Multi-Spectral Imager on the Near Earth Asteroid Rendezvous Mission

A multispectral imager has been developed for a rendezvous mission with the near-Earth asteroid, 433 Eros. The Multi-Spectral Imager (MSI) on the Near-Earth Asteroid Rendezvous (NEAR) spacecraft uses a five-element refractive optical telescope, has a field of view of 2.93 × 2.25°, a focal length of 167.35 mm, and has a spatial resolution of 16.1 × 9.5 m at a range of 100 km. The spectral sensitivity of the instrument spans visible to near infrared wavelengths, and was designed to provide insight into the nature and fundamental properties of asteroids and comets. Seven narrow band spectral filters were chosen to provide multicolor imaging and to make comparative studies with previous observations of S asteroids and measurements of the characteristic absorption in Fe minerals near 1 µm. An eighth filter with a much wider spectral passband will be used for optical navigation and for imaging faint objects, down to visual magnitude of +10.5. The camera has a fixed 1 Hz frame rate and the signal intensities are digitized to 12 bits. The detector, a Thomson-CSF TH7866A Charge-Coupled Device, permits electronic shuttering which effectively varies the dynamic range over an additional three orders of magnitude. Communication with the NEAR spacecraft occurs via a MIL-STD-1553 bus interface, and a high speed serial interface permits rapid transmission of images to the spacecraft solid state recorder. Onboard image processing consists of a multi-tiered data compression scheme. The instrument was extensively tested and calibrated prior to launch; some inflight calibrations have already been completed. This paper presents a detailed overview of the Multi-Spectral Imager and its objectives, design, construction, testing and calibration.     

The X-ray/Gamma-ray Spectrometer on the Near Earth Asteroid Rendezvous Mission

An X-ray/gamma-ray spectrometer has been developed as part of a rendezvous mission with the near-Earth asteroid, 433 Eros, in an effort to answer fundamental questions about the nature and origin of asteroids and comets. During about 10 months of orbital operations commencing in early 1999, the X-ray/Gamma-ray Spectrometer will develop global maps of the elemental composition of the surface of Eros. The instrument remotely senses characteristic X-ray and gamma-ray emissions to determine composition. Solar excited X-ray fluorescence in the 1 to 10 keV range will be used to measure the surface abundances of Mg, Al, Si, Ca, Ti, and Fe with spatial resolutions down to 2 km. Gamma-ray emissions in the 0.1 to 10 MeV range will be used to measure cosmic-ray excited elements O, Si, Fe, H and naturally radioactive elements K, Th, U to surface depths on the order of 10 cm. The X-ray spectrometer consists of three gas-filled proportional counters with a collimated field of view of 5° and an energy resolution of 850 eV @ 5.9 keV. Two sunward looking X-ray detectors monitor the incident solar flux, one of which is the first flight of a new, miniature solid-state detector which achieves 600 eV resolution @ 5.9 keV. The gamma-ray spectrometer consists of a NaI(Tl) scintillator situated within a Bismuth Germanate (BGO) cup, which provides both active and passive shielding to confine the field of view and eliminate the need for a massive and costly boom. New coincidence techniques enable recovery of single and double escape events in the central detector. The NaI(Tl) and BGO detectors achieve energy resolutions of 8.7% and 14%, respectively @ 0.662 MeV. A data processing unit based on an RTX2010 microprocessor provides the spacecraft interface and produces 256-channel spectra for X-ray detectors and 1024-channel spectra for the raw, coincident, and anti-coincident gamma-ray modes. This paper presents a detailed overview of the X-ray/Gamma-ray Spectrometer and describes the science objectives, measurement objectives, instrument design, and shows some results from early in-flight data.     

The ACE Magnetic Fields Experiment

The magnetic field experiment on ACE provides continuous measurements of the local magnetic field in the interplanetary medium. These measurements are essential in the interpretation of simultaneous ACE observations of energetic and thermal particles distributions. The experiment consists of a pair of twin, boom- mounted, triaxial fluxgate sensors which are located 165 inches (=4.19 m) from the center of the spacecraft on opposing solar panels. The electronics and digital processing unit (DPU) is mounted on the top deck of the spacecraft. The two triaxial sensors provide a balanced, fully redundant vector instrument and permit some enhanced assessment of the spacecraft's magnetic field. The instrument provides data for Browse and high-level products with between 3 and 6 vector s−1 resolution for continuous coverage of the interplanetary magnetic field. Two high-resolution snapshot buffers each hold 297 s of 24 vector s−1 data while on- board Fast Fourier Transforms extend the continuous data to 12 Hz resolution. Real-time observations with 1-s resolution are provided continuously to the Space Environmental Center (SEC) of the National Oceanographic and Atmospheric Association (NOAA) for near- instantaneous, world-wide dissemination in service to space weather studies. As has been our team's tradition, high instrument reliability is obtained by the use of fully redundant systems and extremely conservative designs. We plan studies of the interplanetary medium in support of the fundamental goals of the ACE mission and cooperative studies with other ACE investigators using the combined ACE dataset as well as other ISTP spacecraft involved in the general program of Sun-Earth Connections. This revised version was published online in June 2006 with corrections to the Cover Date.     

