Time Delay And Integration Research Articles

Signal-to-noise ratio model in Python for high-resolution space-borne electro-optic imagers

We present a signal-to-noise ratio (SNR) simulator design for high-resolution Earth observation imagers with time-delay and integration (TDI) imaging sensors. SNR is one of the space-borne electro-optic imagers’ driving design and performance evaluation parameters. TDI line imaging sensors are used to meet these imagers’ SNR requirements by increasing effective exposure time during imaging. Determining the optimum number of TDI stages to meet the SNR requirements of the mission is critical for the imaging sensor selection, optical design of the telescope, laboratory radiometric tests, and imaging in orbit. The SNR simulator is based on an end-to-end image chain calculation following the light pathway from the sun to the imager. The simulator comprises radiance, telescope, sensor, and image modules written in the open-access Python environment. The radiance module uses PcModWin/MODerate resolution atmospheric TRANsmission output data of the atmospheric radiance calculation input for the simulator. The simulator computes the SNR for the given telescope and imaging sensor model and simulates associated images to show the radiometric performance. A high-resolution electro-optic imager model is used to demonstrate the performance and versatility of the simulator for the case studies. Experimental simulator validation is demonstrated for the images acquired by the Göktürk-2 space-borne imager.

Assessment of micro-vibrations effect on the quality of remote sensing satellites images

ABSTRACT Recently, there is a growing interest in analysing the degrading effect of satellite micro-vibrations due to the rapid growth in satellite technologies and the urgent need to precisely extract a huge amount of information from satellite images. Different kinds of micro-vibration have a notable effect on the quality of satellite images. The main objective of this paper is to demonstrate and analyse the effect of all types of micro-vibration on the quality of images acquired by high-resolution satellites. An algorithm to simulate micro-vibrations is proposed. A very high-resolution satellite image from the Pleiades-neo satellite is selected as an example to be used in addressing the degrading effects of micro-vibrations. In this paper, the modulation transfer function (MTF) is used as a major function to model the degradation that has been conducted. Also, several quality metrics are used to quantitatively assess the degradation. The key result of this paper is the significant effect of micro-vibrations on the quality of remote sensing satellite images which is attributed to the main influential parameters. These parameters like blur diameter, vibration displacement, number of Time Delay and Integration (TDI) stages of the camera, and the ratio of the integration time to the vibration period.

Stitching and Geometric Modeling Approach Based on Multi-Slice Satellite Images

Time delay and integration (TDI) charge-coupled device (CCD) is an image sensor for capturing images of moving objects at low light levels. This study examines the model construction of stitched TDI CCD original multi-slice images. The traditional approaches, for example, include the image-space-oriented algorithm and the object-space-oriented algorithm. The former indicates concise principles and high efficiency, whereas the panoramic stitching images lack the clear geometric relationships generated from the image-space-oriented algorithm. Similarly, even though the object-space-oriented algorithm generates an image with a clear geometric relationship, it is time-consuming due to the complicated and intensive computational demands. In this study, we developed a multi-slice satellite images stitching and geometric model construction method. The method consists of three major steps. First, the high-precision reference data assist in block adjustment and obtain the original slice image bias-corrected RFM to perform multi-slice image block adjustment. The second process generates the panoramic stitching image by establishing the image coordinate conversion relationship from the panoramic stitching image to the original multi-slice images. The final step is dividing the panoramic stitching image uniformly into image grids and employing the established image coordinate conversion relationship and the original multi-slice image bias-corrected RFM to generate a virtual control grid to construct the panoramic stitching image RFM. To evaluate the performance, we conducted experiments using the Tianhui-1(TH-1) high-resolution image and the Ziyuan-3(ZY-3) triple liner-array image data. The experimental results show that, compared with the object-space-oriented algorithm, the stitching accuracy loss of the generated panoramic stitching image was only 0.2 pixels and that the mean value was 0.799798 pixels, achieving the sub-pixel stitching requirements. Compared with the object-space-oriented algorithm, the RFM positioning difference of the panoramic stitching image was within 0.3 m, which achieves equal positioning accuracy.

