The gravitational redshift monitored with RadioAstron from near Earth up to 350,000 km

Abstract

We report on our efforts to test the Einstein Equivalence Principle by measuring the gravitational redshift with the VLBI spacecraft RadioAstron, in an eccentric orbit around Earth with geocentric distances as small as ∼ 7,000 km and up to 350,000 km. The spacecraft and its ground stations are each equipped with stable hydrogen maser frequency standards, and measurements of the redshifted downlink carrier frequencies were obtained at both 8.4 and 15 GHz between 2012 and 2017. Over the course of the ∼ 9 d orbit, the gravitational redshift between the spacecraft and the ground stations varies between 6.8 × 10 −10 and 0.6 × 10 −10 . Since the clock offset between the masers is difficult to estimate independently of the gravitational redshift, only the variation of the gravitational redshift is considered for this analysis. We obtain a preliminary estimate of the fractional deviation of the gravi- tational redshift from prediction of = −0.016 ± 0.003 stat ± 0.030 syst with the systematic uncertainty likely being dominated by unmodelled effects including the error in accounting for the non-relativistic Doppler shift. This result is consistent with zero within the uncertainties. For the first time, the gravitational redshift has been probed over such large distances in the vicinity of Earth. About three orders of magnitude more accurate estimates may be possible with RadioAstron using existing data from dedicated interleaved observations combining uplink and downlink modes of operation.

Precision measurement of gravitational frequency shift of radio signals using Rao-Cramer estimates

“…The processing results give the boundary of the correspondence of the measured value "redshift" to Einstein's theory improved by an order of magnitude, i.e. at the level of 10 −3 [10]. The rejection of the use of "webinet" and the transition to optimal methods for estimating the frequency of communication signals, in principle, should allow us to compare the measurements even more accurately with the general theory of relativity formula [11].…”

“…In total, about 70 sessions have been accumulated, which were processed in the first approximation. "Single-track data" using synchronization of communication signals according to the onboard standard were processed in a large volume (more than 5,000 sessions, since all sessions were involved, not just specialized gravity ones) and the results are published in [8]. The correspondence of the measured value "redshift" to the general theory of relativity formula is confirmed at the level of 10 −2 .…”

Space VLBI: from first ideas to operational missions

“…Thus, one could conclude that inclusion in the payload of a SVLBI spacecraft operating at centimetre and longer wavelengths such a complicated and expensive device as a space-qualified H-maser is unnecessary provided the PLL option can be used. That said, the presence of the H-maser on board the RadioAstron spacecraft made it possible to conduct the ad hoc Gravitational Redshift Experiment (Litvinov et al, 2018;Nunes et al, 2019). Future advanced SVLBI systems operating at millimetre and sub-millimetre wavelengths (Andrianov et al, 2019;Fish et al, 2019;Kudrishov et al, 2019;Linz et al, 2019;Roelofs et al, 2019) might require new approaches to space-borne telescope heterodyning.…”

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

“…A complementary support by PRIDE to the lander radio science (LaRa) experiment of the ESA-Roscosmos ExoMars-2022 mission has been considered too Dehant et al (2020). PRIDE methodology proved to be useful for fundamental physics experiments on verification of the Einstein Equivalence Principle Litvinov et al (2018); Nunes et al (2020). All these applications rely on the SDtracker software described in this paper.…”