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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Prospective intra-patient evaluation of a shoulder retraction device for radiotherapy in head and neck cancer

Irradiation of tumors in the larynx and pharynx is often technically challenging in patients with a short neck or high shoulders. Shoulder retraction devices can sometimes resolve this problem and allow irradiation via lateral beam directions. This study aimed to measure the proportion of patients who would benefit from such an approach and to quantify the magnitude of the benefit obtained. Twenty patients were studied. Simulator images were obtained before and after intervention. The additional exposure of the cervical spine was measured. Patient comfort and acceptability were assessed with a questionnaire. Improvement of exposure of the cervical spine was observed in 80% of patients. In 20%, there was either no difference or the position was worse. Shoulder retraction exposed a mean of 8.4-10.2 mm more of the cervical spine. Patients in general reported the device as comfortable. The use of a shoulder retraction device produced clinically significant improvements in exposure of the tissues of the cervical spine and neck and should be considered in patients being irradiated for tumors arising in the larynx or hypopharynx.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2013

PROBABILISTIC CATALOGS FOR CROWDED STELLAR FIELDS

We present and implement a probabilistic (Bayesian) method for producing catalogs from images of stellar fields. The method is capable of inferring the number of sources N in the image and can also handle the challenges introduced by noise, overlapping sources, and an unknown point-spread function. The luminosity function of the stars can also be inferred, even when the precise luminosity of each star is uncertain, via the use of a hierarchical Bayesian model. The computational feasibility of the method is demonstrated on two simulated images with different numbers of stars. We find that our method successfully recovers the input parameter values along with principled uncertainties even when the field is crowded. We also compare our results with those obtained from the SExtractor software. While the two approaches largely agree about the fluxes of the bright stars, the Bayesian approach provides more accurate inferences about the faint stars and the number of stars, particularly in the crowded case.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Improving and Assessing the Quality of Uncertainty Quantification in Deep Learning

Deep learning (DL) models have enjoyed increased attention in recent years because of their powerful predictive capabilities. While many successes have been achieved, standard deep learning methods suffer from a lack of uncertainty quantification (UQ). While the development of methods for producing UQ from DL models is an active area of current research, little attention has been given to the quality of the UQ produced by such methods. In order to deploy DL models to high-consequence applications, high-quality UQ is necessary. This report details the research and development conducted as part of a Laboratory Directed Research and Development (LDRD) project at Sandia National Laboratories. The focus of this project is to develop a framework of methods and metrics for the principled assessment of UQ quality in DL models. This report presents an overview of UQ quality assessment in traditional statistical modeling and describes why this approach is difficult to apply in DL contexts. An assessment on relatively simple simulated data is presented to demonstrate that UQ quality can differ greatly between DL models trained on the same data. A method for simulating image data that can then be used for UQ quality assessment is described. A general method for simulating realistic data for the purpose of assessing a model’s UQ quality is also presented. A Bayesian uncertainty framework for understanding uncertainty and existing metrics is described. Research that came out of collaborations with two university partners are discussed along with a software toolkit that is currently being developed to implement the UQ quality assessment framework as well as serve as a general guide to incorporating UQ into DL applications.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2016

Coupling sky images with radiative transfer models: a new method to estimate cloud optical depth

A method for retrieving cloud optical depth ( τ c ) using a UCSD developed ground-based sky imager (USI) is presented. The radiance red–blue ratio (RRBR) method is motivated from the analysis of simulated images of various τ c produced by a radiative transfer model (RTM). From these images the basic parameters affecting the radiance and red–blue ratio (RBR) of a pixel are identified as the solar zenith angle ( θ 0 ), τ c , solar pixel angle/scattering angle ( θ s ), and pixel zenith angle/view angle ( θ z ). The effects of these parameters are described and the functions for radiance, I λ τ c , θ 0 , θ s , θ z , and RBR τ c , θ 0 , θ s , θ z are retrieved from the RTM results. RBR, which is commonly used for cloud detection in sky images, provides non-unique solutions for τ c , where RBR increases with τ c up to about τ c = 1 (depending on other parameters) and then decreases. Therefore, the RRBR algorithm uses the measured I λ meas θ s , θ z , in addition to RBR meas θ s , θ z , to obtain a unique solution for τ c . The RRBR method is applied to images of liquid water clouds taken by a USI at the Oklahoma Atmospheric Radiation Measurement (ARM) program site over the course of 220 days and compared against measurements from a microwave radiometer (MWR) and output from the Min et al. (2003) method for overcast skies. τ c values ranged from 0 to 80 with values over 80, being capped and registered as 80. A τ c RMSE of 2.5 between the Min et al. (2003) method and the USI are observed. The MWR and USI have an RMSE of 2.2, which is well within the uncertainty of the MWR. In conclusion, the procedure developed here provides a foundation to test and develop other cloud detection algorithms.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2015

