Temperature control method and verification for spaceborne atomic clock based on phase change technology

The spaceborne atomic clock, as the time reference for navigation signal generation and system ranging, is the core component of the satellite system payload, and temperature stability is one of the main factors affecting the output frequency stability of the atomic clock. To ensure its continuous and stable operation in orbit, the temperature control system needs to provide a good working temperature environment for it. This article takes the high stability temperature control of the rubidium atomic clock in low-Earth inclined orbit as the research background, and conducts research on temperature control methods and phase change devices based on phase change technology. Simulation analysis and in-orbit telemetry data show that, by utilizing the temperature stability characteristic during the phase change process, the phase change temperature control system effectively suppresses the temperature fluctuations of the rubidium atomic clock caused by drastic changes in the space thermal environment.


Introduction
With the development of aerospace technology, spaceborne payloads have put forward higher requirements for their working temperature control accuracy and temperature stability.The spaceborne atomic clock, as the time reference for navigation signal generation and system ranging, is the core payload of the constellation satellites system, and its performance directly determines the positioning accuracy of users.Temperature stability is one of the main factors affecting the output frequency stability of the atomic clock.In practical applications, the ambient temperature of a spaceborne atomic clock varies greatly.For a spaceborne atomic clock to achieve a higher stability indicator, it is necessary to reduce the temperature sensitivity (frequency temperature coefficient) of the entire machine, which puts forward extremely high requirements for thermal design [1,2] .
Fluctuations in external heat flux and changes in internal heat sources can affect the temperature level of the entire satellite in orbit.The external heat flux absorbed by the satellite will vary with its surface thermal optical properties and changes in the orbital position and attitude.Therefore, for equipment with high-precision temperature control requirements, isolation from external thermal disturbances is usually adopted, supplemented by electric heating active temperature control technology to provide accurate temperature control.China BeiDou navigation satellites have achieved highprecision temperature control of atomic clocks with a temperature control cabin and PID high-precision temperature control design [3][4][5] .
A certain satellite operates in a low-Earth inclined orbit, which is different from the relatively stable heat dissipation condition of a GEO satellite.The satellite's rubidium atomic clock is installed in the sky.The external heat flux undergoes changes from direct sunlight to shadow, with a short period and a large amplitude [6] , which poses a great challenge to the temperature control for the high-stability rubidium atomic clock.This article applies phase change technology for the rubidium atomic clock temperature control system.By suppressing the temperature fluctuations caused by periodic fluctuations in external heat flux through the phase change energy storage characteristic, the temperature stability of the rubidium atomic clock is met, which is ≤ ± 3℃/ orbit period for temperature control.

Phase change temperature control technology and its application
Phase Change Material (PCM), also known as latent heat storage material, refers to the material that can absorb or release heat when undergoes phase change, while the temperature remains unchanged or changes little.Due to the characteristics of phase change material, it can be used in spacecraft with periodic changes in internal heat sources or external environments to maintain relatively stable equipment temperature.Phase change temperature control technology is a technology that utilizes the characteristic of phase change material that stores or releases heat during the phase change process to achieve temperature control for equipment.Phase change device has the functions of energy storage and temperature control [7,8] .
In the thermal control design of the Apollo 15 lunar rover, three-phase change devices were used to control the temperature of important equipment such as signal processing units, batteries, drive controllers, and relays.Mars Exploration Rover lander used phase change devices to control the temperature of the batteries.In 2013, NASA's Johnson Space Research Centre listed energy storage temperature control technology as the annual core temperature control technology.In China Chang'e-1 satellite and Mars-1 exploration missions, phase change devices were used in the thermal control system to suppress the temperature rise of short-term working equipment [9][10][11][12] .The rubidium atomic clock (Figure 1) in this article is installed in the sky in a critical inclination orbit satellite.The rubidium atomic clock requires a temperature range of -10~15℃ for its working environment and a temperature stability requirement of ≤ ±3℃/ orbit period.At the same time, due to the use of secondary temperature control inside the rubidium atomic clock, the working heat consumption varies with the installation plate temperature (Figure 2).If there is no stable boundary condition for the installation plate temperature, the changes in the rubidium atomic clock's heat consumption will further increase the difficulty of temperature control.

Analysis of thermal control task for spaceborne rubidium atomic clock
The satellite adopts a three-axis stable attitude (+Z axis to the earth, +X axis to the flight direction, Figure 3), orbital altitude=1010 km, orbital inclination=63.4°,the angle between the sun and the orbital plane (β angle) varies between -86.9° to+86.9°,and the external heat flow changes are complex.Figure 4 shows the time-variation of transient direct solar heat flux from the satellite to the sky at β=0° and β=86.9°.In extreme cases (β=0°), the transient direct solar heat flux can vary from 0 W/m 2 to 1410 W/m 2 .At β=0°, each orbit of the rubidium atomic clock installation surface undergoes periodic changes from direct sunlight to shadow, with the most severe changes in external heat flux.At β=± 86.9°, the external heat flux is almost zero, and a temperature compensation design is needed to ensure that the temperature of the rubidium atomic clock is not too low.

