Along-track Direction Research Articles

Automated Registration of Images from Multiple Bands of Resourcesat-2 Liss-4 camera

Abstract. Continuous and automated co-registration and geo-tagging of images from multiple bands of Liss-4 camera is one of the interesting challenges of Resourcesat-2 data processing. Three arrays of the Liss-4 camera are physically separated in the focal plane in alongtrack direction. Thus, same line on the ground will be imaged by extreme bands with a time interval of as much as 2.1 seconds. During this time, the satellite would have covered a distance of about 14 km on the ground and the earth would have rotated through an angle of 30”. A yaw steering is done to compensate the earth rotation effects, thus ensuring a first level registration between the bands. But this will not do a perfect co-registration because of the attitude fluctuations, satellite movement, terrain topography, PSM steering and small variations in the angular placement of the CCD lines (from the pre-launch values) in the focal plane. This paper describes an algorithm based on the viewing geometry of the satellite to do an automatic band to band registration of Liss-4 MX image of Resourcesat-2 in Level 1A. The algorithm is using the principles of photogrammetric collinearity equations. The model employs an orbit trajectory and attitude fitting with polynomials. Then, a direct geo-referencing with a global DEM with which every pixel in the middle band is mapped to a particular position on the surface of the earth with the given attitude. Attitude is estimated by interpolating measurement data obtained from star sensors and gyros, which are sampled at low frequency. When the sampling rate of attitude information is low compared to the frequency of jitter or micro-vibration, images processed by geometric correction suffer from distortion. Therefore, a set of conjugate points are identified between the bands to perform a relative attitude error estimation and correction which will ensure the internal accuracy and co-registration of bands. Accurate calculation of the exterior orientation parameters with GCPs is not required. Instead, the relative line of sight vector of each detector in different bands in relation to the payload is addressed. With this method a band to band registration accuracy of better than 0.3 pixels could be achieved even in high hill areas.

Robust stationkeeping and reconfiguration of underactuated spacecraft formations

Feasibility of achieving reliable control for spacecraft formation keeping and reconfiguration without the need for thrust in either the radial or along-track direction is explored in this paper. Analysis of the linearized dynamics without along-track input indicates the presence of an uncontrollable eigenvalue at the origin. A nonlinear controller is designed to indirectly stabilize the uncontrollable modes to a stable manifold around the equilibrium. Conditions for robustness against unmatched uncertainties and disturbances are derived to establish the regions of asymptotic stabilization. The benefits of the proposed control method are also validated via numerical simulations to show that precise formation maintenance can be achieved by dealing with the issues of system nonlinearities, variations in initial conditions, and external disturbances, concurrently.

Physical analysis on improving the recovery accuracy of the Earth's gravity field by a combination of satellite observations in along-track and cross-track directions

The physical investigations on the accuracy improvement to the measurement of the Earth's gravity field recovery are carried out based on the next-generation Pendulum-A/B out-of-plane twin-satellite formation in this paper. Firstly, the Earth's gravity field complete up to degree and order 100 is, respectively, recovered by the collinear and pendulum satellite formations using the orbital parameters of the satellite and the matching accuracies of key payloads from the twin GRACE satellites. The research results show that the accuracy of the Earth's gravity field model from the Pendulum-A/B satellite formation is about two times higher than from the collinear satellite formation, and the further improvement of the determination accuracy of the Earth's gravity field model is feasible by the next-generation Pendulum-A/B out-of-plane twin-satellite formation. Secondly, the Earth's gravity field from Pendulum-A/B complete up to degree and order 100 is accurately recovered based on the orbital parameters of the satellite (e.g., an orbital altitude of 400 km, an intersatellite range of 100 km, an orbital inclination of 89° and an orbital eccentricity of 0.001), the matching accuracies of space-borne instruments (e.g. 10−6 m in the intersatellite range, 10−3 m in the orbital position, 10−6 m/s in orbital velocity, and 10−11 m/s2 in non-conservative force), an observation time of 30 days and a sampling interval of 10 s. The measurement accuracy of the Earth's gravity field from the next-generation Pendulum-A/B out-of-plane twin-satellite formation is full of promise for being improved by about 10 times compared with that from the current GRACE satellite formation. Finally, the physical requirements for the next-generation Pendulum-A/B out-of-plane twin-satellite formation are analyzed, and it is proposed that the satellite orbital altitude be preferably designed to be close to 400±50 km and the matching precision of key sensors from the Pendulum-A/B mission be about one order of magnitude higher than from the GRACE program.