High Resolution Imaging Camera (HiRIC) on China’s First Mars Exploration Tianwen-1 Mission

The High-Resolution Imaging Camera (HiRIC) is one major payload of China’s first Mars exploration mission, and its main objective is to obtain the detailed observation images of the key areas on the Martian surface. In this paper, the leading group of HiRIC shows a full blueprint of the HiRIC. The HiRIC can achieve a high resolution (0.5 m at an altitude of 265 km) with a wide swath width of 9 km. The HiRIC adopts an Off-Axis Three-Mirror Astigmatic (TMA) optical system with a focal length of 4640 mm, an F-number of 12 and a Field of View (FOV) of 2° × 0.693°. In order to reduce the instrument weight, carbon-based material is widely used in the opto-mechanical structure which is in ultra-lightweight design, thus, a light-weight camera with a total mass of 42 kg is obtained. The Time Delay and Integration (TDI) Charge Coupled Devices (CCDs) and Complementary Metal-Oxide-Semiconductor Transistor (CMOS) detectors are all set on the imaging plane to achieve the push-broom imaging and frame imaging, respectively. And the high Signal-to-Noise Ratio (SNR) >100:1 can achieve in multi observation types for various scientific imaging tasks. After 4-year design and fabricate, the HiRIC has been assembly. The testing results show that the instrument is in good condition, and the Modulation Transfer Function (MTF) can achieve 0.18 at Nyquist frequency. The HiRIC can achieve a well image on China first Mars exploration mission.

Effectiveness of Electrical and Optical Detection at Pixel Circuit on Thin-Film Transistors.

The paper presents a typology of electrical open and short defects on thin-film transistors (TFT) using an electrical tester and automatic optical inspection (AOI). The experiment takes the glass 8.5th generation to detect the electrical characteristics engaged with time delay and integration (TDI) charged-coupled-devices (CCDs), a fast line-scan, and a review CCD with five sets of magnification lenses for further inspection. An automatic data acquisition program (ADAP) controls the open/short (O/S) sensor, TDI-CCD, and motor device for machine vision and statistics of substrate defects simultaneously. Furthermore, the quartz mask installed on AOI verified its optical resolution; a TDI-CCD can grab an image of a moving object during transfers of the charge in synchronous scanning with the object that is significant.

Influence of Attitude Parameters on Image Quality of Very High-Resolution Satellite Telescopes

The image quality of very high-resolution satellite telescopes (VHRSTs) is affected by several factors, including optics, detector, pointing accuracy errors, attitude stability errors, and altitude variations. The charge-coupled devices with a time delay and integration (TDI) feature play a significant role along with the satellite attitude parameters in characterizing the VHRST image quality. The imperfection of the satellite control system to maintain the required pointing accuracy and attitude stability can lead to severe degradation in the quality of the acquired imagery. Hence, it is required to investigate their impacts carefully during the satellite preliminary design phase. In this article, the combined effect of the TDI steps, satellite attitude parameters, and orbital altitude variations on the image quality are investigated in detail. The results show that the image quality in the along- and cross-track directions are affected seriously in certain situations by the roll and pitch angular velocity bias. Furthermore, the proposed mathematical model in this article reveals that total modulation transfer function (MTF), as an image quality metric, is affected severely by the satellite altitude variation, particularly when employing a high number of TDI steps. For example, for a given scenario, when employing TDI steps of 64, the mathematical results show that the effect of a satellite altitude variation of 10 km will result in a 65% reduction in total system MTF.

Development of vacuum compatible, multi-mode operation light source

Light sources are used extensively at various stages of development and characterization of an electro-optical (EO) payload like Cartosat, HySIS, Microsat etc. These sources are required to characterize many critical parameters of EO payload like photo response non uniformity (PRNU), noise performance, saturation radiance etc. Currently integrating sphere with quartz tungsten halogen (QTH) lamp is widely used for payload characterization. These lamp sources generally operate in continuous mode in clean room environment and thermo-vacuum. In case of high resolution payloads, time-delay and integration (TDI) detectors are used to improve signal collection. For characterization and testing of such payloads multi-mode (pulsed and continuous) light source (switching at kHz rate in synchronization with payload electronics) with specific spectral range is required. Pulse mode operation requirement cannot be met using QTH lamps. To cater to such need a LED based indigenous source has been developed. This paper delineates circuit design and implementation of driver and characteristics of the source is also discussed. Proposed source is capable to synchronize and operate in multi-mode with external clock pulse with high achieved linearity (>99%) and high stability (>99%) in vacuum condition.

