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At least 1,207 records · Page 67

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NASA NTRS1990

Application of inertial instruments for DSN antenna pointing and tracking

The feasibility of using inertial instruments to determine the pointing attitude of the NASA Deep Space Network antennas is examined. The objective is to obtain 1 mdeg pointing knowledge in both blind pointing and tracking modes to facilitate operation of the Deep Space Network 70 m antennas at 32 GHz. A measurement system employing accelerometers, an inclinometer, and optical gyroscopes is proposed. The initial pointing attitude is established by determining the direction of the local gravity vector using the accelerometers and the inclinometer, and the Earth's spin axis using the gyroscopes. Pointing during long-term tracking is maintained by integrating the gyroscope rates and augmenting these measurements with knowledge of the local gravity vector. A minimum-variance estimator is used to combine measurements to obtain the antenna pointing attitude. A key feature of the algorithm is its ability to recalibrate accelerometer parameters during operation. A survey of available inertial instrument technologies is also given.

Eldred, D. B.↗

NASA NTRS1990

An ice-motion tracking system at the Alaska SAR facility

An operational system for extracting ice-motion information from synthetic aperture radar (SAR) imagery is being developed as part of the Alaska SAR Facility. This geophysical processing system (GPS) will derive ice-motion information by automated analysis of image sequences acquired by radars on the European ERS-1, Japanese ERS-1, and Canadian RADARSAT remote sensing satellites. The algorithm consists of a novel combination of feature-based and area-based techniques for the tracking of ice floes that undergo translation and rotation between imaging passes. The system performs automatic selection of the image pairs for input to the matching routines using an ice-motion estimator. It is designed to have a daily throughput of ten image pairs. A description is given of the GPS system, including an overview of the ice-motion-tracking algorithm, the system architecture, and the ice-motion products that will be available for distribution to geophysical data users.

Kwok, Ronald↗

NASA NTRS1990

The effect of tracking network configuration on GPS baseline estimates for the CASA Uno experiment

The effect of the tracking network on long (greater than 100 km) GPS baseline estimates was estimated using various subsets of the global tracking network initiated by the first Central and South America (CASA Uno) experiment. It was found that best results could be obtained with a global tacking network consisting of three U.S. stations, two sites in the southwestern Pacific, and two sites in Europe. In comparison with smaller subsets, this global network improved the baseline repeatability, the resolution of carrier phase cycle ambiguities, and formal errors of the orbit estimates.

Wolf, S. Kornreich↗

NASA NTRS1989

The effects of practice on tracking and subjective workload

Six college-age male subjects performed one hundred, two-minute trials on a second-order tracking task. After each trial, subjects estimated perceived workload using both the NASA TLX and SWAT workload assessment procedures. Results confirmed an expected performance improvement on the tracking task which followed traditional learning curves within the performance of each individual. Perceived workload also decreased for both scales across trials. While performance variability significantly decreased across trials, workload variability remained constant. One month later, the same subjects returned to complete the second experiment in the sequence which was a retention replication of the first experiment. Results replicated those for the first experiment except that both performance error and workload were at reduced overall levels. Results in general affirm a parallel workload reduction with performance improvement, an observation consistent with a resource-based view of automaticity.

Hancock, P. A.↗

NASA NTRS1989

Performance and modeling of the GSFC pointing acquisition and tracking system for laser communications

In this paper the NASA Goddard Space Flight Center (GSFC) laser communication pointing, acquisition, and tracking laboratory demonstration program is discussed. This program entails the construction of a flexible brass-board simulation tool, and a supporting computer simulation effort. The result of this program will be a viable test bed to support the design and specification of space-based laser communication terminals and, additionally, to address critical performance issues such as tracking robustness in the presence of background (e.g., stellar or earth) noise and/or internally generated thermal noise.

Fox, N. D.↗

NASA NTRS1989

Imaging infrared: Scene simulation, modeling, and real image tracking; Proceedings of the Meeting, Orlando, FL, Mar. 30, 31, 1989

Various papers on scene simulation, modeling, and real image tracking using IR imaging are presented. Individual topics addressed include: tactical IR scene generator, dynamic FLIR simulation in flight training research, high-speed dynamic scene simulation in UV to IR spectra, development of an IR sensor calibration facility, IR celestial background scene description, transmission measurement of optical components at cryogenic temperatures, diffraction model for a point-source generator, silhouette-based tracking for tactical IR systems, use of knowledge in electrooptical trackers, detection and classification of target formations in IR image sequences, SMPRAD: simplified three-dimensional cloud radiance model, IR target generator, recent advances in testing of thermal imagers, generic IR system models with dynamic image generation, modeling realistic target acquisition using IR sensors in multiple-observer scenarios, and novel concept of scene generation and comprehensive dynamic sensor test.

