There are new airports being built in the world. Air Traffic Control (ATC) needs to have failure free operation of the radars to provide effective Air Traffic Services (ATS). However, there are instances, when the radars do not fail, but performance of the radar is hampered due to interference, which can lead to dropping of a track, and this can have high risk.

The sub-optimal performance of radar due to interference is an hazard. The interference can possibly be caused many reasons and one of them can be obstruction in the line of sight of the Primary Surveillance Radar. This hazard has to be mitigated by appropriate safety measures.

The interference can also be due to pressure on the radar bandwidth and the selling of frequencies to mobile phone companies operating in 2G/3G spectrum.

Let us consider the situation when the ATC has to operate in the vicinity of the obstruction which is causing the radar interference and withdrawal of ATS is not acceptable as mitigation for a safety event. The replacement of radar with any other radar is also not deemed necessary.

Keeping in view the constraints, any safety case work to enable ATC to operate in the vicinity of the obstruction will not be a Radar equipment safety case but a Safety Case for the provision of ATS.

Under such circumstances, a Safety Analyst with an engineering qualification becomes the coordinator amongst the ATC Service provider, Regulator, and Radar engineer, to find a cohesive solution.

The major operational risks in Air Traffic Services (ATS) can include the following:-

• Obscuring low level climb out traffic
• Loss of track identity wherein 3 or more consecutive update not visible on the display
• Traffic manoeuvring obscured by clutter
• Additional work in vectoring traffic around the clutter
• Rise in controller work load

One of the possible mitigation for radar interference can be the ue of Secondary Surveillance Radar (SSR). However, it is to be noted that use of transponders is not mandated for all classes of air space, than the reliance on SSR will not be attainable.

The other possibility of mitigation is dependent on the radar performance. Radar performance can be measured based on technical and operational parameters. The parameters relevant in context of hazard under consideration, will include the following:-

• Probability of detection (Pd)
• False Alarm rate
• Seduced tracks
• Dropped plots
• Dropped tracks
• Accuracy
• Resolution

Following is to be considered in respect of the above mentioned parameters:-

• Levels of radar performance deemed appropriate. For example Pd can vary from 80%-90%.
• Principle in measuring an acceptable Pd. Two in twenty concept can be proposed.
• Technically two in twenty dropped tracks meet 90% ratio for Pd.
• Two consecutive dropped tracks fits with the operating procedure where aircraft have to be re-identified after three consecutive dropped tracks.
• How many 2 in 20 drops would be acceptable over the obstacle in the radar's line of sight.