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.     

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...     

Chang’E-1 precision orbit determination and lunar gravity field solution

In this paper we present results assessing the role of Very Long Baseline Interferometry (VLBI) tracking data through precision orbit determination (POD) during the check-out phase for Chang’E-1, and the lunar gravity field solution CEGM-01 based on the orbital tracking data acquired during the nominal phase of the mission. The POD of Chang’E-1 is performed using S-band two-way Range and Range Rate (R&RR) data, together with VLBI delay and delay rate observations. The role of the VLBI data in the POD of Chang’E-1 is analyzed, and the resulting orbital accuracies are estimated for different solution strategies. The final orbital accuracies proved that the VLBI tracking data can improve the Chang’E-1 POD significantly. Consequently, CEGM-01 based on six-month tracking data during Chang’E-1 nominal mission phase is presented, and the accuracy of the model is assessed by means of the gravity field power spectrum, admittance and coherence between gravity and topography, lunar surface gravity anomaly and POD for both Chang’E-1 and Lunar Prospector (LP). Our analysis indicates that CEGM-01 has significant improvements over a prior model (i.e. GLGM-2), and shows the potential of Chang’E-1 tracking data in high resolution lunar gravity field model solution by combining with SELENE and LP tracking data.     

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.     

A priori zenith wet delays in the analysis of VLBI CONT sessions

Tropospheric delay is one of the major sources of error in VLBI (Very Long Baseline Interferometry) analysis. The principal component of this error can be accurately computed through reliable surface pressure data —hydrostatic delay— yet there is also a small but volatile component —wet delay— which is difficult to be modelled a priori. In VLBI analysis, troposphere delay is typically modelled in the theoretical delays using Zenith Hydrostatic Delays (ZHD) and a dry mapping function. Zenith Wet Delay (ZWD) is not modelled but estimated in the analysis process. This work studies inter alia the impact of including external GNSS estimates to model a priori ZWD in VLBI analysis, as well as other models of a priori ZWD.In a first stage, two different sources of GNSS troposphere products are compared to VLBI troposphere estimates in a period of 5 years. The solution with the best agreement to VLBI results is injected in the VLBI analysis as a priori ZWD value and is compared to other options to model a priori ZWD. The dataset used for this empirical analysis consists of the six CONT campaigns.It has been found that modelling a priori ZWD has no significant impact either on baseline length and coordinates repeatabilities. Nevertheless, modelling a priori ZWD can change the magnitude of the estimated coordinates a few millimeters in the up component with respect to the non-modelling approach. In addition, the influence of a priori ZWD on Earth Orientation Parameters (EOP) and troposphere estimates —Zenith Total Delays (ZTD) and gradients—has also been analysed, resulting in a small but significant impact on both geodetic products.     

Analysis of the DORIS contributions to ITRF2008

In its function as an ITRS Combination Centre, DGFI is in charge with the computation of an ITRF2008 solution. The computation methodology of DGFI is based on the combination of datum-free normal equations (weekly or session data sets, respectively) of station positions and Earth orientation parameters (EOP) from the geodetic space techniques DORIS, GPS, SLR and VLBI. In this paper we focus on the DORIS part within the ITRF2008 computations. We present results obtained from the analysis of the DORIS time series for station positions, network translation and scale parameters, as well as for the terrestrial pole coordinates. The submissions to ITRF2008 benefit from improved analysis strategies of the seven contributing IDS analysis centres and from a combination of the weekly solutions of station positions and polar motion. The results show an improvement by a factor of two compared to past DORIS data submitted to ITRF2005, which has been evaluated by investigating the repeatabilities of position time series. The DORIS position time series were analysed w.r.t. discontinuities and other non-linear effects such as seasonal variations. About 40 discontinuities have been identified which have been compared with the results of an earlier study. Within the inter-technique combination we focus on the DORIS contribution to the integration of the different space geodetic observations and on a comparison of the geodetic local ties with the space geodetic solutions. Results are given for the 41 co-location sites between DORIS and GPS.     

Observation level combination of SLR and VLBI with c5++: A case study for TIGO

A multi-technique space geodetic analysis software named c5++ has been developed and allows one to combine data on the observation level. With SLR and VLBI modules being ready and tested, this software has been used to compute coordinate time series of the geodetic fundamental station TIGO, located near Concepción, Chile. It can be shown that the combination of space geodetic data on the observation level leads to a significant improvement of station position repeatability, which is an important measure for the stability of a station in the terrestrial reference frame. Moreover, it could be demonstrated that the geophysical signal of the post-seismic tectonic plate movement is usually more complete than detected by any of the two single-technique solutions. In addition, it has been confirmed that so-called nuisance parameters, which are relying on data from a single technique, are not biased when combing observations from different space geodetic techniques.     

