Chinese VLBI Network Research Articles

Pathfinding Pulsar Observations with the CVN Incorporating the FAST

Abstract The importance of Very Long Baseline Interferometry (VLBI) for pulsar research is becoming increasingly prominent and receiving more and more attention. We present the pathfinding pulsar observation results with the Chinese VLBI Network (CVN) incorporating the Five-hundred-meter Aperture Spherical radio Telescope (FAST). On MJD 60045 (11th April 2023), PSRs B0919+06 and B1133+16 were observed with the phase-referencing mode in the L-band using four radio telescopes (FAST, TianMa, Haoping, and Nanshan) and correlated with the pulsar binning mode of the distributed FX-style software correlator in Shanghai. After further data processing with the NRAO Astronomical Image Processing System (AIPS), we detected these two pulsars and fitted their current positions with accuracy at the milliarcsecond level. By comparison, our results show significantly better agreement with predicted values based on historical VLBI observations than those with previous timing observations, as pulsar astrometry with the VLBI provides a more direct and model-independent method for accurately obtaining related parameters.

Study on Differential Phase Delay of Same‐Beam VLBI in Rendezvous and Docking of Chang'E‐5

AbstractThe Chang'E‐5 (CE5) ascender achieved lunar orbit autonomous rendezvous and docking with orbiter‐returner successfully on 5 December 2020. Same‐beam Very Long Baseline Interferometry (VLBI) (SBI) observations were performed for the first time for autonomous rendezvous and docking in lunar orbit. In this paper, by studying the CE5 rendezvous and docking data observed by the Chinese VLBI network and analyzing the residual differential phase delay of the autonomous control phase, we successfully captured the maneuver of the 5 km holding of the orbiter‐returner. The differential phase delay (DPD) data of the far‐range guidance phase were used to perform orbit determination of the ascender and orbiter‐returner successfully. The results of orbit determination show that the DPD data can improve the relative position accuracy between the ascender and the orbiter‐returner, and the residual root mean square of the DPD data is 5 ps. Furthermore, when one of the probes has only DPD data, its orbit can also be determined.

VLBI astrometry on the white dwarf pulsar AR Scorpii

ABSTRACT AR Scorpii (AR Sco), the only-known radio-pulsing white dwarf binary, shows unusual pulsating emission at the radio, infrared, optical, and ultraviolet bands. To determine its astrometric parameters at the radio band independently, we conducted multi-epoch Very Long Baseline Interferometry (VLBI) phase-referencing observations with the European VLBI Network at 5 GHz and the Chinese VLBI Network plus the Warkworth 30-m telescope (New Zealand) at 8.6 GHz. By using the differential VLBI astrometry, we provide high-precision astrometric measurements on the parallax ($\pi =8.52_{-0.07}^{+0.04}$ mas) and proper motion ($\mu _{\alpha }=9.48_{-0.07}^{+0.04}$ mas yr−1, $\mu _{\delta }=-51.32_{-0.38}^{+0.22}$ mas yr−1). The new VLBI results agree with the optical Gaia astrometry. Our kinematic analysis reveals that the Galactic space velocities of AR Sco are quite consistent with that of both intermediate polars and polars. Combined with the previous tightest VLBI constraint on the size, our parallax distance suggests that the radio emission of AR Sco should be located within the light cylinder of its white dwarf.

A High-Accuracy, High Anti-Noise, Unbiased Frequency Estimator Based on Three CZT Coefficients for Deep Space Exploration Mission.

Deep space exploration navigation requires high accuracy of the Doppler measurement, which is equivalent to a frequency estimation problem. Because of the fence effect and spectrum leakage, the frequency estimation performances, which is based on the FFT spectrum methods, are significantly affected by the signal frequency. In this paper, we propose a novel method that utilizes the mathematical relation of the three Chirp-Z Transform (CZT) coefficients around the peak spectral line. The realization, unbiased performance, and algorithm parameter setting rule of the proposed method are described and analyzed in detail. The Monte Carlo simulation results show that the proposed method has a better anti-noise and unbiased performance compared with some traditional estimator methods. Furthermore, the proposed method is utilized to process the raw data of MEX and Tianwen-1 satellites received by Chinese Deep Space Stations (CDSS). The results show that the Doppler estimation accuracy of MEX and Tianwen-1 are both about 3 millihertz (mHz) in 1-s integration, which is consistent with that of ESA/EVN/CDSN and a little better than that of the Chinese VLBI network (CVN). Generally, this proposed method can be effectively utilized to support Chinese future deep space navigation missions and radio science experiments.

