WP-1.1 –Safety encounter model based on French radar data
ACASA WP-1.1/116
26-09-2000
Version 2.1
WP-1.1
Safety encounter model
based on the
French radar data
1 Introduction
1.1.1
This paper describes the set of encounters (Set A as defined in [1]) extracted from five months
of French radar data recordings, and to be used within ACASA/WP1 to build the European
encounter model [3].
1.1.2
The encounters extracted from French radar data have been selected using the capture criteria
defined in [2] for the safety encounter model. They more or less correspond to the set of
encounters issuing a Resolution Advisory with TCAS II version 6.04a or version 7.0.
1.1.3
The most significant parameters of the safety encounter model based on the set of French
encounters are then described. The various distributions presented in the document have been
derived by the back-end software [4].
1.1.4
Finally, the NMAC rate implied by the safety encounter model [5] is estimated based on the
number of encounters with small HMD (lower than 0,1 NM) and small VMD (lower than 100
feet). As expected, this NMAC rate is greater than the target NMAC rate for flight operations.
2 French radar data recordings
2.1
2.1.1
Radar data coverage
The French radar data are mono-radar data recordings from different mono-pulse SSR :

Auch (43 :34 :37.18 North ; 01 :08 :09.49 East)

Chaumont (48 :26 :56.93 North ; 05 :23 :55.86 East)

Bordeaux (44 :41 :53.24 North ; 00 :22 :13.19 West))

Mont Ventoux (44 :10 :27.25 North ; 05 :16 :08.12 East)

