domain under the Gaussian assumption is also acceptable.

(c)     Horizontal Position Latency – AMC1 ACNS.D.ADSB.070(a)(1) and (a)(2)(vi)

Time of Measurement to Time of Applicability

Applicability: ETSO-C129a (JTSO-C129a)

The intent of this qualification is to ensure that position and related quality indicator information are related to the same time of applicability in a consistent manner.

Based on the particular receiver design, GNSS equipment manufacturers should use a manufacturer-defined test, and/or analysis to determine the latency between the time satellite measurements are collated for processing and the time the equipment calculates a filtered (impulse response) position solution. The equipment should meet a 500-millisecond time of measurement to time of applicability requirement and account for the impulse response of the position solution.

Note: Whilst CS ACNS.D.ADSB does not establish requirements on the time of measurement, the above qualification has been incorporated to ensure consistency with FAA AC 20-165A.

Time of Applicability to Time of Output

Applicability: ETSO-C129a (JTSO-C129a)

The GNSS equipment manufacturer should document the position source latency from time of applicability to time of position output. If this latency exceeds 0.4 seconds, it may not support the 1.5-second total ADS-B transmission latency at the aircraft level (refer also to AMC1 ACNS.D.ADSB.115).

Time Mark

Applicability: ETSO-C129a (JTSO-C129a), ETSO-C196a, ETSO-C145()/C146()

If the use of the time mark to reduce latency is implemented in the ADS-B Out system, GNSS equipment manufacturers should provide installation instructions describing how the time mark relates to the time of applicability of the position, velocity, and related quality indicator information.

(d)     Horizontal Velocity Accuracy – AMC1 ACNS.D.ADSB.070(a)(1) and (a)(2)(vii)

Environmental Noise Test Conditions:

Applicability: ETSO-C129a, ETSO-C145( )/C146( ) (JTSO-C145/C146)

For equipment that was not required to meet the environmental noise standard prescribed by DO-235B, the velocity tests in AC 20-138B, Appendix 4 use environmental noise test conditions that may cause the equipment to stop functioning, i.e. to lose satellite acquisition and tracking capability that causes the equipment to stop outputting velocity. Whilst this contributes to an ADS-B availability issue for operators, this loss of function will not prevent the equipment from being used as an ADS-B velocity input, provided:

(1)     the equipment does not output misleading velocity information at or after the onset of the triggering interference levels; and

         Note: A method to accomplish this is first running the test at the higher noise level to ensure there is no misleading velocity information at loss of function before running the complete test at the lower noise level.

(2)     the equipment manufacturer should state that the equipment meets the noise requirements in DO-235B.

If the above conditions are met, the velocity tests in Appendix 4 of AC 20-138B (see below for NACv=1 and NACv=2 cases) can be run using an interference level that does not cause the equipment to lose acquisition and tracking.

ADS-B Out system installations intending to support NACv = 1:

Applicability: ETSO-C129a (JTSO-C129a), ETSO-C196a, ETSO-C145()/146()

The GNSS equipment manufacturer should perform the velocity tests in Appendix 4 of AC 20‑138B associated with NACv = 1 to substantiate the equipment’s velocity output.

The GNSS equipment manufacturer should indicate that the equipment satisfies the requirements for NACv =1 in the instructions for the ADS-B integration.

ADS-B Out system installations intending to support NACv = 2:

Applicability: ETSO-C129a (JTSO-C129a), ETSO-C196a, ETSO-C145()/146()

The GNSS equipment manufacturer should substantiate that the equipment dynamically outputs HFOMv and VFOMv and perform the velocity tests in AC 20-138C Appendix 4 associated with NACv = 1 and NACv = 2 to substantiate the equipment’s velocity output.

The GNSS equipment manufacturer should indicate that the equipment satisfies the requirements for NACv = 2 in the instructions for ADS-B Out system integration.

Track Angle Validity:

Applicability: ETSO-C129a (JTSO-C129a), ETSO-C196a, ETSO-C145()/146()

Using test and/or analysis for substantiation data, GNSS manufacturers should provide instructions for the ADS-B Out system integrator indicating when the track angle 95 % accuracy, when derived from north/east velocity, exceeds plus/minus ‘eight’ degrees. It is acceptable for the instructions to state that the track angle does not meet the required accuracy below a specified speed.

Note 1: Track Angle Validity is only an issue at taxiing speeds. Thereby, only along-track acceleration (0.58g) and jerk (0.25g/sec) are assumed to apply.

Note 2: Use should be made of the test environment specified in Appendix 4 of AC 20-138B. The interference levels used to demonstrate velocity accuracy compliance can be used for true track angle validity testing as well.

