The Global Ecosystem Dynamics Investigation (GEDI) instrument was designed, built, and tested in-house at NASA’s Goddard Space Flight Center and launched to the International Space Station (ISS) on December 5, 2018. GEDI is a multibeam waveform LiDAR (light detection and ranging) designed to measure the Earth’s global tree height and canopy density using 8 laser beam ground tracks separated by roughly 600 meters. Given the ground coverage required and the 2 year mission duration, a unique optical design solution was developed. GEDI generates 8 ground sampling tracks from 3 transmitter systems viewed by a single receiver telescope, all while maximizing system optical efficiency and transmitter to receiver boresight alignment margin. The GEDI optical design, key optical components, and system level integration and testing are presented here. GEDI began 2 years of science operations in March 2019 and so far, it is meeting all of its key optical performance requirements and is returning outstanding science.
NASA’s Global Ecosystem Dynamics Investigation (GEDI) instrument was launched Dec. 5, 2018, and installed on the International Space Station 419 km from Earth. The GEDI is a Light Detection and Ranging (LIDAR) instrument; measuring the time of flight of transmitted laser beams to the Earth and back to determine altitude for geospatial mapping of forest canopy heights. The need for very dense cross track sampling for slope measurements of canopy height is accomplished by using three individual laser transmitter systems, where each beam is split into two beams by a birefringent crystal. Furthermore, one transmitter is equipped with a diffractive optical element splitting the two beams into four, for a total of 8 beams. The beams are reflected off of the features and imaged by an 800 mm diameter Receiver Telescope Assembly, composed of a Ritchey-Chrétien telescope, a refractive aft optics assembly and focal plane array which collects and focuses the light from the 8 probe beams into the 8 science fibers, each with a field of view on the Earth subtending 300 μrad. The dense cross-track sampling mandated a custom designed dual-fiber interface. The science fibers had to be aligned to the nominal, projected laser spots. The alignment was highly dependent on optimization and co-positioning of the fibers pair-wise due to mechanical constraints. This paper presents the end-to-end alignment and metrology of the full optical system from transmitter elements through receiver telescope, aft-optics, focal plane and receiver fibers performed at NASA Goddard Space Flight Center.
A high-precision ultra-lightweight 0.5m mirror with ultraviolet grade tolerances on surface figure quality has been
measured from its delivery to the Goddard Space Flight Center, through the coating and mounting process, and shown to
survive component vibration testing. This 4.5kg, 0.5m paraboloid mirror is the prime optic of two sounding-rocket
telescopes: SHARPI (solar high angular resolution photometric imager) and PICTURE (planet imaging concept testbed
using a rocket experiment). By integrating the analysis of interferometer data with finite element models, we
demonstrate the ability to isolate surface figure effects comparable to UV diffraction limited tolerances from much larger
gravity and mount distortions. The ability to measure such features paired with in situ monitoring of mirror figure
through the mirror mounting process has allowed for a diagnosis of perturbations and the remediation of process errors.
In this paper, we describe the technical approach used to achieve nanometer scale measurement accuracy, we report and
decompose the final mounted surface figure of 12.5 nm RMS, and we describe the techniques that were developed and
employed in the pursuit of maintaining UV diffraction-limited performance with this aggressively lightweighted mirror.
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