GUIDELINES FOR THE APPLICATION OF THE ECAC RADAR SEPARATION(23)

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The French approach is pragmatic. It only takes into consideration the 5NM which we are aiming to use. In short, one can say that the French method is a comparative method based on the fact that a 5NM radar separation minima 1) has already been used in France for several years in the case of mono-radar display. So if sensible comparative means can be found between multi and mono-radar situations, and if multi-radar proves to be as good as or better than mono-radar, then a 5NM radar separation minima can also be applied in the multi-radar case.
In other words, we do not try to measure a target level of safety for the system. We assume that the target level of safety for the mono-radar situation is acceptable, without being able of measuring it, and we try to compare multi-radar with mono-radar. So the question is : What are the comparative means between mono and multi that we use ?
3. COMPARISON BETWEEN MONO AND MULTI-RADAR SYSTEM
One must remind that the separation minima is applied by the controllers at the level of the radar display. Figure 1 (resp. 2) gives a very simple overview on a mono- (resp. multi-) radar chain.
1)
In France, in the mono-radar case (both for primary radars and monopulse SSR), the radar separation minima is 3 NM for a range less than 40 NM and 5 NM for greater ranges.

- Figure 1 - - Figure 2 -
Therefore, the following elements have to be taken into account for a multi-radar system, because they all influence the accuracy in the horizontal plane :
1  the intrinsic accuracy of the multi-radar tracker (including the loop for on line correction of systematic errors).
2  the errors which may exist in the slant range correction in case of poor knowledge of the altitude (because usually, multi-radar positions are displayed in a common plane, using co-ordinate transform with slant range correction). This mainly concerns MRDPS 1) dedicated to approach control.
3  the loss of precision which results from the extrapolation at the display time.
4  the accuracy proper to the display screen (which depends on the precision of the coding of the distances inside the display, on the pitch of the screen, on the size of the symbols, etc ..., and which vary with the scale).
Some of these elements are common between the multi-radar and the mono-radar situations. Actually, the fourth one (4) impacts the same way for both situations if the same display is used for multi and mono-radar display (this is true in France since analogue display has been abandoned). So the same precautions have to be taken. The very first concerns the precision of the coding : the less significant bit (LSB) of the distances in the display must be equal or smaller than the one used for the broadcast of the tracks (either mono-radar tracks or multi-radar tracks).
1)
MRDOS : Multi-Radar Data Processing System
Item 3 does not directly concern mono-radar display as usually mono-radar data are displayed in (rho, theta) just as they are sent by the radar 1) (except that they are corrected from the radar systematic errors). However, mono and multi-radar display can be compared. Indeed, both display methods have advantages and drawbacks :
-the direct display used for mono-radar shows the position at the time of detection. So it may not show the positions of close aircraft at the same time. And anyway, the positions are a little obsolete when shown on the display. There is a temporal inaccuracy.
- on the opposite, the extrapolation at the display time used for multi-radar shows the positions which are estimated at the time when the display refreshes the screen. There is a geographical inaccuracy due to the estimation process.
Both inaccuracies are about the same provided a certain quality of the speed vector 2) elaborated by the tracker in case of multi-radar, and provided that the extrapolations are made as short as possible.
This demonstrates that there is a need for evaluating the quality of the whole state vector, not only the position, in relation with separation minima.
　
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