Triplett, Milton J.↗

NASA NTRS1990

Mars gravity field error analysis from simulated radio tracking of Mars Observer

Results are presented on the analysis of the recovery of the Martian gravity field from tracking data in the presence of unmodeled error effects associated with different orbit orientations. The analysis was based on the mission plan for the Mars Observer (MO) radio tracking data from the Deep Space Network. From the analysis, a conservative estimate of the gravitational accuracy for the entire mission could be obtained. The results suggest that, because the atmospheric drag is the dominant error source, the spacecraft orbit could possibly be raised in altitude without a significant loss of gravitational signal. A change in altitude will also alleviate the large effects seen in the spectrum the satellite resonant orders.

Smith, D. E.↗

NASA NTRS1990

Object-oriented feature-tracking algorithms for SAR images of the marginal ice zone

An unsupervised method that chooses and applies the most appropriate tracking algorithm from among different sea-ice tracking algorithms is reported. In contrast to current unsupervised methods, this method chooses and applies an algorithm by partially examining a sequential image pair to draw inferences about what was examined. Based on these inferences the reported method subsequently chooses which algorithm to apply to specific areas of the image pair where that algorithm should work best.

Daida, Jason↗

NASA NTRS1990

Comparisons between Geosat and Seasat tracking over non-ocean surfaces

The differences in both the quantity and quality of the nonocean measurements of the Geosat and Seasat altimeters due to the tracker modifications and satellite and antenna design changes are presented. The Geosat tracker exhibits more agility over the rougher surfaces and therefore is able to maintain lock better than Seasat. Large off-nadir attitude excursions created differences in Geosat tracking between cycles. The Geosat altimeter was at times able to track over surfaces Seasat could not, but sometimes the acquisition problem was worse. It is observed that Geosat yields denser coverage, but over land Seasat measurements yield a better overall picture of the general topography. It is concluded that the Geosat elevation data set over land can best be utilized to augment the information obtained from the Seasat data and is most useful when investigating topography at scales less than 100 km.

Brenner, A. C.↗

NASA NTRS1990

The perturbation of ground tracks of periodic orbits

Some geometric characteristics of the ground tracks of periodic orbits are examined. In particular, two theorems are proved concerning points on the ground tracks that are invariant with respect to two periodic orbits. An application of the results to the Topex/Poseidon mission is presented.

Lo, Martin W.↗

NASA NTRS1990

Orbiter-orbiter and orbiter-lander tracking using same-beam interferometry

Two spacecraft orbiting Mars may be tracked simultaneously by a single earth-based antenna. Same-beam interferometric techniques, using two widely separated antennas, produce a spacecraft-spacecraft measurement in the plane of the sky, complementary to the line-of-sight Doppler information. This paper presents an overview of the same-beam interferometric measurement technique, a measurement error analysis, and examples of the application of same-beam interferometry to orbit determination. For the case of Mars Observer and the Soviet Mars '94 mission, orbit determination improvement up to an order of magnitude is found. Relative tracking between a Mars orbiter and a lander fixed on the surface of Mars is also studied. The lander location may be determined to a few meters, while the orbiter ephemeris may be determined with accuracy similar to the orbiter-orbiter case.

Folkner, W. M.↗

NASA NTRS1990

Geographically correlated orbit error for the Topex satellite using GPS tracking

This paper reports on ongoing investigations of geographically correlated orbit error performed in support of precise orbit determination for the Topex/Poseidon mission. Tracking data is assumed to be generated with the Global Positioning System. Geographically correlated orbit error due to errors in modeling the gravity field is described and demonstrated. A method for rational selection of the gravity model coefficients which produce significant orbit error is also presented. The effects of these coefficient errors and other error sources is evaluated using a consider covariance approach. Finally, the covariance analysis is used to evaluate the orbit accuracy for Topex using dynamic, reduced dynamic, and kinematic GPS tracking. The reduced dynamic and kinematic approaches to orbit determination are found to have significant advantages with respect to reducing the level of geographically correlated orbit errors.

