Experiments with a Low Polarization Error Loop Array Direction Finder

A previously reported HF loop antenna array for multipolarization direction finding consisting of two spaced loops is described. The method is compatible with twin channel Watson-Watt operation as an Adcock so that the direction finding process is independent of polarization characteristics of the element pattern. Experimentally measured direction finder (DF) bearings were compared with near simultaneous polarimeter data and conventional crossed loops bearings. A major reduction in polarization error was obtained at the expense of full 360° azimuth response. It is theoretically possible to cover 360° with two similar arrays of the type investigated or with crossed loops elements with quadrature phase shifters.     

Attitude and Oribit Estimation Using Stars and Landmarks

An extended Kalman filter is used to process line-of-sight measurements to stars and known landmarks providing a statistical indication of performance in estimating spacecraft attitude, orbital ephemeris, and the bias drift of a set of three strapdown gyros. The landmark measurements were assumed to have been taken from the imagery of an Earth-observing multispectral scanner. It is shown that filtering of these noisy measurements results in highly accurate estimates of the above parameters. Results are given showing the sensitivity of performance to various system parameters such as star tracker accuracy, errors in the knowledge of landmark position, and number of stars and landmarks processed.     

IBEX Education and Public Outreach

The IBEX mission includes a comprehensive Education and Public Outreach (E/PO) program that develops programs and products in exciting themes from astronomy and space physics. With the active involvement of the Principal Investigator and several science team partners, it is overseen and implemented by the Adler Planetarium & Astronomy Museum. The program includes an internationally distributed planetarium show with accompanying informal education materials that are also accessible to individuals with special needs; a national Space Science Core Curriculum for grades 6–8 in collaboration with other NASA missions; a professional development program for teachers; and workshops that engage Hispanic and Native American students. Materials are made available for download or for order via the IBEX website: http://www.ibex.swri.edu/. Our program is developed, implemented and monitored for effectiveness by organizations with proven capabilities and experience in their respective areas of E/PO. This paper describes these program elements in detail and includes the rationale and design process for this E/PO program.     

单推力航天器交会对接轨迹规划及跟踪控制

针对单推力航天器交会对接问题,提出一种轨迹规划及跟踪算法。首先,考虑到追踪航天器只沿本体X轴安装推力器,且推力方向固定,为了实现从起始位置转移至期望位置并满足姿态要求,基于三维螺旋线设计两阶段转移轨迹,根据初末位置以及末端速度方向要求,求解螺旋线参数。该螺旋线可以保证在初末速度方向固定情况下,曲率积分最小。其次,为了降低轨迹跟踪难度并减小初始时刻的位置跟踪控制力,需要将转移轨迹初始速度与追踪星X轴重合。传统螺旋线无法满足该约束条件。本文对传统螺旋线进行改进,提出一种旋转螺旋线轨迹设计方法。通过引入姿态旋转矩阵,将螺旋线在三维空间旋转,在不改变曲线形状的前提下满足初末位置及速度方向要求。然后,为了跟踪转移轨迹以及跟踪期望推力方向,提出基于CLF（Control Lyapunov Function）的滑模控制策略,当追踪星X轴与期望推力方向夹角较大时,采用CLF,保证最优性;当姿态误差收敛至滑模面附近时,切换为滑模控制,以提升系统鲁棒性。最后,通过仿真验证旋转螺旋线相比于传统螺旋线的优势。     

Integrated acoustic monitoring system for jet fuel tanks

The paper describes a device and a technique of forming the information processing algorithms used in the integrated acoustic monitoring system for jet fuel tanks, intended for monitoring the level, density and mass of liquid products.     

Assessment study of ionosphere correction model using single- and multi-shell algorithms approach over sub-Saharan African region

It is a known fact that ionosphere is the largest and the least predictable among the sources of error limiting the reliability and accuracy of Global Navigation Satellite Systems (GNSS) and its regional augmentation systems like Satellite Based Augmentation System (SBAS) in a safety-of-life application. The situation becomes worse in the Equatorial Ionization Anomaly (EIA) region, where the daytime ionization distribution is modified by the fountain effect that develops a crest of electron density at around ±15° to ±20° of the magnetic equator and a trough at the magnetic equator during the local noon hours. Related to this phenomenon is the appearance of ionosphere irregularities and plasma bubbles after local sunset. These may degrade further the quality of service obtained from the GNSS/SBAS system of the said periods. Considering the present operational augmentation systems, the accuracy and integrity of the ionosphere corrections estimate decreases as the level of disturbances increases. In order to provide a correct ionosphere correction to the user of GNSS operating in African EIA region and meet the integrity requirements, a certified ionosphere correction model that accurately characterizes EIA gradient with the full capacity to over-bound the residual error will be needed. An irregularities detector and a decorrelation adaptor are essential in an algorithm usable for African sub-Saharan SBAS operation. The algorithm should be able to cater to the equatorial plasma vertical drifts, diurnal and seasonal variability of the ionosphere electron density and also should take into account the large spatial and temporal gradients in the region. This study presents the assessment of the ionosphere threat model with single and multi-layer algorithm, using modified planar fit and Kriging approaches.     

The Freja wave and plasma density experiment

The Wave Experiment, F4, on the Swedish/German satelliteFreja, is designed to measure the electric wave fields up to 4 MHz, the magnetic wave fields up to 16 kHz and the plasma density and its relative variations up to 2 kHz. Six wave signals and four density probe signals can be measured simultaneously. The wave forms of all signals are transmitted to ground without any analysis onboard. The limited TM allocation does not allow continuous sampling of the wave signals, so normally the measurements are made in snapshots of various lengths dependent on sampling frequency, etc. Continuous sampling can be made for shorter time periods by using a 6 Mbyte memory as a buffer.