Dual-target Signal Simulation of Very Long baseline Interferometry for Chang’E-5

Very long baseline interferometry (VLBI) is an important deep-space tracking technology from radio astronomy. A mission that uses VLBI, China’s lunar sample-return mission Chang’E-5 (CE-5) collects soil samples on the lunar surface, arrives at a rendezvous point, and docks in the lunar orbit through cooperation of the Orbiter and the Ascender. During rendezvous and docking, the Chinese VLBI network (CVN) used same beam VLBI (SBI) to observe two high-velocity probes simultaneously. However, before the launching of the CE-5, CVN lacked the capability to examine dynamic dual-target tracking ability of SBI mode because there were no similar space probes can be used in tests of upgraded VLBI data-processing system. This study proposes a method to simulate VLBI signals and acquire relevant data from the dual-target probe and calibration quasar signals recorded by each station according to the orbits and beacons of probes, station coordinates, etc. The real-time data-processing capability of the system is verified by processing the simulated data during rendezvous and docking. The consistency between simulation and VLBI data-processing was consistent to better than 0.05 ns. This method is universal and can also be used in VLBI simulation and orbital analysis of other deep-space probes.

The Precision Analysis of the Chinese VLBI Network in Probe Delay Measurement

We propose a Very Long Baseline Interferometry (VLBI) precision evaluation method for probe delay measurement, so as to investigate the error contributions from different components in the Chinese VLBI Network (CVN). This method takes the idea of traditional closure delay analysis for distant radio sources. It focuses on the VLBI closure delay only and therefore excludes the influence of probe orbit determination, which makes it very suitable to evaluate the capability of VLBI probe delay measurement. In this paper, we first introduce the principles of closure delay analysis. Then the statistical results of typical CE5 (Chinese Chang’e 5 lunar exploration mission) and HX1 (Chinese Mars exploration mission) observations are presented, including the comparison of the closure delay precisions between CE5 and HX1 for four closed baseline triangles in CVN. According to the result, we realize that the precision discrepancy between CE5 and HX1 in the closure delay analysis is less than that of residual delay after orbit determination, which reflects the precision level of the VLBI delay measurement.

Orbit determination of China’s first mars probe Tianwen-1 during interplanetary cruise

Tianwen-1 is China’s first deep space probe to explore Mars with the goal of orbiting, landing and roving. This paper presents the orbit determination process and result of the probe in the phase of Earth-Mars trajectory in detail. Orbit determination is carried out based on the tracking data obtained by CDSN (Chinese Deep Space Network) and CVN (Chinese VLBI Network). The results show that the residual RMS of range, Doppler, VLBI delay and delay rate data are about 0.28 m, 0.10 mm/s, 0.12 ns and 0.34 ps/s, respectively. Compared with the previous missions in Chinese Lunar Exploration Program (CLEP), the accuracy of VLBI data is significantly improved. The analysis shows that the prediction error of EOP (Earth Orientation Parameter) and the planetary ephemerides lead to the difference of orbit solution. With the increasing distance between the spacecraft and Earth, the EOP prediction error has a certain influence to the orbit determination accuracy. The difference in orbit solutions due to EOPs’ prediction error before MOI (Mars Orbit Insertion) is about 300 m. Moreover, the orbit difference is about 200 m using different planetary ephemerides (DE421/DE436). Orbit overlapping results show the position and velocity error of Tianwen-1 are less than 2 km and 1 cm/s, respectively. During the approaching phase, the error ellipse in B-plane shows that σ SMAA = 389.40 m, σ SMIA = 140.88 m (1σ).

