Abstract
We have investigated how the quality of stereoscopically measured topography degrades with varying illumination, in particular the ranges of incidence angles and illumination differences over which useful digital topographic models (DTMs) can be recovered. Our approach is to make high-fidelity simulated image pairs of known topography and compare DTMs from stereoanalysis of these images with the input data. Well-known rules of thumb for horizontal resolution (>3–5 pixels) and matching precision (~0.2–0.3 pixels) are generally confirmed, but the best achievable resolution at high incidence angles is ~15 pixels, probably as a result of smoothing internal to the matching algorithm. Single-pass stereo imaging of Europa is likely to yield DTMs of consistent (optimal) quality for all incidence angles ≤85°, and certainly for incidence angles between 40° and 85°. Simulations with pairs of images in which the illumination is not consistent support the utility of shadow tip distance (STD) as a measure of illumination difference, but also suggest new and simpler criteria for evaluating the suitability of stereopairs based on illumination geometry. Our study was motivated by the needs of a mission to Europa, but the approach and (to first order) the results described here are relevant to a wide range of planetary investigations.
Highlights
One of the primary objectives of a mission to Europa (e.g., NASA’s notional Europa Clipper: Thomas and Klaasen, 2013) would be to determine the thickness and structure of Europa’s icy shell by probing it with an ice penetrating radar (IPR)
This evaluation is supported by measurements of the local contrast, which increases monotonically with incidence angle, closely approximating the function tan(i) that would be expected for a Lambertian scatterer
The key result of this investigation is that the EIS wide-angle camera (WAC) will be able to produce stereo digital topographic models (DTMs) of Europa with near-optimal quality for incidence angles between about 40° and 85°, and very probably for incidence angles between 0° and 85°
Summary
One of the primary objectives of a mission to Europa (e.g., NASA’s notional Europa Clipper: Thomas and Klaasen, 2013) would be to determine the thickness and structure of Europa’s icy shell by probing it with an ice penetrating radar (IPR). A high resolution topographic model of a strip straddling the ground track is essential to “declutter” the radar echoes, i.e., to determine which features in them arise from subsurface reflectors and which are generated by surface reflections to the sides of the ground track. Clipper’s notional payload included a Topographical Imager (TI) that would operate simultaneously with the IPR to obtain the needed stereo image coverage. The quality of the images will vary along track because neither the range (image resolution) nor the illumination is constant. We aim to estimate what fraction of stereo coverage from a typical flyby is likely to be useful
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