Adjuvant radiation therapy for bladder cancer: A dosimetric comparison of techniques

Trials of adjuvant radiation after cystectomy are under development. There are no studies comparing radiation techniques to inform trial design. This study assesses the effect on bowel and rectal dose of 3 different modalities treating 2 proposed alternative clinical target volumes (CTVs). Contours of the bowel, rectum, CTV-pelvic sidewall (common/internal/external iliac and obturator nodes), and CTV-comprehensive (CTV-pelvic sidewall plus cystectomy bed and presacral regions) were drawn on simulation images of 7 post-cystectomy patients. We optimized 3-dimensional conformal radiation (3-D), intensity-modulated radiation (IMRT), and single-field uniform dose (SFUD) scanning proton plans for each CTV. Mixed models regression was used to compare plans for bowel and rectal volumes exposed to 35% (V{sub 35%}), 65% (V{sub 65%}), and 95% (V{sub 95%}) of the prescribed dose. For any given treatment modality, treating the larger CTV-comprehensive volume compared with treating only the CTV-pelvic sidewall nodes significantly increased rectal dose (V{sub 35%} {sub rectum}, V{sub 65%} {sub rectum}, and V{sub 95%} {sub rectum}; p < 0.001 for all comparisons), but it did not produce significant differences in bowel dose (V{sub 95%} {sub bowel}, V{sub 65%} {sub bowel}, or V{sub 35%} {sub bowel}). The 3-D plans, compared with both the IMRT and the SFUD plans, had a significantly greater V{sub 65%} {sub bowel} and V{sub 95%} {sub bowel} for each proposed CTV (p < 0.001 for all comparisons). The effect of treatment modality on rectal dosimetry differed by CTV, but it generally favored the IMRT and the SFUD plans over the 3-D plans. Comparison of the IMRT plan vs the SFUD plan yielded mixed results with no consistent advantage for the SFUD plan over the IMRT plan. Targeting a CTV that spares the cystectomy bed and presacral region may marginally improve rectal toxicity but would not be expected to improve the bowel toxicity associated with any given modality of adjuvant radiation. Using the IMRT or the SFUD plans instead of the 3-D conformal plan may improve both bowel and rectal toxicity.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2025

Mitigating imaging systematics for DESI 2024 emission Line Galaxies and beyond

Emission Line Galaxies (ELGs) are one of the main tracers that the Dark Energy Spectroscopic Instrument (DESI) uses to probe the universe. However, they are afflicted by strong spurious correlations between target density and observing conditions known as imaging systematics. In this paper, we present the imaging systematics mitigation applied to the DESI Data Release 1 (DR1) large-scale structure catalogs used in the DESI 2024 cosmological analyses. We also explore extensions of the fiducial treatment. This includes a combined approach, through forward image simulations (Obiwan) in conjunction with neural network-based regression, to obtain an angular selection function that mitigates the imaging systematics observed in the DESI DR1 ELGs target density. We further derive a line of sight selection function from the forward model that removes the strong redshift dependence between imaging systematics and low redshift ELGs. Combining both angular and redshift-dependent systematics, we construct a three-dimensional selection function and assess the impact of all selection functions on clustering statistics. We quantify differences between these extended treatments and the fiducial treatment in terms of the measured 2-point statistics. We find that the results are generally consistent with the fiducial treatment and conclude that the differences are far less than the imaging systematics uncertainty included in DESI 2024 full-shape measurements. We extend our investigation to the ELGs at 0.6 < z < 0.8, i.e., beyond the redshift range (0.8 < z < 1.6) adopted for the DESI clustering catalog, and demonstrate that determining the full three-dimensional selection function is necessary in this redshift range. Our tests showed that all changes are consistent with statistical noise for BAO analyses indicating they are robust to even severe imaging systematics. Specific tests for the full-shape analysis will be presented in a companion paper.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Dislocations With Edge Components in Nanocrystalline bcc Mo

