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the TRM to perform threat evaluation. This is done through a call to GENERATESTMREPORT.
It is expected that such requests will be made on a 1 Hz cycle, however, the algorithms in this
document do not preclude requests from being made at other rates. The data structure, known as the
STMREPORT, also contains fields that are specifically intended for other external modules such as
the pilot display and the aircraft’s transponder. The process of report generation iterates through the
stored TARGETs and determines which, if any, Track File for a given TARGET is most appropriate
to append to the STMR EPORT. If necessary, the target’s relative position will be extrapolated to the
time of the request.
2.2 Active Surveillance
This section defines the algorithms to process surveillance data that have been derived from a
transponder based interrogation and reply sequence. Several major functions performed by these
1. The association of a measurement to an existing Track File.
2. The initialization of a new Track File.
3. The update of an existing Track File.
4. The correlation of a Track File to another existing Track File.
5. The update step for active validation.
This section covers reply data that has been elicited from a Mode S or Mode C Transponder. The
algorithms in this section do not include the functions that select, prepare and form the intruder
information that is provided to the TRM and other external modules. That process is defined in Sec-
tion 2.6, specifically ADDMODESTRACKTOREPORT (Algorithm 98) and ADDMODECTRACK-
TOR EPORT (Algorithm 109).
2.2.1 Input Processing
126.96.36.199 Mode S Surveillance
The entry point to the STM for Mode S surveillance is given by RECEIVEDF0 (Algorithm 1). This
algorithm takes as input the variables described in Table 8, which are the required measured values
and reported data fields associated with the reception of a Downlink Format 0 (DF0) reply message
that has been elicited from an intruder’s Mode S transponder.
The R ECEIVEDF0 algorithm first ensures that an estimate of ownship barometric altitude and own-
ship heading are available. Next, the provided slant range (r_slant), relative bearing (Chi_rel), and
barometric altitude (z_baro) are checked to determine whether all are NaN. This special input pat-
tern indicates a request to update the surveillance mode (surv_mode) of a Target already in memory
without requiring a specific DF0 reply. Under this condition, if a Target is found using ASSOCI-
ATEICAOTOTARGET and a MODESTRACKFILE exists, the surveillance mode is updated and the
If any of the provided slant range (r_slant), relative bearing (Chi_rel), and barometric altitude (z_-
baro) are not NaN a check is performed to handle replies from a target under Hybrid Surveillance.
When a reply was elicited via a Hybrid Surveillance validation interrogation, and a target is found in
memory through a call to ASSOCIATEICAOTOTARGET, a flag (adsb_qual_override) is set to over-
ride further ADS-B quality checks by the STM for that target. The ADS-B quality check override
delegates responsibility for ensuring adequate ADS-B quality of targets under Hybrid Surveillance
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ACAS Xa/Xo MOPS, Vol. II
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