The antenna phase center motion effect in high-accuracy spacecraft tracking experiments

We present an improved model for the antenna phase center motion effect for high-gain mechanically steerable ground-based and spacecraft-mounted antennas that takes into account non-perfect antenna pointing. Using tracking data of the RadioAstron spacecraft we show that our model can result in a correction of the computed value of the effect of up to 2×10-14 in terms of the fractional frequency shift, which is significant for high-accuracy spacecraft tracking experiments. The total fractional frequency shift due to the phase center motion effect can exceed 1×10-11 both for the ground and space antennas depending on the spacecraft orbit and antenna parameters. We also analyze the error in the computed value of the effect and find that it can be as large as 4×10-14 due to uncertainties in the spacecraft antenna axis position, ground antenna axis offset and misalignment, and others. Finally, we present a way to reduce both the ground and space antenna phase center motion effects by several orders of magnitude, e.g. for RadioAstron to below 1×10-16, by tracking the spacecraft simultaneously in the one-way downlink and two-way phase-locked loop modes, i.e. using the Gravity Probe A configuration of the communications links.Astrodynamics & Space Mission

Gravitational redshift test of EEP with RadioAstron from near Earth to the distance of the Moon

The Einstein Equivalence Principle (EEP) is a cornerstone of general relativity and predicts the existence of gravitational redshift. We report on new results of measuring this shift with RadioAstron (RA), a space very long baseline interferometry (VLBI) spacecraft launched into an evolving high eccentricity orbit around Earth with geocentric distances reaching 353 000 km. The spacecraft and ground tracking stations at Pushchino, Russia, and Green Bank, USA, were each equipped with a hydrogen maser frequency standard allowing a possible violation of the predicted gravitational redshift, in the form of a violation parameter ɛ, to be measured. By alternating between RA’s frequency referencing modes during dedicated sessions between 2015 and 2017, the recorded downlink frequencies can essentially be corrected for the non-relativistic Doppler shift. We report on an analysis using the Doppler-tracking frequency measurements made during these sessions and find ϵ = ( 2.1 ± 3.3 ) × 10 − 4 . We also discuss prospects for measuring ɛ with a significantly smaller uncertainty using instead the time-domain recordings of the spacecraft signals and envision how 10−7 might be possible for a future space VLBI mission.Astrodynamics & Space Mission

High spectral resolution multi-tone Spacecraft Doppler tracking software: Algorithms and implementations

We present a software package for single-dish data processing of spacecraft signals observed with VLBI-equipped radio telescopes. The Spacecraft Doppler tracking (SDtracker) software allows one to obtain topocentric frequency detections with a sub-Hz precision and reconstructed and residual phases of the carrier signal of any spacecraft or landing vehicle at any location in the Solar System. These data products are estimated using the ground-based telescope's highly stable oscillator as a reference, without requiring an a priori model of the spacecraft dynamics nor the downlink transmission carrier frequency. The software has been extensively validated in multiple observing campaigns of various deep space missions and is compatible with the raw sample data acquired by any standard VLBI radio telescope worldwide. In this paper, we report the numerical methodology of SDtracker, the technical operations for deployment and usage, and a summary of use cases and scientific results produced since its initial release. Astrodynamics & Space Mission