ATC Radar Question

"Bob Noel" wrote

the elevation radar determines altitude the same way the azimuth radar
determines bearing.

There are actually two radars running for PAR, right? One sweeping left to
right and one sweeping up and down, I think.
--
Jim in NC

Stealth Pilot wrote:

Mode C only provides 100 foot altitude resolution. bzzzzt!

Mode C provides a height eg 1252ft which is deemed only accurate to
100ft because the calibration errors are limited to 100ft.

I believe that Mode C provides 100 foot altitude resolution because
the least significant bit is 100'

"Jackie" wrote in message
...
Matt Barrow wrote:
"Jackie" wrote in message
...
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)?

Asquared + B squared = C squared, I'd guess.


And how does it separate A and B?

"A" is the radar range, "B" is garnered from the encoding altimiter.

"Jackie" wrote in message
...
Jim Logajan 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)?

Without altitude information from the pilot or a Mode C transponder, the
radar system can't extract the horizontal distance from the slant distance.
Where did you read that their systems correct for altitude without Mode C
transponders?

I don't believe I indicated I read this.

If they could do that then they wouldn't have ever needed the
transponders to report altitude.

Does precision approach radar require a transponder to report altitude?

At the altitudes involved during approach, it's not so critical (altitude is,
but not the angle), but during
approach the heading data is as critical as the altitude reporting.

"Stealth Pilot" wrote in message
...
On Fri, 12 Jun 2009 03:54:29 +0000 (UTC), Clark
wrote:

Jackie wrote in
:

[snip]

Does precision approach radar require a transponder to report altitude?

PAR uses a height finder radar to provide sufficient vertical resolution for
precision approaches.

Mode C only provides 100 foot altitude resolution. bzzzzt!

Mode C provides a height eg 1252ft which is deemed only accurate to
100ft because the calibration errors are limited to 100ft.

And when local BARO is not available, the minimums go up.

And why the slop (calibration error) is taken into account when developing the
approach procedure.

"Matt Barrow" wrote in message
...

"Jackie" wrote in message
...
Matt Barrow wrote:
"Jackie" wrote in message
...
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)?

Asquared + B squared = C squared, I'd guess.


And how does it separate A and B?

"A" is the radar range, "B" is garnered from the encoding altimiter.



Interesting thread!

But, for the life of me, I can't figure any reason that atc would really
care about the exact map location of an aircraft--especially when it is both
close enough and high enough for the error to be significant.

Given the two most common uses, surveilance approaches to an airport at
nearly the same elevation as the radar and collision avoidance, the map
distance should be a trivial issue.

Peter

"Peter Dohm" wrote in message
.. .
"Matt Barrow" wrote in message

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)?

Asquared + B squared = C squared, I'd guess.


And how does it separate A and B?

"A" is the radar range, "B" is garnered from the encoding altimiter.



Interesting thread!

But, for the life of me, I can't figure any reason that atc would really care
about the exact map location of an aircraft--especially when it is both close
enough and high enough for the error to be significant.

Given the two most common uses, surveilance approaches to an airport at nearly
the same elevation as the radar and collision avoidance, the map distance
should be a trivial issue.


Given the effort put into the significantly more accurate WAAS infrastructure,
I'd suspect that getting a more accurate fix would be correspondingly desirable.

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

In article ,
Jackie wrote:

Does precision approach radar require a transponder to report altitude?

No.

The beauty of the PAR approach is the aircraft only needs a
functioning radio.

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.