On-Orbit Characterization of MODIS Modulation Transfer Function Using the Moon

The high-contrast edge of the Moon has been used for the on-orbit measurement of the modulation transfer function (MTF) of remote sensing instruments with a lunar observation capability. With the lunar edge as a target, the classical edge method is applied to the National Aeronautics and Space Administration's MODerate resolution Imaging Spectroradiometer (MODIS) on board the Terra and Aqua satellites. One of the major difficulties encountered during the calculation is that the spatial resolution of MODIS is too coarse to capture the fine structure of the edge spread function (ESF), which is required to calculate the MTF. To produce the MODIS ESF in high resolution, lunar images of a selected edge acquired by multiple instrument scans need to be superposed by aligning the edge positions accurately. In this paper, an algorithm is developed to perform the alignment, based on the lunar position data generated by the MODIS geolocation algorithm and recorded scan by scan. The positions of the lunar edges at the focal plane are calculated scan by scan, allowing the construction of a high-quality ESF for MTF derivation. The algorithm is applied to all MODIS bands with 250-m, 500-m, and 1-km spatial resolutions in both along-scan and along-track directions. The along-track MTF results are particularly valuable because the onboard SpectroRadiometric Calibration Assembly can only monitor the along-scan MTF. The trending results show that the along-track MTF of MODIS has been stable throughout the MODIS lifetime and is well above the design specification for all bands. The limitation of the algorithm is analyzed. The algorithm developed in this paper can be applied to other instruments with similar design features.

Assessment of the spectral stability of Libya 4, Libya 1, and Mauritania 2 sites using Earth Observing One Hyperion

The objective of this paper is to formulate a methodology to assess the spectral sta- bility of the Libya 4, Libya 1, and Mauritania 2 pseudo-invariant calibration sites (PICS) using Earth Observing One (EO-1) Hyperion sensor. All the available Hyperion collections, down- loaded from the Earth Explorer website, were utilized for the three PICS. In each site, a reference spectrum is selected at a specific day in the vicinity of the region of interest (ROI) defined by Committee on Earth Observation Satellites (CEOS). A series of ROIs are predefined in the along-track direction with 196 spectral top-of-atmosphere reflectance values in each ROI. Based on the reference ROI spectrum, the spectral stability of these ROIs is evaluated by average deviations (ADs) and spectral angle mapper (SAM) methods in the specific ranges of time and geo-spatial locations. Time and ROI location-dependent SAM and AD results are very stable within � 2 deg and � 1.7% of 1σ standard deviations. Consequently, the Libya 4, Mauritania 2, and Libya 1 CEOS selected PICS are spectrally stable targets within the time and spatial swath ranges of the Hyperion collections. © 2014 Society of Photo-Optical Instrumentation Engineers (SPIE) (DOI: 10.1117/1.JRS.8.083618)

VIIRS On-Orbit Spatial Characterization Using the Moon

The VIIRS instrument onboard the Suomi-NPP satellite was launched in October 2011. The design and operation of its on-orbit calibration system are strongly based on MODIS heritage. However, VIIRS has no onboard calibrator similar to the spectro-radiometric calibration assembly (SRCA) on MODIS to perform the sensor on-orbit spatial characterization of band-to-band registration (BBR) and modulation transfer function (MTF). The Moon has been known as a spectrally, radiometrically, and geometrically stable source that can be used for sensor on-orbit calibration and characterization. In this letter, the algorithms developed and validated for MODIS spatial characterization using the Moon are briefly summarized and extended to VIIRS. The BBR in both along-scan and along-track directions and the MTF in the along-track direction are calculated with the scheduled VIIRS lunar observations and presented. These early results confirm that the VIIRS spatial characterization parameters have been stable since launch and are within the performance specification. The along-scan MTF cannot be accurately determined using the same algorithm and is not included in this study because the movement of the Moon in this direction is too limited to construct an edge spread function for MTF derivation. As part of the VIIRS on-orbit calibration and validation effort, the BBR and MTF parameters will be continuously monitored and evaluated throughout its mission lifetime.

