For the design of scientific experiments using DESIS data it is important to understand the specifics of the ISS orbit. The ISS orbit is using a non-sun-synchronous orbit resulting in observation and illumination conditions that are not reproducible. ISS moves on an orbit with an inclination of 51.6° from west to east approximately 400km above the Earth surface. ISS covers areas from 55° N to 52° S. Based on the tilting capabilities of the platform and the instrument, DESIS data can view from -45° (backboard) to +5° (starboard) and between -40° to +40° (along track). The tilting specification allows the observation of up to 90 % of the populated land masses.
In principle, ISS orbital tracks are repeated every 3 – 5 days. However, the times of overpasses vary widely resulting in different illumination conditions during the acquisition. Further, the prediction of the orbit is very difficult and only possible approximately few days in advance. This is because the ISS position is strongly affected by effects such as sun activity, debris manoeuvres and other orbit manoeuvres. It results in different heights of the ISS above the Earth and can vary in very extreme cases up to ~100 km difference. Figure 1 shows the mean semi-major orbital axis for the year 2016.
Huemmrich et al. (2017) have analysed the possibilities of the HICO instrument onboard of the ISS for Ecosystem Carbon Flux analyses. They found out that the ISS orbit provides longer periods (up to weeks) without any daytime acquisition possibilities and periods providing multiple observations at different times of the day within a period of a few days.
In Figure 2, all possible acquisition for Berlin, Germany, are shown for the DOY on x-axis and the off-nadir angle on Y-axis with the following restrictions:
For the year 2016, theoretically 170 DESIS acquisition would have been possible given the above listed constraints. The distribution of the acquisitions show a periodical grouping with weeks with nearly daily tasking opportunities followed by weeks with no tasking options. Further, the off-nadir angle (ONA; Figure 2) and the acquisition time of day (Figure 3) vary widely. The complex viewing and illumination conditions of each task need to be considered especially for multitemporal and comparative data analyses.
TBE have analysed theoretical imaging opportunities for various places across latitude (Figure 4) and by considering sun elevation > 30° and and Off-Nadir angle of <25°. The orbit analysis confirms the results of Huemmrich et al. (2017) that the distribution of tasking opportunities is dependent on latitude (Figure 5). Grouping of tasking possibilities increase with latitude while tasking opportunities closer to the equator get more dispersed.
Huemmrich, F., Campbell, P., Gao, B., Flanagan, B., Goulden, M., 2017. ISS as a Platform for Optical Remote Sensing of Ecosystem Carbon Fluxes: A Case Study Using HICO. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing. PP. 1-16. 10.1109/JSTARS.2017.2725825.