A new JPL project requires that the center of the images of a rising or setting Sun be determined with certain accuracy — for example, within 1 km — when the Sun is observed from the International Space Station (ISS). This center-finding technique can be used in applications such as a Sun-Tracker. To meet such needs, a simulation tool was developed for the generation of Sun images observed either on the ground or from space. The new technique enables one to find the center of a Sun image based on simulated images. The technique does not rely on ellipse-fitting to the boundary of a Sun image or other calibration techniques, so the accuracy is not affected by the distortion of Sun images.
When observing the Sun image from the ISS, the Sun exhibits an equivalent diameter of approximately 21.5 km. So the 1-km accuracy of the Sun image center translates into a pixel-based accuracy. The longer axes of the asymmetric Sun images are not aligned with the horizontal or the vertical axis. Before finding the center of an image, the angle of rotation of an image is found such that the image’s longer axis aligns with the horizontal or x-axis.
This was accomplished in the following way. A tilted image is normalized such that its peak = 1. A threshold is applied to the image to generate a binary mask and find the approximate center of the image by calculating its center of mass. The binary mask is rotated, and a – b is calculated, where a and b are the larger and smaller widths of the mask, respectively. The angle of rotation is determined from the location of the maximum of a – b .
The widths of the simulated image with ray vertex height hv = 22.5 km have the same [Width Height] values as the measured image. Because the exact center of this simulated image is known, it can be assumed that the center of the measured image is located in the same location, measured from the left vertex to the center of the mask, as well as from the top vertex to the center of the mask.
This work was done by Erkin Sidick of Caltech for NASA’s Jet Propulsion Laboratory.
The software used in this innovation is available for commercial licensing. Please contact Dan Broderick at
This Brief includes a Technical Support Package (TSP).
Technique for Finding the Center of an Image of a Rising or Setting Sun Based on Simulated Images
(reference NPO-49177) is currently available for download from the TSP library.
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Overview
The document presents a Technical Support Package from NASA's Jet Propulsion Laboratory (JPL) detailing a novel technique for accurately determining the center of images of a rising or setting sun. This technique is particularly relevant for observations made from the International Space Station (ISS) and aims to achieve an accuracy of within 1 kilometer. The research is spearheaded by Erkin Sidick and is part of a broader project that seeks to enhance solar tracking capabilities.
The introduction highlights the significance of accurately locating the sun's center in images, which is crucial for applications such as sun-tracking systems. The sun's angular diameter is approximately 0.5333 degrees, translating to an equivalent diameter of about 21.5 kilometers when viewed from the ISS at an average height of 415 kilometers. The document emphasizes the need for a center-finding technique that can operate effectively under various atmospheric conditions.
The methodology described involves a comprehensive simulation tool that generates sun images based on real data collected from the SAGE II satellite at a wavelength of 1020 nm. The process begins with normalizing a tilted image to align its longer axis with the horizontal axis. This is achieved through a series of steps: normalizing the image, applying a threshold to create a binary mask, calculating the center of mass, and determining the angle of rotation to align the image properly. The technique allows for precise calculations of the sun's center, which is essential for further analysis and applications.
Figures included in the document illustrate the process, showing examples of real sun images and the steps taken to analyze them. The document also acknowledges contributions from various researchers and institutions, highlighting collaborative efforts in advancing solar imaging technology.
In conclusion, this Technical Support Package outlines a significant advancement in the field of solar imagery, providing a robust method for accurately determining the center of sun images. The technique has potential applications in various scientific and technological domains, particularly in improving solar tracking systems and enhancing our understanding of solar phenomena. The research is supported by NASA and reflects ongoing efforts to leverage aerospace-related developments for broader applications.
