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 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.

"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.

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)?
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.

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.

"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)?

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.

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

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.