Dynamic dual-target VLBI correlator technology

The dynamic dual-target very long baseline interferometry (VLBI) processing technology applied to the CVNScorr software processor is introduced. Chang’e 5 (CE-5) has achieved China’s first sample return mission from an extraterrestrial body. The characteristics of VLBI are that there are several target probes and the telemetry tracking and command (TT&C) technical requirements are high in CE-5. In the rendezvous and docking process, a real-time in-beam VLBI technology is required to perform the high-precision orbit measurement of two dynamic targets, the orbiter and the ascender. In addition, the VLBI correlator (CVNScorr) should tackle two-phase centers from two probes simultaneously. In the special power flight phases such as rendezvous and docking, near-moon braking, powered descending, and lunar takeoff, the VLBI high-precision delay model cannot be derived owing to the low accuracy of the predicted orbit. If the conventional data processing method is adopted, the VLBI fringe cannot be obtained. Therefore, the VLBI correlator should have the function of real-time dual-target fringe search and delay model reconstruction. CVNscorr adopts the principle of FX-type software correlation and has the function of dual-target correlation module, multibeacon fringe search, and delay model reconstruction. By taking advantage of the characteristics of TT&C signals from different probes, multiple beacons fringes are automatically searched and the delay values of the two targets are obtained simultaneously to reconstruct the high-precision delay models. CVNScorr uses MPI, GPU, and other technologies to realize parallel computing, and it is run on a CPU+GPU parallel cluster platform. In the CE-5 mission, the real-time processing capability is up to four stations of the Chinese VLBI Network and the postprocessing capability is up to six stations. The real-time data rate reaches 128 Mbps per station, the VLBI delay accuracy reaches 0.4 ns, and the real-time performance is better than 25 s.

Precise positioning of Chang’e 3 lander based on Helmert-VCE-aided weighting method using phase delay data from Chinese VLBI Network

The precise positioning of a lander is a key issue during in situ planetary exploration. This paper proposes an optimal weighting algorithm based on a Helmert-variance component estimation applied to VLBI phase delay data. We validated our algorithm on Chang’e 3 lander VLBI phase delay from the Chinese VLBI Network. We consider that the lander positioning accuracy achieved with our new algorithm is at least 53% better, compared to previous estimations based on the same observations. The position of the lander based on continuous 4-day observations was estimated to be 44.1215°N (±21.9 m), −19.5135°E (±4.2 m) and −2632.7 m (±18.9 m) with respect to a moon reference system. This result agrees well with the position derived from images taken by the Narrow Angle Camera on the Lunar Reconnaissance Orbiter.

Promoting Earth-Based Radar Astronomical Observations of the Moon.

Earth-based radar astronomical observations provide information on surface characteristics, orbits, and rotations for a wide variety of solar system objects. Based on compound radio telescopes, both the Chinese VLBI (Very Long Baseline Interferometry) network (CVN) and the Russian VLBI network (Quasar), in cooperation with the Chinese radar transmitters, we present the current ground radar astronomical observations of the moon. The spectrum of the reflected radio signals was obtained and the Doppler frequency shift in bi-static radar mode was measured. Radar albedo of the observed region and power ratios of the reflected signals with left- and right-hand circular polarizations were determined, allowing us to study the radar reflectivity and near-surface wavelength-scale roughness of the moon. Future developments on radar astronomy are also discussed in the paper.

Design and implementation of PFB module with 10G network transmission system in CDAS2

Digital baseband converters (DBBC) are among the core equipment in a very long baseline interferometry (VLBI) system, which undertakes missions of data acquisition, analog-to-digital conversion, and frequency selection. In order to enhance the measurement resolution of the Chinese VLBI Network (CVN), a new generation of DBBC named CDAS2 (the second-generation Chinese VLBI data acquisition system) has been developed. CDAS2 uses polyphase filter banks (PFB) and fast Fourier transform to convert signals from intermediate frequency to baseband. A high-speed sampling analog-to-digital converter is adopted to guarantee CDAS2’s broadband observation ability. In order to facilitate the broadband observation ability of CDAS2, a high-speed data transmission system is designed to transmit data from CDAS2 to a data recording server. The data transmission system includes a Mark5B formatter and a 10G network transmission system. We describe the detailed design of PFB and the data transmission system. We installed two CDAS2s in two radio astronomical telescopes of Shanghai Astronomical Observatory to carry out observation experiments for the radio source of 4C39.25. After processing with DiFX, we got interference fringes successfully. Our experiment demonstrates the validity and performance of CDAS2.

Interferometry imaging technique for accurate deep-space probe positioning

Very long baseline interferometry (VLBI) is a radio astronomy tool with very high spatial resolution. It uses two or more radio telescopes to track the faraway object and gets its visibility. The intensity distribution image of radio source can be obtained by the inverse Fourier transformation of the visibilities sampled on UV plane perpendicular to the line of sight. Chinese VLBI Network (CVN) consists of 5 radio telescopes, and its highest spatial resolution is equivalent to that of a ∼3000km diameters single dish antenna. This paper introduces the interferometry imaging principle, the imaging results of ChangE lunar and Mars Express probes. The measured ChangE-3 (CE-3) Rover relative position accuracy is about 1m by this method. The 1m accuracy is verified by comparisons with Rover null position and the onboard stereo vision measurement results. The successful imaging of spacecraft indicates that the interferometry imaging technology can be used for accurate spacecraft positioning in the future.

