Obtaining both early and late time AEM data: the pros and cons of almost perfect systems and almost perfectly known systems

Both 'almost perfect' and 'almost perfectly monitored' approaches to getting good early time AEM data work, but have several pros and cons. Common factor is the need for accurate removal of a bias response from residual transmitter and / or bucking coil currents. To achieve this bias correction, low noise systems need either much higher altitude data as references (1600 to 2400 m rather than the usual 700 to 800 m presently collected), or in theory could use 2 or more lower altitudes to predict the primary field response at extreme altitude.