ATC Radar Question

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)?

"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.

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?

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.

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.

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?

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?

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?

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.

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.