Initial result of the Chinese Deep Space Stations’ coordinates from Chinese domestic VLBI experiments

China’s Lunar Exploration Program (CLEP) prompted the design and construction of the globally distributed Chinese Deep Space Network (CDSN). This network consists of Jiamusi and Kashi stations in China, and Zapala station in Argentina. However, the positions of Jiamusi and Kashi are not accurate enough for future Chinese deep space missions, and geodetic Very Long Baseline Interferometry (VLBI) is the most effective way to determine their positions. Since the CDSN stations are equipped with narrow-band receivers, they cannot participate in current international VLBI sessions in which wide-band frequencies are utilized. Thus a cooperative geodetic program of the CDSN and Chinese VLBI Network (CVN, the VLBI tracking subsystem of the CLEP) was initiated to determine their positions, in which specially designed frequencies can be utilized, and some CVN stations can act as position reference stations owing to their precise positions from long-term international VLBI observations. Primary results have been obtained from the CDSN–CVN combined domestic VLBI experiments from September 28, 2014, through December 10, 2015. The positions of Jiamusi and Kashi are determined to be better than 10-mm precision in the X, Y, and Z directions, which are improved by a factor of approximately 20 over their a priori values.

A study of VLBI imaging techniques for deep space probes

By observing deep space probes and adjacent extragalactic radio sources alternatively, VLBI array acquire data which can be used for phase referencing radio imaging of deep space probes. The offset of the target source to the image center corresponds to the relative position of the probe to the reference source. Since the position of the reference source is well known, the position of the probe can be derived in a very high precision. In this work, we study the technique of VLBI imaging for deep space probes, including the reconstruction of dirty image, the algorithm of deconvolution and the development of the specialized program ProbeMap for these purposes. With this program, we have obtained VLBI images of CE-3 Rover and Mar Express probe by utilizing observation data from Chinese VLBI Network. The comparison between our results and DIFMAP output demonstrates great consistency and therefore verifies the program. This technique can be used for high precision positioning of lunar probes and other deep space probes in the future.

Precise three-way Doppler of spacecraft based on VLBI data and its application

We have researched the software phase-locked loop (SPLL) technique to extract three way Doppler of spacecraft and the propagation medium error correction method. The method is used on Chang’E 3 X-band DOR main carrier observed by Chinese VLBI Network. The three way Doppler noise is 0.64 mm/s. Orbit determination experiment results of using three way Doppler and range data show that the corresponding Doppler and range rms are about 0.55 mm/s and 0.41 m, and there is ~17 m difference away from the reference precise ephemeris. The difference is within the inner precision of the precise ephemeris. The SPLL has also been used to research the interplanetary scintillation using European Space Agency’s spacecraft MEX. Our results are in good agreement with the results from JIVE with high and low solar elongation. The result also reflects the effect of solar wind plasma on Doppler from MEX. The SPLL based on VLBI is useful for the future deep space exploration projects and radio science.

Relative position determination between Chang’E-3 lander and rover using in-beam phase referencing

Chang’E-3 is China’s first lunar surface exploration mission, as a part of the second phase of China’s Lunar Exploration Program (CLEP). It landed successfully in the northwest of Mare Imbrium on December 14, 2013, incorporating two probes, a lander and a rover named Yutu. The execution of the cooperative science tasks required an accurate determination of the relative position between the lander and the rover. In-beam phase referencing experiments have been conducted with the Chinese VLBI Network (CVN) to accurately image the rover in reference to the stationary lander in order to obtain their offsets in Right Ascension and Declination, which were then transformed into the lunar local coordinate system to acquire their relative position on the lunar surface. The observations and the data processing procedures are described in this paper. Images of the rover in six epochs between December 15 and 21, 2013 were made, resulting in accurate positions of the rover that are consistent with the visual localization approach. Through measurement of the locations of the rover’s omnidirectional antenna with respect to the lander’s directional antenna before their separation, and the recognition of the rover’s turning actions during its exploratory moves, we demonstrate that the accuracy of relative position determined in this way could reach the meter level and the resolution is better than 1 meter.

