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3.2.1 Intruder Prep
INTRUDERP REP (Algorithm 133) is used to prepare the intruder inputs for processing. It sets fields
in the intruder state (st_int), as well as returning values to be stored in processing arrays.
The value of st_int.no_alerts is set to refect whether the Designated No Alerts (DNA) Xo mode is
active for this intruder. Since DNA suppresses traffic advisories as well as resolution advisories,
it is not registered as a processing mode. Instead, the DNA setting (st_int.no_alerts) is checked
whenever special processing is required for DNA.
Next the value of st_int.is_threat is cleared to indicate that this intruder is not recognized as a threat
for this cycle. The output variable belief_vert_int is initialized with the contents of input_int.belief_-
The first conditional block sets the processing mode based on the quality of the surveillance for this
intruder and the ACAS-X operational mode. The TRM processing is adjusted if the quality of the
surveillance is inadequate for producing resolution advisories. The quality of the surveillance from
the STM is indicated by input_int.degraded_surveillance, which is a bit vector. Each bit represents
a specific type of degraded surveillance. If no bits are set, the value of input_int.degraded_surveil-
lance is 0 (DEGRADED_SURVEILLANCE_NONE) indicating no degradation. TRM processing for
this intruder is based on the setting of the degraded_surveillance bits as follows:
• A non-zero value for the DEGRADED_SURVEILLANCE_NAR bit indicates Non-Altitude Re-
porting (NAR) surveillance. This type of surveillance is inadequate for producing resolution
advisories (RAs); it is adequate for producing traffic advisories (TAs) only. The intruder is
flagged by INTRUDER PREP for degraded surveillance processing: (st_int.processing is set to
RA_PROCESSING_DEGRADED_SURVEILLANCE). The vertical beliefs for own aircraft are
used to populate belief_vert_int, replacing the intruder vertical beliefs missing in the input.
An intruder with this type of surveillance degradation will be excluded from determination of
the global RA.
• A non-zero value for the DEGRADED_SURVEILLANCE_ADSB_ONLY bit indicates ADS-B
only surveillance, with no active validation. This type of surveillance has been deemed inade-
quate for producing resolution advisories (RAs); it is adequate for producing traffic advisories
(TAs) only. The intruder is flagged by INTRUDERPREP for degraded surveillance processing:
(st_int.processing is set to RA_PROCESSING_DEGRADED_SURVEILLANCE). An intruder
with this type of surveillance degradation will be excluded from determination of the global
• A non-zero value for the DEGRADED_SURVEILLANCE_NO_BEARING bit indicates surveil-
lance with no valid bearing estimate. This type of surveillance is adequate for producing both
resolution advisories (RAs) and traffic advisories (TAs). There is no special processing in
INTRUDERPREP or the rest of the TRM for this type of surveillance degradation. Because no
special processing is required, this case is not called out in the conditional.
• A non-zero value for the DEGRADED_SURVEILLANCE_REDUCED bit indicates surveil-
lance with an update rate of less than 1 Hz. This type of surveillance is inadequate for produc-
ing vertical resolution advisories (RAs) and traffic advisories (TAs). The intruder is invalid for
vertical TRM processing. An intruder with this type of degraded surveillance must be omitted
from the inputs to INTRUDERPREP. For that reason, this value for degraded_surveillance is
not called out in the conditional.
• A value of DEGRADED_SURVEILLANCE_ALL indicates every degraded surveillance bit is
set. This setting flags the input for dropped intruder processing in the TRM when the intruder
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ACAS Xa/Xo MOPS, Vol. II
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