[Paper Review] RadioAstron gravitational redshift experiment: status update

Д. А. Литвинов, U. Bach+26 more|arXiv (Cornell University)|May 19, 2016

Astrophysics and Cosmic Phenomena6 references3 citations

TL;DR

This paper reports on the RadioAstron gravitational redshift experiment, which tests general relativity by measuring frequency shifts in the spacecraft's ultra-stable H-maser signal as it moves through Earth's gravitational field. Using ground radio telescopes and precise Doppler tracking, the team achieved a relative accuracy of $4 \times 10^{-4}$, with projections to reach $2.5 \times 10^{-5}$ by 2016—nearly an order of magnitude better than the 1976 GP-A mission.

ABSTRACT

A test of a cornerstone of general relativity, the gravitational redshift effect, is currently being conducted with the RadioAstron spacecraft, which is on a highly eccentric orbit around Earth. Using ground radio telescopes to record the spacecraft signal, synchronized to its ultra-stable on-board H-maser, we can probe the varying flow of time on board with unprecedented accuracy. The observations performed so far, currently being analyzed, have already allowed us to measure the effect with a relative accuracy of $4 imes10^{-4}$. We expect to reach $2.5 imes10^{-5}$ with additional observations in 2016, an improvement of almost a magnitude over the 40-year old result of the GP-A mission.

Motivation & Objective

• To test the gravitational redshift effect predicted by general relativity with higher precision than previous experiments.
• To probe potential violations of the equivalence principle by measuring deviations in the gravitational redshift formula.
• To leverage the RadioAstron spacecraft’s highly elliptical orbit to maximize gravitational potential differences.
• To improve upon the 40-year-old GP-A mission result by a factor of nearly 10 using enhanced clock stability and repeated observations.
• To validate the use of deep-space H-maser signals and ground-based radio telescopes for high-precision relativistic experiments.

Proposed method

• The experiment uses two-way and one-way Doppler tracking of the RadioAstron spacecraft’s 8.4 GHz downlink signal, synchronized with an ultra-stable H-maser on board.
• Frequency shifts are measured using ground radio telescopes (e.g., Green Bank, Effelsberg, Onsala) and dedicated tracking stations (Pushchino, Green Bank), with data processed via the PRIDE software framework.
• The gravitational redshift is isolated from other effects such as Doppler shifts, ionospheric and tropospheric delays, and clock drifts through precise modeling and interleaved measurement techniques.
• Systematic and random errors are quantified using an error budget that accounts for clock instability, atmospheric noise, orbit inaccuracies, and residual biases.
• The gravitational redshift is parameterized as $ \Delta f / f = (\Delta U / c^2)(1 + \varepsilon) $, where $ \varepsilon $ quantifies potential violations of general relativity.
• Data from 6 experiments (Oct–Dec 2015) were analyzed, with 30 independent measurements expected to reduce uncertainty to $ 8 \times 10^{-15} $ in $ \Delta f / f $.

Experimental results

Research questions

• RQ1 Can the gravitational redshift effect be measured with higher precision than the 1976 GP-A mission using modern space-based H-masers and ground-based VLBI networks?
• RQ2 What is the achievable accuracy in testing the equivalence principle via the gravitational redshift using the RadioAstron mission’s highly elliptical orbit?
• RQ3 How do atmospheric and clock-related systematic errors affect the measurement of relativistic frequency shifts in deep-space radio tracking?
• RQ4 To what extent can the interleaved measurement technique reduce noise and improve stability in one-way Doppler tracking?
• RQ5 Can the combination of ground radio telescopes and dedicated tracking stations yield comparable or better results than traditional tracking systems for relativistic experiments?

Key findings

• The experiment achieved a relative accuracy of $ 4 \times 10^{-4} $ in measuring the gravitational redshift using data from RadioAstron’s tracking stations.
• Data from high-quality ground radio telescopes (e.g., Effelsberg, Onsala) are expected to improve the accuracy to $ 8 \times 10^{-5} $ after full processing.
• The projected accuracy of $ 2.5 \times 10^{-5} $ for the full experiment surpasses the GP-A mission result by nearly a factor of 10.
• The frequency stability of the 8.4 GHz downlink signal reached $ 2 \times 10^{-14} $ over 1000 seconds, meeting the requirements for high-precision measurements.
• The error budget shows that random and systematic errors in $ \Delta f / f $ are both at the level of $ 4 \times 10^{-14} $ and $ 2 \times 10^{-15} $, respectively, for a single experiment.
• The most sensitive experiments, planned for summer 2016, are expected to achieve a gravitational redshift modulation of up to $ 3 \times 10^{-10} $, maximizing the signal-to-noise ratio.