Advanced Design of Scanning Infrared Focal Plane Arrays

Modern designs of time delay and integration (TDI) IR linear scanning focal plane arrays (IR FPAs) are analyzed. Advanced designs of linear IR FPAs with increased sensitivity and spatial resolution are proposed. The analysis is based on Monte-Carlo simulation of the diffusion of photogenerated charge carriers in photodiode arrays based on mercury–cadmium–telluride epitaxial layers taking into account the main photoelectric and design parameters of the detectors and optical system.

Multi-Row Photodetectors for the Short Wavelength IR Region Based on HgCdTe Heteroepitaxial Structures

Parameters of multi-row photodetectors (PDs) based on HgCdTe heteroepitaxial structures of different formats, including 288 × 4, 480 × 6, 576 × 4, and 576 × 6, with a step of 28 to 14 microns are studied. Owing to the choice of a N+/P-/р-architecture, PDs operate at elevated temperatures in the time delay and integration (TDI) mode with the implementation of the analog mode of TDI and the replacement of defective elements directly in the readout LSI. The PDs are capable of forming high-definition images of the 768 × 576 format at a frame rate of 50 Hz in real time. For the multi-row PDs, high photoelectric parameters were obtained: the detection capacity at the maximum of the spectral sensitivity D* ≥ 5 × 1012 cm W–1 Hz1/2 at temperatures Т~ 170–200 K and the number of working channels is not less than 99.0%.

Development of method for active alignment of multiple time delay and integration detectors in the optically butted focal plane assembly of high-resolution spaceborne imaging systems

Time delay and integration (TDI) detectors are preferred image sensors for high-resolution spaceborne imaging systems for cartographic applications as they provide higher sensitivity by integrating charges over multiple TDI stages in push broom mode. Large swath requirements are met using multiple TDI detectors in optical butting configuration in the focal plane assembly (FPA) to form a single image line on the ground. These detectors need to be aligned with subpixel accuracy to avoid motion smear and errors in the image mosaicking process. Alignment of the multiple TDI detectors in the FPA is a challenging task and is accomplished using custom methods and setups. Passive alignment techniques rely on very high-resolution microscopes for achieving desired accuracies. Active method requires TDI detectors to be operated in staring mode, which is architecturally and functionally not supported by TDI detectors. We developed a scheme for operating the TDI detectors in staring mode without any smear effects to aid in the active alignment process. Based on this, we propose an active alignment approach. The proposed approach is highly advantageous as it can be used throughout the imaging system development phase to demonstrate stability of the alignment and also helps in cross validation with the passive alignment. The developed method was test validated on qualification model FPA, and alignment accuracy of the order of 0.4 μm in tilt and 1.2 μm in line shift has been demonstrated, against the requirements of ±1.5 μm. This method has been extensively used in Cartosat-2S series imaging systems and will be very useful for upcoming high-resolution missions of the Indian Space Research Organisation.

First CCD-in-CMOS-Based Multispectral TDI Image Sensor

A new approach to charge-coupled device (CCD)-based time-delay-and-integration (TDI) imaging is presented that implements the CCD TDI pixels together with advanced CMOS drivers and readout in one single chip. This CCD-in-CMOS technology is compatible with backside illumination for enhanced sensitivity, and multispectral filters can be added for increased imaging performance. With this highly sensitive and high-speed imaging solution, high-end applications such as remote sensing, life sciences and machine vision are targeted.

Implementation of the time delay and integration mode in a scanning 576 × 6 focal-plane array of the long-wave infrared range

The results of the development of a focal-plane array (FPA) of the 576 × 6 format with the time delay and integration (TDI) mode are presented. The comparative analysis of different variants of implementation of the TDI mode in ROIC is conducted. The expedience of the upgrade of existing scanning photodetectors of the 288 × 4 format on the basis of the developed 576 × 6 FPA is justified. The result of this upgrade will be simplification of the optical–mechanical scanning unit and improvement of the quality of thermal image.

