Boresight Alignment Research Articles

Adaptive Reduced-Attitude Control for Spacecraft Boresight Alignment with Safety Constraints and Accuracy Requirements

Adaptive Reduced-Attitude Control for Spacecraft Boresight Alignment with Safety Constraints and Accuracy Requirements

Long-Periodic Analysis of Boresight Misalignment of Ziyuan3-01 Three-Line Camera

The Ziyuan3-01 (ZY3-01) satellite is China’s first civilian stereo surveying and mapping satellite to meet the 1:50,000 scale mapping requirements, and has been operated in orbit for 10 years. The boresight misalignment of the three-line camera (TLC) is an essential factor affecting the geolocation accuracy, which is a principal concern for stereo mapping satellites. However, the relative relationships of TLC are often regarded as fixed for the same ground scene in most traditional geometric calibrations, without considering the on-orbit long-periodic changes. In this paper, we propose a long-periodic method to analyze and estimate the boresight misalignments between three cameras, with the attitude estimation of a nadir (NAD) camera as the benchmark. Offsets and drifts of the three cameras were calculated and calibrated with different compensation models using scale invariant feature transform (SIFT) points as the ground control. Ten simultaneous NAD–Forward (FWD)–Backward (BWD) imagery of the ZY3-01 satellite acquired from 2012 to 2020 were selected to verify the long-periodic changes in TLC boresight misalignments. The results indicate that the boresight alignment angles of ZY3-01 TLC are dynamic during the long-periodic flight, but the structure of TLC is stable for the misalignments of both FWD and BWD within only 7 arc seconds, which can provide a positive reference for subsequent satellite design and long-periodic on-orbit geometric calibration.

Case Study: Rigorous Boresight Alignment of a Marine Mobile LiDAR System Addressing the Specific Demands of Port Infrastructure Monitoring

Accurate infrastructure monitoring of ports and harbors is a vital operation conducted by the port’s authority. To operate regularly in this highly dynamic environment, we explore the potential of the cutting-edge mobile LiDAR systems (MLS) mounted on a vessel. To generate a high-quality 3 D point cloud that would satisfy the expected accuracy required in the monitoring task, the LiDAR scanner and the positioning and orientation system (POS) must be angularly aligned also known as boresight alignment. In this research, we introduce a boresight alignment methodology adapted to the port infrastructure surveillance based on prefabricated planar targets. After an analysis of planar target simulated data, we propose a boresight alignment site design. Then, we apply this boresight alignment site design in a real-world scenario. Obtained results estimate accurately roll and yaw angles errors with standard deviations of less than 0.002 degrees and pitch angle error with standard deviation less than 0.015 degrees. Finally, we defined a validation site and described the procedure that uses these features to validate the quality of the estimated parameters. The relative comparison of the georeferenced point clouds, before and after boresight alignment demonstrates the mitigation of the boresight systematic error impact on the final point cloud.

The boresight alignment of the DArk Matter Particle Explorer

The DArk Matter Particle Explorer (DAMPE) can measure γ-rays in the energy range from a few GeV to about 10 TeV. The direction of each γ-ray photon is reconstructed in the DAMPE payload reference system. To convert this to celestial coordinates, we require the celestial orientation of the payload system, which, however, may slightly deviate from that of the satellite system provided by the star-tracker. In this paper, we adopt a maximum likelihood method and use the γ-rays centered around several bright point-like sources to measure and correct the angular deviations between the DAMPE payload and satellite system, the so-called “boresight alignment”. We also check our method of boresight alignment for some sets of simulation data with artificial orientations and obtain consistent results. The time-dependent boresight alignment analysis does not show evidence for significant variation of the parameters.

Reduced Attitude Control for Boresight Alignment With Dynamic Pointing Constraints

The problem of boresight alignment control is studied when multiple dynamic pointing constraints and angular velocity constraints exist during the maneuver. The tracking errors and dynamic pointing constraints are defined using the reduced attitude, which has only two degrees of freedom and avoids the singularity and redundancy description. An artificial potential function and a barrier Lyapunov function are employed to derive the feedback control law for attitude tracking scenario with no violation of the constraints and limitations. The stability analysis shows that the proposed control law guarantees the asymptotical convergence of the tracking errors. Numerical simulations are conducted to demonstrate the efficacy and performance of the presented control design under several different scenarios.