Three-Dimensional Imaging Algorithm for Forward-Moving ROSAR

In this letter, we present the mode of forward-moving rotor synthetic aperture radar (ROSAR) to achieve 3-D imaging for the front area of the low-altitude aircraft. First, the geometric model is given, and the signal property is analyzed. Based on which, spatial offsets and coupling in the azimuth and along-track directions are corrected in the frequency domain, and then, the forward-moving ROSAR is simplified as a “stop-and-go” mode in the along-track direction. After 2-D imaging in the range-azimuth direction, a 3-D image can be obtained by range-along-track focusing with the improved range-Doppler algorithm. Moreover, azimuth depth of focusing is also given to maintain the imaging quality. Finally, simulation results prove the feasibility and effectiveness of the proposed method for this new imaging mode.

Evaluating sub-pixel offset techniques as an alternative to D-InSAR for monitoring episodic landslide movements in vegetated terrain

Spaceborne Synthetic Aperture Radar (SAR) sensors obtain regular and frequent radar images from which ground motion can be precisely detected using a variety of different techniques. The ability to measure slope displacements remotely over large regions can have many uses, although the limitations of the most commonplace technique, differential InSAR (D-InSAR), must be considered prior to interpreting the final results. One such limitation is the assumption that different rates of movement over a given distance cannot exceed a threshold value, dependent upon the pixel spacing of the SAR images and the radar wavelength. Characteristic features of landslides (i.e. the sharp boundary between stable/active ground and the range of temporally-variable velocities) can exhibit high spatial displacement gradients, breaking a fundamental assumption for reliable D-InSAR analysis. Areas of low coherence are also known to hinder the exploitation of InSAR data. This study assesses the capability of TerraSAR-X Spotlight, TerraSAR-X Stripmap and Envisat Stripmap images for monitoring the slow-moving Shuping landslide in the densely vegetated Three Gorges region, China. In this case study, the episodic nature of movement is shown to exceed the measurable limit for regular D-InSAR analysis even for the highest resolution 11-day TSX Spotlight interferograms. A Sub-Pixel Offset Time-series technique applied to corner reflectors (SPOT-CR) using only the SAR amplitude information is applied as a robust method of resolving time-varying displacements, with verifiable offset measurements presented from TSX Spotlight and TSX Stripmap imagery. Care should be exercised when measuring potentially episodic landslide movements in densely vegetated areas such as the Three Gorges region and corner reflectors are shown to be highly useful for SPOT techniques even when the assumptions for valid D-InSAR analysis are broken. Finally the capability to derive two-dimensional movements from sub-pixel offsets (in range and along-track directions) can be used to derive estimates of the vertical and northwards movements to help infer the landslide failure mechanism.

Tomographic Profiling—A Technique for Multi-Incidence-Angle Retrieval of the Vertical SAR Backscattering Profiles of Biogeophysical Targets

