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At least 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

On the verification of the planetary system around PSR 1257 + 12

The magnitude and nature of residuals expected in the times of pulsar arrival (TOA) of the millisecond pulsar PSR 1257 + 12 as a function of the observational time span is estimated so that an observing progam may be planned to optimize the detection of a perturbation signature of a possible planetary system. The numerical and analytic procedures for evaluating the residuals are described. Calculations results are given in which a recognizable signature is shown to be a modulation of the amplitude of the TOA residual differences as a function of both epoch and observational interval. A supplementary approach involving intensive closely spaced observations over selected time intervals to find the time of a particular zero crossing of the TOA residuals is also discussed.

Peale, S. J.↗
NASA NTRS2017

Current Safety Nets Within the U.S. National Airspace System

There are over 70,000 flights managed per day in the National Airspace System, with approximately 7,000 aircraft in the air over the United States at any given time. Operators of each of these flights would prefer to fly a user-defined “4D” trajectory (4DT), which includes arrival and departure times; preferred gates and runways at the airport; efficient, wind-optimal routes for departure, cruise and arrival phase of flight; and fuel efficient altitude profiles. To demonstrate the magnitude of this achievement a single flight from Los Angeles to Baltimore, accesses over 35 shared or “constrained” resources that are managed by roughly 30 air traffic controllers (at towers, approach control and en route sectors); along with traffic managers at 12 facilities, using over 22 different, independent automation system (including TBFM, ERAM, STARS, ASDE-X, FSM, TSD, GPWS, TCAS, etc.). In addition, dispatchers, ramp controllers and others utilize even more systems to manage each flight’s access to operator-managed resources. Flying an ideal 4DT requires successful coordination of all flight constraints among all flights, facilities, operators, pilots and controllers. Additionally, when conditions in the NAS change, the trajectories of one or more aircraft may need to be revised to avoid loss of flight efficiency, predictability, separation or system throughput. The Aviation Safety Network has released the 2016 airliner accident statistics showing a very low total of 19 fatal airliner accidents, resulting in 325 fatalities1. Despite several high profile accidents, the year 2016 turned out to be a very safe year for commercial aviation, Aviation Safety Network data show. Over the year 2016 the Aviation Safety Network recorded a total of 19 fatal airliner accidents [1], resulting in 325 fatalities. This makes 2016 the second safest year ever, both by number of fatal accidents as well as in terms of fatalities. In 2015 ASN recorded 16 accidents while in 2013 a total of 265 lives were lost. How can we keep it that way and not upset the apple cart by premature insertion of innovative technologies, functions, and procedures? In aviation, safety nets function as the last system defense against incidents and accidents. Current ground-based and airborne safety nets are well established and development to make them more efficient and reliable continues. Additionally, future air traffic control safety nets may emerge from new operational concepts.

Smith, Brian E.↗
NASA NTRS2010

Targeting Low-Energy Ballistic Lunar Transfers

Numerous low-energy ballistic transfers exist between the Earth and Moon that require less fuel than conventional transfers, but require three or more months of transfer time. An entirely ballistic lunar transfer departs the Earth from a particular declination at some time in order to arrive at the Moon at a given time along a desirable approach. Maneuvers may be added to the trajectory in order to adjust the Earth departure to meet mission requirements. In this paper, we characterize the (Delta)V cost required to adjust a low-energy ballistic lunar transfer such that a spacecraft may depart the Earth at a desirable declination, e.g., 28.5(white bullet), on a designated date. This study identifies the optimal locations to place one or two maneuvers along a transfer to minimize the (Delta)V cost of the transfer. One practical application of this study is to characterize the launch period for a mission that aims to launch from a particular launch site, such as Cape Canaveral, Florida, and arrive at a particular orbit at the Moon on a given date using a three-month low-energy transfer.

