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Item 2 (co-ordinate transform) has the same influence for mono and multi-radar. Indeed, mono-radar positions are usually sent in (rho, theta) by the radar's but are transformed within the display in the common plane so as to be used with maps. Therefore, if the co-ordinate transform is adequate for mono-radar, it is for multi-radar. The exception is for primary, as in the mono-radar case, primary data are often displayed without slant range correction. So there is a need to take special care in the multi-radar case for tracks with no mode C (primary only tracks, P+S or secondary tracks corresponding to aircraft only equipped with mode A transponders).
The final accuracy of the slant range correction has to be studied for every implementation, as it depends on factors which are site dependant, such as :
- the method being used for mode C estimation (default height, triangulation, deduction from speed, etc. ...)
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the distance between radar's and aircraft

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the distance between radar's and centre of projection for the co-ordinate transform.

Last but not least, item 1 is to be measured very cautiously, directly out of the tracker. At this level, one can easily think of means for comparing mono and multi radar accuracy. So this is the easiest thing to do. This comparison is needed to check that the tracker output is not precluding the use of the separation minima. But this is not sufficient.
1)
In France, the plots out of the radar’s are called “tracked plots” which means that these are raw plots (raw positions and extracted mode A and mode C codes) with a track number given by the local tracking whose role is the filtering of the false plots.
2)
I suppose here that the extrapolation at the display time is linear. If it is not the case, considerations about longitudinal and transversal accelerations should be added.
As the performance of the tracker in position can in the end be jeopardised by either an imperfect extrapolation to the display time or an inadequate co-ordinate transform, we think that all of the above mentioned items must be evaluated before assessing the quality of a multi-radar system, in relation with an objective of separation minima.
4. FRENCH METHODOLOGY
4.1 Evaluation of the tracker accuracy
This sort of evaluation is always done at CENA with the use of recorded life data. The sample must be big enough so as to be representative from a statistical point of view. Several hours of opportunity traffic are therefore recorded at busy hours. The tools needed for reconstructing the true aircraft trajectories are :
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a chainer : so as to link in time the successive positions which corresponds to the same target. This can possibly be the Object Correlator from RASS.

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a smoother : so as to filter out the noise of the measured radar positions and get reference positions to which the tracker updates can be compared. This can possibly be the MURATREC software from RASS, but France has no experience yet in using it.

So as to assess the quality of a multi-radar tracker in comparison with mono-radar, we compare in France, for each radar being used in the multi-radar system, the radar accuracy and the tracker updates accuracy for this radar. This implies that some additional processing is done either to take back the tracker updates for this particular radar in the local co-ordinate system or to convert the radar measurements in the common plane used by the tracker.
Special attention is then given to the tail of the distribution of the positional errors both for mono-radar and for multi-radar. A systematic analysis of all the cases where the error is greater than x NM5 1)is carried out for the multi-radar system so as to correct them if possible. Then the percentage of residual errors for multi-radar is compared to that of the mono-radar. It is expected to be smaller. If it is not, then the multi-radar tracker has to be improved. If it is smaller, then the multi-radar tracker is considered as possibly allowing the target separation minima and the work goes on with the evaluation of the other items.
　
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