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3.1.1 Inputs and Outputs to the TRM
One of the primary ways in which ACAS X improves on previous collision avoidance efforts is
in explicitly considering uncertainty about the conditions of the local airspace. Instead of single-
sample measurements of various physical quantities, the system works with collections of weighted
samples (called beliefs), where every sample of inputs is assigned a weight representing the proba-
bility that the actual values are close to the measurements in the sample.
Among the inputs to the TRM, there are three such beliefs: INTRUDERHORIZONTALBELIEF (p.E-
19), INTRUDERVERTICALBELIEF (p.E-19), and OWNVERTICALBELIEF (p.E-19). INTRUDER-
HORIZONTALBELIEF contains relative horizontal distance and rate beliefs from own aircraft to an
intruder aircraft. INTRUDERVERTICALBELIEF and OWNVERTICALBELIEF contain altitude and
vertical rate beliefs; they are used for intruder aircraft and own aircraft, respectively.
The entire input to the TRM, which includes the own and intruder beliefs, is defined by a nested
type TRMINPUT (p.E-16). TRMINPUT contains own aircraft data, TRMOWNINPUT (p.E-17), and
an array of intruder aircraft data of type TRMINTRUDERINPUT (p.E-18), one per known intruder.
Most TRM input is produced by the STM as output from GENERATESTMREPORT (Algorithm
93). However, in order to utilize the most recent coordination inputs to the STM, the TRM calls
back to the STM using UPDATEINTRUDERVRC (Algorithm 85). This call updates the vrc field in
TRMINTRUDERINPUT for all intruders being processed by the TRM and returns the coordination
received_vrcs, for output as the Resolution Advisory Complements (RAC).
The output from the TRM is defined by another nested type TRMREPORT (p.E-20). TRMREPORT
contains information for an onboard pilot display, TRMDISPLAYDATA (p.E-21); an array of per-
intruder data for coordination with other ACAS X-equipped aircraft, TRMCOORDINATIONINTER-
ROGATIONDATA (p.E-22); and information about conformant own vertical rates and acceleration
and internal threat processing TRMDEBUGDATA (p.E-26). It also contains information for air-to-
air broadcast, TRMRABROADC ASTDATA (p.E-24) and information for transmission to a ground
station, TRMGROUNDMSGDATA (p.E-24).
In addition, the TRM inputs and outputs state information, T R MSTATE, details of which are
provided in Section 3.1.2. VERTICALTRMUPDATE uses a helper function, VERTICALTRMUP-
DATEPREP (Algorithm 347), to check the validity of the inputs, create the intruder state structures
for new intruders, and create arrays for processing intruder data. VERTICALTRMUPDATEPREP is
not described in this document but is included in Appendix I.
3.1.2 System Architecture
An effort was made to make the data flow in the diagrams explicit. However, the TRM has some
state, or memory, which is updated each time the main algorithm loop is run.
This memory is captured by TRMSTATE, which is passed as an argument into the main VERTICAL-
TRMUPDATE method. TRMSTATE holds state information on previous advisories for own aircraft
(TRMOWNSTATE), state information on previous advisories and online cost settings for each in-
truder (TRMINTRUDERSTATE), and configuration parameters (PARAMSFILE_TYPE). Within TR-
MINTRUDERSTATE, state information for each online cost is stored in a data structure of type
ONLINEC OSTS TATE. These online cost settings are described more fully in Appendix E.6.
The dynamics, and consequently the online cost settings and best individual action with respect to
own aircraft, are different for each intruder aircraft. Thus, updates to the state variables for one
intruder should not affect those of another intruder. To that end, the system maintains a vector of
ACAS Xa/Xo MOPS, Vol. II
© 2018 RTCA, Inc.
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