Palaiseau (48 :43 :00.25 North; 02 :14 :12.41 East)
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26-09-2000
Version 2.1
2.1.2
The two last radars, Mont Ventoux in the South of France and Palaiseau near Orly, were the
default SSRs used during the radar data recording. However, when not available, other SSRs
were used (Chaumont instead of Palaiseau, Auch or Bordeaux instead of Mont Ventoux).
2.1.3
The figure below shows that the overall radar coverage overlaps almost all the French Area
Control Centers, and also part of the traffic in Belgium, Luxembourg, UK (with Palaiseau),
Germany (with Chaumont), Switzerland and Italy (with Chaumont and Mont Ventoux).
Figure 1 : French radar data coverage
Note : In the figure, the black lines represent the boundaries of European states and the blue and red
circles indicate the different radar coverage. The scale is in nautical miles centred on 47:00:00 North
and 00:00:00 West using stereographic projection.
2.1.4
When required (i.e. encounters within overlap of radar coverage in space and time), only one
source of radar data has been used to extract the encounters.
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2.2
2.2.1
Number of flight hours
The overall radar data recordings represent a total of 692 243 flight hours. When compared to
the radar availability during the recording period (total of 5585 recording hours over all SSR),
the ratio of flight hours has increased over the last 3 months of higher traffic density.
Auch
Mont Ventoux
Flight
hours
26-09-2000
Version 2.1
Bordeaux
Palaiseau
Chaumont
Recording hours
250000
2500
200000
2000
150000
1500
100000
1000
50000
500
0
Recording
hours
0
January
February
March
April
May
Figure 2 : Number of flight hours in French radar data
2.2.2
2.3
2.3.1
Almost all the flight hours (95%) are related to IFR flights with only 3% of VFR flight hours
and 2% of military flight hours.
Number of flights
Taking into account all sources of radar data, the number of flights (including IFR, VFR and
military flights) is proportional to the number of flight hours recorded during period (about
half an hour for each flight per radar).
Auch
Mont Ventoux
Bordeaux
Palaiseau
Chaumont
Flight hours
500000
250000
400000
Number
of flights 300000
200000
200000
100000
100000
50000
Flight
150000 hours
0
0
January
February
March
April
May
Figure 3 : Number of flights in French radar data
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26-09-2000
Version 2.1
3 Set of encounters extracted from French radar data
3.1
3.1.1
Number of encounters
In order to build the European encounter model, the overall radar data recordings have been
used to capture a set of encounters in accordance with [2]. The following figure presents the
number of encounters (total of 1243) extracted from the French radar data over the five
months of radar data recordings:
Auch
Mont Ventoux
Bordeaux
Palaiseau
400
303
Number of
encounters
Chaumont
Flight hours
345
400000
315
300
300000 Flight
hours
200000
166
200
114
100
100000
0
0
January
February
March
April
May
Figure 4 : Number of encounters per month extracted from French radar data
3.1.2
The number of selected encounters per flight hour (all IFR, VFR and military flights) is more
or less constant during the period of radar data recordings. However, it is much more
important with the radar data from SSR Palaiseau near Orly.
Auch
Mont Ventoux
Bordeaux
Palaiseau
Chaumont
All SSR
4
3
Nb / 1000
Flight hours 2
1
0
January
February
March
April
May
Figure 5 : Number of selected encounters per flight hours
3.1.3
The high proportion of encounters extracted from Palaiseau is highlighted in the following
figure, which presents the geographical distribution of the selected encounters over France.
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Figure 6 : Geographical distribution of selected encounters
3.1.4
These encounters include neither military/military encounters, nor VFR/VFR encounters and
VFR/military encounters. Furthermore, IFR training/VFR encounters at Toussus-Le-Noble
(TSU), a GA airfield near CDG-Orly, have also been discarded. Indeed, most of these
encounters resulted in small HMD (less than 1 NM) and VMD (less than 500 feet) in Visual
Meteorological Conditions, and would have artificially enriched the safety encounter model.
Figure 7 : Discarded encounters below 3000 feet at Toussus (TSU near CDG-ORLY)
Note : In the figure, the red trajectories are associated with VFR flights, while the green ones stand for
IFR flights.
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3.2
3.2.1
26-09-2000
Version 2.1
Altitude distribution
Furthermore, most of the selected encounters occurred below FL115. This characteristic is
highlighted by the distribution of the highest altitude at CPA over the selected encounters.
This distribution presents some particular flight levels with high proportion of encounters,
especially at FL20 and at FL110.
All SSR
210
FL110
180 FL20
150
Number of
120
encounters
90
FL280
FL200
60
30
40
0
FL
37
0
FL
34
0
FL
31
0
FL
28
0
FL
25
0
FL
22
0
FL
19
0
FL
16
0
FL
13
0
FL
10
0
70
FL
FL
40
FL
FL
10
0
Figure 8 : Distribution of highest altitude at CPA for the selected encounters
3.2.2
The peak of encounters (12%) at FL20 occurred around all the airports within the radar
coverage, and can be explained by the high proportion of IFR/VFR encounters in these areas.
