Presented at 2 nd ISSI Workshop “Space4me Metrology, Clocks and Rela4vis4c Geodesy”,25-28 March 2019, Bern Formulation of testing gravitational redshift based on frequency links between ACES on board ISS and ground station Wen-Bin Shen, Xiao Sun Wuhan University School Of Geodesy And Geomatics
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Formulation of testing gravitational redshift based on ...€¦ · Presented ndat 2 ISSI Workshop “Space4me Metrology, Clocks and Relavis4c Geodesy” ,25-28 March 2019, Bern
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Fig1.Testing of gravitational redshift is one of the critical part of testing the Einstein equivalence principle (EEP, Will 2014), which has been confirmed by various physicists (Pound and Rebka 1959, 1960a, b, 1965; Hafele and Keating 1972; Alley 1979; Vessot et al. 1980; Turneaure et al. 1983; Krisher el al. 1993).
Recently, experiments performed by two Galileo satellites show better results (~ 2×10-5) (Delva et al. 2018; Herrmann et al. 2018). ACES mission: expected to be ~ 2×10-6.
A hydrogen maser (SHM) with medium-term frequency stability; A cold cesium atoms (PHARAO) with long-term frequency stability; Frequency stability: 1×10−13τ−1/2, with accuracy 2×10−16.
Time signals on board are compared to ground clocks using microwave two-way time-transfer system (MWL) operating in the Ku band as well as a laser time-transfer system ELT.
There are already a few literatures addressing to test gravitational redshift based on time comparison (Cacciapuoti and Salomon 2009; Duchayne et al. 2009; Meynadier et al. 2018), but there are short of publications related to frequency comparison.
Concerning ISS’ orbit altitude of 400 km, for each pass over a given ground station, the observation time period is limited to 300-400 s. We aim at the frequency precision better than 2×10-16 in 300s
Introduction
We consider a downlink from satellite A to the ground B. In a general relativistic framework, accurate to 1/c3, the proper frequency shift of the photon from A to B is expressed as (Blanchet et al. 2001)
Due to the velocity of source, the wavelength λ is expressed as
0 1 A A Anc
λ λ⋅⎛ ⎞= −⎜ ⎟
⎝ ⎠
T v(7)
Then, with eqs. (6) and (7), the received relative frequency due to Doppler effect is expressed as
1 cos
1
BB B B
Adopdop
A A A
n n dsf f c tA nfc
α⋅ ∂
− −+ Δ ∂= =⋅
−
∫T v
T v(8)
One-way frequency transfer
2
In atmosphere, electromagnetic waves undergo direction change or refractive bending during their propagations. Hence, the direction of a refracted ray at the space station differs slightly from the un-refracted line-of-sight direction (Millman & Arabadjis 1984), as Figure 9 shows.
Figure 9. Principle of ray refraction through atmosphere for GNSS satellite. (Hoque et al. 2008)
With ISS’ height of around 400 km (Cacciapuoti & Salomon 2009), the ACES-ground links signals propagations experience atmospheric influences, therefore we have (Shen et al. 2016)
Ionospheric refraction index is relevant to carrier frequency, while tropospheric refraction index is not relevant to it. Therefore, for all links f1, f2 and f3, the bending effects of troposphere are almost same, which makes it easy to calculate the tropospheric part.
For ionospheric part, to the order of f-2, we have (Hajj and Roman 1998; Hoque et al. 2008)
21 40.3 ennf
= −
where ne is the electron density per cubic meter
(12)
One-way frequency transfer
For ISS ACES mission, the planned frequency links are as follows:
The frequencies of these three links are denoted as f1, f2, f3 2ndISSIWorkshop“Space4meMetrology,ClocksandRela4vis4cGeodesy”
Formulation for testing gravitational redshift 3
p Ku band uplink, with carrier frequency 13.475 GHz, and frequency shift is known afterwards;
p Ku band downlink, with carrier frequency 14.70333 GHz;
p S band downlink, with carrier frequency 2248 MHz.
A Ku band uplink signal f1 is emitted from ground station (B) at coordinate time t1 and received by space station (A) at t2. Meanwhile, two downlink signals f2, f3 are emitted from A at times t3 and t5 respectively, and received by B at times t4 and t6 respectively. If we definea coordinate time interval by Tij=tj-ti, T23 and T35 will be synchronized to be zero in theory, but actually they may not be zero.
For ACES links: f1=13.475 GHz, f2=14.70333 GHz and f3=2248 MHz, 、 The third link is of low frequency, which suffers much from ionospheric effects. If the value of T35 is extremely small (<1 us), the only different error between links 2 and 3 is the ionospheric effect.
Combining the three observed frequencies, we may determine the gravitational potential difference between space station and ground station. Details are referred to Sun and Shen (2019 in preparation for submission)
If the Einstein’s GRT is correct, coefficient β should be zero.
Formulation for testing gravitational redshift
z = ΔUCM / c2 = 1+β( )ΔUGR / c
2
Discussion 4
It is promising to test gravitational redshift at a level of 3×10-6
by frequency comparison. To achieve that accuracy level, the parameters accuracies should be carefully controlled, which need further investigations. At present accuracy requirement, it is not necessary to consider Earth’s tidal effects and plate motion effect. Simulation experiments are in process.