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NASA NTRS1994

Determination of copper in tap water using solid-phase spectrophotometry

A new application of ion exchange films is presented. The films are used in a simple analytical method of directly determining low concentrations of Cu(2+) in aqueous solutions, in particular, drinking water. The basis for this new test method is the color and absorption intensity of the ion when adsorbed onto the film. The film takes on the characteristic color of the adsorbed cation, which is concentrated on the film by many orders of magnitude. The linear relationship between absorbance (corrected for variations in film thickness) and solution concentration makes the determinations possible. These determinations agree well with flame atomic absorption determinations.

Hill, Carol M.↗

NASA NTRS1994

Methodologies to determine forces on bones and muscles of body segments during exercise, employing compact sensors suitable for use in crowded space vehicles

A complete description of an instrumented ergometer system, including the sensors, the data acquisition system, and the methodologies to calculate the kinematic parameters were initially developed at Tulane University. This work was continued by the PI at NASA Johnson Space Center, where a flight ergometer was instrumented and tested during a KC-135 Zero-Gravity flight. The sensors that form part of the system include EMG probes and accelerometers mounted on the subject using the ergometer, load cells to measure pedal forces, and encoders to measure position and orientation of the pedal (foot). Currently, data from the flight test is being analyzed and processed to calculate the kinematic parameters of the individual. The formulation developed during the initial months of the grant will be used for this purpose. The system's components are compact (all sensors are very small). A salient feature of the system and associated methodology to determine the kinematics is that although it uses accelerometers, position is not determined by integration. Position is determined by determining the angle of two frames of reference for which acceleration at one point is known in coordinates of both frames.

Figueroa, Fernando↗

NASA NTRS1994

Precision orbit determination of altimetric satellites

The ability to determine accurate global sea level variations is important to both detection and understanding of changes in climate patterns. Sea level variability occurs over a wide spectrum of temporal and spatial scales, and precise global measurements are only recently possible with the advent of spaceborne satellite radar altimetry missions. One of the inherent requirements for accurate determination of absolute sea surface topography is that the altimetric satellite orbits be computed with sub-decimeter accuracy within a well defined terrestrial reference frame. SLR tracking in support of precision orbit determination of altimetric satellites is significant. Recent examples are the use of SLR as the primary tracking systems for TOPEX/Poseidon and for ERS-1 precision orbit determination. The current radial orbit accuracy for TOPEX/Poseidon is estimated to be around 3-4 cm, with geographically correlated orbit errors around 2 cm. The significance of the SLR tracking system is its ability to allow altimetric satellites to obtain absolute sea level measurements and thereby provide a link to other altimetry measurement systems for long-term sea level studies. SLR tracking allows the production of precise orbits which are well centered in an accurate terrestrial reference frame. With proper calibration of the radar altimeter, these precise orbits, along with the altimeter measurements, provide long term absolute sea level measurements. The U.S. Navy's Geosat mission is equipped with only Doppler beacons and lacks laser retroreflectors. However, its orbits, and even the Geosat orbits computed using the available full 40-station Tranet tracking network, yield orbits with significant north-south shifts with respect to the IERS terrestrial reference frame. The resulting Geosat sea surface topography will be tilted accordingly, making interpretation of long-term sea level variability studies difficult.

Shum, C. K.↗

NASA NTRS1994

Highly transparent and rugged sensor for velocity determinations of cosmic dust particles

In order to understand the evolution of interplanetary dust, numerous dust particles have been collected and analyzed. An analysis of the composition often provides information on the particle's origin. So does its origin. Composition and orbit data complement each other and should be determined together. If the last orbit of a particle can be determined, its orbital history can often be calculated backward in time and associated with its parent body. To determine the last orbit, the velocity needs to be measured before the particle is collected. The precision required in determining the velocity components relative to the spacecraft should be 1 percent or better. A sensor for naturally charged cosmic dust particles is discussed. Two models of the sensor were tested, one with a free-falling steel ball and the other with particles accelerated to high speed. Analytic expressions of the sensor signals are presented and compared with the test results. The errors in speed and angle were estimated to be about 0.3 percent and 0.2 degrees respectively.

