China's Space Activities In 2007

THE FOURTH BEIDOU NAVIGATION EXPERIMENTAL SATELLITE LAUNCHED The fourth Beidou experimental navigation satellite was launched into snace aton a LM-3A launch vehicle at Xichang Satellite Launch Center on February 3 at 00:28(Beijing Time).24 minutes later,the satellite separated from the launch vehicle and     

Debut Flight Of LM-3C:Launching Tianlian-1 Successfully

FY-3A,the first satellite of China's new generation of polar-orbiting meteorological satellites,was launched into space atop a modified LM-4C launch vehicle.The satellite separated from the rocket 19 minutes after the takeoff.Flying at an altitude of 807km with an inclination of 98.8 degrees,the satellite circles in polar orbit 14 times everyday,covering the whole globe twice a day. FY-3A has a design lifetime of three years.Weighing 2295kg,the three-axis stabilized satellite is equipped with 11 remote s...     

The mass loss of spacecraft polyimide films under the action of atomic oxygen and vacuum ultraviolet radiation

The threshold values of the annual fluence of atomic oxygen (F _AO ≈ 10²⁰ cm^?2), as well as the ratios of the energy-flux density of vacuum ultraviolet radiation of the solar spectrum to the flux density of atomic oxygen (Φ _ν /Φ _AO ≈ 8 × 10^?15 mJ) were determined, which are characterized the influence of the synergistic effect on the mass loss of Kapton-H, PM-A, and PM-1E polyimide films, which are spacecraft materials.     

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。     

Shock protection of penetrator-based instrumentation via a sublimation approach

It is often necessary for space-borne instrumentation to cope with substantial levels of shock acceleration both in the initial launch phase, as well as during entry, descent and landing in the case of planetary exploration. Current plans for a new generation of penetrator-based space missions will subject the associated on-board instrumentation to far greater levels of shock, and ways must therefore be found to either ruggedize or else protect any sensitive components during the impact phase. In this paper, we present an innovative method of shock protection that is suited for use in a number of planetary environments, based upon the temporary encapsulation of said components within a waxy solid which may then be sublimated to return the instrument back to its normal operation. We have tested this method experimentally using micromachined silicon suspensions under applied shock loads of up to 15,000g, and found that these were able to survive without incurring damage. Furthermore, quality factor measurements undertaken on these suspensions indicate that their mechanical performance remains unaffected by the encapsulation and subsequent sublimation process.     

Comparison between observed ionospheric foF2 and IRI-2001 predictions over periods of severe geomagnetic activities at Grahamstown,South Africa

The observed ionospheric F2 critical frequency (foF2) values over a South Africa mid-latitude station, Grahamstown, (geographic coordinates: 33.3°S, 26.5°E), were analysed and compared with International Reference Ionosphere (IRI) model, using the CCIR (Comite´ Consultatif International des Radio communications) and URSI (Union Radio-Scientifique Internationale) coefficients, during four geomagnetically disturbed days in the year 2000. These days are April 5, May 23, August 10 and September 15. The data were analysed for five days around the storm day. Comparisons between the IRI-2001 predicted foF2 values, using both CCIR and URSI coefficients and the observed values are shown with their root-mean-square error (RMSE) and the relative deviation module mean (rdmm) for the various storm periods. The CCIR option performed more accurately than the URSI option.     

Comparison of ionospheric F2 peak parameters foF2 and hmF2 with IRI2001 at Hainan

Monthly median values of foF2, hmF2 and M(3000)F2 parameters, with quarter-hourly time interval resolution for the diurnal variation, obtained with DPS4 digisonde at Hainan (19.5°N, 109.1°E; Geomagnetic coordinates: 178.95°E, 8.1°N) are used to investigate the low-latitude ionospheric variations and comparisons with the International Reference Ionosphere (IRI) model predictions. The data used for the present study covers the period from February 2002 to April 2007, which is characterized by a wide range of solar activity, ranging from high solar activity (2002) to low solar activity (2007). The results show that (1) Generally, IRI predictions follow well the diurnal and seasonal variation patterns of the experimental values of foF2, especially in the summer of 2002. However, there are systematic deviation between experimental values and IRI predictions with either CCIR or URSI coefficients. Generally IRI model greatly underestimate the values of foF2 from about noon to sunrise of next day, especially in the afternoon, and slightly overestimate them from sunrise to about noon. It seems that there are bigger deviations between IRI Model predictions and the experimental observations for the moderate solar activity. (2) Generally the IRI-predicted hmF2 values using CCIR M(3000)F2 option shows a poor agreement with the experimental results, but there is a relatively good agreement in summer at low solar activity. The deviation between the IRI-predicted hmF2 using CCIR M(3000)F2 and observed hmF2 is bigger from noon to sunset and around sunrise especially at high solar activity. The occurrence time of hmF2 peak (about 1200 LT) of the IRI model predictions is earlier than that of observations (around 1500 LT). The agreement between the IRI hmF2 obtained with the measured M(3000)F2 and the observed hmF2 is very good except that IRI overestimates slightly hmF2 in the daytime in summer at high solar activity and underestimates it in the nighttime with lower values near sunrise at low solar activity.     

Cluster observations showing the indication of the formation of a modified-two-stream instability in the geomagnetic tail

This study presents several observations of the Cluster spacecraft on September 24, 2003 around 15:10 UT, which show necessary prerequisites and consequences for the formation of the so-called modified-two-stream instability (MTSI). Theoretical studies suggest that the plasma is MTSI unstable if (1) a relative drift of electrons and ions is present, which exceeds the Alfvèn speed, and (2) this relative drift or current is in the cross-field direction. As consequences of the formation of a MTSI one expects to observe (1) a field-aligned electron beam, (2) heating of the plasma, and (3) an enhancement in the B-wave spectrum at frequencies in the range of the lower-hybrid-frequency (LHF). In this study we use prime parameter data of the CIS and PEACE instruments onboard the Cluster spacecraft to verify the drift velocities of ions and electrons, FGM data to calculate the expected LHF and Alfvèn velocity, and the direction of the current. The B-wave spectrum is recorded by the STAFF instrument of Cluster. Finally, a field aligned beam of electrons is observed by 3D measurements of the IES instrument of the RAPID unit. Observations are verified using a theoretical model showing the build-up of a MTSI under the given circumstances.     

Study of maximum electron density NmF2 at Karachi and Islamabad during solar minimum (1996) and solar maximum (2000) and its comparison with IRI

The monthly hourly medians of maximum electron density, N_mF₂, at two Pakistani ionospheric stations, Karachi and Islamabad, have been determined for solar minimum (1996) and solar maximum (2000) and compared with IRI predictions using the URSI coefficients. At night and pre-noon period the N_mF₂ values at both stations are almost equal during the 2 years. However, at post-noon the values at Karachi are considerably larger than those at Islamabad due to the equatorial or geomagnetic anomaly. Karachi (geomag. coord. 16.44°N, 139.08°E) lies near the region of the equatorial anomaly (+20 and −20 geomagnetic latitude), so most of the N_mF₂ values at Karachi are larger than those at Islamabad (geomag. coord. 24.46°N, 145.67°E). The maximum monthly values of N_mF₂ show a semi-annual variation at Karachi and Islamabad both during 1996 and 2000 as predicted by IRI.