# Error Modeling of Fiber Optic Gyroscope Universal Time Measurement
## 4 Measurement Experiment  
To validate the proposed theoretical model based on FOG error correction and the methodology for the solution of UT1, we conducted static laboratory tests. Measurement experiments were conducted with a giant FOG installed in an underground measurement base station (40.24\({}^{\circ}\) N, 116.19\({}^{\circ}\) E) at Peking University's Changping campus. Figure 10 shows the testing system of the giant FOG, which is divided into two rooms, separating the optical configuration from the electrical part. It is helpful to isolate the vibration and heat generated by these devices from the fiber optic ring. In the optical probe, the light source is an amplified spontaneous emission (ASE) broadband light source. It has an average light intensity of 23 dBm and a central wavelength of 1550 nm. A multifunctional integrated optical device is used for polarization filtering, beam splitting and combining, and phase modulation. Polarization-maintaining circulators enable the transmission direction and polarization of the light waves. The polarization-maintaining fiber coil material was made of silica and coated with acrylic resin. Using a quadrupolar (QAD) winding pattern with 8 layers reduces the phase drift caused by the Shupe effect. In the electrical part, the signal generator generates the sinusoidal modulation signal. The interference light was detected by a photodetector, and the acquired interference light intensity was converted to a digital signal with an NI demodulator including a digitizer and an integrated remote controller PXI chassis. The fiber optic ring is 2 m in diameter, with 20 km of fiber optic length, using a large aluminum alloy skeleton and a circular frame body. The FOG is placed horizontally on a solid rigid concrete foundation built 2 m below the surface soil and anchored to bedrock. The tiltmeter is placed next to the FOG foundation with a resolution of 1 nrad. The room temperature in the isolated measurement chamber is around 286 K. For the FOG exterior, acrylic sheets and Styrofoam were used to achieve passive thermal stability and to mitigate  
Figure 10: Testing system of the giant FOG at Peking University. air convection-induced vibrations, without any vibration isolation and magnetic shielding devices. The entrance is equipped with three isolation doors and a separate control room to minimize environmental impact. Any data segment of raw data from the FOG is selected to analyze its noise characteristics, as shown in Figure 11, with a sampling frequency of 10 Hz and data units of rad/s. The main performance evaluation indicators for FOG raw data are the angular random walk (ARW), bias instability (BI) and sensitivity. The ARW describes the magnitude of short-term white noise in the output of FOG. BI is the degree of deviation when the FOG detects an angular velocity; it represents long-term stability. Both can be obtained using Allan standard deviation analysis. The sensitivity is the self-noise level of the FOG, obtained through PSD analysis. By calculating the PSD of the FOG raw data, one can obtain its noise at various frequencies. Figure 12 shows Allan deviation analysis [35] and power spectral density (PSD) analysis. The slopes of 0 and \(-1/2\) in the Allan curve are the corresponding BI and ARW, respectively. The ARW and BI in the Allan curve are \(3.525\times 10^{-9}\) rad/\(\surd\)s and \(2\times 10^{-10}\) rad/s, respectively. The angular error generated after the 1-day integration operation is calculated to be about \(0.26^{\prime\prime}\) by ARW. If the UT1 solution accuracy is to reach 1 ms/day, the angular error after 1-day integration needs to be less than \(0.015^{\prime\prime}\), so the influence of FOG noise on the UT1 solution is also non-negligible. The PSD analysis in Figure 12b shows that it has a good performance at low frequencies, corresponding to long-term stability, while it exhibits high noise at frequencies higher than 1 Hz. The entire band is relatively flat except for high frequencies that are subject to environmental interference.