Tomographic profiling (TP) is a new imaging technique designed to provide vertical backscatter profiles through biophysical and geophysical target volumes, such as snow, ice, vegetation, and forest canopies. Data is collected as for normal synthetic aperture radar (SAR) imaging, but with the antennas aligned along the scan or along-track direction. The real antenna provides a wide beam in the along-track direction, which is sharpened by the addition of elemental measurements across a subaperture using a SAR-like processing scheme. A novelty of the scheme is the ability to produce an image transect in which the incidence angle is constant at every point. This is accomplished by incrementally sliding the subaperture across the full aperture, and utilizing the appropriate subaperture to provide the necessary viewing geometry at each pixel. This is in contrast to the SAR case, in which the angle of incidence varies across a scene. By suitable phasing between the subaperture elements, the synthesized beam can be steered in angle within the wide angular extent of the real beam, allowing post-measurement retrieval of the backscattering properties of the scene over a continuous range of incidence angles from a single scan. In the across-track direction, a narrow real beam is required to maintain good vertical resolution and limit the size of the horizontal footprint. Example TP experimental fieldwork results are provided for a 42-cm-deep snowpack, collected with a ground-based SAR system. Although the scheme was developed for ground-based applications, its application to the airborne and satellite cases is also discussed.

Ground-based C-band tomographic profiling of a conifer forest stand

We provide a demonstration of the new tomographic profiling (TP) technique, here applied to forestry for the first time. The portable ground-based synthetic aperture radar (GB-SAR) system was used to capture profiles of the vertical polarimetric backscattering patterns through a ∼7 m tall stand of Norway spruce trees. The TP scheme collects data as for normal SAR imaging, but with the antennae aligned in the along-track direction. Adaptive post-processing meant that each TP scan simultaneously captured along-track image transects over the incidence angle range 0°–60°. An important feature of the derived image products is that incidence angle is constant at every point within an image. The measured HH–VV height backscatter profiles were very similar, whilst the cross-/co-polarization ratio showed very little variation with height through the stand. Backscattering profiles showed closest agreement with the branch biomass distribution through the canopy, rather than with trunk or branch + trunk biomasses. Equivalent interferometric tree heights were estimated from the centre of mass of the backscatter-height distribution, which displayed increasing height with increasing incidence angle. There was no significant vertical separation between the cross- and co-polarization returns.

Comparison about Stitching Algorithms for Sub-Images of Multiple TDI CCD Chips Based on Image Matching

Due to the limited pixel numbers in single TDI CCD chips, some TDI CCD chips are combined in the focal plane to reach a sufficient swath width. Basic imagery is delivered as one image data file per strip and how to convert those sub-images to a larger, accuracy equivalent image is an essential problem in satellite images’ processing. Some key factors that affect the imaging quality of TDI CCD images are discussed firstly. It shows that discrepancies in the along-track direction which caused by different viewing angle and topographic relief are the main problem to improve the stitching accuracy. Three basic image-oriented stitching strategies are discussed. Experimental results show that the RMS of shift transformation, single affine transformation and piecewise affine transformation is 1.4 pixels, 1.3 pixels and 0.2 pixels. Piecewise affine transformation fully meets the requirements of further processing.

Study on Multiple-Input Multiple-Output Synthetic Aperture Sonar Composite Acoustic Imaging Technique

The azimuth resolution of imaging sonar lies on the aperture of the sensor array. In the paper, the Multiple-input Multiple-output Synthetic Aperture Sonar (MIMO-SAS) composite acoustic imaging technique was applied to imaging sonar equipped on the movable platform. The azimuth resolutions of both cross-track direction and along-track direction were obtained. The simulation results demonstrate that comparing with the conventional imaging technique Single-Input Multiple-output (SIMO), the azimuth resolution of cross-track direction is improved since the virtual array aperture of MIMO technique, and the azimuth resolution of along-track direction is obtained since the synthetic aperture of SAS technique. The simulation results are consistent with the theoretic value of the azimuth resolution.

TANDEM-X MISSION: OVERVIEW, STATUS AND OUTLOOK

Abstract. TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) opens a new era in spaceborne radar remote sensing [1]–[3], [9], [12]. Realized in the framework of a public-private partnership between the German Aerospace Center (DLR) and EADS Astrium GmbH, a single-pass SAR interferometer with adjustable baselines in across- and in along-track directions was formed by adding a second (TDX), almost identical spacecraft to TerraSAR-X (TSX) and flying the two satellites in a closely controlled formation on average orbit height of 514 km. This allows the acquisition of highly accurate cross-track and along-track interferograms without the inherent accuracy limitations imposed by repeat-pass interferometry due to temporal decorrelation and atmospheric disturbances. With typical across-track baselines of 150 to 500 m a global Digital Elevation Model (DEM) with 2 m relative height accuracy at a 12 m posting is being generated.

