Divergence: Why can't Centers use it?

WickedPenguin Junior Member Pensacola

I've been asking around and have been getting conflicting signals as to why En Route Centers can't use divergence.

I'm thinking it has to do with A) the slower radar rotation rate, B) larger airspace involved - which makes aircraft appear closer together due to scope resolution, and C) the increased distance of aircraft from their radar scopes.

Can anyone clarify this?

Roddy_Piper Resident Knucklehead Vegas baby

it has to do with the mosaic radar they use. the radar return u see is an estimate of where the different radars in use sees the aircraft. it's not a raw radar return like the ASR's. the 5 mile separation also comes from this phenomenon. the radar guess of where the plane is or should be has a variation of approximately 2 miles, so they add those 2 miles to the required 3 miles that tracons get to use.

any single sensor center sectors are allowed to use 3 miles because the radar return is similar to a tracons. ZOA sector 42 is one such sector.

Matty13 Senior Member

Every center (I don't know about CERAPs) uses a mosaic radar display. In theory, the two aircraft you're separating could each be tracked by two different radar sites. This is less accurate than a single-site display. Also contributes to the need for five miles instead of three.

Whoops, Rowdy Roddy Piper beat me to it.

DairyCreamer Senior Member Elsewhere

This is just an educated guess and is in no way official.

First, I would guess it mostly has to do with the mosaic radar. Since we must use 5 miles as we can never guarantee which radar site is picking up a target, there is an inherently larger error radius for where the aircraft actually is in relation to the digitized target. This leads to the passing rules for us in 5-5-7.b where we have to get an aircraft to report passing.

As well, since we are in mosaic, targets all tend to move simultaneously at a 12 second interval, regardless of how long ago they were swept by the radar site reporting their position. For instance, we pipe in terminal radar at a few places in ZAB to supplement ARSRs. Obviously ASR updates much faster than ARSR... but when in normal NAS operations, the planes still make their normal jump every 12 seconds.

Also, frankly, for a lot of our traffic, diverging/passing wouldn't do us any good. Normally, by the time we see the targets passing and issue the instruction per the above paragraph, it usually takes a couple more hits before the Mode C shows a change in altitude and they're outside of 5 miles anyway. Maybe low/slow guys, sure, but even then, they are prone to the same error in mosaic, and the lower they are the more prone to loss of radar coverage too with as widely spaced as most of our sites are.

Single site radar lends itself to the precision and update rate that allows a controller to be more assured of an aircraft's particular position than our mosaic presentation. The faster update rate and the ability to see both primary and beacon targets at the same time (we only see the beacon slash when someone is transponder equipped) may well be what allows terminal radar controllers to have assurity in aircraft passing/diverging without a pilot's report. Our potential position error is too large to allow for that kind of certainty, and safety dictates protecting to that margin of error and then some.

~Nate