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5.2.1 General
From a processing point of view, the radar data processing chain is undoubtedly the most complex element of the ATM system. This is due to its many complex constituent elements and also to the important influence of the radar environment, be it mono- or multi- radar environment. Obviously the final accuracy of the displayed position is a function of the performance characteristics of all radar sub - systems.
Classically, the radar chain consists of primary (PR) and secondary surveillance radar (SSR) sensor(s). The PR sensor system performance is mainly a function of the aircraft reflection characteristics, the environment (surrounding terrain, objects, weather conditions, interference, etc.); the antenna characteristics, the characteristics of the transmitter and the receiver and the extractor processing stages. For the SSR performance, the same elements play a role. Due to the so-called “co-operative” nature of this system, the performance of the SSR is, however, not affected by the reflection characteristics of the aircraft, but rather by the characteristics of the on-board transponder and corresponding on-board antenna(s) (including the effect of
e.g. the impact of the presence of transponder antenna diversity to avoid antenna screening in case of manoeuvring aircraft, which can be very important in the approach phase of an aircraft). For many sensors, both a PR and an SSR are collocated at one site and a logic is present to produce combined target reports.
The target report produced by a radar site is generally referred to as a plot. The performance of a plot is generally measured in the form of a number of plot performance characteristics, categorised as follows (no full definition of all the following notions is given here) :
Coverage  The volume of airspace within which the radar system detects and displays aircraft position plots (irrespective of performance)
Reliability  1. Probability of Correct Plot Detection
2.
Probability of Correct Code Validation (for the various SSR modes)

3.
Probability of the presence of various categories of false PR and SSR plots (not detailed in this paper)

4.
Probability of False SSR Code Validation

Accuracy  The characteristics of the deviations of the measured radar positions compared to the real aircraft position.
1. Various categories of position errors are measured (3-dimensional), i.e.:
.  
Random Errors

.  
Correlated Errors, where the errors in range, azimuth and/or altitude on a correlation in position and/or time ( multi-path, partial resolution loss)

.  
Systematic Errors (various types of systematic range, azimuth, time-stamp and radar position errors).

2.
Possible use of filters

3.
Ageing inaccuracy (time past since last update)

4.
Errors in the display system

The above general reliability and accuracy performance characteristics could in the case of aircraft in close 2-dimensional proximity be affected considerably by the resolution capabilities of the radar system.
Although having a higher overall accuracy modern monopulse SSR-systems suffer particularly from 2-dimensional aircraft proximity.
Overall measured reliability and accuracy of radar plot information are, therefore, not always representative for situations where a radar separation minimum has to be applied. These characteristics should be considered as a function of the degree of proximity of the aircraft. Of particular importance here are cases of correlated position errors, which have been observed for certain radar stations, where, for aircraft in proximity, the “apparent separation” from the radar plot information is larger than the real separation. This behaviour can, in certain conditions, lead to very dangerous situations, and has in the past been the reason for excluding the use of 5 NM separation minimum for certain radar systems suffering from this problem. Separation minima of 8 NM and preferably 10 NM were then applied.
　
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