ATC Radar Question

[Jackie]

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)?

[Matt Barrow[_8_]]

"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)?

A<squared> + B <squared> = C <squared>, I'd guess.

[Jackie]

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)?
>
> A<squared> + B <squared> = C <squared>, I'd guess.


And how does it separate A and B?

[[email protected]]

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)?

Absent something else, it doesn't.

All a primary paint radar knows is azimuth and slant range.


--
Jim Pennino

Remove .spam.sux to reply.

[Jim Logajan]

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

[Jackie]

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

So if a pilot turns his Mode C from off to on, does the radar then
correct the distance plotted (less distance from the antenna) based on
the altitude reported?

[Jackie]

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?

[Jackie]

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.

Could you elaborate on how the "height finder" works?

[Bob Noel[_2_]]

Jackie wrote:
> 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.
>
> Could you elaborate on how the "height finder" works?

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

[Stealth Pilot[_2_]]

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.

[Morgans[_2_]]

"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

[Bob Noel[_2_]]

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'

[Matt Barrow[_8_]]

"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)?
>>
>> A<squared> + 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.

[Matt Barrow[_8_]]

"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.

[Matt Barrow[_8_]]

"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.

[Peter Dohm]

"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)?
>>>
>>> A<squared> + 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

[Matt Barrow[_8_]]

"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)?
>>>>
>>>> A<squared> + 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.

[Brian Whatcott]

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

[Bob Noel[_6_]]

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.

[Jackie]

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.

[Jackie]

Matt Barrow wrote:
> "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)?
>>> A<squared> + 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.

Huh? In the example I provided, "Let's forget about Mode C for a moment
and switch to Mode A only..." there is *no encoding altimeter* so your
point doesn't make sense here.

[John Clear]

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/

[Brian Whatcott]

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

[Jim Logajan]

"Matt Barrow" > wrote:
> "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)?
>>>
>>> A<squared> + 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.

Nit: Actually "C" would be radar range and either "A" or "B" would be from
the altimeter, solving then for "B" or "A".

[BT]

It does not correct.. Radar Range is slant range.. so using RAW Radar and
Skin Paint.. or even a Transponder Mode3 only, 4 codes, you will get a ring
around the mainbang (radar location) as the aircraft approaches the radar
site. It may disappear if smack on directly overhead until exiting the donut
hole on the other side.

I believe even Mode3C with digital displays have the same problems, but with
extensive radar coverage in most areas, the digital display is a mosaic
display of multiple radar feeds with the best target for a given location
coming from a certain radar. So another radar will "paint" the area over the
"mainbang" of the other radar.

This does not always happen depending on multiple radar coverage available.

BT

"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)?

[BT]

I don't think that altitude from the ModeC is ever factored into where to
plot the target. The target was always slant range.
BT

"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? If they could do that then they wouldn't
>> have ever needed the transponders to report altitude.
>
> So if a pilot turns his Mode C from off to on, does the radar then correct
> the distance plotted (less distance from the antenna) based on the
> altitude reported?

[BT]

and that has nothing to do with Mode C accuracy

"Matt Barrow" > wrote in message
...
> "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.
>
>
>

[BT]

reported altitude from a ModeC transponder is never used for vertical
guidance on a PAR approach.

"Matt Barrow" > wrote in message
...
> "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.
>
>
>

[Mike]

"Clark" > wrote in message
...
> "Morgans" > wrote in :
>
>>
>> "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.
>
>
> As far as I know you are entirely correct. The installations I've seen
> (years
> ago and far away) have two independent radar systems, one for the
> horizontal
> and one for the vertical. Of course I've only seen ancient technology and
> I'm
> sure that these days it could be done with one steerable beam.

Older PARs (which there are many still around) don't really have two
independent systems. They basically separate the radar signal between two
antenna (one for azimuth and one for elevation). Newer designs (which are
still quite old) use a phase array which steers the beam. In both designs
the transponder is neither required or used.

[Mike]

"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)?

It's pretty simple. The radiated energy from the transmitted pulse takes
12.36 microseconds to travel one mile and back. The radar uses that timing
to determine range. No corrections for altitude are made. The system that
receives transponder signals (which is an independent system which is
correlated later) works exactly the same way.

[Mike]

"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/wikipedia/commons/2/20/ASR-9_Radar_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.