Auer, Siegfried↗

NASA NTRS1994

Relative elemental abundance and heating constraints determined for the solar corona from SERTS measurements

Intensities of EUV spectral lines were measured as a function of radius off the solar limb by two flights of Goddard's Solar EUV Rocket Telescope and Spectrograph (SERTS) for three quiet sun regions. The density scale height, line-ratio densities, line-ratio temperatures, and emission measures were determined. The line-ratio temperature determined from the ionization balances of Arnaud and Rothenflug (1985) were more self-consistent than the line-ratio temperatures obtained from the values of Arnaud and Raymond (1992). Limits on the filling factor were determined from the emission measure and the line-ratio densities for all three regions. The relative abundances of silicon, aluminum, and chromium to iron were determined. Results did agree with standard coronal relative elemental abundances for one observation, but did not agree for another. Aluminum was overabundant while silicon was underabundant. Heating was required above 1.15 solar radii for all three regions studied. For two regions, local nonconductive heating is needed for any filling factor, and in all three regions for filling factor of 0.1.

Falconer, David A.↗

NASA NTRS1995

Development of a Robust star identification technique for use in attitude determination of the ACE spacecraft

The Advanced Composition Explorer (ACE) spacecraft is designed to fly in a spin-stabilized attitude. The spacecraft will carry two attitude sensors - a digital fine Sun sensor and a charge coupled device (CCD) star tracker - to allow ground-based determination of the spacecraft attitude and spin rate. Part of the processing that must be performed on the CCD star tracker data is the star identification. Star data received from the spacecraft must be matched with star information in the SKYMAP catalog to determine exactly which stars the sensor is tracking. This information, along with the Sun vector measured by the Sun sensor, is used to determine the spacecraft attitude. Several existing star identification (star ID) systems were examined to determine whether they could be modified for use on the ACE mission. Star ID systems which exist for three-axis stabilized spacecraft tend to be complex in nature and many require fairly good knowledge of the spacecraft attitude, making their use for ACE excessive. Star ID systems used for spinners carrying traditional slit star sensors would have to be modified to model the CCD star tracker. The ACE star ID algorithm must also be robust, in that it will be able to correctly identify stars even though the attitude is not known to a high degree of accuracy, and must be very efficient to allow real-time star identification. The paper presents the star ID algorithm that was developed for ACE. Results from prototype testing are also presented to demonstrate the efficiency, accuracy, and robustness of the algorithm.

Woodard, Mark↗

NASA NTRS1994

The determination of the mass and mean density of Enceladus from its observed shape

