Liu QingHui

Relative position determination of a lunar rover using the biased differential phase delay of same-beam VLBI

When only data transmission signals with a bandwidth of 1 MHz exist in the rover, the position can be obtained using the differential group delay data of the same-beam very long baseline interferometry (VLBI). The relative position between a lunar rover and a lander can be determined with an error of several hundreds of meters. When the guidance information of the rover is used to determine relative position, the rover’s wheel skid behavior and integral movement may influence the accuracy of the determined position. This paper proposes a new method for accurately determining relative position. The differential group delay and biased differential phase delay are obtained from the same-beam VLBI observation, while the modified biased differential phase delay is obtained using the statistic mean value of the differential group delay and the biased phase delay as basis. The small bias in the modified biased phase delay is estimated together with other parameters when the relative position of the rover is calculated. The effectiveness of the proposed method is confirmed using the same-beam VLBI observation data of SELENE. The radio sources onboard the rover and the lander are designed for same-beam VLBI observations. The results of the simulations of the differential delay of the same-beam VLBI observation between the rover and the lander show that the differential delay is sensitive to relative position. An approach to solving the relative position and a strategy for tracking are also introduced. When the lunar topography data near the rover are used and the observations are scheduled properly, the determined relative position of the rover may be nearly as accurate as that solved using differential phase delay data.

Analysis of VLBI observation for Tianma radio telescope in Chang’E-3 orbit determination

天马望远镜(上海65 m射电望远镜)于2012年10月完成了第一阶段建设并具备了S/X, L, C波频段的VLBI观测能力. 2013年12月全程参加了嫦娥三号着陆器和月球车X频段的VLBI测定轨测定位任务. 在嫦娥三号中, 利用天马望远镜代替上海佘山25 m射电望远镜, 使中国VLBI观测网的灵敏度提高至1.67倍. 同时, 利用2比特采样代替嫦娥二号的1比特采样, 使灵敏度提高至1.38倍. 上述两项措施使定轨后的ΔDOR型VLBI时延残差由嫦娥二号时的1. 77 ns降至嫦娥三号时的0.67 ns, 着陆器和月球车同波束VLBI差分相时延随机误差降至0.011 ps rms. 本文在介绍天马望远镜及其X频段致冷接收和数据采集系统的基础上, 对其在嫦娥三号测定轨VLBI观测中的应用进行分析.

Mars rover positioning technology basedon same-beam VLBI

Position measurement of a Mars rover is important for engineering and scientific researches. This paper reports a method to carry out high precision positioning of the rover and orbit determination of the orbiter by using the same- beam VLBI between the rover and the orbiter and the orbiter’s ranging data. The calculation method of the differential phase delay and signal link of a Mars rover is analyzed. The measurement error of the differential phase delay caused by solar plasma, atmosphere, ionosphere and instruments is estimated to lower than 10 ps. As a successful example, in Chang’E-3, the differential phase delay data between the rover and lander are obtained with a random error of 1 ps, and the relative position of the rover is determined with an accuracy of 1 m by using the same beam VLBI, which demonstrates the effectiveness of the method. In the case of the Mars rover and the orbit, the simulation analysis results show that the Mars rover position can be determined with an error of hundreds of meters and the orbit of the orbiter can be determined with an error of tens of meters by using the orbiter ranging and the same-beam VLBI differential phase delay data.

Sub-reflector model depending on elevations and performance evaluation for TM65 m radio telescope

A sub-reflector system of the TianMa 65 m radio telescopes has been installed in order to compensate for the gravitational deformation of the sub-reflector support and the main reflector. The position and attitude of the sub-reflector are variable in order to improve the pointing performance and the efficiency at different elevations. A model has been constructed to determine the position and attitude of the sub-reflector with elevation. Test results show that the sub-reflector model can effectively improve the efficiency of the 65 m radio telescope at both high and low elevations, where the efficiency can be improved by 20% and 15%, respectively. At X band, the aperture efficiency of the radio telescope reaches more than 60% over the entire elevation range. Pointing deviations caused by the sub-reflector model are consistent with the grasp simulation results.

A new frequency synthesis approach to precisely track the spacecrafts using Same-Beam Interferometry

This article describes a new frequency synthesis approach to realize extremely accurate relative position determination for two spacecrafts with phase measurements at a variety of frequency separations. Both of the spacecrafts transmit one carrier wave phase modulated by two DOR sine tones at X-band, and one wave at S-band. These frequencies are set to resolve the cycle ambiguity of carrier wave at X-band from two group delays corresponding to the four DOR sub-carriers and one phase delay of the carrier wave at S-band. The procedure to resolve the cycle ambiguity is analysed and discussed in detail, and the corresponding conditions, such as prediction of light time, the transmission media, are also clarified based on mathematical analysis and the same-beam tracking data from SELENE mission. The results show that all the conditions to resolve the cycle ambiguity can be satisfied in Same-Beam Interferometry. Thus, the accuracy of the differential phase delay could be achieved within several picoseconds. This method can be used in the missions with challenging navigation requirements, such as planet-relative targeting, as well as rendezvous and docking around Moon and planets beyond.