Low-energy transfers↗
NASA NTRS2011

Evaluation of the Monotonic Lagrangian Grid and Lat-Long Grid for Air Traffic Management

The Air Traffic Monotonic Lagrangian Grid (ATMLG) is used to simulate a 24 hour period of air traffic flow in the National Airspace System (NAS). During this time period, there are 41,594 flights over the United States, and the flight plan information (departure and arrival airports and times, and waypoints along the way) are obtained from an Federal Aviation Administration (FAA) Enhanced Traffic Management System (ETMS) dataset. Two simulation procedures are tested and compared: one based on the Monotonic Lagrangian Grid (MLG), and the other based on the stationary Latitude-Longitude (Lat- Long) grid. Simulating one full day of air traffic over the United States required the following amounts of CPU time on a single processor of an SGI Altix: 88 s for the MLG method, and 163 s for the Lat-Long grid method. We present a discussion of the amount of CPU time required for each of the simulation processes (updating aircraft trajectories, sorting, conflict detection and resolution, etc.), and show that the main advantage of the MLG method is that it is a general sorting algorithm that can sort on multiple properties. We discuss how many MLG neighbors must be considered in the separation assurance procedure in order to ensure a five-mile separation buffer between aircraft, and we investigate the effect of removing waypoints from aircraft trajectories. When aircraft choose their own trajectory, there are more flights with shorter duration times and fewer CD&R maneuvers, resulting in significant fuel savings.

Kaplan, Carolyn↗
NASA NTRS2017

Batch Sequential Estimation with Non-Uniform Measurements and Non-Stationary Noise

Sequential estimation using the traditional discrete Kalman filter typically assumes the measurement time and state update time are coincident. This is often a poor assumption in realistic measurement scenarios where the data can be received from multiple sources at differing times. This paper develops the necessary algorithm adjustments needed for the Kalman filter to readily process measurement data that arrive at varying times and with non-stationary noise. The algorithm is applied to a relevant problem of orbit determination using one-way uplink radiometric tracking of a spacecraft (in the present case a Mars orbiter).

Seubert, Jill↗
NASA NTRS2005

Fast-Time Evaluations of Airborne Merging and Spacing in Terminal Arrival Operations

NASA researchers are developing new airborne technologies and procedures to increase runway throughput at capacity-constrained airports by improving the precision of inter-arrival spacing at the runway threshold. In this new operational concept, pilots of equipped aircraft are cleared to adjust aircraft speed to achieve a designated spacing interval at the runway threshold, relative to a designated lead aircraft. A new airborne toolset, prototypes of which are being developed at the NASA Langley Research Center, assists pilots in achieving this objective. The current prototype allows precision spacing operations to commence even when the aircraft and its lead are not yet in-trail, but are on merging arrival routes to the runway. A series of fast-time evaluations of the new toolset were conducted at the Langley Research Center during the summer of 2004. The study assessed toolset performance in a mixed fleet of aircraft on three merging arrival streams under a range of operating conditions. The results of the study indicate that the prototype possesses a high degree of robustness to moderate variations in operating conditions.

Krishnamurthy, Karthik↗
NASA NTRS2006

UWB Tracking System Design with TDOA Algorithm

This presentation discusses an ultra-wideband (UWB) tracking system design effort using a tracking algorithm TDOA (Time Difference of Arrival). UWB technology is exploited to implement the tracking system due to its properties, such as high data rate, fine time resolution, and low power spectral density. A system design using commercially available UWB products is proposed. A two-stage weighted least square method is chosen to solve the TDOA non-linear equations. Matlab simulations in both two-dimensional space and three-dimensional space show that the tracking algorithm can achieve fine tracking resolution with low noise TDOA data. The error analysis reveals various ways to improve the tracking resolution. Lab experiments demonstrate the UWBTDOA tracking capability with fine resolution. This research effort is motivated by a prototype development project Mini-AERCam (Autonomous Extra-vehicular Robotic Camera), a free-flying video camera system under development at NASA Johnson Space Center for aid in surveillance around the International Space Station (ISS).

