Integer phase ambiguity estimation in next-generation geodetic Very Long Baseline Interferometry

Next-generation Very Long Baseline Interferometry (VLBI) system designs are aiming at 1 mm global position accuracy. In order to achieve this, it is not only necessary to deploy improved VLBI systems, but also to develop analysis strategies that take full advantage of the observations taken. Since the new systems are expected to incorporate four independent radio frequency bands, it should be feasible to resolve phase ambiguities directly from post-correlation data, providing roughly an order of magnitude improvement in precision of the delay observable. As the unknown ambiguities are of integer nature, it is discussed here how they the can be resolved analytically using algorithms which have been developed for Global Navigation Satellite System (GNSS) applications. Furthermore, it will be shown that ionosphere contribution and source structure effects, so-called core-shifts, can be solved simultaneously with the delay, which is the main geodetic observable for follow-on analysis. In order to verify the proposed algorithm, simulated observations were created using parameters from actual design studies. It is shown that, even in the case of low signal-to-noise ratio observations, reliable phase ambiguity resolution can be achieved and it is discussed how the integer ambiguity recovery depends on the number of observations and signal-to-noise ratio.     

WMAP extragalactic point sources as potential Space VLBI calibrators

The point source list of the Wilkinson Microwave Anisotropy Probe (WMAP) is a uniform, all-sky catalogue of bright sources with flux density measurements at high (up to 94 GHz) radio frequencies. We investigated the five-year WMAP list to compile a new catalogue of bright and compact extragalactic radio sources to be potentially studied with Very Long Baseline Interferometry at millimeter wavelengths (mm-VLBI) and Space VLBI (SVLBI). After comparing the WMAP data with the existing mm-VLBI catalogues, we sorted out the yet unexplored sources. Using the 41, 61 and 94 GHz WMAP flux densities, we calculated the spectral indices. By collecting optical identifications, lower-frequency radio flux densities and VLBI images from the literature, we created a list of objects which have not been investigated with VLBI at 86 GHz before. With total flux density at least 1 Jy and declination above −40°, we found 37 suitable new targets. It is a nearly 25% addition to the known mm-VLBI sources. Such objects are also potentially useful as phase-reference calibrators for the future Japanese SVLBI mission ASTRO-G at its highest observing frequency (43 GHz). The phase-referencing capability of ASTRO-G would allow long integrations and hence better sensitivity for observing faint target sources close to suitable phase calibrators in the sky.     

一种用于同步静止卫星监测的微型VLBI网

针对地球静止轨道（GEO）卫星全天时全天候高精度的监测需求，考虑传统甚长基线干涉（VLBI）测站高成本、高投入和GEO卫星专用观测时段有限等制约条件，研发了简易型VLBI观测系统，并组建了包括上海、都匀和乌鲁木齐三站的微型VLBI网（micro VLBI network，MVN），开展了并置站测试以及对GEO卫星亚太6C的连续监测，并评估了当前MVN的观测能力。结果表明MVN扣除系统差后的单站接收精度为2ns，各基线观测时延拟后残差约几纳秒，GEO目标实测位置精度为百米级（内外符精度分别约100m和400m）。不同于传统VLBI和其他GEO监测手段，MVN还具备全天时、全天候、低造价、易布设及易推广等特点，充分表明了其在GEO卫星监测领域的应用价值。     

Application of time-variable process noise in terrestrial reference frames determined from VLBI data

In recent years, Kalman filtering has emerged as a suitable technique to determine terrestrial reference frames (TRFs), a prime example being JTRF2014. The time series approach allows variations of station coordinates that are neither reduced by observational corrections nor considered in the functional model to be taken into account. These variations are primarily due to non-tidal geophysical loading effects that are not reduced according to the current IERS Conventions (2010). It is standard practice that the process noise models applied in Kalman filter TRF solutions are derived from time series of loading displacements and account for station dependent differences. So far, it has been assumed that the parameters of these process noise models are constant over time. However, due to the presence of seasonal and irregular variations, this assumption does not truly reflect reality. In this study, we derive a station coordinate process noise model allowing for such temporal variations. This process noise model and one that is a parameterized version of the former are applied in the computation of TRF solutions based on very long baseline interferometry data. In comparison with a solution based on a constant process noise model, we find that the station coordinates are affected at the millimeter level.     

