Mission Concept

This mission combines two sensors that, taken together, provide observations important for solid-Earth (surface deformation), ecosystems (terrestrial biomass structure) and climate (ice dynamics). The sensors are: 1) an L-band Interferometric Synthetic Aperture Radar (InSAR) system with multiple polarization, and 2) a multiple beam lidar operating in the infrared (~ 1064 nm) with ~ 25 m spatial resolution and 1 m vertical accuracy. The mission using InSAR to meet the science measurement objectives for surface deformation, ice sheet dynamics, and ecosystem structure has been extensively studied. It requires a satellite in 700-800 km sun-synchronous orbit in order to maximize available power from the solar arrays. An eight day revisit frequency balances temporal decorrelation with required coverage. Onboard GPS achieves cm-level orbit and baseline knowledge to improve calibration. The mission should have a 5 year life time to capture time-variable processes and achieve measurement accuracy.

For ecosystem structure, L-Band InSAR measurements allow estimating forest height with meters accuracy; polarimetry allows estimation of three-dimensional forest structure. The sensitivity of backscatter measurements at different wave polarizations to woody components and their density makes UHF radar sensors suitable for direct measurements of live above ground woody biomass (carbon stock) and structural attributes such as volume and basal area. The multi-beam laser altimeter (lidar) system would accurately measure the distance between the canopy top and bottom elevation, the vertical distribution of intercepted surfaces, and the size distribution of vegetation components within the vertical distribution. Multiple beams measure different size components of vegetation. Although this measurement is the most direct estimate of the height and the vertical structure of forests, the lidar measurement samples the Earth's surface at discrete points, rather than imaging the entire surface. DESDynI combines the two approaches, taking advantage of the precision and directness of the lidar to calibrate and validate the InSAR, especially in ecosystem types where field campaigns have not occurred. These two measurements do not need to be made simultaneously, but could be separated by up to a few weeks because ecosystem structure typically does not evolve significantly on shorter time scales. Whether both instruments are flown on the same platform or separate platforms should be determined by a more thorough study. For example, it might be possible to upgrade the ICESat-II mission to include multi-beam performance in order to meet the ecosystem requirements so long as the two missions are launched within the same time frame and take measurements within a few weeks.

The InSAR instrument consists of an L-band (1.2 GHz) radar to minimize temporal decorrelation in regions of appreciable ground cover. Two sub-bands separated by 70 MHz allow correction of ionospheric effects. The viewable swath width must be larger than 340 km to obtain complete global access. Other parameters include ground resolution better than 35 m to characterize fault geometries, noise equivalent less than -24 dB to map radar-dark regions, electronic beam steering to minimize spacecraft interactions for acquisition and allow ScanSAR operation, and a data rate less than 140 Mbps. Multiple polarization is required for the canopy density profiles needed for ecosystem structure. As noted above, the lidar in DESDynI is a multi-beam laser ranger operating in the infrared.