Technical Provisions for Mode S Services and Extended Squitter

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DRAFT - Working Paper ASP TSGWP11-01 for review by the TSG during the meeting in June 2011 in Paris C-10 Technical Provisions for Mode S Services and Extended Squitter C.2.3.1.2.4 Surface Position Message TYPE Code During Fault Detection and Exclusion Condition Under normal operating conditions, the R C can be directly determined from Horizontal Protection Limit (HPL) or Horizontal Integrity Limit (HIL) inputs to the ADS-B Transmitting Subsystem from the GPS/GNSS receiver. However, there are times when the Fault Detection and Exclusion (FDE) function of the GPS/GNSS receiver has detected a satellite failure but has not excluded the satellite from the navigation data solution. For the purposes of this Manual, the condition just described will be referred to as an “FDE Fault” which is typically annunciated by the GPS/GNSS receiver by an appropriate method. For example, ARINC 743A compliant GPS/GNSS receivers will set Label “130” bit “11” to ONE (1) to indicate the “FDE Fault.” If an “FDE Fault” does not exist, then bit “11” is set to ZERO (0). Of importance is the situation that even though an “FDE Fault” indication has been set, the GPS/GNSS typically continues to provide HPL or HIL data as well as Latitude, Longitude, and Velocity data while continuing to declare them to be valid on the interface. As the “FDE Fault” condition represents a condition where the position and accuracy data cannot be guaranteed by the GPS/GNSS receiver, the ADS-B Transmitting Subsystem shall apply the following process upon detection of the “FDE Fault” annunciation: Note 1.— If position sources can be demonstrated to overcome this degraded state of integrity when the “FDE Fault” condition occurs, then the following requirements do not apply. a. The “TYPE” Code of Surface Position Messages shall be set to “8” in order to indicate that RC is UNKNOWN. Note 2.— The “TYPE” Code is not required to be set to ZERO (0), as to do so would indicate that there is NO Position Data which will result in the ADS-B Transmitting Subsystem declaring an ADS-B Function Failure (see RTCA DO-260B, §2.2.11.6). b. Valid Latitude and Longitude Position data shall continue to be processed and reported in the Surface Position Message as required. c. The NIC Supplement-A and NIC Supplement-C subfields in the Surface Aircraft Operational Status Message (TYPE=31, Subtype=1) shall be set to ZERO (0) in accordance with §C.2.3.10.6 and Table C-28 for a NIC Value of ZERO (0) when “TYPE” Code is set to “8.” d. The NACP subfield in the Surface Aircraft Operational Status Message (TYPE=31, Subtype=1) shall be set to ZERO (0) in accordance with §C.2.3.9.9 and Table C-13 in order to specify that the accuracy is UNKNOWN. e. The NACV CC subfield in the Surface Aircraft Operational Status Message (TYPE=31, Subtype=1) shaill be set to ZERO (0) in accordance with §C.2.3.5.4 and Table C-5. Draft Note 3.— Factors such as surface multi-path have been observed to cause intermittent annunciation of “FDE Faults” by the GPS/GNSS receiver. Such occurrences will result in the intermittent settings of the subfields addressed in subparagraphs “a,” “c” and “d” above. These intermittent conditions should be taken into account by ADS-B and Air Traffic Services that are using the data provided by the ADS-B Transmitting Subsystems. C.2.3.2 AIRBORNE POSITION FORMAT The Airborne Position squitter shall be formatted as specified in the definition of Register 0516 in Figure C-1 and described in the following paragraphs. C.2.3.2.1 Compact Position Reporting (CPR) Format (F) In order to achieve coding that is unambiguous world wide, CPR shall use two format types, known as “even” and “odd.” This one-bit field (“ME” bit 22, Message bit 54) shall be used to define the CPR Format (F) type. A CPR Format equal to ZERO (0) shall denote an “even” format coding, while a CPR Format equal to ONE (1) shall denote an “odd” format coding (§C.2.6.7). DRAFT - Working Paper ASP TSGWP11-01 for review by the TSG during the meeting in June 2011 in Paris
