Formulation to test gravitational redshift based on the tri-frequency combination of ACES frequency links

TL;DR

This paper proposes a tri-frequency combination method using ACES frequency links to improve the testing accuracy of gravitational redshift, surpassing current methods by over an order of magnitude.

Contribution

It introduces a novel tri-frequency combination approach based on frequency measurements, enhancing gravitational redshift testing accuracy with detailed consideration of various environmental effects.

Findings

Achieves testing accuracy of at least 2×10⁻⁶

Improves over current accuracy level of 7×10⁻⁵

Provides a formulation considering multiple atmospheric and relativistic effects

Abstract

Atomic Clock Ensemble in Space (ACES) is an ESA mission mainly designed to test gravitational redshift with high-performance atomic clocks in space and on the ground. A crucial part of this experiment lies in its two-way Microwave Link (MWL), which uses the uplink of carrier frequency 13.475 GHz (Ku band) and downlinks of carrier frequencies 14.70333 GHz (Ku band) and 2248 MHz (S band) to transfer time and frequency. The formulation based on the time comparison has been studied for over a decade. However, there are advantages of using frequency comparison instead of time comparison to test gravitational redshift. Hence, we develop a tri-frequency combination (TFC) method based on the measurements of the frequency shifts of three independent MWLs between ACES and a ground station. The potential scientific object requires stabilities of atomic clocks at least $3\times10^{-16}$/day, so we must consider various effects, including the Doppler effect, second-order Doppler effect, atmospheric frequency shift, tidal effects, refraction caused by the atmosphere, and Shapiro effect, with accuracy levels of tens of centimeters. The ACES payload will be launched as previously planned in the middle of 2021, and the formulation proposed in this study will enable testing gravitational redshift at an accuracy level of at least $2\times10^{-6}$, which is more than one order higher than the present accuracy level of $7\times10^{-5}$.