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-54 Technical Provisions for Mode S Services and Extended Squitter The southern hemisphere value is the above value minus 90 degrees. To determine the correct latitude of the target, it is necessary to make use of the location of the receiver. Only one of the two latitude values will be consistent with the known receiver location, and this is the correct latitude of the transmitting aircraft. d. The first step in longitude decoding is to check that the even-odd pair of messages do not straddle a transition latitude. It is rare, but possible, that NL(Rlat 0) is not equal to NL(Rlat 1). If so, a solution for longitude cannot be calculated. In this event, abandon the decoding of this even-odd pair, and examine further receptions to identify another pair. Perform the decoding computations up to this point and check that these two NL values are equal. When that is true, proceed with the following decoding steps. Note.— When performing the NL function, the encoding process must ensure that the NL value is established in accordance with Note 5 of §C.2.6.2.d. This is more important in the Global Unambiguous Decode because large longitude errors are induced if the decode function is not selecting the NL value properly as discussed in Note 5 of §C.2.6.2.d. e. Compute the longitude zone size Dloni, according to whether the most recently received surface position message was encoded with the even format (i=0) or the odd format (i=1): f. Compute m, the longitude index: ° Dloni = n 90 , where ni is the greater of [NL(Rlati ) - i] and 1. ⎛ XZ m = floor⎜ ⎝ i where NL = NL (Rlat i ) 0 ⋅ ( NL −1) − XZ ⋅ NL) 2 17 1 1 ⎞ + ⎟ 2 ⎠ g. Longitude. The following formulas will yield four mathematical solutions for longitude (for each value of i), one being the correct longitude of the aircraft, and the other three separated by at least 90 degrees. To determine the correct location of the target, it will be necessary to make use of the location of the receiver. Compute the longitude, Rlon0 or Rlon1, according to whether the most recently received surface position message was encoded using the even format (that is, with i=0) or the odd format (i=1): Draft Rloni = Dloni ⋅ MOD( m,ni)+ XZi ⎛ ⎞ ⎜ ⎟ ⎝ ⎠ 2 17 where n i is the greater of [NL(Rlat i ) - i] and 1. This solution for Rloni will be in the range 0° to 90°. The other three solutions are 90°, 180°, and 270° to the east of this first solution. h. A reasonableness test shall be applied to the resulting decoded position in accordance with §C.2.6.10.2. To then determine the correct longitude of the transmitting aircraft, it is necessary to make use of the known location of the receiver. Only one of the four mathematical solutions will be consistent with the known receiver location, and this is the correct longitude of the transmitting aircraft. 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 C C-55 Note.— Near the equator the minimum distance between the multiple longitude solutions is more than 5000 NM, so there is no question as to the correct longitude. For locations away from the equator, the distance between solutions is less, and varies according to the cosine of latitude. For example at 87 degrees latitude, the minimum distance between solutions is 280 NM. This is sufficiently large to provide assurance that the correct aircraft location will always be obtained. Currently no airports exist within 3 degrees of either pole, so the decoding as specified here will yield the correct location of the transmitting aircraft for all existing airports. C.2.6.9 CPR DECODING OF RECEIVED POSITION REPORTS C.2.6.9.1 Overview Note.— The techniques described in the preceding paragraphs (locally and globally unambiguous decoding) are used together to decode the lat/lon contained in airborne, surface intent and TIS-B position reports. The process begins with globally unambiguous decoding based upon the receipt of an even and an odd encoded position squitter. Once the globally unambiguous position is determined, the emitter centered local decoding technique is used for subsequent decoding based on a single position report, either even or odd encoding. C.2.6.9.2 Emitter Centered Local Decoding In this approach, the most recent position of the emitter shall be used as the basis for the local decoding. Note.— This produces an unambiguous decoding at each update, since the transmitting aircraft cannot move more than 360 NM between position updates. C.2.6.10 REASONABLENESS TEST FOR CPR DECODING OF RECEIVED POSITION MESSAGES C.2.6.10.1 Overview Note.— Although receptions of Position Messages will normally lead to a successful target position determination, it is necessary to safeguard against Position Messages that would be used to initiate or update a track with an erroneous position. A reasonableness test applied to the computed position resulting from receipt of a Position Message can be used to discard erroneous position updates. Since an erroneous globally unambiguous CPR decode could potentially exist for the life of a track, a reasonableness test and validation of the position protects against such occurrences. Draft C.2.6.10.2 Reasonableness Test Applied to Position Determined from Globally Unambiguous Decoding A reasonableness test shall be applied to a position computed using the Globally Unambiguous CPR decoding per §C.2.6.7 for Airborne Participants, or per §C.2.6.8 for Surface Participants. Upon receipt of the “even” or “odd” encoded Position Message that completes the Globally Unambiguous CPR decode, the receiver shall perform a reasonableness test on the position decode by performing the following: If the receiver position is known, calculate the distance between the decoded position and the receiver position, and verify that the distance is less than the maximum reception range of the receiver. If the validation fails, the receiver shall discard the decoded position that the “even” and “odd” Position Messages used to perform the globally unambiguous CPR decode, and reinitiate the Globally Unambiguous CPR decode process. A further validation of the Globally Unambiguous CPR decode, passing the above test, shall be performed by the computation of a second Globally Unambiguous CPR decode based on reception of a new “odd” and an “even” Position Message as per §C.2.6.7for an Airborne Participant, or per §C.2.6.8 for a Surface Participant, both received subsequent to the respective “odd” and “even” Position Message used in the Globally Unambiguous CPR decode under validation. Upon accomplishing the additional Globally Unambiguous CPR decode, this decoded position and the position from the locally unambiguous CPR DRAFT - Working Paper ASP TSGWP11-01 for review by the TSG during the meeting in June 2011 in Paris
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