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was encoded using the even format (that is, with i = 0) or the odd format ( i = 1):
where ni = greater of [NL(Rlat,) - i] and 1
2.6.8 CPR DECODING OF RECEIVED POSITION REPOR'IY
The techniques described in the preceding paragraphs (locally and globally unambiguous dccoding) shall be used together to decode
the latitude/longitude contained in airborne, surface, and TCP or TCP+I position reports. The process shall begin with globally
unambiguous decoding based upon the receipt of an even and an odd encoded position quitter. Once the globally unambiguous
position is determined, either of two approaches shall be used to support subsequent decoding based upon a single position report,
either even or odd encoding. The two techniques shall be range monitoring and emitter centered local decoding.
Annex 10 - Aeronautical Telecommunicatiotts Volume ZZZ
2.6.8.2 RANGE MONITORING LOCAL DECODING
2.6.8.2.1 Range monitoring technique
In this approach, local decoding for the airborne format (2.6.4) shall be performed based upon the current position of the receiver.
This shall providc the position of a transmitting aircraft that is unambiguous to plus or minus 180 NM.
Note 1.- I f the transmitting uircraft is within 180 NM, the local decoding technique will correctly decode the locution of the
uircraj?.
The range of the transmitting aircraFt shall be checked at detection and tracks shall only be initiated if the range is less than
180 NM. Once initiated, the range of the tracked aircraft shall be checked at each update and the track shall be dropped if the range
becomes equal to or greater than 180 NM.
For the surface format, the same process shall be used except that the transmitting aircraft must be within 45 NM for detection and
tracking.
Note 2.- The runge limits are reduced since the ambiguity linzit for the surface position reports is one-fourth that of the airborne
cuse.
2.6.8.2.2 Range monitoring example
2.6.8.2.2.1 Decoding of airborne position
2.6.8.2.2.1.1 Detection. At detection, a globally unambiguous decode shall be performed. If range is greater than 160 NM, the
detection attempt shall be discontinued and the track information discarded. Detection shall be attempted if squitters continue to
be received. If the globally decoded range remains greater than 160 NM, the track information shall continue to be discarded.
Note.- I f the aircraft is upproaching, detection will succeed when the range decreases to less than or equal to 160 NM.
2.6.8.2.2.1.2 Track monitoring. After detection, range shall bc monitored during each surveillance update. If range is greater
than 170 NM, the track shall be dropped.
Note.- The use of 160 NM for detection and 170 NM for track drop provides hysteresis that avoids reacquiring a track that
was just dropped due to long range. Thus a track dropped at 170 NM would not be reacquired unless its range dropped to less
than or equal to 160 NM.
2.6.8.2.2.2 Decoding of suqace position. Using the range monitoring technique for decoding squitters in the surface format,
the same process as above shall be used except that the track shall be initiated at 40 NM and dropped at 42.5 NM.
2.6.8.2.3 Emitter centered locul decoding
In this approach, the most recent position of the emitter shall bc used as the basis for the local decoding.
Note.- This produces an unambiguous decoding at each update, since the transmitting uinraft cannot move more than 360 NM
between position updates.
2.6.8.2.4 Technique application
The range monitoring technique shall only be used for ranges less than or equal to 180 NM, far example in air-to-air applications.
For ground stations (i.e. non-aircraft implementations) that are required to operate at ranges in excess of 180 NM, only the emitter
centred technique can shall be used.
Note.- The emitter ce11,tered technique cun be used for both airborne receivers and ground stations.
2811 1102
No. 77
Annex 10 - Aeronautical Telecommunications
TABLES FOR SECTION 2
Tables are numbered 2-X where "X" is the decimal equivalent of the BDS code Y,Z where Y is the BDS1 code and Z is the BDS2
code, used to access the data format for a particular register. The following tables are not included:
2- 1
2-2 to 2-4 (Used by the linked Comm-B protocol)
2-13 to 2-14 (Reserved for airlair state information)
2- 15 (Reserved for ACAS)
2-17 to 2-22
2-35 (Reserved for antenna position)
2-36 (Reserved for aircraft parameters)
2-38 to 2-47
2-49 to 2-63
2-70 to 2-71
2-73 to 2-79
2-87 to 2-94
2-102 to 2-1 11 (Reserved for extended squitter)
2- 1 12 to 2-224
2-225 to 2-226 (Reserved for Mode S byte)
2-227 to 2-240
2-243 to 2-255
Annex 10 - Aeronautical Telecommunications Volume ZZZ
　
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