6 A Highly Integrated Single Unit CubeSat Solar Panel
6.1 Current Integration Density of CubeSat Solar Panels
The primary function of CubeSat solar panels is to host solar cells on the outside of the satellite structure and connect them to the power control and distribution unit, which is commonly found in the board stack that makes up the main satellite bus. A growing number of panels is equipped with additional components and functionality, like sun sensors, magnetic coils, or maximum power point trackers. To compare characteristics of body-mount single unit CubeSat solar panels currently in use, their properties and components are gathered in tables 6.1 and 6.2. The upper part of each table holds panels offered by CubeSat component vendors, the lower part lists a selection of custom made panels which are relevant in the scope of this work. The lists do not claim for completeness, as many university and also commercial CubeSats feature custom made panels.
6.1.1 Mechanical Properties
CubeSat solar panels are based on PCB, aluminum sheet, or carbon-fiber carriers onto which solar cells and electronic components are mounted. Ad-vantages of using PCBs as carrier are, that multiple copper layers and vias can be used to conduct the solar cell current, all electronic components can be soldered directly in place, and boards can be procured at low cost. Aluminum sheet and carbon fiber carriers require additional thin-film conductive layers, increasing complexity and cost of the panels. Aluminum and carbon-fiber carriers, however, can carry higher structural loads than PCBs. Therefore, utilization of such boards can reduce the mass of CubeSat primary structure.
All single unit solar panels have maximum dimensions of 100×83 mm, driven by the requirement to fit between the CubeSat rails (cf. table 6.1, [25]). The de-facto standard for PCB carrier thickness is 1.6 mm. If magnetorqers are embedded into the carrier board, the thickness might deviate from the standard.
For aluminum or carbon-fiber based panels, thicknesses are commonly kept at the same value in order to stay compatible with the standardized frames and CubeSat kits of major vendors. Masses of the panels are between 30–60 g (cf.
table 6.1). CubeSat designs with six panels therefore need to reserve about 20–25 % of total satellite mass for solar panels, if heavier panels are required.
Table 6.1: Single unit CubeSat solar panel properties
Supplier Name Mechanical Solar Cells
𝑀 𝑙 𝑤 ℎ No Eff. 𝑃
g mm mm mm # % W
Clyde Space 25-02869 n/a n/a n/a 1.6 2 28.3 2.1 [118]
DHV Tech. SPC-CS10 39 n/a n/a n/a 2 30 2.4 [119]
EnduroSat XY /MTQ 50 98 82.6 2 2 29.5 2.4 [81]
GomSpace P110UA 57 98 82.6 1.6 2 30 2.3 [80]
ISIS 50 98 82.6 1.6 2 30 2.3 [120]
nano avionics 1U-S 35 100 82 1.6 2 28.7 2 [121]
nano avionics 1U-SMS 40 100 82 1.6 1 28.7 1 [121]
Poli. Torino CubePMT n/a 98 82.5 1.6 2 26 n/a [93]
Uni Würzburg UWE-3 [122]
TU Berlin BEESAT 34.4 98 82 1.6 2 29.6 2.4 [54]
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i i Most solar panels feature mounting holes in the four corners of the panel.
Presence of the holes along the panel’s outer perimeter reduces the enclosed area for embedded air coils and hence the maximum achievable magnetic dipole. To avoid this loss, BEESAT solar panels feature two mounting holes close to their center.
6.1.2 Power Generation
Usually, two large-area, triple-junction solar cells with 28 % and above efficiency are used for generating about 2 W power per single unit solar panel (cf.
table 6.1). Thus, panel surface area is covered more or less completely by solar cells. Components like precision sun sensors, patch antennas, RBF pins, umbilical connectors, and others reduce the available surface area. This either leads to a reduction in number of large-area solar cells, or usage of a larger number of small-area cells. Due to their electrical properties, combinations of different shapes or types of solar cells on a single panel are usually avoided.
