The ATC value is probably the value that has been encoded and sent by the transponder (secondary radar information is based on the information the aircraft sends to a ping). If the static source/s involved are giving the transponder duff info (a combination of sensor fault, icing, water ingress, FMS super brain fart, unlucky Tuesday), then ATC is not telling the crew anything useful. If the satellite signal generated altitude and standby altimeter correspond, then I‘d trust that, especially as nobody would be expecting to talk to ATC at 200ft.. or suddenly find themselves there!
The last sentence In the article about the ADS-B data saying that the aircraft did not get above 250ft for the entire flight is just a reflection of the same info that ATC was receiving and reporting, low just a reflection of the fault.
Last edited by Torquetalk; 12th Oct 2020 at 15:27.