Application of limb-fitting methods to the 11 best Voyager 2 images of Enceladus has shown that the shape of this satellite is closely represented by a triaxial ellipsoid. The observed ratio of the differences of the principal axes F = (b - c)/(a - c) is 0.23 (sup +0.04 sub - 0.01), consisting with the value F = 0.23 expected for a synchronously rotating satellite in hydrostatic equilibrium. We also deduce from the Voyager observations, after allowing for limb topography, that the mean radius of the satellite is 249.4 +/- 0.2 km. For satellites of known mass, measurement of the size and shape leads to a determination of the satellite's mean density and moment of inertial. We have used this method to determine the moments of inertia of Mimas (1988) and Tethys (1991). Enceladus appears to be hydrostatically relaxed, making it an ideal candidate for this type of analysis. However, none of the Pioneer or Voyager spacecraft had a close encounter with this satellite and thus its mass is effectively unknown. Enceladus is trapped in a 2:1 orbit-orbit resonance with Dione, but the amplitudes of libration are too small to allow a useful mass determination. Using the observed shape alone, without any other assumptions other than that the satellite is in hydrostatic equilibrium at its present orbital radius, we place an upper bound on the mean density of 1.12 +/- 0.05 g/cu cm. Thus, the mean density of Enceladus is probably little more than that of water-ice and we conclude that this satellite is markedly deficient in rock. If the mass of a satellite is unknown, but the satellite is differentiated and has a deep mantle of known composition, then we show that measurement of the shape alone can lead to a determination of the satellite's mass, mean density, and moment of inertia. Application of this method to Enceladus, assuming that the satellite has a deep mantle of water-ice of density 0.93 g/cu cm, gives the result that the mean density of the satellite is 1.00 +/- 0.03 g/cu cm. This result fills the one remaining gap in our knowledge of the structure of the Saturnian satellite system. We now know the mean densities of all the primary Saturnian satellites in the sequence from the coorbital satellites, Janus and Epimetheus, through to the outer satellite Iapetus (the densities of the small, secondary satellites in Trojan-type orbits are still unknown). The Saturnian system possesses two striking features. (1) Because of significant porosity, the mean material densities of the satellites Janus, Epimetheus, and Mimas could be substantially greater than the apparent mean densities of these satellies. (2) The densities of the satellites are not correlated with their distances from the planet; in particular, the satellites Enceladus and Tethys have lower mean densities than their interior and exterior neighbors, Mimas and Dione. This may be the result of gross postformation redistribution of rock and ice, possibly due to satellite disruptions are suggested by Smith et al. (1982).

Dermott, Stanley F.↗

NASA NTRS1994

Dynamic orbit determination using GPS measurements from TOPEX/POSEIDON

The GPAS data acquired by the TOPEX/POSEIDON (T/P) Demonstration Receiver (DR) have been used in a dynamic orbit determination, which was based on the description of the gravitational and nongravitational forces in the equations of motion. The GPS carrier phase data were processed in a double difference mode to remove clock errors, including the effects of Selective Availability. Simultaneous estimation of the T/P orbit and GPS orbits was performed using five 10-day cycles in the interval between December (1992) and April (1993). The resulting T/P orbits have been compared with the orbits determined from Satellite Laser Ranging, the French one-way Doppler tracking system, DORIS, and with the JPL reduced dynamic orbit determination strategies and force models with the GPS/DR to those used with SLR/DORIS, the radial component of the T/P orbit (based on JGM-2) was found to agree better than 30 mm (rms) and 35 mm with the JPL reduced dynamic orbit. An experiment gravity tuning was accomplished using four cycles of GPS/DR data. The resulting GPS./DR-orbits, determined by the dynamic technique with the experimental gravity field, are in better agreement with the JPL reduced dynamic orbits in both the radial component (21-25 mm) and altimeter crossover residuals than the JGM-2 orbits. (21-25 mm) and altimeter crossover residuals than the JGM-2 orbits.

Schultz, B. E.↗

NASA NTRS1995

Determination of plasma temperatures and luminosities using multiple extreme-ultraviolet and X-ray filters

We carefully examine the techniques used to infer temperatures of stellar coronal plasmas from the count rates of several broadband instruments in the X-ray and extreme-ultraviolet spectral ranges. In particular, we determine to what extent temperatures can be constrained and the corresponding uncertainties in the luminosities and emission measures lowered by fitting simultaneously count rates from the Einstein imaging proportional counter (IPC), the ROSAT Position Sensitive Proportional Counter (PSPC), the ROSAT Wide Field Camera (WFC) (both filters), and the EXOSAT Low Energy Telescope (LET) with the 3-Lex filter. We use published plasma emissivities with solar photospheric abundances. Since it has been found that single-temperature plasmas do not fit IPC data well, we assume a two-temperature plasma model. We find that, even with count rates from all of the above filters and overly optimistic error estimates, it is still not possible to determine a unique two-temperature solution. However, since the use of count rates from many filters can reduce substantially the number of possible solutions, temperature solutions determined by other means can be tested. We carry out such an analysis on a set of 18 nearby late-type stars to determine possible two-temperature solutions using multifilter photometry, and we compare these results with the temperature solutions derived by Schmitt et al. (1990) using IPC spectral data. In general, the two-temperature fits derived from the IPC spectral data are inconsistent with our results, with our data implying that, for many stars, the two temperatures derived by the IPC may be too low by about a factor of 2. The EXOSAT transmission grating Spectrometer (TGS) spectra of capella and sigma(exp 2) CrB support this conclusion. For Procyon and 70 Oph, though, the presence of a temperature component cooler than a million degress (not detected by the IPC) is deduced. While our analysis suggests the existence of more than one temperature in the coronae of late-type stars, in many instances our WFC data appear to be inconsistent with the presence of significant emission measure over a broad temperature distribution. This, together with the success of two-temperature plasmas in fitting IPC and TGS data, implies that for many stars, the coronal emission measure distribution may in fact be dominated by two distinct temperature regimes.