Improving Positioning Accuracy of the Mobile Laser Scanning in GPS-Denied Environments: An Experimental Case Study

The positioning accuracy of the mobile laser scanning (MLS) system can reach the level of centimeter under the conditions where GPS works normally. However, in GPS-denied environments this accuracy can be reduced to the decimeter or even the meter level because the observation mode errors and the boresight alignment errors of MLS cannot be calibrated or corrected by the GPS signal. To bridge this research gap, this paper proposes a novel technique that appropriately incorporates the robust weight total least squares (RWTLS) and the full information maximum likelihood optimal estimation (FIMLOE) to improve the positioning accuracy of the MLS system under GPS-denied environment. First of all, the coordinate transformation relationship and the observation parameters vector of MLS system are established. Secondly, the RWTLS algorithm is used to correct the 3D point observation model; then the uncertainty propagation parameter vector and the boresight alignment errors between the laser scanner frame and the IMU frame are calibrated by FIMLOE. Lastly, experimental investigation in indoor scenarios was performed to evaluate the effectiveness of the proposed method. The experimental results demonstrate that the proposed method is able to significantly improve the positioning accuracy of an MLS system in GPS-denied environments.

Modeling and reality analysis of beaconless spatial acquisition for intersatellite laser communication system

For an optical satellite link, the error sources that influence the pointing accuracy of the terminals include the attitude estimation error, ephemeris accuracy, and the boresight alignment error. It means an uncertainty cone (UC) exists for the spatial acquisition process. The acquisition process is a continuous adjustment for the visual axis of the two communication terminals, to enable them to detect the light signal from the other end, and finally realize the light closed loop. Beaconless acquisition algorithm and the corresponding acquisition process, modeling for spiral scan pattern and the simulation results of staring-scanning mode are presented. The UC scan range is 5 mrad, through the overlap with a spot divergence angle of 80 μrad, the total acquisition time is 98 s.

On the Design of Radar Corner Reflectors for Deformation Monitoring in Multi-Frequency InSAR

Trihedral corner reflectors are being increasingly used as point targets in deformation monitoring studies using interferometric synthetic aperture radar (InSAR) techniques. The frequency and size dependence of the corner reflector Radar Cross Section (RCS) means that no single design can perform equally in all the possible imaging modes and radar frequencies available on the currently orbiting Synthetic Aperture Radar (SAR) satellites. Therefore, either a corner reflector design tailored to a specific data type or a compromise design for multiple data types is required. In this paper, I outline the practical and theoretical considerations that need to be made when designing appropriate radar targets, with a focus on supporting multi-frequency SAR data. These considerations are tested by performing field experiments on targets of different size using SAR images from TerraSAR-X, COSMO-SkyMed and RADARSAT-2. Phase noise behaviour in SAR images can be estimated by measuring the Signal-to-Clutter ratio (SCR) in individual SAR images. The measured SCR of a point target is dependent on its RCS performance and the influence of clutter near to the deployed target. The SCR is used as a metric to estimate the expected InSAR displacement error incurred by the design of each target and to validate these observations against theoretical expectations. I find that triangular trihedral corner reflectors as small as 1 m in dimension can achieve a displacement error magnitude of a tenth of a millimetre or less in medium-resolution X-band data. Much larger corner reflectors (2.5 m or greater) are required to achieve the same displacement error magnitude in medium-resolution C-band data. Compromise designs should aim to satisfy the requirements of the lowest SAR frequency to be used, providing that these targets will not saturate the sensor of the highest frequency to be used. Finally, accurate boresight alignment of the corner reflector can be critical to the overall target performance. Alignment accuracies better than 4° in azimuth and elevation will incur a minimal impact on the displacement error in X and C-band data.