Study on differential phase delay closure of same-beam VLBI

The paper focuses on verifying the differential phase delay model and detecting and correcting the cycle ambiguity existed in the differential phase delay data of same-beam VLBI. We confirmed the methods of calculating the phase delay closure and the differential phase delay closure with the same wave front. We used the method to analyze the differential phase delay data of the same-beam VLBI from SELENE project. The result of the differential phase delay closure was less than several picoseconds. The method can be used to verify the same wave front differential phase delay model and to detect and correct the cycle ambiguity existed in the differential phase delay data. It can also be used to monitor the fluctuations of the differential phase delay. These results are very useful to select the data for science research and to process same-beam VLBI data in real-time for navigation.

Mars spacecraft tracking and analysis of VLBI orbit determination

The Chinese Mars mission will be launched in 2020, which consists of an orbiter and a rover. Mars spacecraft tracking and precise orbit determination are very important for achieving engineering and scientific detection. This paper first analyzes the Mars spacecraft tracking technology, and then introduces VLBI orbit determination results used in the mission of Mars Science Laboratory. Further, for a Mars spacecraft launched in July 2020, we give the angular distance between the spacecraft and the sun from launch to disorbit, the solar plasma delay, and observation elevation, at Shanghai, Beijing, Kunming and Urumqi stations in the arc segments of trajectory correction maneuver, Martian orbit insertion, plane maneuver, and disorbit. Orbit determination and prediction errors in the arc segment of Martian orbit insertion are also presented. Three-dimensional orbit determination error is reduced from 45.7 km to 18.8 km, and the periareon height prediction error (1 sigma) is reduced from 28.2 km to 7.6 km when combined VLBI data with range and Doppler data.

4-way Doppler technology for tracking satellite on lunar farside

飞行或着陆于月球背面的探测器, 无法从地面直接观测, 必须利用更高轨道的探测器中继才能进行跟踪. 作为月球背面探测器的一种有效的跟踪手段, 本文以SELENE为例, 系统性地给出了4程多普勒观测系统的组成、数据处理和应用方法、定轨和重力场解算结果, 并分析了天线相位特性和自旋对4程多普勒测量精度的影响及改正方法. 这些内容对月球背面探测器的跟踪、测控和数传系统的研发和应用提供了参考实例.

The application of the seam beam VLBI techniquefor the orbit determination of CE-5 in the rendezvous and dockingphase

CE-5 will be launched in 2017 – 2018, and it is a lunar sampling return mission. It is the first time for China to carry out the rendezvous and docking in the Moon. How to achieve rendezvous and docking successfully in the Moon is very important for CE-5 project. When the ascender is about 70 km farther away from the orbiter, the ground based tracking technique including range, Doppler and VLBI will be used to track the orbiter and the ascender. Later the ascender will approach the orbiter automatically. Here the application of the same beam VLBI for the orbit determination of the orbiter and the ascender in the long range of the rendezvous and docking phase is discussed. The same beam VLBI technique can be used to track the orbiter and the ascender simultaneously when they are in the same beam. Delta delay of the two probes can be derived, and the measurement accuracy is much higher than that of the traditional VLBI data because of the cancelation of common errors. Theoretically it can result in a more accurate relative orbit between the two probes. The simulation results show that the relative position accuracy of the orbiter and ascender can reach about 1 m in CE-5 project with delta delay data of 10 ps.

VLBI phase delay and its application in orbit determination of spacecraft

As one of the key technologies for orbit determination, very long baseline interferometry (VLBI) will play an important role in deep space explorations. The traditional group delay of VLBI is affected by the signal to noise ratio (SNR). As the spacecraft going to outer space, the random error of group delay increases quickly because of the SNR getting worse and worse. In this paper, we introduce the phase delay whose random error is much smaller than that of group delay at the same SNR. We deduce the conditions and method to determine the phase ambiguity, and then give the formula of calculating phase delay. The phase delay is used to determine the orbit of Change 2 (CE2) satellite with Doppler/ranging data when it flies to the Sun-Earth Lagrangian point L2 which is 1 500 000 km away from the Earth. It proves that the phase delay is useful for orbit determination.