Ni, Jianjun↗
NASA NTRS2012

Radiative Transfer Through Clouds and Its Applications in Support of the Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) Mission

The Greenland and Antarctic ice sheets, which contain enough ice to raise sea level by about 7 and 60 m, respectively, are losing mass at an increasing rate. To acquire continuous information of the cryosphere, after the Ice, Cloud, and land Elevation Satellite (ICESat) (2003-2010), NASA is actively planning for the ICESat-2 mission. Both ICESat and ICESat-2 are space-borne lidar altimetry systems. The systems measure the time of flight of the arriving photons that are reflected by the surface to deduce the elevation of the underlying terrain. As one of NASA's top priority missions, ICESat-2 is scheduled to launch in 2016. One of the major science goals of ICESat-2 is to quantify the ice sheet mass balance to determine its contributions to the sea level change and its impacts on ocean circulation (Abdalati et al. 2010). Compared to ICESat, which operates at 40 Hz and records the reflected laser energy as a waveform, the significantly improved ICESat-2 lidar employs a 532 nm micro-pulse photon counting system that operates at a high frequency of 10kHz with single photon detectability (Yang et al. 2012). To achieve its science goals, ICESat-2 requires the ability of detecting the elevation change with an accuracy of 0.2 cm/year over the entire ice sheet. Since every photon emitted by the lidar system will travel through the atmosphere, clouds can certainly affect the flight time of the arriving photons. Forward scattering by cloud particles increases the photon path length, thus resulting in biases in ice sheet elevation measurements known as atmospheric path delay (Duta et al. 2001, Yang et al. 2010, 2011). To ensure the accuracy of ICESat-2 surface elevation measurements, it is critical to understand how clouds would affect the travel time of arriving photons. In this talk, we will first present a framework that simulates the behavior of a space-borne 532 mn micro-pulse photon counting lidar in cloudy and clear atmospheres. To investigate the process of laser propagation through clouds, a 3-D Monte Carlo radiative transfer model is used to simulate the photon path distribution and the Poisson distribution is adopted for the number of photon returns. Since the photon counting system only registers the time of the first arriving photon within the detector "dead time", the retrieved average surface elevation tends to bias towards higher values. This is known as the first photon bias. With the scenarios simulated here, the first photon bias for clear sky is about 6.5 cm. Clouds affect surface altimetry in two ways: (1) cloud attenuation lowers the average number of arriving photons and hence reduces the first photon bias; (2) cloud forward scattering increases the photon path length and makes the surface appear further away from the satellite. Compared to clear sky, the average surface elevation detected by the photon counting system for cloudy sky with optical depth 1.0 is 4.0 to 6.0 cm lower for the simulations conducted. The effect of surface roughness on the accuracy of elevation retrievals will also discussed.

Yang, Yuekui↗
NASA NTRS2012

Downsampling Photodetector Array with Windowing

In a photon counting detector array, each pixel in the array produces an electrical pulse when an incident photon on that pixel is detected. Detection and demodulation of an optical communication signal that modulated the intensity of the optical signal requires counting the number of photon arrivals over a given interval. As the size of photon counting photodetector arrays increases, parallel processing of all the pixels exceeds the resources available in current application-specific integrated circuit (ASIC) and gate array (GA) technology; the desire for a high fill factor in avalanche photodiode (APD) detector arrays also precludes this. Through the use of downsampling and windowing portions of the detector array, the processing is distributed between the ASIC and GA. This allows demodulation of the optical communication signal incident on a large photon counting detector array, as well as providing architecture amenable to algorithmic changes. The detector array readout ASIC functions as a parallel-to-serial converter, serializing the photodetector array output for subsequent processing. Additional downsampling functionality for each pixel is added to this ASIC. Due to the large number of pixels in the array, the readout time of the entire photodetector is greater than the time between photon arrivals; therefore, a downsampling pre-processing step is done in order to increase the time allowed for the readout to occur. Each pixel drives a small counter that is incremented at every detected photon arrival or, equivalently, the charge in a storage capacitor is incremented. At the end of a user-configurable counting period (calculated independently from the ASIC), the counters are sampled and cleared. This downsampled photon count information is then sent one counter word at a time to the GA. For a large array, processing even the downsampled pixel counts exceeds the capabilities of the GA. Windowing of the array, whereby several subsets of pixels are designated for processing, is used to further reduce the computational requirements. The grouping of the designated pixel frame as the photon count information is sent one word at a time to the GA, the aggregation of the pixels in a window can be achieved by selecting only the designated pixel counts from the serial stream of photon counts, thereby obviating the need to store the entire frame of pixel count in the gate array. The pixel count se quence from each window can then be processed, forming lower-rate pixel statistics for each window. By having this processing occur in the GA rather than in the ASIC, future changes to the processing algorithm can be readily implemented. The high-bandwidth requirements of a photon counting array combined with the properties of the optical modulation being detected by the array present a unique problem that has not been addressed by current CCD or CMOS sensor array solutions.