Investigation of boresight offsets and co-registration of HiRISE and CTX imagery for precision Mars topographic mapping

Images from two sensors, the High-Resolution Imaging Science Experiment (HiRISE) and the Context Camera (CTX), both on board the Mars Reconnaissance Orbiter, were used to generate Digital Elevation Models (DEMs) of the Martian surface. However, the DEMs generated from the images acquired by these two sensors show discrepancies for various reasons, such as variations in the boresight alignment between the two sensors while flying in the complex environment of space. This paper presents a systematic investigation of the discrepancies between the DEMs generated from the HiRISE and CTX images and further the boresight offsets between the HiRISE and CTX sensors. A bundle adjustment approach is presented for the co-registration of HiRISE and CTX images. The experimental analyses were carried out using eight sets of HiRISE and CTX images collected from different regions. The results indicate that the systematic offsets between the DEMs derived from HiRISE and CTX images reach several hundred meters in object space and dozens of pixels in image space along the north-east direction. After co-registration, the offsets were reduced to about 10m in object space and to sub-pixel level in image space. From the co-registration results, relatively consistent angular boresight offsets were found, which deviated from the default values. The findings are significant for the synergistic use of HiRISE and CTX images for precision Mars topographic mapping.

Fast Autonomous Three-Axis Constrained Attitude Pathfinding and Visualization for Boresight Alignment

A new algorithm suitable for fast onboard implementation is presented for designing attitude maneuver paths that achieve desired boresight alignment in the presence of multiple three-axis attitude constraints. It is based on a recently proposed algorithm in which possible attitudes are discretized in a three-dimensional distortion-minimizing projected space. In that space, grid points are enumerated and graph search pathfinding algorithms are employed to find the shortest constrained path between selected initial and target grid points. The new algorithm extends this approach from a single target point to a continuum of target points, representing all possible attitudes that maintain the desired boresight alignment. It augments the original algorithm with the transformation that makes it possible to assign a single value to estimated distances for the whole continuum of target points. This, in turn, allows the pathfinding algorithms to efficiently determine the shortest overall path to the whole continuum, which corresponds to the shortest attitude maneuver path that achieves the desired boresight alignment.

COMPARISON AND CO-REGISTRATION OF DEMS GENERATED FROM HiRISE AND CTX IMAGES

Images from two sensors, the High-Resolution Imaging Science Experiment (HiRISE) and the Context Camera (CTX), both on-board the Mars Reconnaissance Orbiter (MRO), were used to generate high-quality DEMs (Digital Elevation Models) of the Martian surface. However, there were discrepancies between the DEMs generated from the images acquired by these two sensors due to various reasons, such as variations in boresight alignment between the two sensors during the flight in the complex environment. This paper presents a systematic investigation of the discrepancies between the DEMs generated from the HiRISE and CTX images. A combined adjustment algorithm is presented for the co-registration of HiRISE and CTX DEMs. Experimental analysis was carried out using the HiRISE and CTX images collected at the Mars Rover landing site and several other typical regions. The results indicated that there were systematic offsets between the HiRISE and CTX DEMs in the longitude and latitude directions. However, the offset in the altitude was less obvious. After combined adjustment, the offsets were eliminated and the HiRISE and CTX DEMs were co-registered to each other. The presented research is of significance for the synergistic use of HiRISE and CTX images for precision Mars topographic mapping.

COMPARISON AND CO-REGISTRATION OF DEMS GENERATED FROM HiRISE AND CTX IMAGES

Abstract. Images from two sensors, the High-Resolution Imaging Science Experiment (HiRISE) and the Context Camera (CTX), both on-board the Mars Reconnaissance Orbiter (MRO), were used to generate high-quality DEMs (Digital Elevation Models) of the Martian surface. However, there were discrepancies between the DEMs generated from the images acquired by these two sensors due to various reasons, such as variations in boresight alignment between the two sensors during the flight in the complex environment. This paper presents a systematic investigation of the discrepancies between the DEMs generated from the HiRISE and CTX images. A combined adjustment algorithm is presented for the co-registration of HiRISE and CTX DEMs. Experimental analysis was carried out using the HiRISE and CTX images collected at the Mars Rover landing site and several other typical regions. The results indicated that there were systematic offsets between the HiRISE and CTX DEMs in the longitude and latitude directions. However, the offset in the altitude was less obvious. After combined adjustment, the offsets were eliminated and the HiRISE and CTX DEMs were co-registered to each other. The presented research is of significance for the synergistic use of HiRISE and CTX images for precision Mars topographic mapping.

