Technical description

• External Power: For the FM, this is the power coming from the solar panels feeding the back-end electronics and the internal power distribution. We assume the panels to be non articulating. Hence to optimize the battery recharge on the FM, we plan to use the inertial measurement unit (IMU) for positioning the solar panels to the ideal position relative to the sun, when the cubesat is not performing target mode (e.g. over ocean). For the EM the external power is simply an AC-input from the power grid going into the tracker.

• Tracker: The EM will be mounted on a tracking device, which allows pointing of the instrument in any direction, thus simulating a situation in orbit (figure B6). For this purpose we will adapt the existing LuftBlick TR1 tracker, which was developed for Pandora with support of ESA in 2018 [Müller et al., 2018] and has since then been successfully used in the PGN. Since the EM will weigh more than the Pandora headsensor, we will probably use the next larger size of stepper motors in the adapted version (see section 2.1 of Müller et al. [2018]). Another modification will be switching to a better pointing resolution of 0.001° instead of the 0.01° used in the Pandora operation. The tracker is powered by 220VAC from the grid.

• Mechanical Interface: This is the mounting bracket to hold the EM on the tracker. The one shown in figure B6 is the bracket for “azimuthal scanning”, where one image is a thin vertical slice of figure C3 and the tracker moves in azimuth from image to image, which is the regular scanning mode. We will also build another bracket, which holds the EM tilted by 90°. That one can be used for “zenithal scanning”, where one image is a thin horizontal (partial) slice of figure C3 and the tracker moves in elevation angle from image to image.

• 2U for FM: The section inside the blue frame represents the 2U section in the upper third of figure B1 and is only used for the FM. It contains the Back-end Electronics, the inertial measurement unit (IMU) and the communication elements (COMM), i.e. transmitter, GPS, antenna, etc., which are all powered by the solar panels.

• 4U Frame: The section inside the dashed frame represents the 4U section in the bottom 2 thirds of figure B1. The parts inside this frame are the payload for the cubesat, which we estimate to weigh <6kg (figure B3). With the frame we estimate it to stay <7kg, but the 4U-size is slightly exceeded in each dimension. This is because for the EM we will keep each element of the payload in its original housing and the frame is built around it. In this way we can keep a certain protection for each element even when the system is opened, or for the case of some moisture intrusion. This makes the handling or troubleshooting of the system during field operation much easier than if each part was stripped naked and we would have to bring it inside a building each time we work on it. For a FM, the housing of the internal parts would be removed or reduced, so that the size of the frame would not exceed the 4U.

• EM Enclosure: The EM will have a weatherproof enclosure around the 4U frame (figure B5 in green). With the enclosure the total weight will probably be above 7kg. The enclosure holds 3 ports on an interface panel (figures B3 in gray): two USB ports (right), one RS485 port (center) and one connector for a WiFI antenna (left).

• Internal Power Distribution Unit: This element receives DC power from the tracker (EM) or the solar panels (FM) and distributes it to the other elements of the payload (yellow part in figure B3).

• Front-end Electronics: This computer (figure B3 in green) operates the camera, and stores and pre-processes the measured images, which is described in section 1.1.3.3. It is powered with 12VDC through the internal power distribution unit. In the FM version, it interacts with the Back-end Electronics, from which it receives the commands for a new measurement sequence to be started, and to which it sends the data to be transmitted to the ground. In the EM version it gets its commands and transmits the data over the internet (WiFi) from and to the LuftBlick server. In the EM the computer will also be accessible via 2 USB ports. In order to store and pre-process the measurements, the operating computer will have significantly larger storage space and a more powerful CPU than the back-end computer, which typically only has <10GB storage space.

• Camera: For the camera we plan on using a system based on a high quantum efficiency back-illuminated CCD array with UV coating. For budget reasons we plan to get an off-the-shelf camera system in its original packaging, which includes an active temperature controller (TEC) with a peltier element and a fan (figure B3 in blue). Our currently preferred choice has 1024 x 1024 rectangular pixels, each 13μm wide and with a well-depth of 100000 electrons. The optimized operation range of the camera is between -45 and -30°C. We will not remove the fan for the EM, although it is obviously not needed for the FM. The camera is powered with 12VDC through the internal power distribution unit. It consumes <1A with the TEC off and 4.4A when the TEC runs at 100%. The final choice for the camera will be made in case the project manifests.

• Spectrometer: We plan to use an Offner imaging spectrometer design [Prieto-Blanco et al., 2006] and will purchase a system that matches as closely as possible our needs, i.e. covering a wavelength range of 270 to 520nm at a resolution of ~0.6nm (see also figure C4). We do not expect that such an off-the-shelf system exists and will therefore either modify one ourselves or pay the manufacturer to apply the necessary custom modifications. At this moment we plan on using a system with dimensions as shown in figure B3 (brown parts). The final choice for the spectrometer will be made in case the project manifests.

• Depolarizer: “Depol” stands for a polarizer added in front of the spectrometer (figure B3 in dark green). We plan to use a quartz-silica wedged depolarizer that fits to the spectrometer.

• Telescope: “Telesc” stands for the telescope assembly (figure B3 in red). In order to fulfill the requirement in the SoW (“The EM shall be flexible in terms of allowing an adaptation of the FOV thus being capable of changing the horizontal resolution.”), there will be two different telescopes, one giving a narrow FOR of 7° full width and the other giving a wide FOR of ~75° full width (see also figure C3). One will be able to switch the telescopes mechanically by opening up the system. The telescopes will need to be custom made, since to our knowledge no existing solutions meet our requirements. The two telescopes will differ in length and possibly stick partially out of the 4U frame as seen in figure B5.

• Collimator: The EM will have a collimator attached on top of the telescope (a separate one for the narrow and wide view; not shown in figures B3 to B6), which is not necessary for the FM. This is needed to avoid the direct solar beam scattering off the first lens (more details about this are in items R1, R2 and R3 of section 1.2).

• Passive Heat Control: For both the EM and FM we will use passive temperature control techniques for the payload enclosure and an active control of the camera. For the EM, the heat sink, which is connected to the front-end computer and the camera, is placed on the bottom side of the system, since this part is hardly ever exposed to the direct sun beam (figure B5 in gray). For the FM the heat sink will be placed on the front side of the system, next to the lens. The passive thermal control can typically keep the temperature of the cubesat payload below 30°C in the hot case, i.e. during the half-orbit in sunlight [Mason et al., 2017; Totani et al., 2013]. We expect the same to be true for the EM on the mountaintop location. At full power the TEC draws 4.4A and consumes 53W, which is about half of what the solar panels can deliver. This means that depending on the total energy balance of the FM in orbit, the TEC might not be switched on all the time.