Principle of temperature control system
The design of the rubidium atomic clock temperature control system is shown in Figure 5.The rubidium atomic clock installation plate is an aluminum honeycomb panel.Two "U" shaped heat pipes are embedded inside the installation plate to improve the temperature uniformity, and the embedded position passes through the rubidium atomic clock installation surface.The inner side of the installation plate is pasted with 4 thin film electric heaters and thermistors to form 4 compensation temperature control circuits.The temperature control range is [3℃, 4℃], and the single circuit temperature control power is 24 W. The satellite temperature controller performs high-precision closed-loop temperature control.5 mm polyimide insulation pads are used between the installation plate and the satellite structure for insulation.The rubidium atomic clock and the inner side of the installation plate are covered with MLI components to reduce thermal disturbance to the rubidium atomic clock by other equipment on the satellite.
The outer side of the installation plate installs four phase change devices.These devices are filled with solid-liquid phase change material, and paraffin phase change material is selected.The installation plate and the phase change devices are filled with thermal conductive grease, and the devices are pasted with low solar absorption ratio and high emissivity OSR secondary surface mirrors on the side facing the cold space, serving as the radiation and heat dissipation surface of the entire rubidium atomic clock temperature control system.

Phase change device design
The key to designing a phase change device is the selection of phase change material and the enhancement of heat transfer performance.At present, paraffin phase change material is the most widely used.The comprehensive properties of paraffin phase change material, such as phase change temperature, latent heat, density, and compatibility, are superior to the other phase change materials.However, its disadvantage is low thermal conductivity, only about 0.2 W/mꞏK, and its thermal diffusion performance is only about one-thousandth of copper.During the phase change process, a large temperature gradient is easily formed inside the paraffin [13][14][15] .Therefore, strengthening the thermal conductivity of phase change material is the research focus on the performance of phase change devices.
C 14 H 30 , which matches the working temperature range of the rubidium atomic clock, is selected as the phase change material, with its phase change temperature T m =5.7℃ and latent heat r=228 kJꞏkg -1 .To increase the thermal conductivity of the phase change device, foam metal, foam carbon/graphite, metal fins, and other materials are generally selected.In this paper, aluminum fins are designed to increase their thermal conductivity.
A single phase change device is a fan-shaped structure with a radius of 350 mm, thickness=8 mm, center angle=90°.The product and structural breakdown are shown in Figure 6.The structure of the phase change device consists of an upper plate, lower plate, cushion blocks, sealing frame, fins, and liquid-filled tube.The shell material is made of 3A21 aluminum, and the liquid-filled tube is made of 1060 aluminum.The selected aluminum materials have good fusion welding, corrosion resistance, and good compatibility with the phase change material C 14 H 30 .The overall brazing of the phase change device shell structure can withstand an internal pressure of 0.3 MPa, with a leakage rate ≤ 1.0 × 10 -7 Paꞏm 3 /s, filled with phase change material of approximately 330 g.The internal fins of the device can not only enhance the structural strength but also its heat transfer ability.The measured equivalent thermal conductivity is better than 2.8 W/mꞏK, which is 14 times that of the phase change material C 14 H 30 .In this formula: ℎ -Equivalent latent heat of phase change device, J/kg.According to Formula (2), although heat transfer occurs during the material phase change process, the temperature of the phase change material does not change.
To predict the temperature of the phase change device in orbit and the temperature control effect for the rubidium atomic clock, based on the external heat flow and the temperature boundary of the satellite platform, this article selects two typical cases for thermal analysis and calculation of the phase change device.The calculation cases are shown in Table 1 The structure of the phase change device has been simplified in the Thermal Desktop thermal analysis software [16] .The shell of the phase change device is made of aluminum, and the interior is filled with C 14 H 30 (as shown in Figure 7).In the thermal simulation model, it is simplified as a three-layer shell unit, with the outer two layers of aluminum, each 1 mm thick, and the middle layer of C 14 H 30 .The equivalent thickness is determined by the filling amount, which is 4 mm, and the equivalent heat transfer coefficient of the phase change device is measured by a thermal performance test.In the thermal simulation model, the heat transfer relationship between the satellite platform and the rubidium atomic clock temperature control system is simplified as the radiation heat transfer inside the rubidium atomic clock cabin.The Monte-Carlo method is used for internal and external radiation calculation.Except for the rubidium atomic clock installation plate, other cabin plates of the satellite are treated with fixed temperature boundaries (in Table 1).The rubidium atomic clock thermal simulation model is shown in Figure 8.The initial temperature for the thermal simulation model calculation is set at 20℃, with a calculation time of 5000 s (approximately 13.9 h).The thermal analysis results of the phase change device are shown in Figures 9 to 10.Under the low-temperature case (β=86.9°), the temperature of C 14 H 30 is between -7.8 and -6.9℃, and the phase change material is in the solid state.The external heat flow is relatively stable in this case.The temperature of working rubidium atomic clock A is between 0.7 and 1.0℃, and the temperature fluctuation is less than ± 0.2℃/orbit period.The temperature of non-working rubidium atomic clock B is between -4.7 and -4.3℃.Under the high-temperature case (β=0°), the temperature of C 14 H 30 ranges from -2.0℃ to 5.7℃, and the phase change material undergoes phase change near the phase change point 5.7℃, effectively suppressing the temperature fluctuations caused by external heat flux changes.The working rubidium atomic clock A has a temperature range of 6.4 to 10.3℃, and the temperature fluctuations are less than ± 2℃/orbit period.The non-working rubidium atomic clock B has a temperature range of 1.6 to 6.4℃.The thermal simulation results indicate that using the "cut top and fill in valley" effect of phase change material, the rubidium atomic clock temperature control system effectively meets the temperature index requirement.