High spatial resolution shortwave infrared imaging technology based on time delay and digital accumulation method

Shortwave infrared (SWIR) imaging technology attracts more and more attention by its fascinating ability of penetrating haze and smoke. For application of spaceborne remote sensing, spatial resolution of SWIR is lower compared with that of visible light (VIS) wavelength. It is difficult to balance between the spatial resolution and signal to noise ratio (SNR). Some conventional methods, such as enlarging aperture of telescope, image motion compensation, and analog time delay and integration (TDI) technology are used to gain SNR. These techniques bring in higher cost of satellite, complexity of system or other negative factors. In this paper, time delay and digital accumulation (TDDA) method is proposed to achieve higher spatial resolution. The method can enhance the SNR and non-uniformity of system theoretically. A prototype of SWIR imager consists of opto-mechanical, 1024×128 InGaAs detector, and electronics is designed and integrated to prove TDDA method. Both of measurements and experimental results indicate TDDA method can promote SNR of system approximated of the square root of accumulative stage. The results exhibit that non-uniformity of system is also improved by this approach to some extent. The experiment results are corresponded with the theoretical analysis. Based on the experiments results, it is proved firstly that the goal of 1m ground sample distance (GSD) in orbit of 500km is feasible with the TDDA stage of 30 for SWIR waveband (0.9–1.7μm).

Investigation of 1024 × 10 multirow focal plane arrays based on the solid solution of mercury–cadmium–telluride

A technique for measuring photoelectric parameters of focal plane arrays (FPAs) with time delay and integration (TDI) operation mode without optical-mechanical scanning is presented. A feature of the FPA is the presence of disconnectable high-pass filters in the input cells of the large-scale integration circuit (LSI), which makes it possible to subtract the inconclusive constant component of the signal. A blackbody (BB) with modulator is used as an optical signal source. The criterion of selection of the modulation frequency is the TDI period and the amplitude–frequency characteristics of the FPA. In order to perform correct measurements of signal values, parameters of a nonrecursive TDI filter are calculated. Values of the FPA noise are obtained by subtracting the periodic amplitude modulation. The dependence of background irradiances on the photodetector is calculated and the dependence of the FPA noise on the background temperature is constructed. The internal FPA noise is calculated and numerical values of measured parameters are presented.

Digital readout integrated circuit (DROIC) implementing time delay and integration (TDI) for scanning type infrared focal plane arrays (IRFPAs)

This paper presents a digital readout integrated circuit (DROIC) implementing time delay and integration (TDI) for scanning type infrared focal plane arrays (IRFPAs) with a charge handling capacity of 44.8Me− while achieving quantization noise of 198e− and power consumption of 14.35mW. Conventional pulse frequency modulation (PFM) method is supported by a single slope ramp ADC technique to have a very low quantization noise together with a low power consumption. The proposed digital TDI ROIC converts the photocurrent into digital domain in two phases; in the first phase, most significant bits (MSBs) are generated by the conventional PFM technique in the charge domain, while in the second phase least significant bits (LSBs) are generated by a single slope ramp ADC in the time domain. A 90×8 prototype has been fabricated and verified, showing a significantly improved signal-to-noise ratio (SNR) of 51dB for low illumination levels (280,000 collected electrons), which is attributed to the TDI implementation method and very low quantization noise due to the single slope ADC implemented for LSBs. Proposed digital TDI ROIC proves the benefit of digital readouts for scanning arrays enabling smaller pixel pitches, better SNR for the low illumination levels and lower power consumption compared to analog TDI readouts for scanning arrays.

Smear analysis for multi phase TDI CCD in panoramic remote sensing systems

Multi-phase Time Delay and Integration (TDI) remote sensing systems have the potential to introduce large amount of smear. Image resolution in the scan direction is usually lower than that in the cross scan direction. In designing these systems, it is helpful to understand how smear in the scan direction degrades the image quality. This paper presents a thorough understanding of the causes of along scan smear and a mathematical model that describes the smear in the along scan direction. Also the Modulation Transfer Function (MTF) losses are analyzed based on the novel model. At last a series of image simulations are performed in which along scan smear (with stage number N equal to 16, 32 and 96) is introduced.