Rosborough, George↗

NASA NTRS1991

A comparison of Manchester symbol tracking loops for block 5 applications

The linearized tracking errors of three Manchester (biphase coded) symbol tracking loops are compared to determine which is appropriate for Block 5 receiver applications. The first is a nonreturn to zero (NRZ) symbol synchronizer loop operating at twice the symbol rate (NRZ x 2) so that it operates on half symbols. The second near optimally processes the mid-symbol transitions and ignores the between symbol transitions. In the third configuration, the first two approaches are combined as a hybrid to produce the best performance. Although this hybrid loop is the best at low symbol signal to noise ratios (SNRs), it has about the same performance as the NRZ x 2 loop at higher SNRs (greater than 0-dB E sub s/N sub 0). Based on this analysis, it is tentatively recommended that the hybrid loop be implemented for Manchester data in the Block 5 receiver. However, the high data rate case and the hardware implications of each implementation must be understood and analyzed before the hybrid loop is recommended unconditionally.

Holmes, J. K.↗

NASA NTRS1991

Software manual for operating particle displacement tracking data acquisition and reduction system

The software manual is presented. The necessary steps required to record, analyze, and reduce Particle Image Velocimetry (PIV) data using the Particle Displacement Tracking (PDT) technique are described. The new PDT system is an all electronic technique employing a CCD video camera and a large memory buffer frame-grabber board to record low velocity (less than or equal to 20 cm/s) flows. Using a simple encoding scheme, a time sequence of single exposure images are time coded into a single image and then processed to track particle displacements and determine 2-D velocity vectors. All the PDT data acquisition, analysis, and data reduction software is written to run on an 80386 PC.

Wernet, Mark P.↗

NASA NTRS1991

Particle image velocimetry for the surface tension driven convection experiment using a particle displacement tracking technique

The Surface Tension Driven Convection Experiment (STDCE) is a Space Transportation System flight experiment to study both transient and steady thermocapillary fluid flows aboard the USML-1 Spacelab mission planned for 1992. One of the components of data collected during the experiment is a video record of the flow field. This qualitative data is then quantified using an all electronic, two-dimensional particle image velocimetry technique called particle displacement tracking (PDT) which uses a simple space domain particle tracking algorithm. The PDT system is successful in producing velocity vector fields from the raw video data. Application of the PDT technique to a sample data set yielded 1606 vectors in 30 seconds of processing time. A bottom viewing optical arrangement is used to image the illuminated plane, which causes keystone distortion in the final recorded image. A coordinate transformation was incorporated into the system software to correct this viewing angle distortion. PDT processing produced 1.8 percent false identifications, due to random particle locations. A highly successful routine for removing the false identifications was also incorporated, reducing the number of false identifications to 0.2 percent.

Wernet, Mark P.↗

NASA NTRS1991

Optical joint correlator for real-time image tracking and retinal surgery

A method for tracking an object in a sequence of images is described. Such sequence of images may, for example, be a sequence of television frames. The object in the current frame is correlated with the object in the previous frame to obtain the relative location of the object in the two frames. An optical joint transform correlator apparatus is provided to carry out the process. Such joint transform correlator apparatus forms the basis for laser eye surgical apparatus where an image of the fundus of an eyeball is stabilized and forms the basis for the correlator apparatus to track the position of the eyeball caused by involuntary movement. With knowledge of the eyeball position, a surgical laser can be precisely pointed toward a position on the retina.

Juday, Richard D.↗

NASA NTRS1990

The Tracking and Data Relay Satellite System - The next decade

As currently envisioned, NASA's Tracking and Data Relay Satellite System (TDRSS) will support the tracking and telecommunications requirements of LEO user satellites until the late 1990s, when existing TDRSS satellites will reach the end of their service lives. Spacecraft replacement is conceived as inseparable from network expansion for the accommodation of user population growth and the improvement of user services. The objective is to achieve a cost-effective/low-risk transition from TDRSS to Advanced TDRSS without interruption of user support. Attention is presently given to Advanced TDRSS architectural candidates studied and the choices which have emerged through technical tradeoff assessments.

Guion, William S.↗

NASA NTRS1990

Real time visual tracking of targets in six dimensions

Six dimensional target information is produced by a target tracking vision system for use in real time target tracking by a robotic system. The vision system described in this paper produces three axes of position data and three axes of orientation data using a single camera which views a three dimensional target. The system performs target detection, target discrimination and determines the target position and orientation relative to the camera. As the target information is determined, it is communicated to a computer which is controlling the robot motion in real time. Details of the image processing algorithms and image processing hardware used in the vision system are discussed in the paper.

Walsh, Peter M.↗