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NASA NTRS1992

Method of separation of celestial gamma-ray bursts from solar flares

We recently discovered 217 'new' celestial gamma-ray burst candidates from the 'new' burst search of the PVO real time data base. The burst search covered the time period from September 1978 to July 1988. Sixty were confirmed by at least one other spacecraft, e.g., ISEE-3, V-II, V-12, etc. None triggered the PVO high time resolution memory. In this paper we describe a new algorithm based on the relationship between time width T(w) and hardness ratio HR, to distinguish cosmic gamma-ray bursts from solar flares without knowing the directions of the events. The algorithm was tested with 83 triggered and 60 nontriggered confirmed gamma-ray bursts and 30 confirmed solar flares from PVO.

Chuang, K. W.↗

NASA NTRS2010

Operational Dynamic Configuration Analysis

Sectors may combine or split within areas of specialization in response to changing traffic patterns. This method of managing capacity and controller workload could be made more flexible by dynamically modifying sector boundaries. Much work has been done on methods for dynamically creating new sector boundaries [1-5]. Many assessments of dynamic configuration methods assume the current day baseline configuration remains fixed [6-7]. A challenging question is how to select a dynamic configuration baseline to assess potential benefits of proposed dynamic configuration concepts. Bloem used operational sector reconfigurations as a baseline [8]. The main difficulty is that operational reconfiguration data is noisy. Reconfigurations often occur frequently to accommodate staff training or breaks, or to complete a more complicated reconfiguration through a rapid sequence of simpler reconfigurations. Gupta quantified a few aspects of airspace boundary changes from this data [9]. Most of these metrics are unique to sector combining operations and not applicable to more flexible dynamic configuration concepts. To better understand what sort of reconfigurations are acceptable or beneficial, more configuration change metrics should be developed and their distribution in current practice should be computed. This paper proposes a method to select a simple sequence of configurations among operational configurations to serve as a dynamic configuration baseline for future dynamic configuration concept assessments. New configuration change metrics are applied to the operational data to establish current day thresholds for these metrics. These thresholds are then corroborated, refined, or dismissed based on airspace practitioner feedback. The dynamic configuration baseline selection method uses a k-means clustering algorithm to select the sequence of configurations and trigger times from a given day of operational sector combination data. The clustering algorithm selects a simplified schedule containing k configurations based on stability score of the sector combinations among the raw operational configurations. In addition, the number of the selected configurations is determined based on balance between accuracy and assessment complexity.

Lai, Chok Fung↗

NASA NTRS2010

Advances in Landslide Hazard Forecasting: Evaluation of Global and Regional Modeling Approach

A prototype global satellite-based landslide hazard algorithm has been developed to identify areas that exhibit a high potential for landslide activity by combining a calculation of landslide susceptibility with satellite-derived rainfall estimates. A recent evaluation of this algorithm framework found that while this tool represents an important first step in larger-scale landslide forecasting efforts, it requires several modifications before it can be fully realized as an operational tool. The evaluation finds that the landslide forecasting may be more feasible at a regional scale. This study draws upon a prior work's recommendations to develop a new approach for considering landslide susceptibility and forecasting at the regional scale. This case study uses a database of landslides triggered by Hurricane Mitch in 1998 over four countries in Central America: Guatemala, Honduras, EI Salvador and Nicaragua. A regional susceptibility map is calculated from satellite and surface datasets using a statistical methodology. The susceptibility map is tested with a regional rainfall intensity-duration triggering relationship and results are compared to global algorithm framework for the Hurricane Mitch event. The statistical results suggest that this regional investigation provides one plausible way to approach some of the data and resolution issues identified in the global assessment, providing more realistic landslide forecasts for this case study. Evaluation of landslide hazards for this extreme event helps to identify several potential improvements of the algorithm framework, but also highlights several remaining challenges for the algorithm assessment, transferability and performance accuracy. Evaluation challenges include representation errors from comparing susceptibility maps of different spatial resolutions, biases in event-based landslide inventory data, and limited nonlandslide event data for more comprehensive evaluation. Additional factors that may improve algorithm performance accuracy include incorporating additional triggering factors such as tectonic activity, anthropogenic impacts and soil moisture into the algorithm calculation. Despite these limitations, the methodology presented in this regional evaluation is both straightforward to calculate and easy to interpret, making results transferable between regions and allowing findings to be placed within an inter-comparison framework. The regional algorithm scenario represents an important step in advancing regional and global-scale landslide hazard assessment and forecasting.

