Back-Side Illuminated CMOS Star Tracker for Dim Object Detection
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Solution Overview
Problem
Star trackers are limited to detecting relatively bright stars for attitude information and struggle to detect dim objects due to sensitivity issues and radiation susceptibility in space environments.
Innovation Solution
A multiple mode star tracker with a global shutter CMOS image sensor and attitude quaternion registration enables precise alignment and stacking of image frames, boosting signal-to-noise ratio to detect dim objects without requiring a gyroscope or complex pixel registration algorithms.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If front-side illuminated CMOS sensors are used, then device complexity is reduced, but sensitivity deteriorates significantly
Solution Approach 1:
The patent inverts the traditional front-side illuminated CMOS sensor structure by using back-side illuminated sensors. This inversion allows light to enter the sensor from the back side, bypassing the metal interconnect layers that block light in FSI sensors, thereby achieving high sensitivity while maintaining device simplicity.
2Measurement precision
If micro-lenses are added to FSI CMOS sensors, then sensitivity is improved, but accuracy of stellar image centroids deteriorates
Solution Approach 1:
By inverting to back-side illumination, the patent eliminates the need for micro-lenses entirely. The light enters the photodiode directly from the back side without passing through metal interconnect layers, achieving both high sensitivity and high centroid accuracy without the trade-off inherent in FSI sensors.
3Device complexity
If rolling-shutter readout is used, then device complexity is reduced, but measurement accuracy deteriorates due to skewed readout time
Solution Approach 1:
The patent transitions from a static rolling-shutter readout to a dynamic global shutter readout system. The global shutter captures all pixels simultaneously at a given moment, eliminating the temporal skew inherent in rolling-shutter systems, thereby achieving accurate readout timing while maintaining reasonable device complexity through integrated circuit design.
4Measurement precision
If CCD sensors are used, then sensitivity is improved, but reliability deteriorates due to susceptibility to space radiation and transient effects
Solution Approach 1:
The patent changes the material and structural parameters by using back-side illuminated CMOS sensors with optimized photodiode designs. This parameter change enables the sensor to achieve CCD-level sensitivity while maintaining the radiation hardness and transient immunity inherent to CMOS technology, thereby improving reliability in the space radiation environment.
5Device complexity
If simple pixel designs with rolling-shutter readout are used, then device complexity is reduced, but sensitivity deteriorates
Solution Approach 1:
By inverting to back-side illumination, the patent fundamentally changes how light interacts with the pixel structure. This inversion enables the use of sophisticated pixel designs with pinned photodiodes and transfer gates that achieve high sensitivity, while the global shutter readout ensures all pixels integrate signal for the same absolute time period, further enhancing sensitivity without excessive complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables the detection of dim objects by enhancing the signal-to-noise ratio through frame stacking, allowing for simultaneous attitude determination and full-frame imaging, improving sensitivity and reducing the need for external gyroscopic data.
Implementation Method 1
back-side illuminated (BSI) CMOS imaging sensors, which feature fundamentally improved sensitivity. BSI sensor designs result in the entire surface of the imaging sensor being light sensitive, greatly improving the sensor's quantum efficiency
Implementation Method 2
The pixels are operated according to a global shutter, ensuring that each pixel of the sensor integrates signal for the same absolute time period
Implementation Method 3
the attitude quaternion is used to register each pixel in the collected series of image frames or video with respect to an IRF during some spatial motion of the focal plane
Data Source
AI summary
Multiple mode star tracker methods and systems in which attitude information and image information is generated are provided. The multiple mode star tracker includes a detector having a plurality of pixels arranged in a focal plane array. The detector is operated to obtain multiple image frames from within a field of view containing a plurality of stars. For each of the image frames, the attitude of the detector and in turn the attitude of each pixel is determined. Based on the attitude quaternion of the individual pixels within a plurality of frames, image data from the plurality of frames is co-added or stacked to form a composite image. The co-addition of multiple frames of image data enables or facilitates the detection of dim objects by the multiple mode star tracker. Moreover, embodiments of the present disclosure enable the attitude quaternion for individual pixels within individual frames to be determined using the multiple mode star tracker function of the instrument, and without requiring attitude information provided by a separate device, such as a gyroscope.