Wood, Brian E.↗

NASA NTRS1993

Short arc orbit determination for altimeter calibration and validation on TOPEX/POSEIDON

TOPEX/POSEIDON (T/P) is a joint mission of United States' National Aeronautics and Space Administration (NASA) and French Centre National d'Etudes Spatiales (CNES) design launched August 10, 1992. It carries two radar altimeters which alternately share a common antenna. There are two project designated verification sites, a NASA site off the coast at Pt. Conception, CA and a CNES site near Lampedusa Island in the Mediterranean Sea. Altimeter calibration and validation for T/P is performed over these highly instrumented sites by comparing the spacecraft's altimeter radar range to computed range based on in situ measurements which include the estimated orbit position. This paper presents selected results of orbit determination over each of these sites to support altimeter verification. A short arc orbit determination technique is used to estimate a locally accurate position determination of T/P from less than one revolution of satellite laser ranging (SLR) data. This technique is relatively insensitive to gravitational and non-gravitational force modeling errors and is demonstrated by covariance analysis and by comparison to orbits determined from longer arcs of data and other tracking data types, such as Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) and Global Positioning System Demonstration Receiver (GPSDR) data.

Williams, B. G.↗

NASA NTRS1993

Evaluation of Landsat-4 orbit determination accuracy using batch least-squares and sequential methods

The Goddard Space Flight Center (GSFC) Flight Dynamics Division (FDD) commissioned Applied Technology Associates, Incorporated, to develop the Real-Time Orbit Determination/Enhanced (RTOD/E) system on a Disk Operating System (DOS)-based personal computer (PC) as a prototype system for sequential orbit determination of spacecraft. This paper presents the results of a study to compare the orbit determination accuracy for a Tracking and Data Relay Satellite (TDRS) System (TDRSS) user spacecraft, Landsat-4, obtained using RTOD/E, operating on a PC, with the accuracy of an established batch least-squares system, the Goddard Trajectory Determination System (GTDS), operating on a mainframe computer. The results of Landsat-4 orbit determination will provide useful experience for the Earth Observing System (EOS) series of satellites. The Landsat-4 ephemerides were estimated for the May 18-24, 1992, timeframe, during which intensive TDRSS tracking data for Landsat-4 were available. During this period, there were two separate orbit-adjust maneuvers on one of the TDRSS spacecraft (TDRS-East) and one small orbit-adjust maneuver for Landsat-4. Independent assessments were made of the consistencies (overlap comparisons for the batch case and covariances and the first measurement residuals for the sequential case) of solutions produced by the batch and sequential methods. The forward-filtered RTOD/E orbit solutions were compared with the definitive GTDS orbit solutions for Landsat-4; the solution differences were generally less than 30 meters after the filter had reached steady state.