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(a) Same cell orientation with mounting holes in the corners
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(b) Opposite cell orientation with mounting holes in the center
Figure 6.3: Solar cell orientation and mounting hole placement
For mounting large-area solar cells, two general methods are applicable. If cells are oriented in the same direction with respect to the chamfered side (cf. figure 6.3(a)), electrical connection between cells is simplified, but free surface area is reduced to a narrow brim at the panel’s edges and a narrow gap between the cells. Hence, additional components on the outside need to have a small footprint area.
If chamfered sides of the cells are pointing in opposite directions, electric routing is less simple, but a significant gain in connected area for additional components is observed (cf. figure 6.3(b)). Precision sun sensors or RBF pins might be located between the solar cells while mounting holes are sitting at the chamfered cell edges. Yet, inter-cell area is not large enough to host patch antennas for high-throughput communication (cf. chapter 3).
Apart from solar cells, the two bottommost entries in table 6.2 feature MPPT capabilities implemented in hardware and software on the panel. Panels not featuring built-in MPPT capabilities have to rely on this being provided by the PCDU. Busch in [56] states that having MPPT implemented directly on the panel, "optimal performance can be achieved for each panel separately" while single panels are exposed to different irradiation and temperature conditions.
6.1.3 Attitude Determination Sensors
Simple Solar panels feature photodiodes acting as coarse sun sensors, which require temperature sensors to compensate thermal offsets (cf. table 6.2).
The advantage of using photodiodes is their small size. Precision sun sensors, e.g. based on position-sensitive devices (PSDs) or CMOS technology, require more electrical power and more area on both sides of the panel.
MEMS magnetic field sensors and gyroscopes, usually located on ADCS boards, are rarely found on commercial CubeSat solar panels (cf. table 6.2). Those devices exhibit a pronounced temperature dependency of magnetic field and gyroscopic measurements. Their application on solar panels, which experience the largest temperature swings during satellite life time, requires additional effort for temperature calibration. Placing magnetic field sensors close to components carrying large currents, like solar cells or magnetorquers, further
Table 6.2:Single unit CubeSat solar panel components
Supplier Name Sensors Actuators
Sun Temp. Magn. Gyro Torquer mA m2
Clyde Space 25-02869 X X – – X 80 [118]
DHV Tech. SPC-CS10 X X X – – – [119]
EnduroSat XY /MTQ X X – X X 131 [81]
GomSpace P110UA X X – – X 38 [80]
ISIS X X – – – – [120]
nano avionics 1U-S X X – – X 70 [121]
nano avionics 1U-SMS X X – – X 70 [121]
Poli. Torino CubePMT X X X X X n/a [93]
Uni Würzburg UWE-3 X X X X X 28 [122]
TU Berlin BEESAT X X – – X 46 [54]
impairs sensor readings. Using a microcontroller local to the solar panel for correction of temperature effects and filtering of sensor data is advantageous for precise attitude determination [56].
6.1.4 Attitude Control Actuators
Owing to their simplicity, magnetorquers are considered to be the best choice for coarse CubeSat pointing. Most vendors offer their panels with the option for an embedded air coil (cf. table 6.2 and section 4.1). Some vendors offer optional connectors for additional, customized magnetorquers on the panel.
The advantage of using embedded air coils is that they do not occupy additional volume inside the satellite and there is no reduction in available area for placing electronic components on the solar panel back. Disadvantages are the reduced area for placing vias, high manufacturing costs for multi-layer boards, and the widely-held assumption of small provided magnetic dipoles (cf. section 4.1).
Advantages of board-external magnetorquers are the unrestricted choice of solar panel carrier material, reduced production cost for two or four-layer PCBs, and flexibility in terms of late magnetic actuator selection. Also, magnetorquers and solar panels could be sourced from different suppliers. The biggest among the disadvantages of using board-external magnetorquers is the
increased volumetric consumption of this solution. This effect is less severe using wound air coils, as they can be attached to the primary structure or the solar panels. If they are attached to solar panels, available area for electronic components on the rear of the panel is though reduced (cf. section 4.1).