Appendix C C-11 C.2.3.2.2 Time Synchronization (T) This one-bit field (“ME” bit 21, Message bit 53) shall indicate whether or not the Time of Applicability of the message is synchronized with UTC time. “T” equal to ZERO (0) shall denote that the time is not synchronized to UTC. “T” equal to ONE (1) shall denote that Time of Applicability is synchronized to UTC time. When T=1, the time of validity in the Airborne Message format shall be encoded in the 1-bit “F” field which (in addition to CPR format type) shall indicate the 0.2 second time tick for UTC Time of Position Validity. The “F” bit shall alternate between 0 and 1 for successive 0.2 second time ticks, beginning with F=0 when the Time of Applicability shall be an exact even-numbered UTC second. C.2.3.2.3 CPR Encoded Latitude/Longitude The CPR Encoded Latitude/Longitude field in the Airborne Position Message shall be a 34-bit field (“ME” bits 23 – 56, Message bits 55 – 88) containing the Latitude and Longitude of the Aircraft’s Airborne Position. The Latitude and Longitude shall each occupy 17 bits. The Airborne Latitude and Longitude encoding shall contain Airborne CPR-encoded values in accordance with §C.2.6. The unambiguous range for the local decoding of Airborne Messages shall be 666 km (360 NM). The positional accuracy maintained by the Airborne CPR encoding shall be approximately 5.1 meters. Notes.— DRAFT - Working Paper ASP TSGWP11-01 for review by the TSG during the meeting in June 2011 in Paris 1. The Latitude/Longitude encoding is also a function of the CPR format value (the “F” bit) described above. 2. Although the positional accuracy of the airborne CPR encoding is approximately 5.1 meters in most cases, implementers should be aware that the longitude position accuracy may only be approximately 10.0 meters when the latitude is either –87.0 ±1.0 degrees, or +87 ±1.0 degrees. C.2.3.2.3.1 Extrapolating Position (When T=1) If “T” is set to one, Airborne Position Messages shall have Times of Applicability that are exact 0.2 UTC second epochs. In that case, the “F” bit shall be ZERO (0) if the Time of Applicability is an even-numbered 0.2 second UTC epoch, or ONE (1) if the Time of Applicability is an odd-numbered 0.2 second epoch. Note 1.— Here, an “even-numbered 0.2 second epoch” means an epoch that occurs an even number of 200-millisecond time intervals after an even-numbered UTC second. An “odd-numbered 0.2 second epoch” means an epoch that occurs an odd number of 200-millisecond time intervals after an even-numbered UTC second. Examples of even-numbered 0.2 second UTC epochs are 12.0 s, 12.4 s, 12.8 s, 13.2 s, 13.6 s, etc. Examples of odd-numbered UTC epochs are 12.2 s, 12.6 s, 13.0 s, 13.4 s, 13.8 s, etc. Draft The CPR-encoded Latitude and Longitude that are loaded into the Airborne Position register will comprise an estimate of the A/V position at the Time of Applicability of that Latitude and Longitude, which is an exact 0.2 second UTC epoch. The register shall be loaded no earlier than 150 ms before the Time of Applicability of the data being loaded, and no later than 50 ms before the Time of Applicability of that data. This timing ensures that the ADS-B Receiving Subsystem may easily recover the Time of Applicability of the data in the Airborne Position Message, as follows: • If F=0, the Time of Applicability shall be the nearest even-numbered 0.2 second UTC epoch to the time that the Airborne Position Message is received. • If F=1, the Time of Applicability shall be the nearest odd-numbered 0.2 second UTC epoch to the time that the Airborne Position Message is received. Note 2.— If the Airborne Position register is loaded every 200 ms, the ideal time to load that register would be 100 ms before the Time of Applicability of the data being loaded. The register would then be reloaded, with data applicable at the next subsequent 0.2 second UTC epoch, 100 ms before that next subsequent 0.2 second epoch. That way, the time of transmission of an Airborne Position Message would never differ by more than 100 ms from the Time of Applicability of the data in that message. By specifying “100 ms ± 50 ms” rather than 100 ms exactly, some tolerance is allowed for variations in implementation. DRAFT - Working Paper ASP TSGWP11-01 for review by the TSG during the meeting in June 2011 in Paris
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Appendix A A-99 This subfield shall
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Appendix A A-109 A.4. MODE S BROADC
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Appendix B B-3 TYPE code Subtype co
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Appendix CD CD-3 ACTIONS EVENTS DRA
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Appendix CD CD-23 CD.2.4.7.5 BINARY
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Technical Provisions for Mode S Services and Extended Squitter

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