Followup to Memo 26

SDP has released my followup memo to memo 26. This one is SDP memo 27. I've addressed comments and criticisms, corrected some errors, and invented a new, more efficient approach to ionospheric calibration. The conclusion remains the same: SKA1-LOW cannot be calibrated to the level required for EOR imaging within the lifetime of the telescope.

Going further than this would be a good PhD topic! I don't intend to do much more.