BBM/EM design of the thermal hyperspectral imager: An instrument for remote sensing of earth's surface, atmosphere and ocean, from a microsatellite platform

The Thermal Hyperspectral Imager (THI) is a low cost, low mass, power efficient instrument designed to acquire hyperspectral remote sensing data in the long-wave infrared. The instrument has been designed to satisfy mass, volume, and power constraints necessary to allow for its accommodation in a 95kg micro-satellite bus, designed by staff and students at the University of Hawai'i. THI acquires approximately 30 separate spectral bands in the 8–14μm wavelength region, at 16 wavenumber resolution. Rather than using filtering or dispersion to generate the spectral information, THI uses an interferometric technique. Light from the scene is focused onto an uncooled microbolometer detector array through a stationary interferometer, causing the light incident at each detector at any instant in time to be phase shifted by an optical path difference which varies linearly across the array in the along-track dimension. As platform motion translates the detector array in the along-track direction at a rate of approximately one pixel per frame (the camera acquires data at 30Hz) the radiance from each scene element can be sampled at each OPD, thus generating an interferogram. Spectral radiance as a function of wavelength is subsequently obtained for each scene element using standard Fourier transform techniques. Housed in a pressure vessel to shield COTS parts from the space environment, the total instrument has a mass of 15kg. Peak power consumption, largely associated with the calibration procedure, is <90W. From a nominal altitude of 550km the resulting data would have a spatial resolution of approximately 300m. Although an individual imaging event yields approximately 1Gbit of raw uncompressed data, onboard processing (to convert the interferograms into a conventional spectral hypercube) can reduce this to tens of Mega bits per scene. In this presentation we will describe (a) the rationale for the project, (b) the instrument design, and (c) how the data are processed. Finally we will present data acquired by THI on a laboratory microscope stage to demonstrate the spectro-radiometric quality of the data that the instrument can provide.

Orbit Boosting Maneuvers for Two-Craft Coulomb Formations

This paper investigates the effectiveness of using active electrostatic charging to perform orbit altitude adjustments of a nominally circular orbit. Coulomb forces are employed to gently pull a charged object, such as space debris or a satellite being repositioned, in the along-track direction. In contrast to prior work, this study uses an enhanced position-dependent electrostatic force model. The charge at a fixed absolute potential is a function of separation distance, size, and charge of the neighboring object. The pulling configuration at a given voltage is shown to provide larger electrostatic forces over a pushing configuration. Further, the pulling configuration provides relative dynamics that are easier to stabilize. Variational equations are employed to estimate the resulting semimajor axis changes. Numerical sweeps are performed illustrating that kilovolt levels of potential are sufficient to achieve kilometer-level radius changes per orbit for geosynchronous orbit regimes. A key new insight is that, with a linear mass to effective radius scaling of the tugged object, a critical mass always exists beyond which the increased capacitance dominates over the mass increase. As a result, using the electrostatic tractor on a multiton geosynchronous orbit object can become easier if the object is larger than this critical mass.

Measurements of In-Flight Cross-Track Antenna Patterns of Radar Depth Sounder/Imager

Antenna arrays with low sidelobes in the cross-track direction are needed for sounding and imaging ice-sheets margins including outlet glaciers. Weak radar signals from the ice-bed interface are often masked by off-vertical surface clutter from extremely rough crevassed surfaces in ice-sheet margins. Synthetic aperture radar (SAR) processing can be used to synthesize a large array for reducing clutter in the along-track direction. Low-side-lobe transmit- and receive-antenna patterns must be generated from a limited size array in the cross-track direction. Airborne antenna pattern measurements are critical to verifying pattern characteristics in the presence of a non-ideal ground plane and neighboring aircraft structures, as well as in-flight operational dynamics. In this paper, we describe a set of airborne measurements performed to determine and optimize antenna patterns for the very high frequency (VHF) array used to sound and image polar ice sheets. We measured antenna patterns by flying over a relatively smooth ice surface at an altitude of about 2700 m. The pattern data were obtained by processing the surface echoes with aircraft rolled from left to right over more than five cycles. We also simulated antenna patterns using a three-dimensional computer model of the entire airborne platform and compared with experimental results. The discrepancies between the measured and simulated results are less than 2.7 dB for 85% of the data samples. The measured pattern data will be used to optimize our array processing algorithms.