On orbit verification status
In April 2021, the satellite was launched into orbit.1) The phase change temperature control system utilizes the temperature stability characteristics of phase change material during the phase change process, effectively suppressing the temperature fluctuations of the rubidium atomic clock caused by drastic changes in the space thermal environment.The temperature fluctuation of the rubidium atomic clock in orbit is better than ±1.5℃/ orbit period.
2) Filling high thermal conductivity aluminum fins inside the phase change device can effectively improve the thermal conductivity.The measured equivalent thermal conductivity is better than 2.8 W/mꞏK, which is 14 times that of the phase change material C 14 H 30 .At the same time, the phase change device is formed as a whole through vacuum brazing, with good pressure resistance and sealing performance, which can meet the requirements of long-term operation in orbit.
3) This article establishes the phase change heat transfer model in a space thermal environment and simplifies the thermal model of a phase change device.The thermal simulation results are consistent with the variety rule and range of temperature changes in orbit, verifying the correctness and effectiveness of the phase change thermal physical model and design method.
In summary, the phase change temperature control system for spaceborne atomic clocks exhibits good temperature control characteristics and also has the advantage of high reliability.This article on temperature control methods based on phase change technology has certain reference significance for the temperature control design of high-temperature stability equipment in the aerospace field.

Figure 2 .
Figure 2. Heat consumption curve of rubidium atomic clock with temperature.

Figure 3 .
Figure 3. Satellite orbit flight attitude and rubidium atomic clock location.

Figure 4 .
Figure 4. Transient solar heat flux variation of installation surface.

4 Figure 5 .
Figure 5. Schematic diagram of rubidium atomic clock phase change temperature control system.

Figure 6 .
Figure 6.Product and structural breakdown of phase change device.4.3.Thermal simulation analysisThe satellite flies in an inclined orbit, orbital altitude=1010 km, orbital inclination=63.4°.When the satellite is in orbit, its temperature is affected by direct solar heat flux, earth's albedo heat flux, and earth's infrared heat flux.Considering the heat transfer inside the phase change temperature control system, when the temperature of the phase change material is above or below the phase change point, the heat balance equation of the phase change device is:

Figure 7 .
Figure 7. Sectional drawing of phase change device.

Figure 8 .
Figure 8. Thermal simulation model of rubidium atomic clock temperature control system.

Figure 9 .
Figure 9. Calculating results of phase change material and rubidium atomic clock A/B under the lowtemperature case.

Figure 10 .Figure 11 .
Figure 10.Calculating results of phase change material and rubidium atomic clock A/B under the hightemperature case.As a comparison, Figure 11 shows the thermal simulation temperature curves of the rubidium atomic clock A/B under the high-temperature case without phase change devices.The temperature of working rubidium atomic clock A ranges from 6.1 to 14.4℃, with a temperature fluctuation ± 4.2℃/orbit period, while the temperature of non-working rubidium atomic clock B ranges from 1.4 to 10.0℃.It can be seen that the temperature of rubidium atomic clock A/B fluctuates significantly with the fluctuation of external heat flux.Without phase change devices, the rubidium atomic clock temperature control system is no longer able to meet the temperature stability index requirement.

Figure 12 .
Figure 12.Temperature curves of rubidium atomic clock temperature control system in orbit.6.Conclusion This article takes the high stability temperature control of the rubidium atomic clock in low-Earth inclined orbit as the research background, and conducts research on temperature control methods and phase change devices based on phase change technology.Through simulation analysis and in-orbit flight verification, the following conclusions are drawn:

Table 1 .
. Definition of calculation cases.