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2.3.1 Input Processing
There are three entry points to the STM for passive surveillance. Each entry point is associated with
the reception of a specific ADS-B Report type as generated by an ADS-B Report Generator. As
such, each of the entry points require a different set of input parameters as indicated in Table 10,
Table 11 and Table 12.
18.104.22.168 State Vector Position Report
RECEIVESTATEVECTORPOSITIONREPORT (Algorithm 38) requires the inputs defined in Table
10. Upon the reception of an ADS-B State Vector Position Report, the STM first checks if the
ownship altitude track has been established (i.e., the track is not NaN), the ownship WGS84 state
has been established (i.e, not a NaN), that this state information has been updated recently (as
determined by own_wgs84_timeout), and that the reported latitude and longitude are valid (i.e, not a
NaN). Next, the predicted location of the ownship is estimated with PROPAGATEOWNSHIPTOTOA
(Algorithm 50) to account for ownship motion in the time between the target observation and the
most recent ownship WGS84 state input. The position information contained within the report (lat,
lon and alt) is then transformed to local East, North, Up (ENU) coordinates, as it is this relative
Cartesian coordinate frame that is used for state estimation. The transformation to ENU coordinates
is performed with CONVERTOBSERVEDTARGETTOENU (Algorithm 41). Next, the reported Mode
S address (mode_s), and whether the message came from ADS-R (rebroadcast) are used to associate
the report to an existing Target (ASSOCIATEADSBREPORTTOTARGET (Algorithm 39)).
If the State Vector Position Report is not associated with an existing Target, a new ADSBTRACK-
FILE is created and initialized through a call to INITIALIZEADSBTRACKFILE (Algorithm 40).
Next, the new ADSBTRACKFILE is stored in memory through ADDADSBTRACKTODB (Algo-
rithm 48) or ADDADSRTRAC KTODB (Algorithm 49) depending on the reception of an ADS-R
message. These algorithms attempt to correlate the new ADSBTRACKFILE to an existing Target
(ASSOCIATEICAOTOTARGET (Algorithm 4)). If correlation is successful, the ADSBTRACKFILE
is added to the existing Target. Otherwise, the ADSBTRACKFILE is added to a new Target. The
horizontal track is defined relative to the predicted location of the ownship, and this ownship loca-
tion is stored as additional data elements in the ADSBTRACKFILE.
If the call to ASSOCIATEADSBREPORTTOTARGET successfully associates to a Target, the corre-
sponding ADSBTRACKFILE is retrieved from memory. If a minimum allowed time (min_obs_-
toa_step) has not passed since the last track update, the ADSBTRACKF ILE is not updated with the
State Vector Position Report. If the minimum time threshold has passed, temporary copies of the
previous horizontal track are created. The relative horizontal track is then redefined in a rotated
coordinate frame based on the predicted latitude and longitude of the ownship with REDEFINEES-
TIMATEINROTATEDFRAME (Algorithm 51). Next, the two tracks in the ADSBTRACKFILE are
progressed through calls to ADVANCEADSBTRACKPOSITION and ADVANCEVERTICALTRACK.
If the horizontal track was updated inside ADVANCEADSBTRACKPOSITION, the ownship location
is stored as additional data elements in the ADSBTRACKFILE; otherwise, the ownship location is
not stored and the horizontal track is reset to the previous horizontal track values.
If either a new Track File was created or an existing Track File was updated, the reported Mode S
Address (mode_s), whether the reported address is a non-standard ICAO address (non_icao), the
integrity of the reported data (NIC), the quantization of the reported altitude (q_int), and the time of
applicability (toa) are stored in the ADSBTRACKFILE.
© 2018 RTCA, Inc.
ACAS Xa/Xo MOPS, Vol. II
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