We report high-resolution transmission electron microscopy (HRTEM) observation of a high density of dislocations with edge components (approximately 1016 m-2) in nanocrystalline (NC) body-centered cubic (bcc) Mo prepared by high-pressure torsion. We also observed for the first time of the 1/2 <111> and <001> pure edge dislocations in NC Mo. Crystallographic analysis and image simulations reveal that the best way using HRTEM to study dislocations with edge components in bcc systems is to take images along <110> zone axis, from which it is possible to identify 1/2 <111> pure edge dislocations, and edge components of 1/2 <111> and <001> mixed dislocations. The <001> pure edge dislocations can only be identified from <100> zone axis. The high density of dislocations with edge components is believed to play a major role in the reduction of strain rate sensitivity in NC bcc metals and alloys.

99 GENERAL AND MISCELLANEOUS↗
NASA NTRS1987

A gridding approach to detect patterns of change in coastal wetlands from digital data

The conversion of land to water or water to land in the Grand Bayou wetlands area from 1972 to 1981 is evaluated on the basis of Landsat MSS band 2 and 4 images. A postclassification (land vs water) procedure is employed, and grids 1, 5, 10, 25, and 50 pixels square are superimposed on the classified data in an effort to reduce the effects of poor georegistration of the images (simulated by shifting one set of grids two pixels vertically and horizontally). The results are presented in tables, maps, and sample images and characterized in detail. Grids 5 or 10 pixels square are found to give percentage land/water change values and locations similar to those obtained in pixel-by-pixel analysis of nearly perfectly matched images.

Pelletier, R. E.↗
DOE OSTI2018

Initial Investigation of the Small Feature Detection Capability of the ORNL Fast Neutron Tomographic Imaging System

A variety of fast neutron imaging simulations and measurements were performed to investigate the spatial resolution and small feature detection capability of a fast-neutron tomographic imaging system developed at Oak Ridge National Laboratory. The system uses an associated particle DT (deuterium-tritium) neutron generator as the source and 25 mm × 25 mm × 100 mm plastic scintillators as the neutron detectors. Two image quality indicators were examined: 1) a variety of long steel pins embedded axially in the interior of a polyethylene cylinder and 2) a variety of short steels pins embedded radially in the surface of a shielded polyethylene cylinder. Measurements demonstrated detectability down to approximately 3 mm. Further data analysis may show improved detection performance and allow a more precise determination of the spatial resolution and small feature detection limit. The current DT generator has a neutron production spot size of about 3.5 mm, which appears to be the limiting factor in the current system configuration.

Wright, Michael C.↗
DOE OSTI2019

Simulated refraction-enhanced X-ray radiography of laser-driven shocks

Refraction-enhanced x-ray radiography (REXR) is used to infer shock-wave positions of more than one shock wave, launched by a multiple-picket pulse in a planar plastic foil. This includes locating shock waves before the shocks merge, during the early time and the main drive of the laser pulse that is not possible with the velocity interferometer system for any reflector. Simulations presented in this paper of REXR show that it is necessary to incorporate refraction and attenuation of x rays along with the appropriate opacity and refractive-index tables to interpret experimental images. Simulated REXR shows good agreement with an experiment done on the OMEGA laser facility to image a shock wave. REXR can be applied to design multiple-picket pulses with a better understanding of the shock locations. Finally, this will be beneficial to obtain the required adiabats for inertial confinement fusion implosions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

SU-G-JeP3-07: Real-Time Image Guided Radiation Therapy for Heterotopic Ossification in Patients After Hip Replacement

Purpose: To demonstrate the feasibility of using CBCT in a real-time image guided radiation therapy (IGRT) for single fraction heterotopic ossification (HO) in patients after hip replacement. In this real-time procedure, all steps, from simulation, imaging, planning to treatment delivery, are performed at the treatment unit in one appointment time slot. This work promotes real-time treatment to create a paradigm shift in the single fraction radiation therapy. Methods: An integrated real-time IGRT for HO was developed and tested for radiation treatment of heterotopic ossification for patient after hip replacement. After CBCT images are acquired at the linac, and sent to the treatment planning system, the physician determines the field and/or draws a block. Subsequently, a simple 2D AP/PA plan with prescription of 700 cGy is created on-the-fly for physician to review. Once the physician approves the plan, the patient is treated on the same simulation position. This real-time treatment requires the team of attending physician, physicist, therapists, and dosimetrist to work in harmony to achieve all the steps in a timely manner. Results: Ten patients have been treated with this real-time treatment, having the same beams arrangement treatment plan and prescription as our clinically regular CT-based 2D plans. The average time for these procedures are 52.9 ±10.7 minutes from the time patient entered the treatment room until s/he exited, and 37.7 ±8.6 minutes from starting CBCT until last beam delivered. Conclusion: The real-time IGRT for HO treatment has been tested and implemented to be a clinically accepted procedure. This one-time appointment greatly enhances the waiting time, especially when patients in high level of pain, and provides a convenient approach for the whole clinical staff. Other disease sites will be also tested with this new technology.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2026

