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for ALTITUDEDEPENDENTCOCCOST. As the traffic advisory determination uses a different factor
than the resolution advisory logic, the DETERMINEALTITUDEDEPENDENTCOCFACTOR is called
twice with different parameters for each.
This algorithm determines the risk of being in clear of conflict at the current state (factor_coc).
This factor varies with altitude so that as altitude increases the risk of having no alert increases
proportionally. In this way, the factor_coc can be thought of as the amount of additional logic
sensitivity to be applied at a given altitude.
At various points in this algorithm the CALCULATETHRESHOLDRAMPUPFACTOR and CALCU-
LATETHRESHOLDRAMPDOWNFACTOR algorithms are used. These methods take an input value,
a threshold parameter, and a roll-off parameter and return a value between 0 and 1. The different
factors combine to determine the additional sensitivity added to the logic.
The algorithm loops over each vertical sample in turn to determine the necessity of the additional
sensitivity. Due to this iterative approach, the intermediate calculations are weighted and summed
during the loop. Two factors (factor_reduction and factor_proximate) are calculated using sub-
functions and components of the vertical sample. The reduction factor reduces the amount of sensi-
tivity increase in circumstances where the own aircraft vertical rate is low. The proximate factor is
the main discriminator of when to increase the sensitivity based on the vertical sample and a forward
projection in time.
Nested within the vertical sample loop is another loop which iterates over the horizontal tau samples.
Since many tau samples have zero weight, a check is performed to determine if further calculations
are required. Furthermore, the tau value itself must be within the maximum tau applicability defined
by the parameters and less than the largest possible tau value, to prevent improper increases in
sensitivity. For each valid tau sample a vertical projection of the value of tau forward in time is
performed. This projected separation (z_rel_actual) represents the expected vertical separation at
the time of loss of horizontal separation. It is used to calculate a factor (factor_actual) which reduces
the applicability of the additional sensitivity in cases where there will be a large vertical separation
when horizontal separation is lost. This condition occurs most frequently in geometries where the
aircraft are expected to cross altitude very early in an encounter. To allow these well-separated
crossings to occur, the additional sensitivity should not be applied. The intermediate calculation of
factor_coc combines the weight, proximate factor, actual factor, and a final factor computed using
the tau sample value. As the value of tau increases, this factor reduces the additional sensitivity to
Once the algorithm finishes looping over the vertical samples, the factor_coc is scaled by the altitude
of the own aircraft. This causes the additional sensitivity to be applied in full above the high altitude
threshold (H_thresold_hi), not at all below the low threshold (H_thresold_lo), and proportionally
between. Finally, the reduction factor is applied here as well to appropriately reduce the additional
sensitivity in the desired cases. The sensitivity due to this factor can be reduced up to a maximum
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ACAS Xa/Xo MOPS, Vol. II
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