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 A-8 Technical Provisions for Mode S Services and Extended Squitter if either the position accuracy or integrity can be verified by other means, or the application has no specific requirements for these parameters. A.2.3.2 AIRBORNE POSITION FORMAT The airborne position squitter shall be formatted as specified in the definition of transponder register 0516. Additional details are specified in the following paragraphs. A.2.3.2.1 COMPACT POSITION REPORTING (CPR) FORMAT (F) In order to achieve coding that is unambiguous worldwide, CPR shall use two format types, known as even and odd. This 1-bit field (bit 22) shall be used to define the CPR format type. F=0 shall denote an even format coding, while F=1 shall denote an odd format coding (see §AC.2.6.7). A.2.3.2.2 TIME SYNCHRONIZATION (T) This 1-bit field (bit 21) shall indicate whether or not the time of applicability of the message is synchronized with UTC time. T=0 shall denote that the time is not synchronized to UTC. T=1 shall denote that the time of applicability is synchronized to UTC time. Synchronization shall only be used for airborne position messages having the top two horizontal position precision categories (format TYPE Codes 9, 10, 20 and 21). When T=1, the time of validity in the airborne position message format shall be encoded in the 1-bit F field which, in addition to CPR format type, indicates 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 is an exact evennumbered UTC second. A.2.3.2.3 LATITUDE/LONGITUDE The latitude/longitude field in the airborne position message shall be a 34-bit field containing the latitude and longitude of the aircraft airborne position. The latitude and longitude shall each occupy 17 bits. The airborne latitude and longitude encodings shall contain the 17 bits of the CPR-encoded values defined in §AC.2.6. Draft Note 1.— The unambiguous range for the local decoding of airborne messages is 666 km (360 NM). The positional accuracy maintained by the airborne CPR encoding is approximately 5.1 metres. The latitude/longitude encoding is also a function of the CPR format value (the “F” bit) described above. Note 2.— Although the positional accuracy of the airborne CPR encoding is approximately 5.1 metres in most cases, the longitude position accuracy may only be approximately 10.0 metres when the latitude is either –87.0 ±1.0 degrees, or +87 ±1.0 degrees. A.2.3.2.3.1 Extrapolating position (when T = 1) If T is set to one, airborne position messages with format TYPE Codes 9, 10, 20 and 21 shall have times of applicability which are exact 0.2s UTC epochs. In that case, the F bit shall be 0 if the time of applicability is an even-numbered 0.2s UTC epoch, or 1 if the time of applicability is an odd-numbered 0.2s UTC epoch. Note.— In such a case, an “even-numbered 0.2s epoch” means an epoch which occurs an even number of 200-ms time intervals after an even-numbered UTC second. An “odd-numbered 0.2s epoch” means an epoch which occurs an DRAFT - Working Paper ASP TSGWP11-01 for review by the TSG during the meeting in June 2011 in Paris
DRAFT - Working Paper ASP TSGWP11-01 for review by the TSG during the meeting in June 2011 in Paris Appendix A A-9 odd number of 200-ms time intervals after an even-numbered UTC second. Examples of even-numbered 0.2s UTC epochs are 12.0s, 12.4s, 12.8s, 13.2s, 13.6s, etc. Examples of odd-numbered UTC epochs are 12.2s, 12.6s, 13.0s, 13.4s, 13.8s, etc. The CPR-encoded latitude and longitude that are loaded into the airborne position register shall comprise an estimate of the aircraft/vehicle (A/V) position at the time of applicability of that latitude and longitude, which is an exact 0.2s 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 shall ensure that the receiving ADS-B system may recover the time of applicability of the data in the airborne position message, as follows: 1) If F = 0, the time of applicability shall be the nearest even-numbered 0.2s UTC epoch to the time that the airborne position message is received. 2) If F = 1, the time of applicability shall be the nearest odd-numbered 0.2s UTC epoch to the time that the airborne position message is received. Recommendation.— If the airborne position register is updated at its minimum (every 200 ms), that register should be loaded 100 ms before the time of applicability. The register should then be reloaded, with data applicable at the next subsequent 0.2s UTC epoch, 100 ms before that next subsequent 0.2s epoch. Note 1.— In this 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. Note 2.— The position may be estimated by extrapolating the position from the time of validity of the fix (included in the position fix) to the time of applicability of the data in the register (which, if T = 1, is an exact 0.2s UTC time tick). This may be done by a simple linear extrapolation using the velocity provided with the position fix and the time difference between the position fix validity time and the time of applicability of the transmitted data. Alternatively, other methods of estimating the position, such as alpha-beta trackers or Kalman filters, may be used. Every 200 ms, the contents of the position registers shall be updated by estimating the A/V position at the next subsequent 0.2s UTC epoch. This process shall continue with new position fixes as they become available from the source of navigation data. A.2.3.2.3.2 Extrapolating position (when T = 0) Draft T shall be set to zero if the time of applicability of the data being loaded into the position register is not synchronized to any particular UTC epoch. In that case, the position register shall be reloaded with position data at intervals that are no more than 200 ms apart. The position being loaded into the register shall have a time of applicability that is never more than 200 ms different from any time during which the register holds that data. Note.— This may be accomplished by loading the airborne position register at intervals that are, on average, no more than 200 ms apart, with data for which the time of applicability is between the time the register is loaded and the time that it is loaded again. (Shorter intervals than 200 ms are permitted, but not required.) If T = 0, receiving ADS-B equipment shall accept airborne position messages as being current as of the time of receipt. The transmitting ADS-B equipment shall reload the airborne position register with updated estimates of the A/V position, at intervals that are no more than 200 ms apart. The process shall continue with new position reports as they become available. DRAFT - Working Paper ASP TSGWP11-01 for review by the TSG during the meeting in June 2011 in Paris
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Appendix B B-3 TYPE code Subtype co
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Msg Bit # “ME” Bit # Appendix C
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Appendix C C-59 Register 0516 DRAFT
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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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