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fonnats. For the TCP, TCP+I format, i shall always be set to 0 (even encoding), whereas the airborne format shall use both even
(i = 0) and odd (i = 1) encoding. For the airborne format, Nb shall equal 17, and for the TCP, TCP+l format, Nb shall equal 14.
a) Alat, is computed from the equation:
b) The latitude zone index number, j, is then computed from the values of lat,, Alati and YZi using the equalion:
la t, 1 MOD(lat,,Alati) YZi
j = floor(-) + floor (? + Alati Alat,
c) The decoded position lalitude, Rlati, is then computed from the values of j, Alat,, and YZ, using the equation:
Rlat, = Alat; . [j + - i?)
dj Alon, (the longitude zone size in the E-W direction) is then computed from Rlat, using the equation:
1 360". when NL(Rlati) - i = 0
ej The longitude zone coordinate rn is then computed from the values of [on,, Alon, and XZi using the equation:
lon 1 MOD(l~n,~,Alo-q 3) ,) m = floor(*) Alon. + floor (i + Alon, zNh
f) The decoded position longitude, Rlon, is then computed from the values of rn, XZ,, and Aloni using the equation:
2.6.6 COMPUTATIONFSO R THE SURFACE MESSAGE
The following computations shall be performed to obtain the decoded latitude and longitude for the surface position format.
a) Alat, is computed fmm the equation:
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No. 77
Annex 10 - Aeronautical Telecommunications
b) The latitude zone index, j, is then computed from the values of l ~ t ,A~la,t ; and YZ,u sing the equation:
j = floor (d;"b; - + floor
c) The decoded position latitude, Rlat,, is then computed from the values of j, Alati, and YZ, using the equation:
d) Alon; (the longitude zone size, in the E-W direction) is then computed from Rlar; using the equation:
Volume III
when NL(REati) - i = 0
e) The longitude zone coordinate rn is then computed from the values of lon,, Alon, and XZi using the equation:
lon 1 MOD(lon,,Aloni) YZi
rn = f l o o rA(lo2n). + floor (? +
Aloni - 72 )
f) The decoded position longitude, Rlon, is then computed from the values of m, XZi, and Aloni using the equation:
2.6.7 GLOBALLYU NAMBIGUOUS AIRBORNE
POSITION DECODING
The CPR algorithm shall utilize one airborne-encoded "even" format reception (denoted XZo, YG), together with one airborneencoded
"odd format reception (denoted XZ,, YZ,), to regenerate the global geographic position latitude, Rlat, and longitude, Rlon.
The time between the "even" and "odd format encoded position reports shall be no longer than 10 seconds.
Note 1.- This algorithm might be used to obtain globally unambiguous position reports for aircraft out of the range of ground
sensors, whose position reports ure coming via satellite data links. It might also be applied ro ensure rhar local positions are being
correctly decoded over long ranges from the receiving sensor:
Note 2.- The time difference limit of I0 seconds between the even- and odd-format position reports is determined by the
maximum pemzitted separation of 3 NM. Positions greater than 3 NM apart cannot be used to solve a unique global position. An
aircraft capable of a speed of 1 850 kmh (1 000 kt) will fly about 5.1 km (2.8 NM) in I0 seconds. Therefore, the CPR algorithm
will be able to unambiguously decode its position over a 10-second delay between position reports.
Given a 17-bit airborne position encoded in the "even" format (Xq, YZo) and another encoded in the "odd format (XZ,, YZ1),
separated by no more than 10 scconds (= 3 NM), the CPR algorithm shall regenerate the geographic position from the encoded
position reports by performing thc following sequence of steps:
a) Compute Alate and A/at, from the equation:
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Part I
b) Compute the latitude index:
Annex 10 - Aeronautical Telecommunications
(59 YZ;;760 YZl
j = floor
+ i)
c) Compute the values of Rlato and Matl using the following equation:
Southern hemisphere values of Rlat; will fall in the range from 270" to 360'. Subtract 360" from such values, thereby
restoring Rlat; to the range from -90' to +90°.
d) If NL(Rlato) is not equal to NL(Rlat,) then the two positions straddle a transition latitude, thus a solution for global longitude
is not possible. Wait for positions where they are equal.
e) If NL(Rlat,) is equal to NL(Rlat,) then proceed with computation of Aloni, according to whether the most recently received
airborne position message was encoded with the even format (i = 0) or the odd format (i = 1) :
where n, = greater of [NL(Rlat,) - i] and 1.
f ) Compute rn, the longitude index:
XZo.(NL-1)-XZI*NL 1
rn = floor
217
where NL = NL (Rlat,)
g) Compute the global longitude, Rlono or Rlonl, according to whcther the most reccntly received airborne position message
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