In the terminal area and for the approach and landing phase, radio provides the most appropriate method of height measurement. That is to say that the altitude reference frame is switched from barometric to terrain clearance measurement. It is at first a little surprising to see from Mr. Davies' requirements for this phase that the accuracies are in line with those currently in use in subsonic jets. But consideration shows that this is because the approach speed and general handling characteristics of the S.S.T. must approximate to those of subsonic jets in the approach and landing phase. Mr. Davies has also stated that automatic landing is essential for the S.S.T., so that the picture of requirements for radio altimetry in the approach and landing phase is fairly clear.