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Aviation => Pilot/Controller Forum => Topic started by: Twocky61 on December 28, 2014, 03:16:26 AM

Title: Flying Below Radar
Post by: Twocky61 on December 28, 2014, 03:16:26 AM
I have often wondered why it is planes can drop below radar

Radar is used both by aviation & shipping

Is it not possible for aviation & shipping radar to overlap?

That way a plane would not be able to drop below as then it would be on shipping radar.
Title: Re: Flying Below Radar
Post by: RonR on December 28, 2014, 10:13:29 AM
I'm not an expert when it comes to radar, but basically, a radar station requires a line-of-sight view to the aircraft in order to be able to "see" the aircraft.  The lower the altitude of an aircraft and the further away it is from the radar station, the more likely it is that an object on the ground could get in between.  These objects could be anything like a hill, a mountain, tall buildings, etc.  Radar is also limited in its range.  A land-based radar station can see aircraft out several hundred miles over the ocean but when that aircraft gets beyond a certain point (300-350 miles?? not really sure), I believe that the curvature of the Earth comes into play.  The aircraft drops below the horizon as viewed from the land-based radar station so the ocean itself becomes the "object" that gets between the aircraft and the radar station.

As for ship-based radar, I can't really comment on that except to say that the principle of how it works is the same as far as I know, but whether it would be compatible or not, I couldn't say.  I also think that shipping radar systems are far less powerful than land-based aircraft radar systems.

If anyone wants to add to this simplified explanation, please feel free!!   :-)

Title: Re: Flying Below Radar
Post by: martyj19 on December 28, 2014, 12:26:32 PM
If the question is, why is there no radar coverage over the ocean, the issue is "out of range" rather than "below", and the secondary answer is "we don't really need it".

Bear in mind there are two radar systems, "primary" that relies only on reflection of the radar signal from the target back to the radar installation, and "secondary" that interrogates the onboard transponder to identify the target, and the target actively responds back to the radar installation.

In addition to the line of sight factor, near the ground the primary radar will be overwhelmed by "ground clutter", reflections from objects on the ground that aren't aircraft.  As to distance, not only is the curvature of the earth a factor, but the radar return is no longer strong enough to be picked up by the ground station and separated from the background noise.

Shipping would not be helpful because there is not enough of it to provide anywhere near reliable service, let alone that they are not interconnected and you would have to rely on some transmission medium such as satellite to get the radar returns to the controllers, and there isn't enough bandwidth even if you had the interconnect.

We use the old fashioned way, position reports and generous separation, to handle oceanic traffic.  It works for the amount of traffic that needs to be handled.  Also, the aircraft have TCAS onboard so they can keep track of one another.
Title: Re: Flying Below Radar
Post by: Twocky61 on December 29, 2014, 03:44:43 AM
Thanks for such a detailed explanation Ron & Marty

The ground clutter re transponders also answers the 9-11 question as to why the terrorists were able to disengage the transponders. I always thought transponders were permanantly engaged as are black boxes.

Thanks again  :-D

Title: Re: Flying Below Radar
Post by: martyj19 on December 29, 2014, 08:06:09 AM
Transponders have on-off switches.  They can be easily turned off.  It is standard procedure to leave the transponder off during ground operations, unless the airport has ground surveillance radar.

Also, ground clutter doesn't affect the secondary radar, because the ground doesn't reply to the inquiries, only the aircraft do that.

Almost anything can be turned off if you go to the trouble of finding its circuit breaker, which sometimes is remote from the flight deck.

Here is an article that goes into more detail on the available facilities.  One section that is relevant here is where they talk about using ACARS to do oceanic position reports.