Home' RTCA Documents for Review : ACAS X MOPS DRAFT Vol. 2 Contents DRAFT
Algorithm 115 TargetIsEmpty
1 function TargetIsEmpty( tgt::Target(p. E -13) )
return (!TrackExists(tgt.modes_track)) && (!TrackExists(tgt.adsb_track)) && (!TrackExists(tgt.
modec_tracks)) && (!TrackExists(p. 15)(tgt.adsr_track)) && (tgt.designation_state.is_designated ==
Referenced In: MergeTargets(p. C-12), RemoveStaleTracks(p. 108)
2.6.6 Active Validation
The STM will provide the TRM (via the STMREPORT) with the estimated relative position and
velocity of a target aircraft that has been derived from ADS-B surveillance data, but only if the
ADS-B Track passes a number of checks. One such check is Active Validation, which compares
the ADS-B track with the Mode S Track on the same target. If the two tracks do not match within a
specified threshold, the ADS-B Track is considered invalid and the Mode S Track is provided to the
The active validation state of a TARGET is stored in its av_state field. The available state values are
defined in CONSTANTS (Appendix D) as follows:
• NOT_VALIDATED: The target’s active validation state has not yet been determined. This
state can transition to all other states.
• PROVISIONAL: The target has passed an initial M of N test and is likely to become fully
validated. A target in this state transitions to VALID once the full M of N tests have passed, or
back to NOT_VALIDATED if any test fails. No transition is available directly to INVALID.
• VALID: The target has been successfully validated. This state can transition to INVALID if
later validations fail the M of N test, or to NOT_VALIDATED if a track file coasts out.
• INVALID: The target has failed more than (N-M) of the previous N tests. Once marked as
INVALID, a target cannot transition back to VALID.
Active Validation is a two step process. The first step (ACTIVEVALIDATIONUPDATE (Algorithm
116)) updates and stores the data structures that contain the track elements that are relevant for the
Active Validation comparison. The second step (ACTIVEVALIDATIONC HECK (Algorithm 117))
compares the stored data structures and determines the validity of the ADS-B Track. This two step
process is necessary as Active Validation utilizes track data at the time of a Mode S Track update,
while the comparison itself is not necessary until the STMREPORT is generated.
Within ACTIVEVALIDATIONUPDATE, active validation will not be performed if there is no MODE-
STRACKFILE for the given target, no ADSBTRACKFILE for the given target, no established range
estimate for the Mode S Track, or no established velocity estimate for the ADS-B Track. Next, if
the number of past elements exceeds the total used by the comparison (N), the oldest stored values
are removed from the active validation history (v) data structure.
The active portion of the validation history data structure is then updated with the M ODESTRACK-
FILE. Before the validation history data structure is updated, the MODESTRACKFILE is extrapo-
lated to the requested time, t. If the vertical track of the MODESTRACKFILE is invalid, a value of
zero is substituted for the vertical dimension. Next, the passive portion of the validation history data
structure is updated with the Target’s ADSBTRACKFILE, which is extrapolated to the requested
© 2018 RTCA, Inc.
ACAS Xa/Xo MOPS, Vol. II
Links Archive ACAS X MOPS DRAFT Vol.1 Navigation Previous Page Next Page