Deep-field analytical calibration

The next generation of imaging surveys, including the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST), Euclid, and the Nancy Grace Roman Space Telescope, will provide unprecedented constraints on cosmology using weak gravitational lensing. To fully exploit this statistical power, shear measurement methods must achieve sub- per cent accuracy while mitigating systematic biases from noise, the point-spread function (PSF), blending, and shear-dependent detection. The analytical calibration framework (AnaCal) has demonstrated such accuracy but requires adding noise to images, reducing effective depth. We introduce Deep-Field Analytical Calibration (DEEP-FIELD AnaCal), an extension of AnaCal that uses deep-field images to compute shear responses while preserving the statistical power of wide-field data. We validate DEEP-FIELD AnaCal on isolated and blended galaxy image simulations with LSST-like conditions, finding it meets the stringent requirement of multiplicative bias $|m| < 3\times 10^{-3}$ at 99.7 per cent confidence. Compared to standard AnaCal applied to wide-field images, DEEP-FIELD AnaCal increases the effective galaxy number density from 17 to 30 arcmin$^{-2}$ for simulated 10-yr LSST data. With deep fields $10\times$ longer than the wide field, we find pixel noise variance in shear estimation is reduced by 30 per cent and overall uncertainty by $\sim 25~{{\ \rm per\ cent}}$. Finally, using the LSST Deep Drilling Fields strategy, we assess sample variance and find an equivalent calibration uncertainty of $\lesssim 0.3~{{\ \rm per\ cent}}$. These results demonstrate that DEEP-FIELD AnaCal offers a promising path to achieve the required shear calibration for upcoming weak lensing surveys.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1996

Searching for Faint Companions to Nearby Stars with the Hubble Space Telescope

A search for faint companions (FC's) to selected stars within 5 pc of the Sun using the Hubble Space Telescope's Planetary Camera (PC) has been initiated. To assess the PC's ability to detect FCs, we have constructed both model and laboratory-simulated images and compared them to actual PC images. We find that the PC's point-spread function (PSF) is 3-4 times brighter over the angular range 2-5 sec than the PSF expected for a perfect optical system. Azimuthal variations of the PC's PSF are 10-20 times larger than expected for a perfect PSF. These variations suggest that light is scattered nonuniformly from the surface of the detector. Because the anomalies in the PC's PSF cannot be precisely simulated, subtracting a reference PSF from the PC image is problematic. We have developed a computer algorithm that identifies local brightness anomalies within the PSF as potential FCs. We find that this search algorithm will successfully locate FCs anywhere within the circumstellar field provided that the average pixel signal from the FC is at least 10 sigma above the local background. This detection limit suggests that a comprehensive search for extrasolar Jovian planets with the PC is impractical. However, the PC is useful for detecting other types of substellar objects. With a stellar signal of 10(exp 9) e(-), for example, we may detect brown dwarfs as faint as M(sub I) = 16.7 separated by 1 sec from alpha Cen A.

Schroeder, Daniel J.↗
DOE OSTI2025

Quasiaperiodic grain boundary phases of Σ⁢5 tilt grain boundaries in refractory metals

We report ground-state structures and phase transitions in Σ⁢5⁢[001] tilt grain boundaries (GBs) in body-centered-cubic (bcc) refractory metals Nb, Ta, Mo, and W. Σ⁢5 tilt GBs have been extensively investigated over the past several decades, with their ground-state structure—composed of kite-shaped structural units—previously thought to be well understood. By performing a rigorous GB structure search that optimizes the number of atoms in the boundary core, we predict different quasiaperiodic “split kite” phases analogous to those previously found in GBs in face-centered-cubic metals. Furthermore, our results suggest that complex aperiodic phases of GBs appear to be a general phenomenon, as validated through density functional theory calculations. Moreover, the atoms in the split kite phase demonstrate distinct collective diffusion dynamics. Phase-contrast image simulations of split kites show better agreement with experimental observations, offering an alternative explanation for previous microscopy results and motivating future atomically resolved imaging of the GB structure.

