Determination of the Geopotential Difference between Atomic Clock Ensemble in Space (ACES) and Ground Station using the Tri-Frequency Combination (TFC) Method

According to general relativity theory, a precise clock runs at different rates at positions with different geopotentials. Atomic Clock Ensemble in Space (ACES) is a mission using high-performance clocks and links to test fundamental laws of physics in space. The ACES microwave link (MWL) will make the ACES clock signal available to ground laboratories equipped with atomic clocks. The ACES-MWL will allow space-to-ground and ground-to-ground comparisons of atomic frequency standards. This study aims to apply the tri-frequency combination (TFC) method to determine the geopotential difference between the ACES and a first order triangulation station in Egypt. The TFC 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. Here we present a simulation experiment. In this experiment, we use the international space station (ISS) orbit data, ionosphere and troposphere models, regional gravitational potential and geoid for Africa, solid Earth tide model, and simulated clock data by a conventionally accepted stochastic noises model. The scientific object requires stabilities of atomic clocks at least 3 × 10 ⁻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 decimetres. This study is supported by the National Natural Science Foundations of China (NSFC) under Grants 42030105, 41721003, 41804012, 41631072, 41874023, Space Station Project (2020)228, and the Natural Science Foundation of Hubei Province of China under Grant 2019CFB611.