Technical Provisions for Mode S Services and Extended Squitter

Appendix CD CD-23
CD.2.4.7.5 BINARY ENCODING
Note.— For the rest of this appendix, 360 NM is not converted.
Once an area cell has been defined, nominally 360 by 360 NM, the encoding within the cell is expressed as a binary
fraction of the aircraft position within the cell. This means that the aircraft latitude and longitude are all zeroes at a point
when the aircraft is at the origin of the cell (the south west corner for the proposed encoding) and all ones at point one
resolution step away from the diagonally opposite corner.
This provides the seamless transition between cells. This technique for seamless encoding is illustrated in Figure CD-5
for the area cells defined above. For simplicity, only two-bit encoding is shown.
CD.2.4.7.6 ENCODING
DRAFT - Working Paper ASP TSGWP11-01 for review by the TSG during the meeting in June 2011 in Paris
The above techniques would be sufficient for an encoding system if the Earth were a cube. However, to be consistent on
a sphere, additional features must be applied to handle the change in longitude extent as latitudes increase away from
the equator. The polar regions must also be covered by the coding.
All lines of longitude must have the same nominal radius, so the latitude extent of an area cell is constant. The use of a
360 NM minimum unambiguous range leads to 15 latitude zones from the equator to the poles.
00
11
10
01
00
11
10
01
00
00
360 NM 360 NM
Draft
01 10 11 00 01 10 11 00
Figure CD-5. CPR seamless encoding
DRAFT - Working Paper ASP TSGWP11-01 for review by the TSG during the meeting in June 2011 in Paris
360 NM
360 NM