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-52 Technical Provisions for Mode S Services and Extended Squitter b. Compute the latitude index: j = ⎛ 59⋅ YZ0 − 60⋅ YZ ⎜ ⎝ 2 floor 17 1 1 ⎞ + ⎟ 2 ⎠ c. Compute the values of Rlat0 and Rlat1 using the following equation: Rlat i ⎛ YZ = Dlati ⋅⎜ MOD 17 ⎝ ⎞ i ( j, 60 − i) + ⎟ 2 ⎠ Southern hemisphere values of Rlati shall fall in the range from 270° to 360°. Subtract 360° from such values, thereby restoring Rlati to the range from −90° to +90°. d. If NL(Rlat0) is not equal to NL(Rlat1) then the two positions straddle a transition latitude—thus a solution for global longitude is not possible. Wait for positions where they are equal. Note 3.— 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. If NL(Rlat0) is equal to NL(Rlat1) then proceed with computation of Dloni, according to whether the most recently received Airborne Position Message was encoded with the even format (i = 0) or the odd format (i = 1) : Dlon i = 360° n i where n i = greater of NL(Rlat i) − i f. Compute m, the longitude index: , ⎛ XZ ⎜ ⎝ ⋅ [ ] and 1. ( NL −1) − XZ1 ⋅ NL 1 ⎞ + ⎟ 2 ⎠ Draft 0 m = floor , 17 where NL NL( Rlat ) 2 = i . g. Compute the global longitude, Rlon0 or Rlon1, according to whether the most recently received Airborne Position Message was encoded using the even format (that is, with i = 0) or the odd format (i = 1): ⎛ Rloni = Dloni ⋅ ⎜ MOD m,ni ⎝ ⎞ ( )+ XZi 2 17 ⎟ , ⎠ [ ] and 1. where n i = greater of NL(Rlat i) − i h. A reasonableness test shall be applied to the resulting decoded position in accordance with §C.2.6.10.2. C.2.6.8 GLOBALLY UNAMBIGUOUS CPR DECODING OF SURFACE POSITION 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. 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-53 As surface-format messages are initially received from a particular aircraft, if there is no prior history of 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 intrack, then it is not necessary to use even-odd decoding. Beginning with the first individual Surface Position 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 §C.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. 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: Draft a. Compute the latitude zone sizes Dlat 0 and Dlat 1 from the equation: Dlati = 90° 60 − i b. Compute the latitude index: ⎛ 59 ⋅YZo − 60YZ1 1 ⎞ j = floor⎜ + ⎟ 17 ⎝ 2 2 ⎠ 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 Rlat 0 and Rlat 1 using the following equation: ⎛ Rlati = Dlati⎜ MOD j,60−i ⎝ ( )+ YZi 2 17 DRAFT - Working Paper ASP TSGWP11-01 for review by the TSG during the meeting in June 2011 in Paris ⎞ ⎟ ⎠
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