ATC Radar Question

Jackie wrote:
I understand that ATC radar uses an encoding altimeter and a transponder
operating with Mode C to determine an aircraft's altitude. Let's forget
about Mode C for a moment and switch to Mode A only or just a primary
return.

If an aircraft is at 18,000 ft (approx 3 nm) and 3 nm away from the
radar antenna, as seen on a map, how does the radar correct for slant
distance when distance is displayed (e.g. using concentric circle
distance markers on the scope or relative to a known distance, such as a
marker on the display)?

In other words how does the radar know that the aircraft is actually 3
nm away laterally and not 4.25 nm (approx slant distance at that altitude)?


I am late into the thread.
But you are presumably talking about an area surveillance radar.
Its fan beam does not typically stick 45 degrees up into the sky. Too
wasteful of energy.
Another concept to ponder: if its beam WERE able to steer up at 45
degrees or more, what do you think its path would look like on a plan
position indicator? (a regular display). You've mentioned its slant
range is 4.25 miles at 3 mile distance horizontally. 30 seconds later,
it might be overhead: where would it paint in terms of range?
Three miles?? A circular range ring at 3 miles, all round the display?

:-)

Brian W

Brian Whatcott wrote:
Jackie wrote:
I understand that ATC radar uses an encoding altimeter and a
transponder operating with Mode C to determine an aircraft's
altitude. Let's forget about Mode C for a moment and switch to Mode A
only or just a primary return.

If an aircraft is at 18,000 ft (approx 3 nm) and 3 nm away from the
radar antenna, as seen on a map, how does the radar correct for slant
distance when distance is displayed (e.g. using concentric circle
distance markers on the scope or relative to a known distance, such as
a marker on the display)?

In other words how does the radar know that the aircraft is actually 3
nm away laterally and not 4.25 nm (approx slant distance at that
altitude)?


I am late into the thread.
But you are presumably talking about an area surveillance radar.
Its fan beam does not typically stick 45 degrees up into the sky. Too
wasteful of energy.

That's interesting. So if what you say is correct, an airport
surveillance radar has very little coverage of the area, say at the top
of a class B airspace because to cover that high an angle is "wasteful."
For example, a VFR plane flying legally just above the B ceiling could
very well be out of coverage of the radar that is supposed to be also
monitoring another high performance aircraft poking through that ceiling
at a high rate of speed. I'm not sure I agree with such an energy
saving measure.

Another concept to ponder: if its beam WERE able to steer up at 45
degrees or more, what do you think its path would look like on a plan
position indicator? (a regular display). You've mentioned its slant
range is 4.25 miles at 3 mile distance horizontally. 30 seconds later,
it might be overhead: where would it paint in terms of range?
Three miles?? A circular range ring at 3 miles, all round the display?

My distances where small to keep the math simple for discussion
purposes, not to suggest what an actual display would include. Geez.

In article ,
Jackie wrote:

That's interesting. So if what you say is correct, an airport
surveillance radar has very little coverage of the area, say at the top
of a class B airspace because to cover that high an angle is "wasteful."
For example, a VFR plane flying legally just above the B ceiling could
very well be out of coverage of the radar that is supposed to be also
monitoring another high performance aircraft poking through that ceiling
at a high rate of speed. I'm not sure I agree with such an energy
saving measure.

Over the top of a class B airport is where the planes taking off
from that airport usually aren't. They go out the sides, not
straight up. There is usually more then one radar feed for a
TRACON, so although the radar at the primary airport for the class
B can't see right above it, other radars in the area provide that
coverage.

For example, SFO has a radar station, as does OAK and NUQ (Moffett
Field). They overlap each other so there is fairly complete
coverage.

John
--
John Clear - http://www.clear-prop.org/

Jackie wrote:
Brian Whatcott wrote:
Jackie wrote:
...
If an aircraft is at 18,000 ft (approx 3 nm) and 3 nm away from the
radar antenna, as seen on a map, how does the radar correct for slant
distance when distance is displayed (e.g. using concentric circle
distance markers on the scope or relative to a known distance, such
as a marker on the display)?....

I am late into the thread.
But you are presumably talking about an area surveillance radar.
Its fan beam does not typically stick 45 degrees up into the sky. Too
wasteful of energy.

That's interesting. So if what you say is correct, an airport
surveillance radar has very little coverage of the area, say at the top
of a class B airspace because to cover that high an angle is "wasteful."
For example, a VFR plane flying legally just above the B ceiling could
very well be out of coverage of the radar that is supposed to be also
monitoring another high performance aircraft poking through that ceiling
at a high rate of speed. I'm not sure I agree with such an energy
saving measure.

Another concept to ponder: if its beam WERE able to steer up at 45
degrees or more, what do you think its path would look like on a plan
position indicator? (a regular display). You've mentioned its slant
range is 4.25 miles at 3 mile distance horizontally. 30 seconds later,
it might be overhead: where would it paint in terms of range?
Three miles?? A circular range ring at 3 miles, all round the display?

My distances where small to keep the math simple for discussion
purposes, not to suggest what an actual display would include. Geez.

Your understanding of the concept of airfield control radars and
surveillance radars is incomplete.
At a particular airfield the area of most interest is the approaches,
and potential conflicts. High flying traffic is of no interest.
The range limit of airfield radars may be in the 30 mile radius range.
If you work out the antenna angle needed to view an aircraft at 10,000ft
at 30 miles, you will see that the desired angle is not large. (4 deg)
The beam's vertical fan may be as low as 5 to 10 degrees.
But this is tiltable.
Area surveillance radars do not operate typically as singletons.
They look much further. They work with secondary transponders.

Brian W

"Jackie" wrote in message
...
Brian Whatcott wrote:
Jackie wrote:
I understand that ATC radar uses an encoding altimeter and a transponder
operating with Mode C to determine an aircraft's altitude. Let's forget
about Mode C for a moment and switch to Mode A only or just a primary
return.

If an aircraft is at 18,000 ft (approx 3 nm) and 3 nm away from the
radar antenna, as seen on a map, how does the radar correct for slant
distance when distance is displayed (e.g. using concentric circle
distance markers on the scope or relative to a known distance, such as a
marker on the display)?

In other words how does the radar know that the aircraft is actually 3
nm away laterally and not 4.25 nm (approx slant distance at that
altitude)?


I am late into the thread.
But you are presumably talking about an area surveillance radar.
Its fan beam does not typically stick 45 degrees up into the sky. Too
wasteful of energy.

That's interesting. So if what you say is correct, an airport
surveillance radar has very little coverage of the area, say at the top of
a class B airspace because to cover that high an angle is "wasteful." For
example, a VFR plane flying legally just above the B ceiling could very
well be out of coverage of the radar that is supposed to be also
monitoring another high performance aircraft poking through that ceiling
at a high rate of speed. I'm not sure I agree with such an energy saving
measure.

The antenna design of surveilance radars allows for a pretty wide coverage
area in the vertical plane due to the beam pattern. If you look at the
antenna, you'll see that it's a modified parabolic for this very reason.
http://upload.wikimedia.org/wikipedi...ar_Antenna.jpg
Notice how the antenna is more focused horizontally than vertically.
Imagine the spray pattern of a pressure washer with a fan tip and you get
the idea.

There is a "cone of silence" above all surveilance radars, but it is not
that big. Many airports locate their radar off the airport a mile or so for
this reason, and because there is also a receiver recovery time of
approximately 1/4 mile.