Home' RTCA Documents for Review : ACAS X MOPS DRAFT Vol.1 Contents 92
ACAS X MOPS
State and Status Message. If the TYPE field has the value of 31, the ME field
contains an Aircraft Operational Status Message.
Note: ACAS X passive surveillance does not make use of Airborne Position Messages
with TYPE field values in the range 20 to 22.
If the TYPE field has the value 1, 2, 3 or 4, the ME field contains an Aircraft
Identification and Type message. This may optionally be decoded as specified in
§22.214.171.124.8 (Extended Squitter With Aircraft Identification Message).
Note: The remaining ME field message types, 19, 23 to 28 and 30 are Airborne
Aircraft Status Messages
(Emergency/Priority Status), Test Messages, or reserved message types. None of
these message types are used by ACAS X passive surveillance.
126.96.36.199.188.8.131.52 Altitude from Airborne Position Message
The Altitude subfield of an Airborne Position Message is located in bit positions 9 to 20
of the ME field (bit positions 41 to 52 of the extended squitter). If the ME field of a valid
extended squitter transmission has a TYPE subfield with the value 0 or with a value in
the range 9 to 18, then the Altitude subfield of the ME field shall (1088) be decoded. If
all bits in the Altitude subfield have the value 0, then the message contains no valid
altitude data, and the Altitude subfield shall (1089) be ignored. Otherwise, the Altitude
subfield contains altitude data encoded as specified in Ref. G, §184.108.40.206.3.4.3 (“Altitude
Encoding” in ADS-B Airborne Position Messages), and shall (1090) be decoded into an
appropriate value for use in initial altitude determination and in passive surveillance.
220.127.116.11.18.104.22.168 CPR Format from Airborne Position Message
The CPR Format subfield of an Airborne Position Message is located in bit position 22 of
the ME field (bit position 54 of the extended squitter). This field indicates whether the
Encoded Latitude and Encoded Longitude subfields of that Airborne Position Message
use the even or odd encoding method specified in Ref. G. If the ME field of a valid
extended squitter transmission has a TYPE subfield with a value in the ranges 9 to 18,
then the CPR Format subfield shall (1091) be used in conjunction with the Encoded
Latitude and Encoded Longitude fields to decode them into latitude and longitude in
accordance with Ref. G, Appendix A, §A.1.7.5 (Computations for Airborne, TIS-B and
Note: After position initialization and the reasonableness tests in §22.214.171.124.3.2.8, a
locally unambiguous CPR decode is used to update the position.
The computed position for any Airborne Position Message received subsequent to
successful reasonableness test validation per §126.96.36.199.3.2.8 shall (1092) be checked for
reasonableness per Ref. G §188.8.131.52.3.
Note: An erroneous local unambiguous CPR decode could result in an incorrect
position that potentially remains incorrect for the life of the track..
184.108.40.206.220.127.116.11 Encoded Latitude from Airborne Position Message
The Encoded Latitude subfield of an Airborne Position Message is located in bit
positions 23 to 39 of the ME field (bit positions 55 to 71 of the extended squitter). If the
ME field of a valid extended squitter transmission has a TYPE subfield with a value in
the ranges 9 to 18, then the Encoded Latitude is valid and shall (1093) be decoded into a
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