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I) If the emergencylpriority status message (2.3.8) is active, it shall be transmitted at the specified rate of once per second. Other
active event-driven messages shall be assigned equal priority for the remaining capacity.
2) If the emergencylpriority status message is not active, transmission priority shall be allocated equally to all active event-driven
messages.
2.5.6 DERIVATIOONF MODE FIELD BITS
FOR AIRCRAFC INTENTION PARAMETERS
For aircraft architectures that do not present the GFM with a dedicated status word (containing the mode field definitions
associated with aircraft intention parameters), the GFM shall derive the status from cach of the appropriate FCC status words in
order lo set the respective bits in each of the mode fields of the register numbcr 4Ol6.
2811 1102
No. 77
An~zex 10 - Aero~iarrtical Teleco~nmunicutinns Volume 111
2.6 LATITUDELONGITUDE CODING USINGCOMPACT POSITION REPORTING (CPR)
2.6.1 PRINCIPI~E OF THE CPK ALGORITHM
The Mode S extended squitters use compact position reporting (CPR) to encode latitude and longitude efficiently into messages.
I. The restilting messages are compact in the sense that several higher-order hits, whiclz are normally constant for long periods of
time, are not trarzsmitred in every message. For aanlple, in a direct binary represenration of latitude, one bit would designate
whether the uircrafr is in the izurtheriz or southern hemisphere. This bir would remain constant for a long time, possibly the
emire life of the uircrft. To repeatedly transmit this bit in every position message would be ineficient.
2. Because the higher-order hits are not transmitted, it follows that multiple locations on the earth will produce the same encoded
position. If' only a single position message were received, the decoding would involve ambiguity as to which of the multiple
solutions is the correct location of the aircraft. The CPR technique includes a provision to enable a receiving system to
unambiguously determine the location of the aircruft. This is done by encoding in two ways that differ slightly. The two formats,
called even-fomat and oddformat, are eaclz transmitted 50 per cent of the time. Upon reception of both types within a short
period (approximately 10 seconds), the receiving sysrem can urzurnhig~touslyd etermine the location of the aircraft.
3. Once tlzis process has been carried out, the higher-order bits are known at the receiving station, so subsequent single message
receptions serve to unanzbiguously indicate the location of the aircrafi as it moves.
4. In certain special cases, a single reception can be decoded into the correct location without an evedodd pair This decoding is
based on the fact that the multiple locations are spaced by at least 360 NM. In addition to the correct locations, the other
locations are separated by integer multiples of 360 NM to the north and south and also integer multiples of 360 NM to the east
and west. In a special case in which it is known that reception is impossible beyond a range of 180 NM, the nearest solution is
the correct location of the aircraft.
5. The parameter values in the preceding paragraph (360 and 180 NM) apply to the airborne CPR encoding. For aircraft on the
sutface, the CPR parameters are srnaller by a factor of 4. This encoding yields better resolution but reduces the spacing of the
multiple solutions.
2.6.2 CPR ALGORITHM PARAMETERS
AND INTERNAL FUNCTIONS
The CPR algorithm shall utilize the following parameters whose values are set as follows for rhc Mode S extended squitter
application:
a) The number of bits used to encode a position coordinate, Nh, is set as follows:
For airborne encoding: Nb = 17
For surface encoding: Nb = 19
For TCP, TCP+I encoding: Nb = 14.
Note I.- The Nb parameter determines the encoded position precision (approximately 5 m for the airborne encoding,
1.25 nz for the sniface encoding, and 41 nz for the TCt.1 TCP+I encoding).
b) The number of geographic latitude zones between the equator and a pole, denoted NZ, is scl to 15.
Note 2.- Tl~e NZ parameter determines the uizambigilous airborne range for decoding (360 NM). The sutfuce
larrtude/lorzgir~lde ncoding oinits the high-order 2 bits of the 19-bit CPR encoding, so the effective urzambiguolls range ,for
surface position reports is 90 NM.
The CPR algorilhm shall define internal functions to be used in the encoding and decoding processes.
Part I Annex 10 - Aeronautical Telecommunicatio~is
c) The notation floor(x) denotes the floor of r, which is defined as the greatest integer value k such that k 5 x.
Note 3.- For example, fhor(3.8) = 3, whilefloor(-3.8) = -4.
d) The nolalion I x I denotes the absolute value of x, which is defined as thc value x when x 2 0 and the value -x when r < 0.
　
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