Body-centered cubic↗
DOE OSTI2015

Using compressive sensing to recover images from PET scanners with partial detector rings

Purpose: Most positron emission tomography/computed tomography (PET/CT) scanners consist of tightly packed discrete detector rings to improve scanner efficiency. The authors’ aim was to use compressive sensing (CS) techniques in PET imaging to investigate the possibility of decreasing the number of detector elements per ring (introducing gaps) while maintaining image quality. Methods: A CS model based on a combination of gradient magnitude and wavelet domains (wavelet-TV) was developed to recover missing observations in PET data acquisition. The model was designed to minimize the total variation (TV) and L1-norm of wavelet coefficients while constrained by the partially observed data. The CS model also incorporated a Poisson noise term that modeled the observed noise while suppressing its contribution by penalizing the Poisson log likelihood function. Three experiments were performed to evaluate the proposed CS recovery algorithm: a simulation study, a phantom study, and six patient studies. The simulation dataset comprised six disks of various sizes in a uniform background with an activity concentration of 5:1. The simulated image was multiplied by the system matrix to obtain the corresponding sinogram and then Poisson noise was added. The resultant sinogram was masked to create the effect of partial detector removal and then the proposed CS algorithm was applied to recover the missing PET data. In addition, different levels of noise were simulated to assess the performance of the proposed algorithm. For the phantom study, an IEC phantom with six internal spheres each filled with F-18 at an activity-to-background ratio of 10:1 was used. The phantom was imaged twice on a RX PET/CT scanner: once with all detectors operational (baseline) and once with four detector blocks (11%) turned off at each of 0 °, 90 °, 180 °, and 270° (partially sampled). The partially acquired sinograms were then recovered using the proposed algorithm. For the third test, PET images from six patient studies were investigated using the same strategy of the phantom study. The recovered images using WTV and TV as well as the partially sampled images from all three experiments were then compared with the fully sampled images (the baseline). Comparisons were done by calculating the mean error (%bias), root mean square error (RMSE), contrast recovery (CR), and SNR of activity concentration in regions of interest drawn in the background as well as the disks, spheres, and lesions. Results: For the simulation study, the mean error, RMSE, and CR for the WTV (TV) recovered images were 0.26% (0.48%), 2.6% (2.9%), 97% (96%), respectively, when compared to baseline. For the partially sampled images, these results were 22.5%, 45.9%, and 64%, respectively. For the simulation study, the average SNR for the baseline was 41.7 while for WTV (TV), recovered image was 44.2 (44.0). The phantom study showed similar trends with 5.4% (18.2%), 15.6% (18.8%), and 78% (60%), respectively, for the WTV (TV) images and 33%, 34.3%, and 69% for the partially sampled images. For the phantom study, the average SNR for the baseline was 14.7 while for WTV (TV) recovered image was 13.7 (11.9). Finally, the average of these values for the six patient studies for the WTV-recovered, TV, and partially sampled images was 1%, 7.2%, 92% and 1.3%, 15.1%, 87%, and 27%, 25.8%, 45%, respectively. Conclusions: CS with WTV is capable of recovering PET images with good quantitative accuracy from partially sampled data. Such an approach can be used to potentially reduce the cost of scanners while maintaining good image quality.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2022

Image segmentation for analyzing galaxy-galaxy strong lensing systems

The goal of this Letter is to develop a machine learning model to analyze the main gravitational lens and detect dark substructure (subhalos) within simulated images of strongly lensed galaxies. Using the technique of image segmentation, we turn the task of identifying subhalos into a classification problem, where we label each pixel in an image as coming from the main lens, a subhalo within a binned mass range, or neither. Our network is only trained on images with a single smooth lens and either zero or one subhalo near the Einstein ring. On an independent test set with lenses with large ellipticities, quadrupole and octopole moments, and for source apparent magnitudes between 17–25, the area of the main lens is recovered accurately. On average, only 1.3% of the true area is missed and 1.2% of the true area is added to another part of the lens. In addition, subhalos as light as 10 8.5 M ⊙ can be detected if they lie in bright pixels along the Einstein ring. Furthermore, the model is able to generalize to new contexts it has not been trained on, such as locating multiple subhalos with varying masses or more than one large smooth lens.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Testing the no-hair theorem with event horizon telescope observations of Sagittarius A*