Patawaran, Ferze D.↗
NASA NTRS1985

A search in the COS-B data base for correlated time variability in regions containing objects of interest

As is well known, association of the gamma-ray sources with celestial objects is, in general, difficult on a pure positional basis, while correlated time variability could obviously provide such proof. This technique can be employed on regions of the gamma-ray sky containing interesting objects of known variability at some wavelength even in the absence of a recognized gamma-ray excess with the aim to extract a weak but predictable signal from the surrounding noise. This technique is applied here on a longer variability time scale, generally of the order of days. Photons coming from the sky regions centered on the various celestial objects considered were selected with energies 100 MeV and with arrival directions within an energy-dependent area of radius of approx 6 deg at 100 MeV. In order to construct a time profile of such photons, their arrival times were grouped in bins of dimensions defined by the available photons number and by the value of the period searched for.

Caraveo, P. A.↗
NASA NTRS2012

Deep Space Habitat Concept of Operations for Extended Duration Transit Missions

NASA's Capability-Driven Framework (CDF) describes an approach for progressively extending human exploration missions farther into the Solar System for longer periods of time as allowed by developments in technology and spacecraft systems. Within this framework design reference missions (DRMs) targeted for several specific destinations are being used to assess different combinations of vehicles, operations, and advanced technologies to help understand which combination will best support expanded human exploration both efficiently and sustainably. Several of the identified destinations have been found to require missions with a round trip duration exceeding one year. These mission durations exceed the capabilities of current human-rated spacecraft if resupply from Earth is not possible. This makes the design of an efficient and reliable Deep Space Habitat (DSH) critical for reaching these destinations. The paper will describe the current understanding of DSH capabilities and functions that must be exhibited by any future habitat design for these missions. This description of the DSH is presented in the form of a concept of operation, which focuses on the functions that any DSH must provide, as opposed to a specific DSH design concept. Development of a concept of operations, based on DRM features, provides a common basis for assessing the viability of design concepts incorporating differing configurations and technologies. A study team with representation from several NASA Centers and relevant engineering and scientific disciplines collaborated to develop this DSH concept of operations for the transit phases of these missions. The transit phase of a mission is defined as the time after leaving Earth but before arrival at the destination and the time after leaving the destination but before arriving back at Earth. These transit phases were found to have many common features across all of the destinations being assessed for the CDF and thus arguing for a common concept of operations for all of the associated DRMs. The paper first describes the CDF and potential destinations being considered within this framework. DRMs representative of those currently being assessed will then be discussed to illustrate the range of capabilities that the DSH must support. The 18 functions identified by the multi-discipline analysis team are then described at a summary level. Several specific examples of the potential use for this concept of operations are described.

