Publication: Magyar Közlöny
Issue: MK-2009-104 (Year: 2009, Number: 104)
Era: 2004-2010
Section: 
Paragraph Index: 2515

4. TYPICAL ALGORITHMS AND PARAMETERS FOR THREAT DETECTION AND GENERATION OF ADVISORIES Note 1.— The characteristics given below describe a reference design for the ACAS II collision avoidance logic. This description, however, does not preclude the use of alternative designs of equal or better performance. . Note 2.— Lower case mathematical symbols are used to represent variables throughout this section. Upper case symbols are used for parameters. The dot notation used for some parameters does not indicate that they are derived quantities but rather that they have the dimensions suggested by the notation, e.g. distance/time for a speed parameter. 4.1 Tracking performance characteristics 4.1.1 RANGE TRACKING Range, range rate, and range acceleration (r, r , r ) are estimated by means of an adaptive α-ß-γ tracker using for its coefficients α, ß, and γ values that are decreasing with each successive range measurement until they reach their minimum values equal to 0.40, 0.10 and 0.01, respectively. The range acceleration estimate is used to estimate the expected miss distance in range at closest approach, m, using the following formula: / r m r rr r = − +   This estimate is not calculated when further calculations indicate that it may not be reliable either because of the magnitude of the estimation errors or because of a possible manoeuvre by one of the aircraft in the horizontal plane. The latter calculations rely on the age of the track, the observed accuracy of the successive range predictions, the observed consistency of the range acceleration estimates, the observed consistency of a second range track based on a linearized trajectory agreeing with the previously estimated miss distance, and the observed consistency of a rough bearing track. 4.1.2 ALTITUDE TRACKING 4.1.2.1 Sources of altitude data. Intruder aircraft’s altitude is obtained from intruder Mode C or Mode S reports. Own aircraft’s altitude is obtained from the source that provides the basis for own Mode C or Mode S reports and is required to be at the finest quantization available. 4.1.2.1.1 Altitude report credibility. Before any altitude report is accepted, a test is made to determine whether the report is credible. A credibility window is calculated on the basis of the previous estimated altitude and altitude rate. The altitude report is discarded and the altitude track updated as though the report was missing (4.1.2.3.7) if the report is outside the credibility window. Annex 10 — Aeronautical Communications Volume IV 22/11/07 ATT-38 4.1.2.2 Own altitude rate. Own ACAS aircraft’s altitude rate is obtained from a source having errors that are as small as possible and in any event no greater than those of the rate output of the tracker described in 4.1.2.3.6. 4.1.2.3 INTRUDER ALTITUDE TRACKING 4.1.2.3.1 Altitude tracking terms’ description Established rate track. An altitude track for which the pattern of the last few altitude reports received from the intruder allows the inference that that intruder is climbing or descending with a constant, non-zero altitude rate. Level track. An altitude track for which the pattern of the last few altitude reports received from the intruder allows the inference that that intruder is level. New track. An altitude track newly initialized. Oscillating track. An altitude track for which the pattern of the last few altitude reports received from the intruder oscillates between two or more values in a way that allows the inference that that intruder is level. Transition. An altitude report for a track that is different from the last credible altitude report for that track. Trend. A trend exists for the altitude rate if the two most recent altitude level transitions were in the same direction. Unconfirmed rate track. An altitude track for which the pattern of the last few altitude reports received from the intruder does not allow the track to be classified in any other way. 4.1.2.3.1.1 On any cycle of tracking, each track is attributed one and only one track classification. 4.1.2.3.1.2 Any track classification is maintained until conditions for another track classification are satisfied. 4.1.2.3.2 The ACAS II tracks the altitudes of intruders. Tracking is based on automatic pressure-altitude reports from their transponders, using altitude reports quantized as received. For every intruder on every cycle the tracker provides altitude and altitude rate estimates. Note.— The function that associates Mode C altitude data with tracks is specified in Chapter 4, 4.3.2.1. The altitude tracker specified below assumes that this function has been performed prior to application of the tracker. 4.1.2.3.2.1 The reference altitude tracking design assumes that, for each track, altitude reports are received at the nominal rate of one altitude report per second. However, it allows for missing reports, in other words, cases in which no altitude report has been received for a given track prior to a tracking cycle. 4.1.2.3.2.2 Intruder altitude tracks of one of two types are created and maintained. So-called 100-ft tracks are obtained when altitude reports are supplied in units of 100 ft. Such tracks are updated by a dedicated tracker referred to as the 100-ft altitude tracker. So-called 25-ft tracks are obtained when altitude reports are supplied in units of 25 ft. Such tracks are updated by a dedicated tracker referred to as the 25-ft altitude tracker. 