Home' RTCA Documents for Review : ACAS X MOPS DRAFT Vol.1 Contents 34
ACAS X MOPS
Note: In multi-threat encounters, the ACAS Xa logic generates individual RAs for each
threat, which are not announced to the flight crew.
Hash Code – A fixed-length number or message obtained by computing a hash function
over an arbitrarily-sized piece of data. Intended to uniquely identify that piece of data.
Horizontal Miss Distance (HMD) - The horizontal range between two aircraft at the point
of closest approach.
Horizontal Position Integrity Bounds – Describes an integrity containment region about
the reported position, within which the true position of the surveillance position referen ce
point is assured to lie at the reported time of applicability.
Horizontal Position Uncertainty – 95% accuracy bound on horizontal position equivalent
to Estimated Position Uncertainty (EPU), which is defined as the radius of a circle,
centered on the reported position, such that the probability of the actual position being
outside the circle is 0.05. When reported by a GPS or GNSS system, EPU is commonly
called Horizontal Figure of Merit (HFOM).
Hybrid Surveillance – The combined use of active and validated ADS-B passive
surveillance data to update a track.
ICAO 24-bit Aircraft Address – A unique 24-bit address assigned to each Mode S
equipped aircraft. It is included in Mode S replies and broadcasts in order to uniquely
identify the aircraft. It allows Mode S interrogators (SSRs or Active CAS) to interrogate
one aircraft at a time by specifying that aircraft's ICAO aircraft address in the
interrogation. Historically this was called the Mode S discrete address or Mode S address
or Mode S ID.
ICAO Aircraft Identification – The three letter ICAO designator for the aircraft operating
agency followed by the flight number (e.g., BAW123); or the Registration Marking of the
aircraft (e.g., FGZCF).
Increased Rate Resolution Advisory - A resolution advisory that advises the pilot to
increase the altitude rate to a value exceeding that of a previous positive RA.
Intent - The intent to pass above or below a particular threat. For ACAS-equipped
threats, this intent is conveyed in coordination messages that amount to an instruction not
to maneuver in a sense that conflicts with ownship’s intent.
Intruder - A transponder-equipped aircraft within the surveillance range of ACAS for
which ACAS has an established track.
Master - The master is the aircraft that has priority in coordination tie-breaking and is
allowed to initiate geometric reversals.
Minimum Trigger Level (MTL) - The minimum input power level that results in a 90%
ratio of decoded to received replies.
Mode A - A type of ATCRBS transmission which requests (via Mode A interrogations)
or reports (via Mode A replies) aircraft identity information.
Mode C - A type of ATCRBS transmission which requests (via Mode C interrogations)
or reports (via Mode C replies) aircraft altitude information. ATCRBS transponders
which do not have Mode C capability respond to Mode C interrogations with standard
ATCRBS framing pulses but no altitude information.
Mode C Altitude – altitude encoded in accordance with ‘SSR automatic pressure-altitude
transmission code’ as defined in ICAO Annex 10.
Mode S - A type of SSR transmission which contains a unique ICAO 24-bit aircraft
address, thus allowing interrogations to be addressed to individual aircraft. Mode S
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