Improving Sensor Fusion: A Parametric Method for the Geometric Coalignment of Airborne Hyperspectral and Lidar Data

Synergistic applications based on integrated hyperspectral and lidar data are receiving a growing interest from the remote-sensing community. A prerequisite for the optimum sensor fusion of hyperspectral and lidar data is an accurate geometric coalignment. The simple unadjusted integration of lidar elevation and hyperspectral reflectance causes a substantial loss of information and does not exploit the full potential of both sensors. This paper presents a novel approach for the geometric coalignment of hyperspectral and lidar airborne data, based on their respective adopted return intensity information. The complete approach incorporates ray tracing and subpixel procedures in order to overcome grid inherent discretization. It aims at the correction of extrinsic and intrinsic (camera resectioning) parameters of the hyperspectral sensor. In additional to a tie-point-based coregistration, we introduce a ray-tracing-based back projection of the lidar intensities for area-based cost aggregation. The approach consists of three processing steps. First is a coarse automatic tie-point-based boresight alignment. The second step coregisters the hyperspectral data to the lidar intensities. Third is a parametric coalignment refinement with an area-based cost aggregation. This hybrid approach of combining tie-point features and area-based cost aggregation methods for the parametric coregistration of hyperspectral intensity values to their corresponding lidar intensities results in a root-mean-square error of 1/3 pixel. It indicates that a highly integrated and stringent combination of different coalignment methods leads to an improvement of the multisensor coregistration.

Calibration of the Mercury Laser Altimeter on the MESSENGER Spacecraft

This paper gives a detailed description of the prelaunch and in-orbit calibrations of the Mercury Laser Altimeter (MLA) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission, which was launched on August 3, 2004 and has been operating in orbit about Mercury since March 2011. A brief summary of the MLA instrument is given, followed by the instrument measurement model and calibration formulas. The prelaunch tests used to determine the values of various calibration coefficients are described. The boresight alignment parameters were verified and recalibrated by special tests, with the MESSENGER spacecraft en route to Mercury. The MLA instrument model and the calibration methods were largely derived from airborne and spaceborne lidar for Earth science observation at the NASA Goddard Space Flight Center and will benefit future space lidar developments for Earth and space science.

Boresight alignment method for mobile laser scanning systems

Mobile laser scanning (MLS) is the latest approach towards fast and cost-efficient acquisition of 3-dimensional spatial data. Accurately evaluating the boresight alignment in MLS systems is an obvious necessity. However, recent systems available on the market may lack of suitable and efficient practical workflows on how to perform this calibration. This paper discusses an innovative method for accurately determining the boresight alignment of MLS systems by employing 3D laser scanners. Scanning objects using a 3D laser scanner operating in a 2D line-scan mode from various different runs and scan directions provides valuable scan data for determining the angular alignment between inertial measurement unit and laser scanner. Field data is presented demonstrating the final accuracy of the calibration and the high quality of the point cloud acquired during an MLS campaign.

Accuracy analysis for DSM and orthoimages derived from SPOT HRS stereo data using direct georeferencing

During the HRS (High Resolution Stereo) Scientific Assessment Program the French space agency CNES delivered data sets from the HRS camera system with high precision ancillary data. Two test data sets from this program were evaluated: one is located in Germany, the other in Spain. The first goal was to derive orthoimages and digital surface models (DSM) from the along track stereo data by applying the rigorous model with direct georeferencing and without ground control points (GCPs). For the derivation of DSM, the stereo processing software, developed at DLR for the MOMS-2P three line stereo camera was used. As a first step, the interior and exterior orientation of the camera, delivered as ancillary data from positioning and attitude systems were extracted. A dense image matching, using nearly all pixels as kernel centers provided the parallaxes. The quality of the stereo tie points was controlled by forward and backward matching of the two stereo partners using the local least squares matching method. Forward intersection lead to points in object space which are subsequently interpolated to a DSM in a regular grid. DEM filtering methods were also applied and evaluations carried out differentiating between accuracies in forest and other areas. Additionally, orthoimages were generated from the images of the two stereo looking directions. The orthoimage and DSM accuracy was determined by using GCPs and available reference DEMs of superior accuracy (DEM derived from laser data and/or classical airborne photogrammetry). As expected the results obtained without using GCPs showed a bias in the order of 5–20 m to the reference data for all three coordinates. By image matching it could be shown that the two independently derived orthoimages exhibit a very constant shift behavior. In a second step few GCPs (3–4) were used to calculate boresight alignment angles, introduced into the direct georeferencing process of each image independently. This method improved the absolute accuracy of the resulting orthoimages and DSM significantly.

