Multi-angular airborne thermal observations: A new hyperspectral setup for simulating thermal radiation and emissivity directionality at the satellite scale

Land Surface Temperature (LST) is a key parameter to the understanding and modelling of many Earth system processes. Viewing and illumination geometry are known to have significant impacts on remotely sensed retrieval of LST, particularly for heterogeneous regions with mixed components. However, it is difficult to accurately quantify these impacts, in part due to the challenges of retrieving high-quality data for the different components in a scene at a variety of different viewing and illumination geometries over a time period where the real surface temperature and sun-sensor geometries are invariant. Previous field studies have attempted this through observations with aircraft-mounted single-band thermal cameras to further understanding of real-world conditions, but these sensors have limited accuracies and cannot be used to consider the angular variability of emissivity or to simulate multi-band satellite observations.

To redress this, the National Centre for Earth Observation’s Airborne Earth Observatory (NAEO) have developed and manufactured a modified mount for their state-of-the-art commercial pushbroom longwave hyperspectral airborne sensor, the Specim AisaOWL (102 narrowband channels across the 7.6 – 12.6 µm region). When mounted in standard mode, the field-of-view of the OWL sensor is 24° (± 12°), however the modified mount enables off-nadir measurements up to 48°. This has the potential to evaluate both thermal radiation and spectral emissivity directionality up to and beyond the view angles of most thermal satellite sensors. With LST now classified as an Essential Climate Variable, this work is particularly relevant as it will help to improve the accuracy of retrievals from current and future satellites (e.g. LSTM, SBG, TRISHNA).

In this presentation, we first present an overview of the design modifications that enable these high-angle observations and preliminary results from test flights before detailing how this setup will be used in an upcoming joint ESA-NASA campaign dedicated to quantifying and simulating thermal radiation directionality over agricultural regions at the satellite scale.