Simulation and results on real-time positioning of Chang'E-3 rover with the same-beam VLBI observations

The real-time and high precision positioning of the lunar rover vehicle is an important step for lunar exploration and science. SBI (same-beam interferometry) is the differential very long baseline interferometry (VLBI) technology, which can be used in lunar exploration with its high precision and stability. In this paper, the relative positioning model of the lunar rover vehicle (LRV) to lunar module (LM) based on the SBI and extended kalman filtering (EKF) is developed and presented. Using the current Chinese VLBI network and the planed Chang'E-3, SBI observation data with an attainable precision of picoseconds and a sample interval of 4s are simulated. The LRV's relative position to the LM is then estimated by the least squares adjustment, EKF and an adaptive EKF, respectively. Results show that the Adaptive EKF performs the best real-time solutions with the accuracy of 1.86m in X direction, 0.33m in Y direction and 0.09m in Z direction, which can provide a good reference for real-time positioning of planed Chang'E-3 rover.

基于ΔDOR信号的高精度VLBI技术

During the Chinese Lunar Project Chang'E-2 mission, a series of experiments on ΔDOR (delta differential one-way ranging) technology was carried out over a two week period starting on April 2, 2011. ΔDOR technology was first adopted in China deep space navigation, and the ΔDOR delay observables were obtained and used for orbit determination. In this paper, we introduce the ΔDOR observing technology, including the ΔDOR measurement principles. We also discuss the experiment design, the rules for selecting the calibration quasars, the switching time choice, the signal recording setup for the channels, the ΔDOR data processing method implemented in the software, and our analysis of the error sources of the ΔDOR delay and its contribution to orbit determination based on the characteristics of the Chinese VLBI network. Through these analyses and verification studies on the orbit determination from the satellite orbits in the CE-1 and CE-2 missions, we conclude that the accuracy of the ΔDOR delay is about 0.5 ns, an improvement of about one order of magnitude compared with that from S-band telemetry signals of less than 1 MHz. This study presents an important technique for high precision orbit determination of lunar and deep space explorations.

Very long baseline interferometry detection of the Galactic black hole binary candidate MAXI J1836—194

Abstract The X-ray transient MAXI J1836—194 is a newly identified Galactic black hole binary candidate. As most X-ray transients, it was discovered at the beginning of an X-ray outburst. After the initial canonical X-ray hard state, the outburst evolved into a hard intermediate state and then went back to the hard state. The existing RATAN-600 radio monitoring observations revealed that it was variable on a time-scale of days and had a flat or inverted spectrum, consistent with optically thick synchrotron emission, possibly from a self-absorbed jet in the vicinity of the central compact object. We observed the transient in the hard state near the end of the X-ray outburst with the European very long baseline interferometry (VLBI) Network (EVN) at 5 GHz and the Chinese VLBI Network (CVN) at 2.3 and 8.3 GHz. The 8.3-GHz observations were carried out at a recording rate of 2048 Mbps using the newly developed Chinese VLBI data acquisition system, twice higher than the recording rate used in the other observations. We successfully detected the low-declination source with a high confidence level in both observations. The source was unresolved (≤0.5 mas), which is in agreement with an au-scale compact jet.

Improvement of orbit determination for geostationary satellites with VLBI tracking

China’s COMPASS satellite navigation system consists of five or more geostationary (GEO) satellites. The roles of GEO satellites are to improve the regional user’s positioning accuracy and provide the continuous Radio Determination Satellite Service. The motion of GEO satellites relative to a ground tracking station is almost fixed, and regular orbit maneuvers are necessary to maintain the satellites’ allocated positions above the equator. These features present difficulties in precise orbit determination (POD). C-band ranging via onboard transponders and the L-band pseudo-ranging technique have been used in the COMPASS system. This paper introduces VLBI tracking, which has been successfully employed in the Chinese lunar exploration programs Chang’E-1 and Chang’E-2, to the POD of GEO satellites. In contrast to ranging, which measures distances between a GEO satellite and an observer, VLBI is an angular measurement technique that constrains the satellite’s position errors perpendicular to the satellite-to-observer direction. As a demonstration, the Chinese VLBI Network organized a tracking and orbit-determination experiment for a GEO navigation satellite lasting 24 h. This paper uses the VLBI delay and delay-rate data, in combination with C-band ranging data, to determine the GEO satellite’s orbit. The accuracies of the VLBI delay and delay rate data are about 3.6 ns and 0.4 ps/s, respectively. Data analysis shows that the VLBI data are able to calibrate systematic errors of the C-band ranging data, and the combination of the two observations improves orbit prediction accuracy with short-arc data, which is important for orbital recovery after maneuvers of GEO satellites. With the implementation of VLBI2010, it is possible for VLBI to be applied in the COMPASS satellite navigation system.