Kirschbaum, Dalia B.↗

NASA NTRS1997

Plural-wavelength flame detector that discriminates between direct and reflected radiation

A flame detector employs a plurality of wavelength selective radiation detectors and a digital signal processor programmed to analyze each of the detector signals, and determine whether radiation is received directly from a small flame source that warrants generation of an alarm. The processor's algorithm employs a normalized cross-correlation analysis of the detector signals to discriminate between radiation received directly from a flame and radiation received from a reflection of a flame to insure that reflections will not trigger an alarm. In addition, the algorithm employs a Fast Fourier Transform (FFT) frequency spectrum analysis of one of the detector signals to discriminate between flames of different sizes. In a specific application, the detector incorporates two infrared (IR) detectors and one ultraviolet (UV) detector for discriminating between a directly sensed small hydrogen flame, and reflections from a large hydrogen flame. The signals generated by each of the detectors are sampled and digitized for analysis by the digital signal processor, preferably 250 times a second. A sliding time window of approximately 30 seconds of detector data is created using FIFO memories.

Hall, Gregory H.↗

NASA NTRS2011

Advances in Landslide Nowcasting: Evaluation of a Global and Regional Modeling Approach

The increasing availability of remotely sensed data offers a new opportunity to address landslide hazard assessment at larger spatial scales. A prototype global satellite-based landslide hazard algorithm has been developed to identify areas that may experience landslide activity. This system combines a calculation of static landslide susceptibility with satellite-derived rainfall estimates and uses a threshold approach to generate a set of nowcasts that classify potentially hazardous areas. A recent evaluation of this algorithm framework found that while this tool represents an important first step in larger-scale near real-time landslide hazard assessment efforts, it requires several modifications before it can be fully realized as an operational tool. This study draws upon a prior work s recommendations to develop a new approach for considering landslide susceptibility and hazard at the regional scale. This case study calculates a regional susceptibility map using remotely sensed and in situ information and a database of landslides triggered by Hurricane Mitch in 1998 over four countries in Central America. The susceptibility map is evaluated with a regional rainfall intensity duration triggering threshold and results are compared with the global algorithm framework for the same event. Evaluation of this regional system suggests that this empirically based approach provides one plausible way to approach some of the data and resolution issues identified in the global assessment. The presented methodology is straightforward to implement, improves upon the global approach, and allows for results to be transferable between regions. The results also highlight several remaining challenges, including the empirical nature of the algorithm framework and adequate information for algorithm validation. Conclusions suggest that integrating additional triggering factors such as soil moisture may help to improve algorithm performance accuracy. The regional algorithm scenario represents an important step forward in advancing regional and global-scale landslide hazard assessment.

Kirschbaum, Dalia Bach↗

NASA NTRS1997

Dynamics Questions Associated with the AERCam Sprint Free-Flyer

The International Space Station will require the development of small robotic vehicles for such tasks as external inspection, monitoring of extravehicular activities (EVA's) and station build-up, and providing additional lighting at EVA worksites. The Autonomous EVA Robotic Camera (AERCam) family of free-flyers is currently being developed at NASA Johnson Space Center to perform these functions; the first member of the family is the AERCam Sprint vehicle. Many interesting dynamical questions are associated with the Sprint free-flyer. For instance, the reaction of a vehicle which is nearly spherically symmetric (such as Sprint) to a stuck-on thruster is significantly more complicated than that obtained for an idealized, perfectly spherical, spacecraft model. In particular, the real spacecraft will experience a form of forced nutation, with convergence towards either its major or minor principal axis, depending on both the applied torque and the mass properties of the vehicle. Furthermore, the body-fixed jet force vector may have a significant component along this principal axis, so giving rise to a considerable net linear acceleration of the spacecraft. The large velocity that can result is very important, as it may lead to collision with the nearby Orbiter, and is completely overlooked in the idealized analysis. This report will firstly briefly describe the stuck-on thruster dynamics of the real vehicle, and outline how the small products of inertia of the spacecraft determine the time constants of the motion. Secondly, the dynamical effects of a failed-off jet on the Sprint free-flyer will be described in more detail, and compared with the stuck-on thruster case. This will help to show whether the two malfunctions should be dealt with differently in flight. Finally, the stuck-on thruster detection software (known as the uncommanded motion algorithm) that is proposed to be flown on the Sprint vehicle will be analyzed, and all possible perturbation sources that may tend to give rise to false stuck-on thruster alarms quantified. It will be shown that false alarms can be triggered by discrete events such as a failed-off thruster, a glancing collision with Orbiter structure, or thruster saturation. They can also be triggered by a combination of errors introduced by constantly present sources such as thrust level errors, thruster misalignment, inertial cross-coupling, angular rate sensor noise, and structural vibrations induced by thruster firings. Based on this fact, the original plan to have the spacecraft enter safe mode whenever the uncommanded motion algorithm is triggered does not appear to be advisable. A better approach seems to be to provide the Sprint pilot with a warning whenever uncommanded motion is detected, and then allow the crew to determine whether safing the vehicle is appropriate.

