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.