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ACAS X MOPS
currently interrogating. The MU field of this interrogation includes the ICAO 24-bit
Aircraft Address of the transponder on the Active CAS aircraft. Each ACAS X that is
currently interrogating shall (1009) spontaneously transmit these interrogations at
nominal 10-second intervals and at full power. Each ACAS X shall (1010) monitor the
receipt of such interrogations by own Mode S transponder to determine the number of
other Active CAS units within detection range (nominally 30 NM). Once each second,
NTA shall (1011) be updated as the number of distinct Active CAS addresses monitored
within the previous 20-second period.
In addition to knowledge of NTA, proper operation of inequality 1 also depends on
knowing the distribution of Active CAS interrogators in close vicinity. Using this
information and NTA, interference limiting inequality 1 can be matched to the local
ACAS X environment. Each scan, ACAS X shall (1012) estimate the distribution of
other Active CAS aircraft by using range information derived from Mode S surveillance
of those aircraft from which TCAS Broadcast Interrogation Messages have been
received. In order to achieve an accurate estimate of the distribution regardless of the
level of interference limiting, ACAS X shall (1013) include only those Active CAS
aircraft that are within 6 NM range.
Each scan, the ACAS X distribution function within 30 NM and within 6 NM shall
(1014) be characterized in the following manner. The distribution of Active CAS within
30 NM shall (1015) be computed as the ratio of the number of Active CAS within 30
NM to the number within 6 NM. This ratio is proportional to an ACAS X distribution
factor, 2, according to the following formula:
where NTA is the Active CAS count derived from monitoring all TCAS Broadcast
Interrogation Messages and NTA6 is the number of aircraft from which TCAS Broadcast
Interrogation Messages have been received that are estimated to be within 6 NM.
The distribution of Active CAS within 6 NM shall (1016) be computed as the ratio of the
number of Active CAS within 6 NM to the number within 3 NM. This ratio is
proportional to an ACAS X distribution factor, 1, according to the following formula:
1 = 1/4 [NTA6/NTA3],
where NTA3 is the number of aircraft from which TCAS Broadcast Interrogation
Messages have been received that are estimated to be within 3 NM.
ACAS X aircraft that are operating on the ground or at or below a radio altitude of 2000
ft AGL shall (1017) include both airborne and on-ground Active CAS in the value for
NTA3 and NTA6. Otherwise, ACAS X shall (1018) include only airborne Active CAS in
the value for NTA3 and NTA6. Identification and surveillance of on-ground Active CAS
can be accomplished by examining the VS field in the DF=0 acquisition reply to a UF=0
acquisition interrogation and by correlating the ICAO 24-bit aircraft address associated
with the reply with the address received in the TCAS Broadcast Interrogation.
The maximum value of 1 and 2 shall (1019) not exceed 1 which defines a uniform-in-
area Active CAS distribution and the minimum value of 1 and 2 shall (1020) not be
allowed to decrease below 0.5 which defines a uniform-in-range Active CAS distribution.
These constraints on 1 and 2 are imposed to ensure that the ACAS X surveillance range
is always adequate for collision avoidance and that the maximum allowable interrogation
power-rate product is always maintained at a reasonable level regardless of the measured
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