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2.4  In computing the HOP, the study reviewed three sets of assumptions about the way in which errors may combine to cause an overlap situation when they appear to be separated. Eventually the study considered only the more cautious assumption. In this instance, aircraft are assumed to be separated only in azimuth, as viewed by the radar. Since radar inaccuracies are greatest in azimuth, the assumption is very conservative. It depends only on the distribution of azimuth errors of a radar. It is independent of any assumptions about controller judgement, and independent of the observed headings of aircraft.
2.5  Currently in the UK, radar data is recorded simultaneously from the radar to be assessed and from other, overlapping radars. A suite of computer programmes is used to reconstruct the tracks of aircraft and from these, the error at each radar plot is estimated. The distribution of the errors is then plotted, and the HOP is computed on the basis of the distribution. If the HOP is less than the ‘critical value,’ (e.g. 2.5X10-10 per flight hour), the radar system is considered suitable for the separation minimum to be applied. It should be noted that comparison of simultaneous radar data recordings is not the only acceptable method to determine errors. A comparison between radar data and satellite navigation or other airborne equipment records could be used, for example.
2.5.1  
Since ‘gross’ radar errors are rare, a relatively large number of plots need to be recorded in order to record enough errors to have sufficient confidence that the radar performance can be assessed. The UK study consisted of 162,000 recorded plots, of which 48 plots were tail errors.  It is stated in the study report that approximately 25 plots in the ‘tail’ of the distribution are sufficient for an assessment. This is expected to require a sample in the order of 50,000 plots for an SSR.

3.  
CONCLUSIONS OF THE STUDY

3.1  The study recommends criteria to determine whether a radar may support given separation minima, based on measurements of performance. For a separation minimum of 5NM applied to a range of 160NM with en-route traffic, the criteria are:
a) the number of errors with absolute value greater than 0.2o must be less than 1% and;
b) the tail of the distribution (beyond 0.4o must have exponential form, or be faster decaying; and
c) the number of errors with absolute value greater than 0.4o must be less than 0.03%; and
d) the mean value of errors with absolute value greater than 0.4o must be less than 0.55o.
3.2  The table below sets out the criteria for a radar to support four possible separation minima. They have been optimised for SSRs operating in combined mode (SSR and PSR).
Criterion  2NM  3NM  5NM  10 NM 
Less than 1% of errors may be greater than:  0.08o  0.12o  0.20o  0.40o 
The tail is defined as starting at: Less than 0.03% of errors may be in the tail, and they must have a negative exponential or faster-decaying form.  0.16o  0.24o  0.40o  0.80o 
The mean of errors in the tail (see previous criterion) must be less than:  0.22o  0.33o  0.55o  1.10o 

ANNEX 2 Attachment B
EXAMPLE METHODOLOGY FOR MULTI-RADAR TRACKING SYSTEMS
French methodology for assessing that a multi-radar system can support a given radar separation minima.
1. FOREWORD
This paper deals with the technical aspects of the topic, in relation with the accuracy of the multi-radar system in the horizontal plane, needed for supporting a given separation minima. Operational aspects related to the practical conditions for the application of a given separation minima, such as system availability, contingency considerations, etc ... and all the technical aspects not related to radar are not taken into account in this paper.
2. FRENCH APPROACH
　
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