Oza, D. H.↗

NASA NTRS1993

Determination of radio-frame position for Earth and Jupiter from Ulysses encounter tracking

Radio metric tracking data acquired from the Ulysses spacecraft about its encounter with Jupiter in February 1992 allow an accurate measurement of some components of the orbital elements describing the positions of the Earth and Jupiter with respect to extragalactic radio sources. Range and Doppler data acquired from the Earth while the spacecraft is far from any planet provide an estimate of the spacecraft trajectory relative to the orbit of the Earth. Doppler data near Jupiter provide an accurate position determination of the spacecraft with respect to Jupiter. Very Long Baseline Interferometry observations of the spacecraft with respect to the distant radio sources provide a direct measure of the spacecraft position in the radio reference frame. Combining these measurements provides a means to estimate the locations of the Earth and Jupiter in the radio reference frame. One of the three Euler angles describing the orientation of the Earth's orbit in the radio frame has been determined to an accuracy of 50 nanoradians; the result agrees with other recent determinations of this orientation. The position of Jupiter at the time of Ulysses encounter has been determined to 15 nanoradians in ecliptic latitude and longitude.

Folkner, W. M.↗

NASA NTRS1993

Orbit determination covariance analysis for the Deep Space Program Science Experiment mission

To define an appropriate orbit support procedure for the DSPSE mission, detailed permission orbit determination covariance analyses have been performed for the translunar and trans-Geographos mission phases. Preliminary analyses were also performed for the lunar mapping mission phase. These analyses are designed to assess the tracking patterns and the amount of tracking data needed to obtain orbit solutions of required accuracy for each mission phase and before and after each major orbit perturbation, such as orbit maneuvers and flybys of the Earth and Moon. In addition to operational orbit determination procedures, these analyses identify major error sources, estimate their contribution to orbital errors, and address possible strategies to reduce orbit determination error. For the lunar orbit phase, several lunar gravity error modeling approaches have been investigated. The covariance analysis results presented in this paper will serve as a guide for providing orbit determination support for the DSPSE mission.

Beckman, M.↗

NASA NTRS1994

Understanding the determinants of problem-solving behavior in a complex environment

It is often argued that problem-solving behavior in a complex environment is determined as much by the features of the environment as by the goals of the problem solver. This article explores a technique to determine the extent to which measured features of a complex environment influence problem-solving behavior observed within that environment. In this study, the technique is used to determine how complex flight deck and air traffic control environment influences the strategies used by airline pilots when controlling the flight path of a modern jetliner. Data collected aboard 16 commercial flights are used to measure selected features of the task environment. A record of the pilots' problem-solving behavior is analyzed to determine to what extent behavior is adapted to the environmental features that were measured. The results suggest that the measured features of the environment account for as much as half of the variability in the pilots' problem-solving behavior and provide estimates on the probable effects of each environmental feature.

Casner, Stephen A.↗

NASA NTRS1996

GPS Attitude Determination Using Deployable-Mounted Antennas

The primary objective of this investigation is to develop a method to solve for spacecraft attitude in the presence of potential incomplete antenna deployment. Most research on the use of the Global Positioning System (GPS) in attitude determination has assumed that the antenna baselines are known to less than 5 centimeters, or one quarter of the GPS signal wavelength. However, if the GPS antennas are mounted on a deployable fixture such as a solar panel, the actual antenna positions will not necessarily be within 5 cm of nominal. Incomplete antenna deployment could cause the baselines to be grossly in error, perhaps by as much as a meter. Overcoming this large uncertainty in order to accurately determine attitude is the focus of this study. To this end, a two-step solution method is proposed. The first step uses a least-squares estimate of the baselines to geometrically calculate the deployment angle errors of the solar panels. For the spacecraft under investigation, the first step determines the baselines to 3-4 cm with 4-8 minutes of data. A Kalman filter is then used to complete the attitude determination process, resulting in typical attitude errors of 0.50.

Osborne, Michael L.↗

NASA NTRS1996

An Evaluation of Attitude-Independent Magnetometer-Bias Determination Methods

Although several algorithms now exist for determining three-axis magnetometer (TAM) biases without the use of attitude data, there are few studies on the effectiveness of these methods, especially in comparison with attitude dependent methods. This paper presents the results of a comparison of three attitude independent methods and an attitude dependent method for computing TAM biases. The comparisons are based on in-flight data from the Extreme Ultraviolet Explorer (EUVE), the Upper Atmosphere Research Satellite (UARS), and the Compton Gamma Ray Observatory (GRO). The effectiveness of an algorithm is measured by the accuracy of attitudes computed using biases determined with that algorithm. The attitude accuracies are determined by comparison with known, extremely accurate, star-tracker-based attitudes. In addition, the effect of knowledge of calibration parameters other than the biases on the effectiveness of all bias determination methods is examined.

