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-36 Technical Provisions for Mode S Services and Extended Squitter A.2.6.8 GLOBALLY UNAMBIGUOUS SURFACE POSITION DECODING This algorithm shall utilize one CPR surface position encoded “even” format message together with one CPR surface position encoded “odd” format message, to regenerate the geographic position of the aircraft or target. As surface-format messages are initially received from a particular aircraft, if there is no established track on this aircraft, then a global decode shall be performed using even and odd format receptions, as described in this section. Note 1.— If the aircraft has been transmitting airborne format messages and their receptions were in track, then it is not necessary to use even-odd decoding. Beginning with the first individual surface message reception, the location can be decoded using the local-decode technique, based on the previous target location as the reference. Note 2.— Even if the aircraft is appearing for the first time in surface format receptions, any single message could be decoded by itself into multiple locations, one being the correct location of the transmitting aircraft, and all of the others being separated by 90 NM or more from the correct location. Therefore, if it were known that the transmitting aircraft cannot be farther away than 45 NM from a known location, then the first received message could be decoded using the locally unambiguous decoding method described in §A.2.6.6. Under some circumstances it may be possible for an aircraft to be first detected when it is transmitting surface position messages farther than 45 NM away from the receiving station. For this reason, even-odd decoding is required when messages are initially received from a particular aircraft. After this initial decode, as subsequent messages are received, they can be decoded individually (without using the even-odd technique), provided that the intervening time is not excessive. This subsequent decoding is based on the fact that the aircraft location has not changed by more than 45 NM between each new reception and the previously decoded location. The even-odd decoding process shall begin by identifying a pair of receptions, one in the “even” format, the other in the “odd” format, and whose separation in time does not exceed the time interval of X seconds, where X=50 seconds, unless the Ground Speed in either Surface Position Message is greater than 25 knots, or is unknown, in which cases X=25 seconds. Note 3.— The limit of 25 seconds is based on the possible change of location within this time interval. Detailed analysis of CPR indicates that if the change of location is 0.75 NM or less, then the decoding will yield the correct location of the aircraft. To assure that the change of location is actually no larger, and considering the maximum aircraft speed of 100 knots specified for the transmission of the surface format, the combination indicates that 25 seconds will provide the needed assurance. For targets on the airport surface when speeds are much less and the transmission rate is as low as one per 5 seconds, the corresponding time limit is 50 seconds. Draft Given a CPR 17-bit surface position encoded in the “even” format (XZ0, YZ0) and another encoded in the “odd” format (XZ1, YZ1), separated by no more than X seconds, the algorithm shall regenerate the geographic position (latitude Rlat, and longitude Rlon) of the aircraft or target by performing the following sequence of steps: a) Compute the latitude zone sizes Dlat0 and Dlat1 from the equation: b) Compute the latitude index: 90° Dlati = 60 − i ⎛59 ⋅YZo −60YZ1 1 ⎞ j = floor ⎜ + 17 2 2 ⎟ ⎝ ⎠ 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-37 c) Latitude. The following formulas will yield two mathematical solutions for latitude (for each value of i), one in the northern hemisphere and the other in the southern hemisphere. Compute the northern hemisphere solution of Rlat0 and Rlat1 using the following equation: ⎛ YZi ⎞ Rlati = Dlati ⎜MOD( j,60− i ) + 17 ⎟ ⎝ 2 ⎠ 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(Rlat0) is not equal to NL(Rlati). 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. 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: 90° Dloni = n , where ni is the greater of [NL(Rlati ) – i] and 1. i ( ) ⎛ XZ0⋅ NL −1) − XZ1⋅NL 1 ⎞ m = floor ⎜ + 17 ⎟ 2 2 ⎟ ⎝ ⎠ where NL = NL (Rlati) Draft 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): ⎛ XZi ⎞ Rloni = Dloni ⋅ ⎜MOD ( m, ni) + 17 ⎟ ⎝ 2 ⎠ where ni = greater of [NL(Rlati) – 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. 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. Note.— Near the equator the minimum distance between the multiple longitude solutions is more than 5 000 NM, so there is no question as to the correct longitude. For locations away from the equator, the distance DRAFT - Working Paper ASP TSGWP11-01 for review by the TSG during the meeting in June 2011 in Paris
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Msg Bit # “ME” Bit # Appendix C
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Technical Provisions for Mode S Services and Extended Squitter

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