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)?