Hashmall, J. A.↗

NASA NTRS1996

Some Considerations in the Determination of the Accuracy of a Measurement in Space of the Newtonian Gravitational Constant (G)

A commonly suggested method for determining the Newtonian constant of universal gravitation (G) is to observe the motion of two bodies of known mass moving about each other in an orbiting laboratory. In low Earth orbit (LEO), bodies constructed of even the densest material available experience a gravitational attraction that is several times smaller than the 'tidal' forces (due to their proximity to the Earth), which tend to pull them apart. While the tidal forces do not preclude stable orbits of the two objects about each other, they and the Coriolis force (in the rotating laboratory) dominate the motion, and the gravitational attraction of the two bodies may be considered a weak (but significant) contribution to the motion. As a result, compared to an experiment that would be performed in a laboratory far from the Earth, greater accuracy of measuring the motion of the two bodies may be required for a given accuracy in the determination of G. We find that the accuracy with which positions must be determined is not much different in an experiment in LEO than in one performed far from the Earth, but that rotational periods must be determined more accurately. Using a curvature matrix analysis, we also find that a value of G may be extracted (with some loss in accuracy, but probably some practical gain) from an analysis of the time dependence of the distance between the bodies rather than of a full specification (distance and direction) of their relative positions. A measurement of the gravitational constant to one part in 10(exp 4) continues to be thinkable, but one part in 10(exp 5) will be very difficult.

Baker, Stephen D.↗

NASA NTRS1997

Determination of Coronal Magnetic Fields from Vector Magnetograms

During the course of the present contract we developed an 'evolutionary technique' for the determination of force-free coronal magnetic fields from vector magnetograph observations. The method can successfully generate nonlinear force- free fields (with non-constant-a) that match vector magnetograms. We demonstrated that it is possible to determine coronal magnetic fields from photospheric measurements, and we applied it to vector magnetograms of active regions. We have also studied theoretical models of coronal fields that lead to disruptions. Specifically, we have demonstrated that the determination of force-free fields from exact boundary data is a well-posed mathematical problem, by verifying that the computed coronal field agrees with an analytic force-free field when boundary data for the analytic field are used; demonstrated that it is possible to determine active-region coronal magnetic fields from photospheric measurements, by computing the coronal field above active region 5747 on 20 October 1989, AR6919 on 15 November 1991, and AR7260 on 18 August 1992, from data taken with the Stokes Polarimeter at Mees Solar Observatory, University of Hawaii; started to analyze active region 7201 on 19 June 1992 using measurements made with the Advanced Stokes Polarimeter at NSO/Sac Peak; investigated the effects of imperfections in the photospheric data on the computed coronal magnetic field; documented the coronal field structure of AR5747 and compared it to the morphology of footpoint emission in a flare, showing that the 'high- pressure' H-alpha footpoints are connected by coronal field lines; shown that the variation of magnetic field strength along current-carrying field lines is significantly different from the variation in a potential field, and that the resulting near-constant area of elementary flux tubes is consistent with observations; begun to develop realistic models of coronal fields which can be used to study flare trigger mechanisms; demonstrated that magnetic nonequilibrium can disrupt sheared coronal arcades, and that helmet streamers can disrupt, leading to coronal mass ejections. Our model has significantly extended the realism with which the coronal magnetic field can be inferred from actual observations. In a subsequent contract awarded by NASA, we have continued to apply and improve the evolutionary technique, to study the physical properties of active regions, and to develop theoretical models of magnetic fields.

Mikic, Zoran↗