Williams, Trevor↗

NASA NTRS2017

Comparison of the Effects of Velocity and Range Triggers on Trajectory Dispersions for the Mars 2020 Mission

Mars 2020, the next planned U.S. rover mission to land on Mars, is based on the design of the successful 2012 Mars Science Laboratory (MSL) mission. Mars 2020 retains most of the entry, descent, and landing (EDL) sequences of MSL, including the closed-loop entry guidance scheme based on the Apollo guidance algorithm. However, unlike MSL, Mars 2020 will trigger the parachute deployment and descent sequence on range trigger rather than the previously used velocity trigger. This difference will greatly reduce the landing ellipse sizes. Additionally, the relative contribution of each models to the total ellipse sizes have changed greatly due to the switch to range trigger. This paper considers the effect on trajectory dispersions due to changing the trigger schemes and the contributions of these various models to trajectory and EDL performance.

Dutta, Soumyo↗

NASA NTRSDate not supplied

Entry Guidance Design and Post-Flight Performance of the Mars 2020 Mission

Like its predecessor Mars Science Laboratory, the Mars 2020 mission successfully utilized a derivative of the Apollo Entry Terminal Point Controller guidance algorithm, whereby bank angle controls range flown along a trajectory. The flight performance of this algorithm in conjunction with a range-trigger for parachute deploy delivered the Perseverance rover to 1.7 km from the expected touchdown location within an ellipse of 7.5 x 5.2 km. This miss distance is largely attributed to atmospheric and aerodynamic modeling uncertainties between the design and as-flown trajectory. This algorithm for guided entry continues to provide a solid basis for Mars missions to improve upon.

Guidance↗

NASA NTRSDate not supplied

Post-Flight EDL Entry Guidance for the Mars 2020 Mission

Guidance↗

NASA NTRS2013

Application of a Smart Parachute Release Algorithm to the CPAS Test Architecture

One of the primary test vehicles for the Capsule Parachute Assembly System (CPAS) is the Parachute Test Vehicle (PTV), a capsule shaped structure similar to the Orion design but truncated to fit in the cargo area of a C-17 aircraft. The PTV has a full Orion-like parachute compartment and similar aerodynamics; however, because of the single point attachment of the CPAS parachutes and the lack of Orion-like Reaction Control System (RCS), the PTV has the potential to reach significant body rates. High body rates at the time of the Drogue release may cause the PTV to flip while the parachutes deploy, which may result in the severing of the Pilot or Main risers. In order to prevent high rates at the time of Drogue release, a "smart release" algorithm was implemented in the PTV avionics system. This algorithm, which was developed for the Orion Flight system, triggers the Drogue parachute release when the body rates are near a minimum. This paper discusses the development and testing of the smart release algorithm; its implementation in the PTV avionics and the pretest simulation; and the results of its use on two CPAS tests.

Bledsoe, Kristin↗

NASA NTRS1977

A continuous/discrete simulation for interplanetary spacecraft

The paper gives a general description of a continuous/discrete simulator of the telemetry flow from an unmanned interplanetary spacecraft in flight. The real time simulation method is based on the trigger method, where a trigger is a set of control words which describe a condition and an action to be taken upon satisfaction of the condition. Simulation consists of finding the points at which the given condition is satisfied and taking the specified action. The paper describes the four kinds of triggers and how they are called. The calculational algorithm used to advance time in the simulator is briefly described.