IMMPDAF for radar management and tracking benchmark with ECM

A framework is presented for controlling a phased array radar for tracking highly maneuvering targets in the presence of false alarms (FAs) and electronic countermeasures (ECMs). Algorithms are presented for track formation and maintenance; adaptive selection of target revisit interval, waveform and detection threshold; and neutralizing techniques for ECM, namely, against a standoff jammer (SOJ) and range gate pull off (RGPO). The interacting multiple model (IMM) estimator in combination with the probabilistic data association (PDA) technique is used for tracking. A constant false alarm rate (CFAR) approach is used to adaptively select the detection threshold and radar waveform, countering the effect of jammer-induced false measurements. The revisit interval is selected adaptively, based on the predicted angular innovation standard deviations. This tracker/radar-resource-allocator provides a complete solution to the benchmark problem for target tracking and radar control. Simulation results show an average sampling interval of about 2.5 s while maintaining a track loss less than the maximum allowed 4%     

Signatures of Coronal Hole Spectra Between 660 Å and 1460 Å Measured with Sumer on Soho

Spectra of the northern polar coronal hole measured with the SUMER spectrometer on SOHO on 25 October 1996 are analyzed. We present spectra taken at locations on the solar disk where part of the spectrometer slit intersects a polar coronal hole region and an area of brighter emission from outside of the coronal hole area. By comparing the line intensities between the parts of the spectrum taken inside the "dark" area of the coronal holes and the brighter regions, we work out the signatures of the specific coronal hole in the chromosphere, transition region and lower corona. We find that emissions of neutral atom lines, of which there are many in the spectrum of SUMER, show no difference between the coronal hole and the bright boundary areas, whereas all ionized species show strong intensity enhancements, including the continuum emissions of carbon and hydrogen. These enhancements are larger than in normal quiet Sun areas. This revised version was published online in June 2006 with corrections to the Cover Date.     

Rendezvous Navigation and Guidance Using a Hand-Held Self-Powered Digital Computer

This paper describes a simplified solution of the space rendezvous problem based on optical sightings taken with a hand-held sextant entered into and processed by a small digital computer. Compared to the manual method described in [1], this approach greatly reduces the astronaut's work load. Also, the computer can provide a more accurate solution, can present more flight plan options (allowing more tradeoffs between fuel usage and time), and can allow greater freedom in the sequencing of measurements and maneuvers. Physical characteristics, input-output parameters, and logic design of the computer are briefly described.     

多股射流横向喷入有约束管流过程的模拟

用基于控制体的有限差分方法对一排射流横向喷入管流的流场进行了数值分析，以建立流场温度分布和上游流动条件及几何参数的关系，可变的参数包括：喷嘴直径，喷嘴数，圆柱管半径，射流与主流的体积流量比，温度比和动压比，结果发现，动压比，喷嘴个数和喷嘴间距是三个重要变量，通过对模拟结果的函数拟合，发现射流喷射深度与动压比除以喷嘴数的平方之间的关系可用一个对数函数很好地描述，给出了适用最佳混合的喷射深度和喷嘴间距，提出了可用于结构实设计的程序。     

First results from the six-axis electron spectrometer on ISEE-1

A survey, using results from the first 25 orbits of ISEE-1, has been made of some aspects of electrons in the dawn magnetosheath. There are indications that the flow of plasma is not uniformly turbulent over this region. The electron heat flux is observed to be directed away from the shock and to have an average value of about twice the interplanetary heat flux. Many magnetopause crossings were observed and usually resemble abrupt transitions from one well-defined plasma state to another. The ejection of plasma from flux tubes converted up against the magnetopause is observed for about half the time, and its thickness and dependence on the solar wind mach number agree with theoretical predictions. A full traversal of the whole forward hemisphere of the magnetosheath is required to fully confirm these deductions.     

Vertical and radial metallicity gradients in high latitude galactic fields with SDSS

We used the ugr magnitudes of 1437467 F-G type main-sequence stars with metal abundance $-2?[\mathrm{Fe}/\mathrm{H}]?+0.2$ dex and estimated radial and vertical metallicity gradients for high Galactic-latitude fields, $50°<b?90°$ and $0°<l?360°$, of the Milky Way Galaxy. The radial metallicity gradient $d[\mathrm{Fe}/\mathrm{H}]/\mathit{dR}=-0.042\pm 0.011$ dex kpc^?1 estimated for the stars with $1.31<z\le 1.74$ kpc is attributed to the thin-disc population. While, the radial gradients evaluated for stars at higher vertical distances are close to zero indicating that the thick disc and halo have not undergone a radial collapse phase at least at high Galactic latitudes. The vertical metallicity gradients estimated for stars with three different Galactic latitudes, $50°<b?65°,65°<b?80°$ and $80°<b?90°$ do not show a strong indication for Galactic latitude dependence of our gradients. The thin disc, $0.5<z?2$ kpc, with a vertical metallicity gradient $d\left[\mathrm{Fe}/\mathrm{H}\right]/\mathit{dz}=-0.308\pm 0.018$ dex kpc^?1, is dominant only in galactocentric distance interval $6<R?10$ kpc, while the thick disc ($2<z?5$ kpc) could be observed in the intervals $6<R?10$ and $10<R?15$ kpc with compatible vertical metallicity gradients, i.e. $d\left[\mathrm{Fe}/\mathrm{H}\right]/\mathit{dz}=-0.164\pm 0.014$ dex kpc^?1 and $d\left[\mathrm{Fe}/\mathrm{H}\right]/\mathit{dz}=-0.172\pm 0.016$ dex kpc^?1. Five vertical metallicity gradients are estimated for the halo ($z>5$ kpc) in three galactocentric distance intervals, $6<R?10,10<R?15$ and $15<R?20$ kpc. The first one corresponding to the interval $6<R?10$ kpc is equal to $d\left[\mathrm{Fe}/\mathrm{H}\right]/\mathit{dz}=-0.023\pm 0.006$ dex kpc^?1, while the others at larger galactocentric distances are close to zero. We derived synthetic vertical metallicity gradients for 2,230,167 stars and compared them with the observed ones. There is a good agreement between the two sets of vertical metallicity gradients for the thin disc, while they are different for the thick disc. For the halo, the conspicuous difference corresponds to the galactocentric distance interval $6<R?10$ kpc, while they are compatible at higher galactocentric distance intervals.