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4.0 Assumptions and Constraints
 
Operational Use of ADS-B In Non Radar Airspace Generic Design Safety Case 

 
 
   Use in areas not ADS-B based surveillance system will only be used in airspace not covered by radar currently covered by radar control. Radar will still be the primary
surveillance system and ADS-B data will not be used when radar data is available. Therefore ADS-B will enhance safety by providing surveillance where there is none today.  The proposed separation minimum for use in airspace where ADS-B surveillance is available will be the same as that used in en-route airspace where SSR radar surveillance exists. The separation minimum for en-route SSR radar is 5 NM. This minimum has been used with previous generations of conventional SSR radars (RSR) operated at rotation rates of 5 rpm with ranges of approximately 160 NM. The same minimum continues to be used today with newer technology monopulse radars.  Safety analysis in support of this safety case will be by conducting a comparative assessment on the performance of conventional SSR radars (and/or monopulse radars) against a typical ADS B system.  The 5 NM separation minimum will only be used when the data from ADS B has an accuracy performance of Navigational Uncertainty Categories (NUC) equal of better than 5.
  

Operational Use of ADS-B In Non Radar Airspace Generic Design Safety Case
 

 
 
5.0 Systematic Risk Assessment  Comparative The risk assessment will be based on: analysis against  SSR a) conducting a comparative analysis of the ADS B system performance with current Secondary Surveillance Radar (SSR)
b) conduct of hazard identification process 
 
5.1 Comparative analysis with SSR  Initial comparative The calculated error for the conventional SSR radar (RSR) used by analysis with one State, at a distance of 160 NM from the radar head was in the conventional SSR order of  + or – 0.77NM. [1] at 3 sigma. used by one State 
The azimuth resolution performance of the RSR at a distance of 160 NM range was 12.6NM.[1]  The accuracy performance provided by the ADS-B surveillance system for an aircraft with Navigational Uncertainty Categories (NUC) level of 5 is <0.25 (95%) or <0.46 NM [2] at 3 sigma for the horizontal error.  The above clearly shows that the accuracy performance of the ADS-B based surveillance system far exceeds the performance of old technology radars used in the en-route environment. Table 1 provides a comparison of the performance between an ADS-B based and conventional SSR radar based surveillance system.    In addition, the ADS-B based surveillance system, broadcasts at a rate of once every second and does not suffer from the ability to resolve more than one aircraft at the same location.   Many States have replaced older conventional SSR radars (RSR) with modern Monopulse SSR radars. These new Monopulse SSR radars provide improved accuracy and operate at higher rotation rates, typically in the order of 16 rpm. However, the separation minimum of 5NM used in en-route radar airspace remains unchanged.  A common operational availability requirement for Monopulse SSR radar is equal to or better than 0.9995.  This figure does not take into account power, air conditioning and other auxiliary systems. The
　
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