Crop producers requiring crop biomass maps to support timely application of in-season fertilisers, pesticides or growth regulators rely on either on-ground active sensors or airborne/satellite imagery. Active crop sensing (for example using Yara N-Sensor^TM, Greenseeker^TM or Cropcircle^TM) can only be used when the crop is accessible by person or vehicle, and extensive, high-resolution coverage is time consuming. On the other hand, airborne or satellite imaging is often hampered by cloud, either in the sensor-image path or by associated non-uniform illumination of ground targets. We have combined the desirable attributes of active optical sensing with the fast, synoptic coverage afforded by aircraft platforms. An ultra low-level aircraft (ULLA) system carrying an active NIR/Red CropCircle^TM sensor was successfully deployed at an altitude of 3-5 m over a 270 Ha field of skip-row sorghum (Sorghum bicolor) to measure and map crop vigour via the simple ratio (SR) index. A comparison to a SR map derived from a metre-resolution airborne digital image was found to reproduce the spatial variability observed in the image of the field. As the sensor contains its own light source, it can be operated irrespective of ambient light conditions. It is relatively cheap, lightweight, small in size and can easily be retro-fitted to aircraft. The ULLA active sensor approach offers crop managers a viable alternative to conventional imaging technologies especially when they have day-to-day access to aircraft already conducting low-level operations, for example crop dusting and reconnaissance, over their agricultural fields. Furthermore, extensive on-ground work worldwide to calibrate active optical sensors to crop N-status/recommendations means whole-of-field N status/recommendation maps can now be rapidly generated using the same sensors mounted in aircraft.