Except for folding mechanisms of deployable solar panels or antennas, other actuators located on the single unit CubeSat panels are unheard of. In the scope of the AraMiS project, attaching miniaturized reaction wheels to the solar panels, so-called intelligent tiles, of larger satellite was proposed by Speretta in [123].
6.1.5 Harness
CubeSat component vendors want their solar panels to be compatible with not only their products, but also with those of other suppliers. This leads to a situation, where every functionality present on a panel features its own connector and harness. Solar cells are connected to the PCDU, attitude sensors and actuators to the ADCS, and so on. Therefore, volumetric utilization rate inside a CubeSat is further reduced if the number of different functionalities were to be increased on traditional CubeSat solar panels, as each would require its own harness and connector.
Custom solar panels may differ from the harness scheme described above.
In case of the BEESAT satellites, IDC connectors and flat ribbon cables are used to connect all functionality of the solar panels to the satellite bus.
This increases the volumetric utilization and eases assembly of the satellite.
However, using flat ribbon cables is challenging, when different signals and supply voltages have different targets to be routed to, as a connector can only be connected to one subsystem board. Splitting one end of the cable and using more than one connector on that end allows to connect a single panel to multiple target systems at slightly increased cost and assembly effort.
Modern consumer electronic products heavily rely on flexible flat cable (FFC) to connect components. Multiple metallic conductors are bonded to a flexible plastics film to form an ultra thin cable. FFCs offers great flexibility in CubeSat harness design, as the harness might be specifically adapted to the satellite
structure and feature several connectors, both mid-cable and at the multiple ends, as stated by Karuza in [124]. Disadvantages are high production cost and long lead times.
One exception in terms of harness are the UWE-3 solar panels, which use direct board-to-board connectors between panels and the backplane (cf. [56]).
This is the most simple and cost effective way of connecting the panels. The use of a backplane, however, is inefficient in terms of volumetric utilization (cf. section 2.1.3.2).
For a modular university CubeSat platform, using FFC as harness is not applicable. Individual connectors and cables are also not recommended for high performance spacecraft due to the poor volumetric utilization they impose (cf. [6]). Flat ribbon cable and IDC connectors are considered to be the best choice. They allow to have multiple connectors mid-cable, and if a common bus is used to connect all subsystems (cf. section 2.2.4), they enable flexible and modular CubeSat design with very good volumetric utilization.
6.1.6 Conclusion
Discussing the integration density of single unit CubeSat solar panels shows, that different qualities of panels exist. On the lowest level, solar panels have simplified designs in order to achieve compatibility between products of different vendors and to be low cost. Basic equipment of those solar panels are solar cells, coarse sun sensors, temperature sensors, and optionally magnetorquers.
Harness is realized using multiple connectors, one per embedded functionality.
On the intermediate level, panels are equipped with additional attitude determi-nation sensors. BEESAT solar panels, for example, are considered intermediate level panels. They feature a precision sun sensor based on a PSD, with eval-uation of sensor raw data and execution of the attitude control algorithms being performed by the ADCS software located on the OBC board.
High level panels incorporate precision attitude sensors, magnetorquers, PCDU functionality, and a microcontroller to execute those tasks locally on the panel. One high level example is the UWE-3 panel that features attitude sensor filtering and MPPT on a local microcontroller (cf. [56]). A second
example is the so-called "intelligent tile" dubbed CubePMT developed in the scope of the AraMiS project at Politecnico Torino [93]. This panel has no precision sun sensor, but includes a gyroscope, a magnetic field sensor, a magnetorquer, MPPT, and voltage regulators for local power supply to the electronics components. It is the panel with the highest integration density up to date, but still awaits IOD. Due to the use of six different connectors and some taller components like gyroscopes and buck-boost converters, the positive effect on overall volumetric utilization for the satellite are believed to be less than optimal. Notable about the CubePMT is the envisaged layout of the proposed AraMiS-C1 single unit CubeSat, where the satellite bus is made from four CubePMT tiles and two communication tiles, which leaves most of the internal volume available for payloads.