(e)     Geometric Altitude Accuracy (VFOM) – AMC1 ACNS.D.ADSB.085

Applicability: ETSO-C129a (JTSO-C129a), ETSO-C196a, ETSO-C145()/146()

GNSS equipment manufacturers should provide substantiation data showing if and how the equipment computes and outputs VFOM. If VFOM is output, the following criteria for an acceptable HAE-referenced geometric altitude output and its associated VFOM accuracy metric are recommended to be applied:

(1)     The HAE output should be calculated using the general least squares position solution of DO-229D Appendix J.1 (or any mathematically equivalent linear combination of range measurements). There is no restriction on the choice of the weight matrix W including non-weighted solutions; the use of the LNAV/VNAV, LP, LPV approach weight (w_i = 1/σ_i²) is optional.

(2)     The HAE accuracy should be tested using the procedure of DO-229D Section 2.5.8.3. The σ_i² used to compute the variance d_U² should be greater or equal to the ones listed in DO‑229D Appendix J when the equipment uses SBAS-provided integrity and greater or equal to the ones listed as an acceptable means for FDE-provided integrity in section 2.1.2.2.2.2 when the equipment does not use SBAS-provided integrity. A fixed sigma of 33.3 m is considered a sufficient over-bound when using FDE-provided integrity. For equipment that uses SBAS-provided integrity, testing only in the highest mode attainable for its declared Operational Class as specified in the test itself is acceptable.

(3)     The accuracy metric should be greater or equal to 1.96 d_U where d_U is computed using the same σ_i employed during the HAE accuracy test procedure. General certification substantiation data that the equipment meets this requirement is sufficient; no specific test is required.

For GPS equipment that outputs altitude references other than HAE whilst the overall ADS-B Out System meets AMC1 ACNS.D.ADSB.085(b), an equivalent data accuracy should be demonstrated.

Note 1: The scaling factors for the vertical position accuracy metrics were rounded to 2 decimal places; there is no intention to prohibit the use of a more accurate number.

Note 2: The vertical position accuracy metrics listed above are the standard metrics used to provide a minimum of 95 % containment (varying from 95 % to approximately 98.5 % for the vertical metrics) under the assumption that a Gaussian distribution with a sigma of σ_i over- bounds the error of the range measurements. The use of a general least squares position solution (or mathematically equivalent) results in a single Gaussian distribution for the components (North, East, Up) of the position error. Any accuracy metric that can be mathematically demonstrated to provide a minimum 95 % containment in the position domain under the Gaussian assumption is also acceptable.

Part 6 – Compliance Matrix BDS Register Fields

This part of Appendix H lists compliance matrices of the BDS register fields transmitted by the 1090 ES ADS-B transmit unit, with respect to the population of the 1090 ES data fields with data from approved sources (CS ACNS.D.ADSB.025(a) applies).

Omitted in the tables are fields containing the subtype codes (for these, refer to Part 1 of this Appendix) and reserved fields.

Reference to ADS-B Out item numbers is made in line with Part 1 of this Appendix respectively. Reference to Definitions is made in line with Part 1 of this Appendix.

Within the requirements (Req’t) column, ‘M’ expresses a mandatory requirement, i.e. the respective fields are populated with data from approved sources. ‘O’ expresses an optional requirement, ‘NA’ expresses non-applicability and ‘C’ expresses a conditional requirement (requirement is mandatory provided that the condition expressed in the remark column is met).

In addition to the 1090 ES data fields (as specified by the respective ‘ME’ Bits conveyed within the downlink format DF 17), the 3-bit ‘Capability (CA)’ field, also conveyed within downlink format DF 17, should be populated for all below registers as follows:

DF 17 – CA Field

Register 05₁₆ – Airborne Position Message

Register 06₁₆ – Surface Position Message

Register 08₁₆ - Aircraft Identification and Category Message

Register 09₁₆ - Airborne Velocity Message - Velocity over Ground (Subtypes 1and2, Normal/Supersonic)

Register 09₁₆ - Airborne Velocity Message - Airspeed (Subtypes 3 and 4, Normal/Supersonic)

Register 61₁₆ - Aircraft Status Message - Emergency Status and Mode A Code

Register 61₁₆ - Aircraft Status Message - ACAS RA Broadcast

Register 62₁₆ - Target State and Status Message

Register 65₁₆ – Aircraft Operational Status Message - While Airborne

Register 65₁₆ – Aircraft Operational Status Message - On the Surface

[Issue: CS-ACNS/4]