Hoffman, Stephen J.↗
NASA NTRS1990

Time domain version of the uniform GTD

The uniform geometrical theory of diffraction (UTD) solutions can be inversely transformed analytically to obtain a time-domain version of the UTD. The time-domain solutions are valid in the early time period where an observation time t is close to the time after the arrival of the first diffracted wavefront. Comparisons with GTD (geometrical theory of diffraction) and also with available rigorous results reveal that the UTD solutions are accurate for substantial early time periods while the GTD results are valid for very early time periods.

Veruttipong, Thavath Watt↗
NASA NTRS1998

Analyzing Space-Based Interferometric Measurements of Stars and Network Measurements of Gamma-Ray Bursts

Since the announcement of the discovery of sources of bursts of gamma-ray radiation in 1973, hundreds more reports of such bursts have now been published. Numerous artificial satellites have been equipped with gamma-ray detectors including the very successful Compton Gamma Ray Observatory BATSE instrument. Unfortunately, we have made no progress in identifying the source(s) of this high energy radiation. We suspected that this was a consequence of the method used to define gamma-ray burst source "error boxes." An alternative procedure to compute gamma-ray burst source positions, with a purely physical underpinning, was proposed in 1988 by Taff. Since then we have also made significant progress in understanding the analytical nature of the triangulation problem and in computing actual gamma-ray burst positions and their corresponding error boxes. For the former, we can now mathematically illustrate the crucial role of the area occupied by the detectors, while for the latter, the Atteia et al. (1987) catalog has been completely re-reduced. There are very few discrepancies in locations between our results and those of the customary "time difference of arrival" procedure. Thus, we have numerically demonstrated that the end result, for the positions, of these two very different-looking procedures is the same. Finally, for the first time, we provide a sample of realistic "error boxes" whose non-simple shapes vividly portray the difficulty of burst source localization.

Taff, L. G.↗
NASA NTRS1986

Ground-based time-guidance algorithm for control of airplanes in a time-metered air traffic control environment: A piloted simulation study

The rapidly increasing costs of flight operations and the requirement for increased fuel conservation have made it necessary to develop more efficient ways to operate airplanes and to control air traffic for arrivals and departures to the terminal area. One concept of controlling arrival traffic through time metering has been jointly studied and evaluated by NASA and ONERA/CERT in piloted simulation tests. From time errors attained at checkpoints, airspeed and heading commands issued by air traffic control were computed by a time-guidance algorithm for the pilot to follow that would cause the airplane to cross a metering fix at a preassigned time. These tests resulted in the simulated airplane crossing a metering fix with a mean time error of 1.0 sec and a standard deviation of 16.7 sec when the time-metering algorithm was used. With mismodeled winds representing the unknown in wind-aloft forecasts and modeling form, the mean time error attained when crossing the metering fix was increased and the standard deviation remained approximately the same. The subject pilots reported that the airspeed and heading commands computed in the guidance concept were easy to follow and did not increase their work load above normal levels.

Knox, C. E.↗
NASA NTRS2012

Network Connectivity for Permanent, Transient, Independent, and Correlated Faults

This paper develops a method for the quantitative analysis of network connectivity in the presence of both permanent and transient faults. Even though transient noise is considered a common occurrence in networks, a survey of the literature reveals an emphasis on permanent faults. Transient faults introduce a time element into the analysis of network reliability. With permanent faults it is sufficient to consider the faults that have accumulated by the end of the operating period. With transient faults the arrival and recovery time must be included. The number and location of faults in the system is a dynamic variable. Transient faults also introduce system recovery into the analysis. The goal is the quantitative assessment of network connectivity in the presence of both permanent and transient faults. The approach is to construct a global model that includes all classes of faults: permanent, transient, independent, and correlated. A theorem is derived about this model that give distributions for (1) the number of fault occurrences, (2) the type of fault occurrence, (3) the time of the fault occurrences, and (4) the location of the fault occurrence. These results are applied to compare and contrast the connectivity of different network architectures in the presence of permanent, transient, independent, and correlated faults. The examples below use a Monte Carlo simulation, but the theorem mentioned above could be used to guide fault-injections in a laboratory.

