Very Long Baseline Interferometry Tracking Research Articles

Spacecraft VLBI tracking to enhance stellar occultations astrometry of planetary satellites

Context. Stellar occultations currently provide the most accurate ground-based measurements of the positions of natural satellites (down to a few kilometres for the Galilean moons). However, when using these observations in the calculation of satellite ephemerides, the uncertainty in the planetary ephemerides dominates the error budget of the occultation. Aims. We quantify the local refinement in the central planet’s position achievable by performing Very Long Baseline Interferometry (VLBI) tracking of an in-system spacecraft temporally close to an occultation. We demonstrate the potential of using VLBI to enhance the science return of stellar occultations for satellite ephemerides. Methods. We identified the most promising observation and tracking opportunities offered by the Juno spacecraft around Jupiter as perfect test cases, for which we ran simulations of our VLBI experiment. Results. VLBI tracking at Juno’s perijove close to a stellar occultation locally (in time) reduces the uncertainty in Jupiter’s angular position in the sky to 250–400 m. This represents up to an order of magnitude improvement with respect to current solutions and is lower than the stellar occultation error, thus allowing the moon ephemeris solution to fully benefit from the observation. Conclusions. Our simulations showed that the proposed tracking and observation experiment can efficiently use synergies between ground- and space-based observations to enhance the science return on both ends. The reduced error budget for stellar occultations indeed helps to improve the moons’ ephemerides, which in turn benefit planetary missions and their science products, such as the recently launched JUICE and upcoming Europa Clipper missions.

Analysis of the Advantages and Disadvantages of Astrometric and Geodetic VLBI Observation at Ka-band

The development and necessity of the frequency settings in astrometric and geodetic Very Long Baseline Interferometry (VLBI) observations are analyzed, especially on the advantages and disadvantages of observations at Ka-band. Compared to X-band, the advantages at Ka-band include more compact extragalactic sources, relatively smaller core shift effect, as well as efficiency to improve the precision in the realization of radio celestial reference frame and in the linkage to the quasi-inertia optical frame of Gaia project. Advantages in terms of deep space exploration are high in telemetry data rate, high in VLBI tracking precision, and helpful to mitigate the detrimental effects of ionosphere and solar plasma on the observed time delay. Disadvantages include relatively weak extragalactic sources, high precision requirement of reflector and antenna pointing, and susceptible to atmospheric absorption and emission effects. But the loss in system sensitivity due to these factors could be mostly compensated by the rapid development in high data rate sampling and recording technology. Finally in accordance with the current status of VLBI technique in China some specific recommendations for further development are recommended for criticism by readers.

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.

New high-accuracy spacecraft VLBI tracking using high data-rate signal: A demonstration with Chang’E-3

As the scientific data volume in deep-space exploration rapidly grows, spacecraft heavily relies on high data-rate signals that span several megahertz to transmit data back to Earth. Employing high data-rate signals for high-accuracy radiometric interferometry can simultaneously deal with data transmission and spacecraft navigation. We demonstrate very long baseline interferometry (VLBI) tracking of the Chang’E-3 lander and rover to determine their relative lunar-surface position using downlink high data-rate signals. A new method based on the VLBI phase-referencing technique is proposed to obtain the differential phase delay, which is much more accurate than the differential group delay acquired by conventional VLBI approaches. The systemic errors among different signal channels have been well calibrated using the new method. The data from the Chang’E-3 mission were then processed, and meter-level accuracy positions of the rover with respect to the lander have been obtained. This demonstration shows the feasibility of high-accuracy radiometric interferometry using high data-rate signals. The method proposed in this paper can also be applied to future deep-space navigation.

Spacecraft VLBI and Doppler tracking: algorithms and implementation

We present the results of several multi-station Very Long Baseline Interferometry (VLBI) experiments conducted with the ESA spacecraft Venus Express as a target. To determine the true capabilities of VLBI tracking for future planetary missions in the solar system, it is necessary to demonstrate the accuracy of the method for existing operational spacecraft. We describe the software pipeline for the processing of phase referencing near-field VLBI observations and present results of the ESA Venus Express spacecraft observing campaign conducted in 2010-2011. We show that a highly accurate determination of spacecraft state-vectors is achievable with our method. The consistency of the positions indicates that an internal rms accuracy of 0.1 mas has been achieved. However, systematic effects produce offsets up to 1 mas, but can be reduced by better modelling of the troposphere and ionosphere and closer target-calibrator configurations.

Orbit determination for Chang’E-2 lunar probe and evaluation of lunar gravity models

The Unified S-Band (USB) ranging/Doppler system and the Very Long Baseline Interferometry (VLBI) system as the ground tracking system jointly supported the lunar orbit capture of both Chang’E-2 (CE-2) and Chang’E-1 (CE-1) missions. The tracking system is also responsible for providing precise orbits for scientific data processing. New VLBI equipment and data processing strategies have been proposed based on CE-1 experiences and implemented for CE-2. In this work the role VLBI tracking data played was reassessed through precision orbit determination (POD) experiments for CE-2. Significant improvement in terms of both VLBI delay and delay rate data accuracy was achieved with the noise level of X-band band-width synthesis delay data reaching 0.2–0.3 ns. Short-arc orbit determination experiments showed that the combination of only 15 min’s range and VLBI data was able to improve the accuracy of 3 h’s orbit using range data only by a 1–1.5 order of magnitude, confirming a similar conclusion for CE-1. Moreover, because of the accuracy improvement, VLBI data was able to contribute to CE-2’s long-arc POD especially in the along-track and orbital normal directions. Orbital accuracy was assessed through the orbital overlapping analysis (2 h arc overlapping for 18 h POD arc). Compared with about 100 m position error of CE-1’s 200 km200 km lunar orbit, for CE-2’s 100 km100 km lunar orbit, the position errors were better than 31 and 6 m in the radial direction, and for CE-2’s 15 km100 km orbit, the position errors were better than 45 and 12 m in the radial direction. In addition, in trying to analyze the Delta Differential One-Way Ranging (DOR) experiments data we concluded that the accuracy of DOR delay was dramatically improved with the noise level better than 0.1 ns and systematic errors better calibrated, and the Short-arc POD tests with DOR data showed excellent results. Although unable to support the development of an independent lunar gravity model, the tracking data of CE-2 provided evaluations of different lunar gravity models through POD. It is found that for the 100 km100 km lunar orbit, with a degree and order expansion up to 165, JPL’s gravity model LP165P did not show noticeable improvement over Japan’s SGM series models (100×100), but for the 15 km×100 km lunar orbit, a higher degree-order model can significantly improve the orbit accuracy. Chang’E-2, VLBI, orbit determination, lunar gravity field

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.

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 [ Rowlands et al., 1997 , Ullman, 1994 ], 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.

The Problem of Gravitational Absorption

In a recent letter to the Editors, Bocchio (1971) has discussed the problem of detecting the possible absorption of gravitation by using observations of artificial satellites. Using an absorption coefficient, R, quite close to the upper limit found by Tomaschek & Groten (1963) and others he gave orbital perturbation data for satellite orbits having perigee heights of about lo5 m. It is quite clear that perturbations of this order of magnitude will hardly be detected even from a drag-free satellite observed by continuous laser or VLBI (very long base line interferometry) tracking using only one orbital period.