全景式TDICCD摆扫航空相机像面旋转的高精度补偿

In the imaging process of scanning camera, measures must be taken to guarantee the synchronous scanning movement between scanning mechanism and the detector of image plane. If there exists angular deviation or speed deviation between the two mechanisms, it gives rise to the image rotation phenomenon. The image is blurred, causing the decline of image quality. The paper takes the panoramic time delay and integration (TDI) CCD scanning aerial camera as an example, establishes the mathematical model of image surface rotation analysis using coordinate transformation method, analyzes the influences of position and speed synchronization error on imaging quality. The synchronous compensation method based on disturbance observer is presented, which ensures the position and speed synchronization between the two mechanisms, realizes the independent design of speed controller and synchronous controller and facilitates the practical application. Through the laboratory static resolution imaging and external imaging experiments, theoretical analysis results are verified. The experimental results show that the position and speed synchronous errors based on the method are less than 0.0043, 0.0695/s, respectively, which meet the requirements of compensation accuracy, in the imaging process the image rotation and distortion have been significantly inhibited and the image quality is greatly improved.

Automatic focus algorithms for TDI X-Ray image reconstruction

Abstract. In food industry, most products are checked by X-rays for contaminations. These X-ray machines continuously scan the product passing through. To minimize the required X-ray power, a Time, Delay and Integration (TDI) CCD-sensor is used to capture the image. While the product moves across the sensor area, the X-ray angle changes during the pass. As a countermeasure, adjusting the sensor shift speed on a single focal plane of the product can be selected. However, the changing angle result in a blurred image in dependance to the thickness of the product. This so-called ''laminographic effect'' can be compensated individually for one plane by inverse filtering. As the plane of contamination is unknown, the blurred image will be inversely filtered for different planes, but only one of these images shows the correctly focussed object. If the correct image can be found, the plane containing the contamination is identified. In this contribution we demonstrate how the correctly focussed images can be found by analyzing the images of all planes. Different characteristics for correctly and incorrectly focussed planes like sharpness, number of objects and edges are investigated by using image processing algorithms.

外同步式时间延迟积分CCD传感器模拟装置

To avoid the damage of Time Delay and Integration(TDI) CCD sensors in the experimental process,a TDICCD simulation instrument synchronized with input driving clocks is introduced.The simulation instrument is designed with a simple method to meet its physical size in a length of 100 mm and a width of 45 mm.As the size is same as that of a real TDI CCD sensor,the instrument can simulate the photonic characteristics of the pixels for TDI CCD sensors exactly.Furthermore,it can simulate the electrical characteristics of pins in the TDI CCD sensor and can synthesize a complete video output signal,by which an important function,selectable TDI stages with 16,32,48 or 64 stage,is available.Moreover,the instrument can be taken to simulate the thermal delay characteristic between the input driving clock and output OS signals in the temperature adjustable range of 0.5~12.5 ns,which is convenient to the sample time adjustment for the signal circuit.

空间相机地心距误差修正

To modify the geocentric core distance error of a Time Delay and Integration(TDI) CCD space camera in the calculation of image motion compensation,and decrease the influence of the error on the match error of image motion velocity,a model of image motion calculation of points bellow satellites was deduced.The influence of the geocentric core distance error on the relative error of image motion velocity was analyzed based on the model,and the geocentric core distance error was modified by two steps according to the error sources.The former adopted the World Geodetic System(WGS-84) to modify the error generated by ellipticity of the error,and the latter used the data source of the earth's height above sea level(United States Geological Survey Digital Elevation Model,USGS DEM) to make a digital elevation map modify the error generated by the deference heights above sea level.After modifying,the image motion model of points bellow satellites was deduced.The calculation and analysis by the model indicate that the maximum error of image motion velocity eliminated are about 2.85% and 1.76% with the modification of geocentric core distance error by the WGS-84 model and digital elevation map,respectively.Those data clearly show that the match error of forward direction(the integral direction of TDI CCD)has been greatly decreased by modifying the geocentric core distance error,and the imaging quality of TDI CCD space camera has improved greatly.