4.1.2.3.2.3 Special logic automatically switches intruder altitude tracks between the 100-ft altitude tracker and the 25-ft altitude tracker following a confirmed change in the units in which altitude reports are supplied. Such a change is considered confirmed when three successive valid altitude reports expressed in the same units have been received. 4.1.2.3.2.4 When an altitude reporting unit change has been observed but not yet been confirmed, the existing track is coasted and the altitude report is temporarily stored. Once the unit change is confirmed, the track is re-initialized using the last altitude rate estimate computed before the change as well as all temporarily stored altitude reports. Attachment Annex 10 — Aeronautical Communications ATT-39 22/11/07 4.1.2.3.2.5 The 25-ft tracker is an adaptive alpha-beta tracker. It is briefly described in 4.1.2.3.5. 4.1.2.3.2.6 The design of the 100-ft altitude tracker is motivated by the need for a stable altitude rate estimate when the true altitude rate of the intruder is less than 100 ft/s, in other words, less than one quantization interval per tracking cycle. This tracker estimates the altitude rate indirectly by estimating the time taken to cross one quantization level. Further details on this design are provided in 4.1.2.3.6. 4.1.2.3.3 Altitude rate confidence. For every intruder on every cycle, the tracker provides an indication of either “high” or “low” confidence in the altitude rate estimate (4.1.2.3.6.9 and 4.1.2.3.6.10). 4.1.2.3.4 Altitude rate reasonableness. The tracker provides a “best estimate” altitude rate and upper and lower bounds for this altitude rate consistent with the received sequence of reports. 4.1.2.3.5 25-ft quantization reports 4.1.2.3.5.1 For altitude reports quantized to 25-ft increments, an adaptive α-ß tracker is used. This tracker is adaptive in the sense that it selects among three sets of α and ß values depending on the magnitude of the prediction error, i.e. the difference between the predicted altitude and the reported altitude, as well as on the magnitude of the rate estimate. These α and ß values are: • α = 0.4 and ß = 0.100 when the current altitude rate estimate is less than 7.0 ft/s; otherwise, • α = 0.5 and ß = 0.167 when the prediction error is less than 22.5 ft; and otherwise, • α = 0.6 and ß = 0.257. 4.1.2.3.5.2 The tracker maintains two distinctive sets of altitude and altitude rate estimates. The first one is derived directly from the standard α-ß smoothing equations. This set is purely internal to the tracker. The second set contains the estimates passed to the collision avoidance logic. It differs from the first set as follows. The altitude estimate passed to the logic is constrained to be within one-half quantization interval of the reported altitude (±12.5 ft). The altitude rate estimate passed to the logic is set equal to zero when the internal estimate decreases below 2.5 ft/s in absolute value and is kept equal to zero until the internal estimate increases beyond 5.0 ft/s in absolute value. 4.1.2.3.5.3 The tracker uses only two of the previously defined track classifications: level track and established rate track (4.1.2.3.1). It declares a track to be a level track when at least seven tracking cycles have elapsed since the last altitude transition (4.1.2.3.1). The internal rate estimate is then reset to zero. It declares the track to be an established rate track when, following two sufficiently closely spaced altitude transitions, the internal rate estimate (and thus also the rate estimate passed to the logic) increases beyond 5.0 ft/s. 4.1.2.3.5.4 Confidence in the estimates is declared “high” when the track has existed for at least four tracking cycles and the prediction error has been no greater than 22.5 ft on at least two successive tracking cycles. It is set to “low” when the prediction error is larger than 22.5 ft. It is also set to “low” when altitude reports have been missing on two successive cycles. 4.1.2.3.6 100-ft quantization reports. For altitude reports quantized to 100-ft increments, the performance of the altitude tracker is, in all respects, equal to or better than that of a reference tracker setting the altitude rate estimate to have an appropriate sign and the magnitude as described in this paragraph. 4.1.2.3.6.1 Tracker variables. The reference tracker uses the following variables: ż altitude rate estimate, m/s (ft/s); Żgu see 4.1.2.3.6.5.1; Annex 10 — Aeronautical Communications Volume IV 22/11/07 ATT-40 Δz altitude difference between the current report and the most recent credible report; Tn 1 s; Q 30.5 m (100 ft); tr time since the most recent credible report, s; tp time between the two most recent altitude level transitions or, for multiple transitions within one cycle, the average time between these transitions, s; tb estimated level occupancy time after the most recent transition, s; tbm calculated lower bound on level occupancy time, s; ß computed smoothing coefficient for tb; ∫β limit for ß based on tb; bt number of altitude levels crossed between the two most recent altitude level transitions; zb number of altitude levels crossed at the most recent rate; ε smoothed error estimate of tb, s; dt sign of the most recent altitude transition (= +1 for an increase in altitude; = –1 for a decrease); and x* value of any variable x before being updated following an altitude level transition. 4.1.2.3.6.2 Report credibility. The altitude report is regarded as being credible if either of the following conditions is satisfied:

Source: https://magyarkozlony.hu/hivatalos-lapok/1f7c6b0e16b4b71a92e5ad24416008bbe2e26aab/dokumentumok/710811d1f7f958a2990684d0cbf918e84f5497e5/letoltes