Mechanism of error propagation from the subdaily Universal Time model into the celestial pole offsets estimated by VLBI

Within the analysis of space geodetic observations, errors of the applied subdaily Earth rotation model can induce systematic effects in different estimated parameters. In this paper, we focus on the impact of the subdaily Universal Time (UT1) model on the celestial pole offsets (CPO) estimated from very long baseline interferometry (VLBI) observations. We provide a mechanism that describes the error propagation from the subdaily UT1 into the daily CPO.In typical 24-h VLBI sessions the observed quasars are well distributed over the sky. But the observations, if looked at from the Earth-fixed frame, are not homogeneously distributed. The amount of observations performed in different terrestrial directions shows an irregularity which can be roughly compared to the case where the observations are collected in only one Earth-fixed direction. This peculiarity leads to artefacts in VLBI solutions, producing a correlation between the subdaily variations in UT1 and the position of the celestial pole. As a result errors in diurnal terms of the subdaily UT1 model are partly compensated by the estimated CPO. We compute for each 24-h VLBI session from 1990 until 2011 the theoretical response of the CPO to an error in the subdaily UT1 by setting up a least-squares adjustment model and using as input the coordinates of the observed quasars and observation epochs. Then real observed response of the estimated CPO derived from the VLBI session solutions is compared to the predicted one. A very good agreement between the CPO values estimated from VLBI and the predicted values was achieved. The presented model of error propagation from the subdaily UT1 into the daily CPO allows to predict and explain the behaviour of CPO estimates of VLBI solutions computed with different subdaily Earth rotation models, what can be helpful for testing the accuracy of different subdaily tidal models.     

Automated and dynamic scheduling for geodetic VLBI – A simulation study for AuScope and global networks

As we move into the next era of geodetic VLBI, the scheduling process is one focus for improvement in terms of increased flexibility and the ability to react with changing conditions. A range of simulations were conducted to ascertain the impact of scheduling on geodetic results such as Earth Orientation Parameters (EOPs) and station coordinates. The potential capabilities of new automated scheduling modes were also simulated, using the so-called ‘dynamic scheduling’ technique. The primary aim was to improve efficiency for both cost and time without losing geodetic precision, particularly to maximise the uses of the Australian AuScope VLBI array.We show that short breaks in observation will not significantly degrade the results of a typical 24?h experiment, whereas simply shortening observing time degrades precision exponentially. We also confirm the new automated, dynamic scheduling mode is capable of producing the same standard of result as a traditional schedule, with close to real-time flexibility. Further, it is possible to use the dynamic scheduler to augment the 3 station Australian AuScope array and thereby attain EOPs of the current global precision with only intermittent contribution from 2 additional stations. We thus confirm automated, dynamic scheduling bears great potential for flexibility and automation in line with aims for future continuous VLBI operations.     

用于太阳系天体VLBI观测的时延模型

VLBI观测技术可以用于对深空航天器的跟踪定位以及测速观测．这类近距离天体发出的射电信号波前是球面波．为此，本文提出了一个1ps精度下近距离射电天体地面VLBI观测时间延迟模型．     

Proposal of application of same beam VLBI measurements in precision orbit determination of lunar orbiter and return capsule and lunar gravity field simulation in Chang’E-3 mission

High accuracy differenced phase delay can be obtained by observing multiple point frequencies of two spacecraft using the same beam Very Long Baseline Interferometry (VLBI) technology. Its contribution in lunar spacecraft precision orbit determination has been performed during the Japanese lunar exploration mission SELENE. In consideration that there will be an orbiter and a return capsule flying around the moon during the Chinese lunar exploration future mission Chang’E-3, the contributions of the same beam VLBI in spacecraft precision orbit determination and lunar gravity field solution have been investigated. Our results show that the accuracy of precision orbit determination can be improved more than one order of magnitude after including the same beam VLBI measurements. There are significant improvements in accuracy of low and medium degree coefficients of lunar gravity field model obtained from combination of two way range and Doppler and the same beam VLBI measurements than the one that only uses two way range and Doppler data, and the accuracy of precision orbit determination can reach meter level.     