Figure 9 : Geographical distribution of the selected encounters below FL50
3.2.3
The number of IFR/VFR and IFR/Military encounters has been estimated using the Mode A
codes of the aircraft involved in the selected encounters. The proportion of IFR/VFR goes up
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to 56% below 5,000 feet, and represents 16% of the encounters between FL50 to FL115.
Below 5,000 feet, there is also a high proportion (21%) of encounters between IFR and
military flights, the majority of which probably involves VFR flights under military control.
400
334
300
258
Number of
encounters 200
100
210
IFR/IFR
155
84
79
IFR/VFR
IFR/Military
64
6
10 2
0
19
13 0 3
0
Below
FL50
FL50FL115
FL115FL195
FL195FL295
Above
FL295
Figure 10 : Types of flight involved in the selected encounters
3.2.4
The peak of encounters (14%) at FL110 can be explained by the high proportion of encounters
in Paris TMA where the arrival/departure procedures for Roissy and Orly create a high
proportion of 1,000 feet level-off encounters at some crossing points inside the TMA.
Figure 11 : Selected encounters between FL100 and FL110 within Paris TMA
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3.3
3.3.1
26-09-2000
Version 2.1
Other features
The following figure presents the altitude, vertical speed and ground speed of the selected
aircraft trajectories over the time in the day (in seconds). The majority (94%) of the
encounters is distributed from 6h to 20h in TU time, with a peak (18%) between 14h and 16h.
FL
Ft/mn
Kts
Figure 12 : From top to bottom, altitude (FL), vertical speed (Ft/mn) and ground speed
(Kts) of selected encounters
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4 Safety encounter model based on French encounters
4.1.1
This section presents the most significant parameters of the safety encounter model based on
the set of encounters extracted from French radar data. The various distributions discussed
hereafter have been derived by the back-end software [4], based on 1237 encounters (against
1243 initial ones) taken into account by the back-end.
4.2
Altitude layers
4.2.1
The following figure presents for each source of radar data, the distribution of encounters per
altitude layers, as defined in the European encounter model [3]:
Layer
1
2
4
5
From
1,000 ft
5,000 ft
FL115
FL195
FL295
To
5,000 ft
FL115
FL195
FL295
FL415
Number of encounters
0,4
3
373
Probabilities
404
400
277
0,3
300
167
0,2
200
0,1
100
16
0
0
Layer 1
Layer 2
Layer 3
Layer 4
Layer 5
Figure 13 : Number of encounters per layer extracted from French radar data
4.2.2
4.3
The derived probabilities of encounter per layer range from 1% in the last layer up to 30% in
the first one and 33% in layer 2. For the last layer (above FL295), the set of encounters is not
considered large enough to obtain relevant probability distributions for the encounter model.
The probabilities associated with this layer are nevertheless provided in the document for
completeness.
Horizontal and vertical manoeuvres
Turn probabilities per layer
4.3.1
As illustrated in the figure below, most of the selected encounters involve aircraft without
horizontal manoeuvres (85% of aircraft without turn) whatever the layer. Nevertheless, the
probabilities of turn range from 13% in the first layer to 28% in layer 2 (and 31% in layer 5).
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26-09-2000
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1000
750
Number of
aircraft 500
Turn
No turn
250
0
Layer 1
Layer 2
Layer 3
Layer 4
Layer 5
Figure 14 : Distribution of horizontal manoeuvres for the set of encounters
4.3.2
Furthermore, in most of the turns (up to 75% in the first layer down to 63% in layer 4), the
bank angle is lower than 25 degrees, except in the last layer where 63% of aircraft with a turn
have a bank angle between 35 and 50 degrees. This high proportion of great bank angle above
FL295 is probably due to military aircraft with fighter performances.
Vertical manœuvres per layer
4.3.3
As shown in the figure below, the vertical profiles of the aircraft involved in the encounters,
consist in a relative majority of level aircraft whatever the layer is. The proportion of level
aircraft ranges from 33% in the layer 3 to 52% in the first layer and 53% in the last layer. This
is not surprising for the upper layers where the aircraft are flying at their cruise flight levels.
For the first layer that includes a majority of IFR/VFR encounters, it can be explained by VFR
flights flying (more or less) level.
Layer 1
Layer 2
Layer 3
Layer 4
Layer 5
0,6
0,5
Proba.
0,4
0,3
0,2
0,1
0
Descent
End of
descent
Undershoot
Start of
descent
Level
Start of
Climb
Overshoot
End of
climb
Climb
Figure 15 : Distribution of vertical manoeuvres per layer
4.3.4
The proportion of climbing or descending aircraft are also relatively high whatever the layer
(up to 25% in layer 3 and around 15% in the other layers of aircraft in descent, and from 11%
in layer 1 up to 23% in layer 2 of climbing aircraft). Finally, the proportions of End of climb
or descent are not insignificant, particularly for the layer 4 (from FL195 to FL295) where it
represents 22% of the vertical manoeuvres.
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4.3.5
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The encounter types A and B, as defined the European encounter model [3], allow to
distinguish between:

Encounters (type A) involving only Level, End of climb (i.e. Level-off on climb) and End
of descent (i.e. Level-off on descent) for which the proportion of large VMD is expected
to be greater, and

Other encounters (type B) for which the VMD should not be correlated to the vertical
separation minima.
Layer 1
Layer 2
Layer 3
Layer 4
Layer 5
0,8
0,6
Probabilities
0,4
0,2
0
Type B, noncrossing
Type B,
crossing
Type A, noncrossing
Type A,
crossing
Figure 16 : Distribution of encounter types per layer
4.3.6
Whatever the layer, the majority of encounters are type B encounters (from 59% in layer 4,
61% in the first layer up to 78% in the third layer). Besides, almost all the encounters involve
non-crossing situations in the vertical plane (only 1% of vertical crossing in layer 3 up to 9%
in layer 5 and 17% in the first layer).
4.3.7
More precisely, the most frequent encounters in each layer are the following:
4.3.8

Below 5,000 feet, Level-Level (27%) and Level-Descent (14%) encounters;

In the [FL50; FL115[ layer, Level-Climb (21%) and Level-End of climb (12%)
encounters;

In the [FL115; FL195[ layer , Level-Descent (26%) Descent-Climb (12%) encounters;

In the [F195; FL295[ layer, Level-Climb (22%), level-Descent (18%) and Level-End of
climb (16%) encounters;