Lesh, F.↗

NASA NTRS2021

Autonomous Off-road Navigation over Extreme Terrains with Perceptually-challenging Conditions

We propose a framework for resilient autonomous navigation in perceptuallychallenging unknown environments with mobility-stressing elements such asuneven surfaces with rocks and boulders, steep slopes, negative obstacles like cliffsand holes, and narrow passages. Environments are GPS-denied and perceptuallydegradedwith variable lighting from dark to lit and obscurants (dust, fog, smoke).Lack of prior maps and degraded communication eliminates the possibility of prioror off-board computation or operator intervention. This necessitates real-time onboardcomputation using noisy sensor data. To address these challenges, we proposea resilient architecture that exploits redundancy and heterogeneity in sensing modalities.Further resilience is achieved by triggering recovery behaviors upon failure.We propose a fast settling algorithm to generate robust multi-fidelity traversabilityestimates in real-time. The proposed approach was deployed on multiple physicalsystems including skid-steer and tracked robots, high-speed RC car and legged robotsand as a part of Team CoSTAR’s effort to theDARPASubterranean Challenge, wherethe team won 2nd and 1st place in the Tunnel and Urban Circuit, respectively.

Agha-mohammadi, Ali-akbar↗

NASA NTRS2003

Occurrence frequencies of IMF triggered and nontriggered substorms

The occurrence of triggered and nontriggered substorm are examined in light of current interest in such issues as substorm identification, IMF By variations, and potentially undetected small-scale solar wind perturbation. Global substorms are identified using a sudden, persistent decrease in the AL index. The onset of this global expansion is taken to be the time of the Pi 2 burst nearest in time to the beginning of the AL, decrease. IMF triggers were identified both subjectively through visual scanning of the data and automatically with a computer algorithm. Both northward turnings of the IMF Bz and decreases in the amplitude of the By component were considered as possible triggers. Two different solar wind monitors were used in the investigation: IMP-8 in a circular orbit with a distance 12 to approx.35 Re to the Earth-Sun line and ISEE-2 in an elliptical orbit with a distance only 5 to approx.10 Re to the Earth-Sun line. The IMP-8 results show that the triggering probability does not depend on the distance of the monitor from the Earth-Sun line in the range 12-35 Re. The ISEE dataset shows that closer than 12 Re the triggering probability is the same as it is in the IMP-8 data set. Thus there appears to be no dependence of triggering on the location of the monitor provided it is within 35 Re of the Earth. We also demonstrate that including the By component does not significantly increase the probability of substorm triggering. Approximately 60% of all substorms appear to be triggered. Of the 40% for which we could not identify a trigger, 10% occurred while the IMF was northward. The data suggest that substorm onset is a consequence of an internal magnetospheric instability that is highly sensitive to changes in magnetospheric convection induced by a sudden change in the IMF, but that these changes are not always necessary.

Hsu, Tung-Shin↗

NASA NTRS2011

On the Use of a Range Trigger for the Mars Science Laboratory Entry Descent and Landing

In 2012, during the Entry, Descent, and Landing (EDL) of the Mars Science Laboratory (MSL) entry vehicle, a 21.5 m Viking-heritage, Disk-Gap-Band, supersonic parachute will be deployed at approximately Mach 2. The baseline algorithm for commanding this parachute deployment is a navigated planet-relative velocity trigger. This paper compares the performance of an alternative range-to-go trigger (sometimes referred to as Smart Chute ), which can significantly reduce the landing footprint size. Numerical Monte Carlo results, predicted by the POST2 MSL POST End-to-End EDL simulation, are corroborated and explained by applying propagation of uncertainty methods to develop an analytic estimate for the standard deviation of Mach number. A negative correlation is shown to exist between the standard deviations of wind velocity and the planet-relative velocity at parachute deploy, which mitigates the Mach number rise in the case of the range trigger.

Way, David W.↗

NASA NTRS2006

Using Satellite Data to Characterize the Temporal Thermal Behavior of an Active Volcano: Mount St. Helens, WA