VLBI observations to the APOD satellite

The APOD (Atmospheric density detection and Precise Orbit Determination) is the first LEO (Low Earth Orbit) satellite in orbit co-located with a dual-frequency GNSS (GPS/BD) receiver, an SLR reflector, and a VLBI X/S dual band beacon. From the overlap statistics between consecutive solution arcs and the independent validation by SLR measurements, the orbit position deviation was below 10?cm before the on-board GNSS receiver got partially operational. In this paper, the focus is on the VLBI observations to the LEO satellite from multiple geodetic VLBI radio telescopes, since this is the first implementation of a dedicated VLBI transmitter in low Earth orbit. The practical problems of tracking a fast moving spacecraft with current VLBI ground infrastructure were solved and strong interferometric fringes were obtained by cross-correlation of APOD carrier and DOR (Differential One-way Ranging) signals. The precision in X-band time delay derived from 0.1?s integration time of the correlator output is on the level of 0.1?ns. The APOD observations demonstrate encouraging prospects of co-location of multiple space geodetic techniques in space, as a first prototype.     

The bistatic radar capabilities of the Medicina radiotelescopes in space debris detection and tracking

An accurate measurement of the position and trajectory of the space debris fragments is of primary importance for the characterization of the orbital debris environment. The Medicina Radioastronomical Station is a radio observation facility that is here proposed as receiving part of a ground-based space surveillance system for detecting and tracking space debris at different orbital regions (from Low Earth Orbits up to Geostationary Earth Orbits). The proposed system consists of two bistatic radars formed by the existing Medicina receiving antennas coupled with appropriate transmitters. This paper focuses on the current features and future technical development of the receiving part of the observational setup. Outlines of possible transmitting systems will also be given together with the evaluation of the observation strategies achievable with the proposed facilities.     

一种VLBI数字基带转换器的设计与研究

数字基带转换器在VLBI系统中主要完成中频信号的基带转换和频段选择,文中采用频域抽取和邻信道合并技术设计一种VLBI数字基带转换器。采用该方法设计的转换器具有比较平坦的通带特性和较大的阻带衰减,能够方便地实现带宽的动态变化。文中对频域抽取进行理论推导论证,并根据邻信道合并的条件,详细说明了原型低通滤波器的设计方法,最后通过仿真验证了新方法的有效性。     

基于我国VLBI网航天器定位归算仿真分析

结合我国VLBI观测网现状,通过仿真计算分析了观测时延噪声与系统差、预报轨道偏差和测距误差等对航天器定位归算的影响。结果表明,仅利用VLBI跟踪时附加飞行器地心距约束有利于压制定位点的弥散。 3站 VLBI跟踪的定位解是有偏估计,4站跟踪时要求所加约束的误差应与预报轨道偏差相当,若过大将起不到压制定位点弥散的良好效果,若过小则会在定位结果中引入系统偏差。时延与测距的观测噪声主要引起定位点序列的弥散。若观测量存在系统差则将引起定位结果的系统性偏差,而且定位点序列会发生渐近线性趋势跳变。同时,这也可以作为观测量是否存在系统差的判据。另外,若某时刻前后VLBI跟踪站存在3、4站变更,定位点序列有时发生趋势变化。分析结论对于我国后续探月和萤火计划等工程实践具有借鉴意义。     

一种用于深空导航定位的新型DOR信标

甚长基线干涉测量(VLBI)是深空探测器导航定位的重要手段之一,深空探测器的VLBI观测通常采用差分单程测距(DOR)信标和双差单程测距(Delta-DOR)测量体制,精确地测量深空探测器相对于河外射电源的角距.相位杂散是影响Delta-DOR测量精度的主要误差源之一,针对这项误差研究了一种基于伪随机噪声调制方式的新型...     

基于同波束干涉的空间三维相对位置测量研究

针对深空探测器相对位置精确测量需求,建立了空间三维相对位置测量模型,研究了基于单基线同波束干涉测量(Same-beam VLBI,SBI)的空间三维位置最小二乘解算方法。利用"嫦娥3号"着陆器的测控天线与定向天线的SBI实测数据,验证了测量模型与解算方法的有效性。结果显示:SBI干涉时延随机误差约0.225 ps(0.07 mm);测控天线与定向天线之间距离误差约0.216 m,方向误差约30.4°。该研究结果有望应用于后续深空探测任务譬如"嫦娥5号"器间高精度相对测量中。