Above FL295, Level-Descent (25%) and Level-Level (18%) encounters.
Except in the first layer, the most frequent encounters are type B encounters involving only
one Level aircraft. In the first layer, although the majority of encounters is composed of type
B ones, the most frequent vertical combination correspond to the Level-Level situation (type
A). Besides, for most of the type A encounters in the first layer, the VMD is distributed below
800 feet, with a peak (33%) around 500 feet.
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4.4
26-09-2000
Version 2.1
Horizontal and Vertical Miss Distance at CPA
VMD per layer
4.4.1
The distribution of VMD depends on the layer, with a high proportion of large VMD (around
1,100 feet) between FL50 and FL295 (layers 2, 3 and 4). For the first layer (below 5,000 feet),
the VMD is mainly distributed between 0 and 800 feet, with peak (25%) around 500 feet. For
the last layer (above FL295), the set of encounters is not large enough to obtain a realistic
VMD distribution.
Layer 1
Layer 2
Layer 3
Layer 4
Layer 5
0,5
0,4
0,3
Proba.
0,2
0,1
0-
10
0
20
0
30
0
40
0
50
0
60
0
70
0
80
0
90
0
10
00
11
00
12
00
13
00
14
00
15
00
16
00
17
00
18
00
19
00
20
00
0
Figure 17 : VMD distribution per layer for the selected encounters
4.4.2
With respect to the probability of small VMD (lower than 100 feet), it should be noted that it
decreases when the layer increases with: 23 encounters (6,2%) in layer 1, 13 encounters
(3,2%) in layer 2, 2 encounters (1,2%) in layer 3 and 2 encounters (0,7%) in layer 4. None
encounter with such small VMD has been observed above FL295.
4.4.3
Furthermore, most of these encounters with small VMD correspond to type B encounters,
except for the first layer where 50% of the encounters with VMD lower than 100 feet involve
only Level, End of climb or End of descent (type A encounters).
HMD per layer
4.4.4
Within the safety encounter model build from the set of selected encounters, HMD will be
assumed to be uniformly distributed between 0 and 500 feet. (~ 0.1 NM). As shown in the
following figure, the initial HMD distribution observed from the set of encounters is quite
uniform within the range of unfiltered HMD for the layer, particularly for layers 2, 3 and 4.
4.4.5
For the first layer (below 5,000 feet), the probability of small HMD is slightly reduced.
Finally, for the last layer, the HMD distribution is not necessarily relevant, as the layer is not
well populated by the set of encounters.
Note: The encounter capture criteria used when processing the radar data, includes an HMD filtering
as follows, which explains that encounters with HMD greater than these thresholds are not taken into
account.
Altitude of the lower a/c
below FL50
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above FL289
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HMD
26-09-2000
Version 2.1
< 1.0 NM
Layer 1
< 1.5 NM
Layer 2
< 2.0 NM
Layer 3
Layer 4
Layer 5
0,2
0,15
Proba.
0,1
0,05
0
0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9
1
1,1 1,2 1,3 1,4 1,5 1,6 1,7 1,8 1,9
2
NM
Figure 18 : HMD distribution per layer for the selected encounters
HMD and VMD correlation
4.4.6
Initial analysis of the HMD and VMD joint distribution seems to indicate that both parameters
are not correlated. Nevertheless, further study is required to check that HMD and VMD
parameters are independent.
5000
Layer 1 (1000 - 5000 ft)
Layer 2 (FL50 - FL115)
4500
Layer 3 (FL115 - FL195)
Layer 4 (FL195 - FL295)
4000
Layer 5 (FL295 - FL415)
Linear regression (Layer 1)
3500
Linear regression (Layer 2)
Linear regression (Layer 3)
Linear regression (Layer 4)
3000
VMD
(Feet)
Linear regression (Layer 5)
2500
2000
layer 5
1500
1000
R2 = 0,0294
layer 3
R2 = 0,0045
layer 4
R2 = 0,0284
layer 2
R2 = 0,0123
layer 1 R2 = 0,0156
500
0
0
2000
4000
6000
8000
10000
12000
HMD (feet)
Figure 19 : Distribution of HMD and VMD per layer for the selected encounters
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4.5
4.5.1
26-09-2000
Version 2.1
Implied NMAC rate
Assuming that HMD and VMD are independent, the number of NMAC implied by the model
[5] depends on the proportion of encounter with small HMD (< 0.1 NM), the proportion of
encounters with VMD lower than 100 feet and the rate of encounters itself.
Rate of NMACs (per flight hour) =
(Rate of encounters) * (Rate of HMD < 0.1NM) * (Rate of VMD < 100 feet)
4.5.2
Taking into account the number of flight hours included in the radar data recordings, the rate
of encounters is as follows:
Rate of encounters (per flight hour)= 1237 / 692 243 = 0,00178694
4.5.3
Taking into account the effect of the HMD filtering which depends on the layer and assuming
the HMD distribution is uniform, the rate of encounters with ‘small HMD’ extracted from
radar data is as follows:
Rate of encounters with HMD < 0.1NM
= ((373 / 10) + ((404 + 167 + 277) / 15) + (16 / 20) ) / 1237
= 0,07650229 (around 7,6%)
4.5.4
The proportion of encounters with VMD lower than 100 feet is not very high, except within
the first layer (below 5,000 feet). In particularly, there is no such encounter in the last layer
(above FL295). Taking into account the altitude and VMD distributions, the probability of
‘small VMD’ is as follows:
Rate of encounters with VMD < 100 feet
= ((28/10)+ (13 + 2 +2)/15+ (0/16)) /
((373 / 10) + ((404 + 167 + 277) / 15) + (16 / 20) ) = 0,04156393 (around 4,2 %)
4.5.5
As a consequence, the NMAC rate implied by the model, without taking into account the
altimetry error, is as follows:
Rate of NMACs (per flight hour) = 5,682 10-6
4.5.6
In order to obtain the target NMAC rate of 3 10-7, the VMD distribution should be modified
(by reducing the proportion of VMD < 100 feet), or the HMD distribution should not be
assumed uniform, particularly for the first layer (with lower proportion of HMD < 0,1 NM).
5 Conclusion
5.1.1
The parameters of the safety encounter model based on French radar data have been presented
and correlated to the traffic characteristics in the radar coverage. The most important
characteristics is may be the great importance of the two first layers (below FL115), which
include two third of the selected encounters.
5.1.2
Another important feature is on one hand, the great proportion of encounters with VMD lower
than 800 feet, with a peak (about one quarter) around 500 feet, below FL50, and on the other
hand the great proportion (about one third) of large VMD (around 1,000 feet) above FL50.
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5.1.3
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Version 2.1
Finally, the set of encounters selected above FL295 is not large enough to obtain relevant
probability distributions for the last layer.
6 References
[1] ‘A new approach for ACAS Risk Ratio models – ACASA WP-10/100 Version 3.0, March
2000.
[2] ‘Radar data processing specifications’– ACASA WP-1.1.5 Version 4.0, November 1999.
[3] ‘Draft specification of the European Encounter Model – ACASA WP-1.1.75/030 Version
1.9, July 2000.
[4] ‘Specification of the back-end software to derive parameters for the European encounter
model’ – ACASA WP-1.1.6/086 Version 1.5, August 2000.
[5] ‘NMAC rate’ – ACASA WP-1.1.7/100 Version 1.0, June 2000.
7 Acronyms
ACAS
Airborne Collision Avoidance System
ACASA
Airborne Collision Avoidance Systems Analysis
CPA
Closest Point of Approach
FL
Flight Level
HMD
Horizontal Miss distance (at horizontal CPA)
NM
Nautical Miles
NMAC
Near Mid Air Collision
VMD
Vertical Miss distance (at horizontal CPA)
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