Originally Posted by
marie paire
This is not so. The ATC tag will show the altitude the aircraft is cleared to (controller manual input processed by the ATC FDPS) and the actual FL and attitude of the aircraft (climbing, descending, level) derived from mode C.
There is a huge amount of information squittered by the aircraft it would be impossible and confusing for the controllers to sort it out and action it. However, it is there and various systems can use it. The Alt Select information came from Mode-S squitter with the Mode-S ID. FR-24 passed what they had to the authorities. ADS-C allows the ground systems to ask the aircraft for more information - and the crew are totally unaware of the information being sent.
All the controllers get displayed to them is what is needed to do their jobs, but what is displayed is just the tip of quite a large iceberg of information.