# UVSQ-SAT, a Pathfinder CubeSat Mission for Observing Essential Climate Variables
## 3 Materials and Methods
### The Space Segment: The UVSQ-SAT CubeSat Payload  
#### 3.2.1 The ERS Sensors  
To measure with accuracy the incoming solar radiation (TSI) and the Earth Outgoing Radiation (EOR = top of atmosphere Outgoing Longwave Radiation (OLR) + Shortwave Radiation (OSR)), we will use new Earth Radiative Sensors (ERS). ERS are sensors based on miniaturized thermopiles (active area of 5 mm \(\times\) 5 mm, responsivity of \(\sim\) 0.2 \(\mu\) V per Wm\({}^{-2}\)) designed to measure the heat flux from a 180\({}^{\circ}\) field of view angle. The passive (cold) junctions of the thermopile are fully protected from radiation and in thermal contact with the UVSQ-SAT structure, which serves as a heat-sink. Figure 3 shows the measurement principle of these sensors. UVSQ-SAT uses an ERS sensor with carbon nanotubes and an ERS sensor with an optical solar reflector on each side of the CubeSat (Figure 2). The ERS sensors with carbon nanotubes will absorb all incoming solar radiation and outgoing terrestrial radiation. They have a flat spectrum from UV up to 100 \(\mu\)m, and have a near-perfect cosine response. The carbon nanotubes used (Vantablack from Surrey NanoSystems) are one of the darkest substances known, absorbing up to 99.964% at 700 nm if the light is perpendicular to the material. These carbon nanotubes show excellent Bidirectional Reflectance Distribution Function (BRDF) performances for different incident angles and different scatter angles (total integrated scatter of 0.94% with an angle of incidence of 70\({}^{\circ}\)). The ERS sensors with an optical solar reflector will absorb mainly top of atmosphere outgoing longwave radiation. They have a high reflectance from UV up to 3 \(\mu\)m. They were selected since the variation of solar absorptance due to environmental exposure (thermal cycling, UV radiation, protons, etc.) is extremely weak. The performances of the coatings (carbon nanotubes and optical solar reflectors) were measured using a spectrophotometer (Agilent Cary 5000 UV-NIS-NIR) to obtain the solar absorption (between 200 and 2500 nm), an IR reflectometer (model DB100) to obtain the IR normal emittance (around 10 \(\mu\)m), \begin{table}
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**Properties** & **Value** & **Comments** \\ \hline Solar cells & 12 & 3G30A solar cells provided by Azurspace \\ Batteries & 22.5 Wh at 8 V & 2 Panasonic batteries (NCR18650B) with heaters \\ Power generated & 2.2 W & Orbit average power per 1U area in LEO \\ Power consumption & 1.6 W & Maximum orbit average with margins \\ \hline ADCS & 3 axis magnetometer & Measurements of the local Earth magnetic field \\ (Appendix A) & 3 axis magnetorquer & 0.2 Am\({}^{2}\) magnetic dipole \\  & 6 SLCD-61N8 photodiodes & Coarse estimation of the Sun’s direction (\(\theta\)) \\ \hline CDHS and OBC & 400 MHz, 32-bit ARM9 & Processor \\ (Appendix A) & 32 MB SDRAM & Synchronous Dynamic Random Access Memory \\  & 2\(\times\)2 GB SD cards & Non-volatile Data Storage (SD card redundancy) \\  & 1 MB NOR flash memory & Code storage \\  & 1\({}^{2}\)C, SPI, UARTs & UART is only used for debugging iOBC \\ \hline Data downlink & 1.2/9.6 kbps & UHF BPSK (437.020 MHz) communication \\ Data uplink & 9.6 kbps & VHF FSK (145.830 MHz) communication \\ Ground contact station & Less than 1 hour per day & LATMOS station \\ Redundancy stations & NCU (TW), ACRI-ST (FR) & Other stations: amateur radio partners \\ Downlink UVSQ-SAT data & 1.8 Mbyte per day & Maximum during a day \\ Uplink UVSQ-SAT data & 0.3 Mbyte per day & Maximum during a day \\ Transponder & Link with amateur radio & Live retransmission of FM signals \\ \hline Payload & 12 ERS & EEI measurements \\  & 4 DEVINS & UV SSI measurements \\  & 1 Teach’ Wear (TW) sensor & Accelerometer, gyroscope, and compass \\ \hline Launch adapter & SIPOD or Quadpack & CubeSat deployer with a satellite mass up to 2 kg \\ \hline \hline \end{tabular}
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