A method for determining the average height of a large topographic ice sheet from observations of the echo received by a satellite altimeter

AbstractIce-sheet surfaces have scales of fluctuation that arc similar to the diameter of the area illuminated by a satellite radar altimeter. The present theory of altimeter, developed to describe scattering from the ocean surface, does not deal properly with the geometry of ice-sheet surfaces. In this paper, the theory of altimeter is extended to cover this geometry. A general relation for the altimeter echo from a surface of unknown geometry is developed, including the effects of the penetration of the surface by the radar waves. This expression is linearized, using the characteristic operating geometry of satellite altimeters and the gentle nature of ice-sheet gradients. From this expression, an integral equation is derived, from whose solution the spatial average of the height of the surface relative to a spherical datum can be determined. The integral equation is of a Volterra type, which permits the uniqueness of the solution for the average height to be investigated simply. The method is extended to provide a solution for the spatial average of the height of a local region of the ice sheet, provided the region remains large in comparison with the area illuminated by the altimeter, and to deal with variations in the antenna bore-sight alignment. The results have a number of implications for the collection and reduction of echoes in an experiment to determine the average height of an ice sheet. The unique determination of the average height requires the echo to be known over a time interval that depends on the extrema of the surface, which therefore must be known a priori. The average height itself can be determined by the operation on the echo of a linear operator whose kernel is derived from the solution of the Volterra-type equation. This marks a change from the procedures currently used in practice to reduce echoes from ice sheets.

A method for determining the average height of a large topographic ice sheet from observations of the echo received by a satellite altimeter

AbstractIce-sheet surfaces have scales of fluctuation that arc similar to the diameter of the area illuminated by a satellite radar altimeter. The present theory of altimeter, developed to describe scattering from the ocean surface, does not deal properly with the geometry of ice-sheet surfaces. In this paper, the theory of altimeter is extended to cover this geometry. A general relation for the altimeter echo from a surface of unknown geometry is developed, including the effects of the penetration of the surface by the radar waves. This expression is linearized, using the characteristic operating geometry of satellite altimeters and the gentle nature of ice-sheet gradients. From this expression, an integral equation is derived, from whose solution the spatial average of the height of the surface relative to a spherical datum can be determined. The integral equation is of a Volterra type, which permits the uniqueness of the solution for the average height to be investigated simply. The method is extended to provide a solution for the spatial average of the height of a local region of the ice sheet, provided the region remains large in comparison with the area illuminated by the altimeter, and to deal with variations in the antenna bore-sight alignment. The results have a number of implications for the collection and reduction of echoes in an experiment to determine the average height of an ice sheet. The unique determination of the average height requires the echo to be known over a time interval that depends on the extrema of the surface, which therefore must be known a priori. The average height itself can be determined by the operation on the echo of a linear operator whose kernel is derived from the solution of the Volterra-type equation. This marks a change from the procedures currently used in practice to reduce echoes from ice sheets.

Hole grating beam samplerâ€

The hole grating beam sampler consists of a uniform array of small circular holes in a highly polished mirror. When used as a turning flat in a high-energy laser (HEL) optical train, it produces a multiplicity of angularly separated low-power replicas of the HEL beam in both the transmitted and reflected directions. It thus provides the opportunity to perform many simultaneous diagnostic tests such as power monitoring, bore-sight alignment, jitter sensing, wavefront sensing, power-in-the-bucket measurements, and spectral purity measurements in real time during HEL operation on a noninterference basis with the intended mission. The principle of the hole grating beam sampler will be discussed in detail, and several relationships useful to the HEL systems engineer will be derived. The capabilities and limitations of this versatile HEL diagnostic tool will then be discussed in the context of several specific applications.

Applied lasers

The laser has been found useful for short-distance communication or for communication across the macrodimensions of space. Two short-distance voice-communication systems are described?one requiring careful boresight alignment and the other involving a megaphone-like broad-beam laser that needs little alignment. A laser range-measuring system has been developed that can make distance checks for aircraft refueling or for measuring the altitude of spacecraft landing on the moon. The principles of ranging can also be applied to selective viewing, maintaining a separation of vehicles moving along a set path, or selectively counting certain-sized objects. The laser also can be used in a simple intrusion alarm since its narrow beam can be reflected over a very long distance with little spreading.