The advent of the Event Horizon Telescope (EHT), a millimeter-wave very long baseline interferometric array, has enabled spatially resolved studies of the subhorizon-scale structure for a handful of supermassive black holes. Among these, the supermassive black hole at the center of the Milky Way, Sagittarius A* (Sgr A*), presents the largest angular cross section. Thus far, these studies have focused on measurements of the black hole spin and the validation of low-luminosity accretion models. However, a critical input in the analysis of EHT data is the structure of the black hole spacetime, and thus these observations provide the novel opportunity to test the applicability of the Kerr metric to astrophysical black holes. Here we present the first simulated images of a radiatively inefficient accretion flow (RIAF) around Sgr A* employing a quasi-Kerr metric that contains an independent quadrupole moment in addition to the mass and spin that fully characterize a black hole in general relativity. We show that these images can be significantly different from the images of an RIAF around a Kerr black hole with the same spin and demonstrate the feasibility of testing the no-hair theorem by constraining the quadrupolar deviation from the Kerr metric with existing EHT data. Equally important, we find that the disk inclination and spin orientation angles are robust to the inclusion of additional parameters, providing confidence in previous estimations assuming the Kerr metric based on EHT observations. However, at present, the limits on potential modifications of the Kerr metric remain weak.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRSDate not supplied

Optical Time Transfer for Bistatic SAR Spacecraft

A spacecraft-to-spacecraft optical time-transfer simulation has been developed as a tool for informing NASA’s Surface Deformation and Change (SDC) mission architecture. The SDC mission will combine radar images from multiple spacecraft to improve understanding of the Earth’s sea-level and landscape changes. Spacecraft must be precisely synchronized in order to create sharp radar images. Simulation of multiple spacecraft time-synchronizing via laser communication can inform technology choices of a mission by providing a picosecond-precision level estimate of clock error. This timing and ranging simulation has been combined with a radar system performance analysis pipeline. The simulated timing errors are used to predict performance of bistatic SAR systems in the presence of oscillator noise and time synchronization in accuracy. This analysis includes both analytic approximation equations from existing literature, and a numerical radar simulation to extract key system performance parameters like phase error and signal-to-noise ratio (SNR)degradation. Precision time-transfer techniques facilitate the accurate synchronization of clocks between any combination of terminals. Most time-transfer technology for comparing two clocks at different terminals use radio frequencies (RF) to measure the time delay between the sending and receiving of signals. Laser technology offers the capability to transmit high data rates with systems that are of smaller size and lower power than comparable RF systems. The clocks on independent spacecraft will have some phase and frequency errors between them that result in clock drift. The two clock models that are included in this bi-directional MATLAB simulation are a cesium-based Chip-Scale Atomic Clock (CSAC) and a rubidium-based Miniature Atomic Clock (MAC). The CSAC has flown as hardware for small satellite missions such as the University of Florida’s CHOMPTT mission. A study of example orbits, including that of NASA NASA-ISRO Synthetic Aperture Radar Mission (NISAR) mission, and lasing rates demonstrate the impact of flight configuration parameters on the synchronization error between two spacecraft. The MATLAB timing simulation uses a Runge-Kutta 4th-order method to propagate spacecraft orbits and computes the light-travel time estimate between them. The simulation outputs the estimated range and estimated clock error based on a user-defined spacecraft cluster configuration. The radar simulation and analytic approximations are applied to evaluate a potential future NASA bistatic SAR constellation architecture. In the proposed architecture, satellites follow each other in the same orbit at 800 km altitude, with a 210 km baseline. We also baseline the CSAC as an ultra stable oscillator, and use NASA’s NISAR for baseline radar system parameters to compute a clock-system introduced phase error of 5.6 degrees without synchronization by frequent time transfer. We build on this base case with a sensitivity analysis of radar performance over a proposed range of constellation and radar system parameters. With this analysis pipeline, we comment on which radar parameters should or should not be changed to minimize synchronization requirements. This analysis technique could be extended or modified to evaluate the timing requirements of other geometries for other future multistatic SAR missions, or other interferometric satellite missions.

Surface Deformation and Change↗