White, Allan L.↗
NASA NTRS1984

A new multifunction acousto-optic signal processor

An acousto-optic architecture for simultaneously obtaining time integration correlation and high-speed power spectrum analysis was constructed using commercially available TeO2 modulators and photodiode detector-arrays. The correlator section of the processor uses coherent interferometry to attain maximum bandwidth and dynamic range while achieving a time-bandwidth product of 1 million. Two correllator outputs are achieved in this system configuration. One is optically filtered and magnified 2 : 1 to decrease the spatial frequency to a level where a 25-MHz bandwidth may be sampled by a 62-mm array with elements on 25-micro centers. The other output is magnified by a factor of 10 such that the center 4 microseconds of information is available for estimation of time-difference-of-arrival to within 10 ns. The Bragg cell spectrum-analyzer section, which also has two outputs, resolves a 25-MHz instantaneous bandwidth to 25 kHz and can determine discrete-frequency reception time to within 15 microseconds. A microprocessor combines spectrum analysis information with that obtained from the correlator.

Berg, N. J.↗
NASA NTRS2003

Runway Operations Planning: A Two-Stage Heuristic Algorithm

The airport runway is a scarce resource that must be shared by different runway operations (arrivals, departures and runway crossings). Given the possible sequences of runway events, careful Runway Operations Planning (ROP) is required if runway utilization is to be maximized. From the perspective of departures, ROP solutions are aircraft departure schedules developed by optimally allocating runway time for departures given the time required for arrivals and crossings. In addition to the obvious objective of maximizing throughput, other objectives, such as guaranteeing fairness and minimizing environmental impact, can also be incorporated into the ROP solution subject to constraints introduced by Air Traffic Control (ATC) procedures. This paper introduces a two stage heuristic algorithm for solving the Runway Operations Planning (ROP) problem. In the first stage, sequences of departure class slots and runway crossings slots are generated and ranked based on departure runway throughput under stochastic conditions. In the second stage, the departure class slots are populated with specific flights from the pool of available aircraft, by solving an integer program with a Branch & Bound algorithm implementation. Preliminary results from this implementation of the two-stage algorithm on real-world traffic data are presented.

Anagnostakis, Ioannis↗
NASA NTRS1995

Contingency maneuver strategies for the Total Ozone Mapping Spectrometer-Earth Probe (TOMS-EP)

The Total Ozone Mapping Spectrometer-Earth Probe (TOMS-EP) is a polar-orbiting spacecraft designed to measure total ozone levels in the Earth's atmosphere. The nominal mission orbit is a 955-kilometer circular Sun-synchronous orbit with an ascending node mean local crossing time (MLT) between 11:02 a.m. and 11:25 a.m. These two mean local ascending node times constitute the boundaries of the MLT box for this mission. The MLT boundaries were chosen to maintain the Sun-to-Earth-to-vehicle orbit-normal (SVN) angle within a preselected set of seasonally independent boundaries. Because the SVN angle is seasonally dependent, but the MLT is not, contingency options for correcting the MLT of orbital states that fall outside of the required MLT range become time dependent. This paper focuses on contingency orbit adjustment strategies developed at the Goddard Space Flight Center (GSFC) Flight Dynamics Division (FDD) during the mission planning phase of TOMS-EP. Time-dependent delta-V strategies are presented for correcting mission orbit states lying outside of the MLT range. Typically, passive control of the MLT drift rate can be used to restore the orbit state to the required MLT before a seasonal violation of SVN angle constraints can occur. Passive control of the MLT drift rate is obtained through adjustment of the semimajor axis and/or the inclination. The time between initial arrival on orbit at an 'out-of-the box' MLT state and violation of the SVN angle constraints is always less than or equal to 1 year. The choice of which parameter(s) to adjust is dictated by the duration of this time period, the desired mission lifetime, the delta-V cost, and operational constraints.

Kestler, James↗