Improved GRACE kinematic orbit determination using GPS receiver clock modeling

Kinematic positions of Low Earth Orbiters based on GPS tracking are frequently used as pseudo-observations for single satellite gravity field determination. Unfortunately, the accuracy of the satellite trajectory is partly limited because the receiver synchronization error has to be estimated along with the kinematic coordinates at every observation epoch. We review the requirements for GPS receiver clock modeling in Precise Point Positioning (PPP) and analyze its impact on kinematic orbit determination for the two satellites of the Gravity Recovery and Climate Experiment (GRACE) mission using both simulated and real data. We demonstrate that a piecewise linear parameterization can be used to model the ultra-stable oscillators that drive the GPS receivers on board of the GRACE satellites. Using such a continuous clock model allows position estimation even if the number of usable GPS satellites drops to three and improves the robustness of the solution with respect to outliers. Furthermore, simulations indicate a potential accuracy improvement of the satellite trajectory of at least 40 % in the radial direction and up to 7 % in the along-track and cross-track directions when a 60-s piecewise linear clock model is estimated instead of epoch-wise independent receiver clock offsets. For PPP with real GRACE data, the accuracy evaluation is hampered by the lack of a reference orbit of significantly higher accuracy. However, comparisons with a smooth reduced-dynamic orbit indicate a significant reduction of the high-frequency noise in the radial component of the kinematic orbit.

Precise Orbit Determination of GRACE-A Satellite with Kinematic GPS PPP

Precise Point Positioning (PPP) has been widely used in navigation and orbit determination applications as we can obtain precise Global Positioning System (GPS) satellite orbit and clock products. Kinematic PPP, which is based on the GPS measurements only from the spaceborne GPS receiver, has some advantages for a simple precise orbit determination (POD). In this study, we developed kinematic PPP technique to estimate the orbits of GRACE-A satellite. The comparison of the mean position between the JPL's orbit product and our results showed the orbit differences 0.18 cm, 0.54 cm, and 0.98 cm in the Radial, in Along-track, and Cross-track direction respectively. In addition, we obtained the root mean square (rms) values of 4.06 cm, 3.90 cm, and 3.23 cm in the satellite coordinate components relative to the known coordinates.

Accuracy of Two-Line-Element Data forGeostationary and High-Eccentricity Orbits

Accuracy of Two-Line-Element Data forGeostationary and High-Eccentricity Orbits

Collaborative Vision-Integrated Pseudorange Error Removal: Team-Estimated Differential GNSS Corrections with no Stationary Reference Receiver

This paper presents an approach for generating Global Navigation Satellite System (GNSS) differential corrections by distributing GNSS and georeferenced vision measurements through a vehicle-to-vehicle (V2V) communications network. Conventionally, high-quality differential GNSS corrections are generated from a stationary reference receiver in close proximity to a set of mobile users. The proposed method, which is called Collaborative Vision Integrated Pseudorange Error Removal (C-VIPER), instead generates differential corrections using data from moving vehicles, thus eliminating the need for an infrastructure of stationary receivers. An important feature of the proposed algorithm is that individual differential corrections are computed for each satellite, so that corrections can be shared among users with different satellites in view. As demonstrated in simulation, measurement sharing significantly improves positioning accuracy in both the cross-track direction, where the quality of visual lane-boundary measurements is high, and the along-track direction, where the quality is low. Furthermore, because measurements are shared among many vehicles, the networked solution is robust to vision-sensor dropouts that may occur for individual vehicles.