According to the AIM, in class B and C airspace separation of VFR traffic is "suspended during CENRAP operations":

Separation of VFR aircraft will be suspended during CENRAP operations. Traffic advisories and sequencing to the primary airport will be provided on a workload permitting basis. The pilot will be advised when center radar presentation (CENRAP) is in use.

What is CENRAP, how are pilots informed that it's in use (i.e. what is the radio phraseology?) and what action is expected from pilots in that case?


CENRAP is when an approach control uses the long-range center radars for the separation of their traffic, instead of their normal radars(maintenance, damage, etc). When in this mode, separation standards increase due to the longer rotation/slower update speeds(which allow the longer range visibility) of the Center Radar.

If you ever hear about CENRAP, it'll usually be on the ATIS, or the radar facility probably won't provide services to VFR aircraft, just due to the degradation of radar quality.


CENRAP means Center ENroute RADar Program. This program was created by ZKC center and then adapted throughout the FAA. The ARTCC transmits radar coverage data on the interfacility INTO/INTO interface. Center surveillance comes from multiplte radars. The Center's coversge area is divided into 16nm x 16nm boxes called Radar Sort Box (RSB). Many of these RSBs create what is called mosaic radar. Each RSB has multiple radar sites adapted, from best to lesst.
FAA 7110.6B Order says 3nm seperation may be applied for single site radar provided the target is within 40 nm of the antenna. Beyond 40nm, then 5nm separation must be used. Because of this order, multiple radars (mosaic) cannot be used. Also center radsr is evaluating the target's merit and will dynamically switch to next best radar. Its possible for the center's software to switch to 4 different radars based on coverage merit.
CENRAP coverage area is adapted by defining which RSBs to use. Typically a 4 x 4 RSB is 4x16 or 84 miles by 84 miles. Also FAA long range radar used 12 second sweeps and targets are reduced to a 1/8 nm accuracy by the common digitalizer. Range accuracy is 1/8th nm and termiinal radar is analog providing a smooth coverage.


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