A VLBI baseline post-adjustment approach for station velocity estimation in Eurasian continent

Baseline lengths and their time-derivatives among 58 geodetic VLBI stations were fitted by using 4439 observing sessions from the International VLBI Service for Geodesy and Astrometry (IVS). First, the velocities of eight stations in Eurasian continent were set as unknown quantities. Then, two standard global solutions from 3523 IVS sessions and 1110 sessions from database code XA, respectively, were applied prior to all-station coordinates and the non-estimated station velocities. Finally, from the relations among the coordinates, velocities, baseline length and its time-derivative, two types of baseline post-adjustment (BPA) were used to estimate the velocities of the eight stations. We discuss the data processing details, including the effect of different prior values for the stations and the optimal solution.     

Improving dUT1 from VLBI intensive sessions with GRAD gradients and ray-traced delays

Exact knowledge of the angle of Earth rotation UT1 with respect to coordinated time UTC, dUT1, is essential for all space geodetic techniques. The only technique which is capable of determining dUT1 is Very Long Baseline Interferometry (VLBI). So-called Intensive VLBI sessions are performed on a daily basis in order to provide dUT1. Due to the reduced geometry of Intensive sessions, there is however no possibility to estimate tropospheric gradients from the observations, which limits the accuracy of the resulting dUT1 significantly. This paper deals with introducing the information on azimuthal asymmetry from external sources, thus attempting to improve the dUT1 estimates. We use the discrete horizontal gradients GRAD and the empirical horizontal gradients GPT3 as well as ray-traced delays from the VieVS ray-tracer for this purpose, which can all be downloaded from the VMF server of TU Wien (http://vmf.geo.tuwien.ac.at). The results show that this strategy indeed improves the dUT1 estimates when compared to reference values from multi-station VLBI stations, namely by up to 15%. When converted to length-of-day (LOD), the estimates can be compared to LODs from global analyses of Global Navigation Satellite Systems (GNSS). Here, the improvement amounts to up to 7% compared to neglecting a priori information on azimuthal asymmetry.     

The simulation of lunar gravity field recovery from D-VLBI of Chang’E-1 and SELENE lunar orbiters

The lunar gravity field is a foundation to study the lunar interior structure, and to recover the evolution history of the Moon. It is still an open and key topic for lunar science. For above mentioned reasons, it becomes one of the important scientific objectives of recent lunar missions, such as KAGUYA (SELENE) the Japanese lunar mission and Chang’E-1, the Chinese lunar mission. The Chang’E-1 and the SELENE were successfully launched in 2007. It is estimated that these two missions can fly around the Moon longer than 6 months simultaneously. In these two missions, the Chinese new VLBI (Very Long Baseline Interferometry) network will be applied for precise orbit determination (POD) by using a differential VLBI (D-VLBI) method during the mission period. The same-beam D-VLBI technique will contribute to recover the lunar gravity field together with other conventional observables, i.e. R&RR (Range and Range Rate) and multi-way Doppler. Taking VLBI tracking conditions into consideration and using the GEODYNII/SOVLE software of GSFC/NASA/USA [8 and 10], we simulated the lunar gravity field recovering ability with and without D-VLBI between the Chang’E-1 and SELENE main satellite. The cases of overlapped flying and tracking period of 30 days, 60 days and 90 days have been analyzed, respectively. The results show that D-VLBI tracking between two lunar satellites can improve the gravity field recovery remarkably. The results and methods introduced in this paper will benefit the actual missions.     

Polarization VLBI observations of AGN jets now and into the future

The history of Very Long Baseline Interferometry (VLBI) observations has been characterized predominantly by an ongoing quest for increasingly high resolution and sensitivity. VLBI monitoring of relatively large samples of Active Galactic Nuclei (AGNs) with uniform quality and linear polarization sensitivity are now available at the moderately high frequencies of 15 and 43 GHz. This has enabled considerable advances in our understanding of the relativistic jets of AGNs, but gaps in the available observational material remain, which must be addressed in future VLBI polarization observations. Linear polarization observations at frequencies above 43 GHz remain non-routine, and the availability of multi-frequency and circular polarization measurements is still limited. It is of interest both to push inward toward the jet base and to study details of the jets themselves on parsec scales, such as magnetic field structures along and across the jets, which are intrinsically related to their formation, launching and propagation. Requirements for future VLBI polarization observations are considered, highlighting the key role that can be played by space VLBI observations. Unique opportunities are offered by relatively low-frequency space VLBI observations that are sensitive to effects such as Faraday rotation, opacity, and low-frequency absorption.     