ASTER thermal infrared data over Mt. St Helens were used to characterize its thermal behavior from Jun 2000 to Feb 2006. Prior to the Oct 2004 eruption, the average crater temperature varied seasonally between -12 and 6 C. After the eruption, maximum single-pixel temperature increased from 10 C (Oct 2004) to 96 C (Aug 2005), then showed a decrease to Feb 2006. The initial increase in temperature was correlated with dome morphology and growth rate and the subsequent decrease was interpreted to relate to both seasonal trends and a decreased growth rate/increased cooling rate, possibly suggesting a significant change in the volcanic system. A single-pixel ASTER thermal anomaly first appeared on Oct 1, 2004, eleven hours after the first eruption - 10 days before new lava was exposed at the surface. By contrast, an automated algorithm for detecting thermal anomalies in MODIS data did not trigger an alert until Dec 18. However, a single-pixel thermal anomaly first appeared in MODIS channel 23 (4 um) on Oct 13, 12 days after the first eruption - 2 days after lava was exposed. The earlier thermal anomaly detected with ASTER data is attributed to the higher spatial resolution (90 m) compared with MODIS (1 m) and the earlier visual observation of anomalous pixels compared to the automated detection method suggests that local spatial statistics and background radiance data could improve automated detection methods.

Advanced Spaceborne Thermal Emission and Reflectio↗

NASA NTRS1978

Flight tests of a simple airborne device for predicting clear air turbulence encounters

An airborne clear-air turbulence detector is being flight-tested on board NASA's C-141 and Learjet aircraft. The device is an infrared (IR) sensor in the water vapor band and is designed to detect changes in vapor concentrations associated with turbulence in shear conditions. Warnings of about 5 min have been demonstrated at flight altitudes from 9.1 to 13.7 km (30,000 to 45,000 ft). Encounter predictions were obtained 80% of the time, and false alarms were given about 6% of the time. Several simple algorithms were studied for use as signal output analyzers and for alert triggering.

Kurkowski, R. L.↗

NASA NTRS2013

Studies in Astronomical Time Series Analysis. VI. Bayesian Block Representations

This paper addresses the problem of detecting and characterizing local variability in time series and other forms of sequential data. The goal is to identify and characterize statistically significant variations, at the same time suppressing the inevitable corrupting observational errors. We present a simple nonparametric modeling technique and an algorithm implementing it-an improved and generalized version of Bayesian Blocks [Scargle 1998]-that finds the optimal segmentation of the data in the observation interval. The structure of the algorithm allows it to be used in either a real-time trigger mode, or a retrospective mode. Maximum likelihood or marginal posterior functions to measure model fitness are presented for events, binned counts, and measurements at arbitrary times with known error distributions. Problems addressed include those connected with data gaps, variable exposure, extension to piece- wise linear and piecewise exponential representations, multivariate time series data, analysis of variance, data on the circle, other data modes, and dispersed data. Simulations provide evidence that the detection efficiency for weak signals is close to a theoretical asymptotic limit derived by [Arias-Castro, Donoho and Huo 2003]. In the spirit of Reproducible Research [Donoho et al. (2008)] all of the code and data necessary to reproduce all of the figures in this paper are included as auxiliary material.

signal detection↗

NASA NTRSDate not supplied

Implementation of a Six Degree of Freedom Precision Lunar Landing Algorithm Using Dual Quaternion Representation

In this study, a powered descent guidance algorithm using a unit dual quaternion represen- tation of the vehicle dynamics is implemented in a high-fidelity simulation and on representative flight hardware. This Dual-Quaternion Guidance (DQG) algorithm is applied to the precision lunar landing problem which levies complex constraints upon the trajectory, including state triggered attitude constraints to enable terrain-relative navigation and hazard detection as well as real-time requirements for landing site re-designation. The investigation explores DQG’s usefulness as a mission design tool as well as a real-time guidance algorithm and defines real-time performance requirements for the hazard detection and avoidance (HDA) re-targeting phase of precision lunar landing. The experiment is presented in two parts. First, DQG is implemented within a high-fidelity Monte Carlo simulation to tune the algorithm’s parameters for the simulated vehicle, to refine the mission design, and to develop guidance update timing requirements to perform the HDA maneuver. DQG generates trajectories online for the divert which are tracked by the vehicle’s inner-loop controllers to the targeted landing site. Second, DQG is run on representative hardware to demonstrate real-time operation through a divert maneuver. These results allow for rapid, flexible, optimal mission design satisfying complex constraints, and for the definition of real-time performance requirements for the HDA operations inherent in precision lunar landing. The HDA divert maneuver is found to require guidance trajectory updates in less than three seconds. DQG is found to be too slow to meet this update timing on the descent and landing computer (DLC) in its current implementation. DQG running on alternative hardware can meet the update rate requirement. Algorithm implementation improvements are also recommended which are expected to speed up computation sufficiently to meet requirements on the DLC.

GN&C↗