面向绕月交会对接的同波束VLBI研究

针对"嫦娥5号"(CE-5)探测器间高精度相对测量需求,设计了我国深空干涉测量处理中心框架下的同波束VLBI处理算法,分析了X波段同波束VLBI相位解模糊条件和结果;通过引入群时延辅助的相位干涉技术,大幅抑制了干涉时延随机误差,为同波束VLBI中相位解模糊提供了先验条件;利用CE-5对接实测数据验证了本文工作的有效性,为CE-5任务同波束VLBI的实施奠定了基础。     

一种基于探测器下行信号融合处理的无线电干涉测量方法

针对深空探测器常用下行信号体制,结合无线电干涉测量特点,提出了一种基于探测器DOR信标信号和数传信号融合处理的干涉测量方法。该方法首先通过相关处理得到DOR信号和数传信号的差分相位,利用DOR信号进行带宽综合得到时延估计,并构建时延模型;然后,利用时延模型得到DOR信号与数传信号在数传载波处的相位差,并以此对数传信号差分相位进行补偿;最后,利用DOR信号差分相位和补偿后的数传信号差分相位进行带宽综合,实现高精度干涉测量。深空探测网数据处理结果表明融合处理后时延估计随机误差明显降低;但受介质时延误差影响,融合处理对系统时延精度的改善幅度有所减小。该方法仅通过改进干涉测量数据处理方法即实现了时延估计随机误差的改善,不仅提高了信号使用效率,而且增强了航天测控系统的鲁棒性,在应急测控背景下具有特殊意义。     

Analysis of the brightness temperature features of the lunar surface using 37 GHz channel data from the Chang'E-2 microwave radiometer

Compared with other remote observations, brightness temperatures (T_B) derived from microwave emission measurements provide a unique means to characterize the physical properties of the lunar surface. Using Chang’E-2 microwave radiometer data, we produced 12 global T_B images of the lunar surface during a diurnal cycle with different local times separated by approximately 2?h. There are two types of remarkable T_B units on the lunar surface, the “hot regions” occurring during the lunar day in the lunar Maria regions and the “microwave cold spots” occurring during the nighttime typically related to young craters. Compared with their surroundings, the hot regions are much warmer during the lunar day and slightly colder at night, while the microwave cold spots are much colder during the lunar night and slightly warmer in the daytime. Moreover, the T_B heating and cooling rates of these two units are larger than others at the same average latitude where they are located during the lunar day, especially after sunrise and before sunset. The hot regions have a good agreement with the mare regions with high TiO₂ abundance. Besides, brightness temperatures in the lunar Maria correlate closely with their TiO₂ abundance. For most microwave cold spots, they agree with the young craters, and their brightness temperature distributions have a significant negative correlation with the lunar surface nighttime temperature and rock abundance.     

QSO number counts to B < 24.5 from hubble space telescope WFPC2 images

We have conducted a survey for faint quasars to B < 24.5, z < 2.1 based on archival high-latitude Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) images covering 0.04 deg² of the sky in three filters spanning 2300–9600 Å. Taking advantage of the superb 0.1″ resolution of WFPC2, we have developed an approach to select quasar candidates based on their morphology and position in multicolor space. We derive cumulative surface densities and compare them with our previous survey in the Groth–Westphal Strip and other faint quasar surveys that utilize very different selection approaches. In most cases the agreement with these surveys is good, however, we find approximately twice as many quasar candidates to B = 24.5 as the COMBO-17 survey. Although we are confident that our approach eliminates most contaminating stars and galaxies, spectroscopic follow-up will be required to determine the level of residual contamination and confirm or refute this excess.     

A balloon-borne very long baseline interferometry experiment in the stratosphere: Systems design and developments

The balloon-borne very long baseline interferometry (VLBI) experiment is a technical feasibility study for performing radio interferometry in the stratosphere. The flight model has been developed. A balloon-borne VLBI station will be launched to establish interferometric fringes with ground-based VLBI stations distributed over the Japanese islands at an observing frequency of approximately 20?GHz as the first step. This paper describes the system design and development of a series of observing instruments and bus systems. In addition to the advantages of avoiding the atmospheric effects of absorption and fluctuation in high frequency radio observation, the mobility of a station can improve the sampling coverage (“uv-coverage”) by increasing the number of baselines by the number of ground-based counterparts for each observation day. This benefit cannot be obtained with conventional arrays that solely comprise ground-based stations. The balloon-borne VLBI can contribute to a future progress of research fields such as black holes by direct imaging.