The HYPSO-1 bus and payload were assembled separately by the NanoAvionics and NTNU teams, respectively. This led to considerable attention on the communication and interfaces between the separate components. Therefore, team organization and documentation became critical factors early on in the project. The focus of this section is on the NTNU’s development since the NanoAvionics bus already has flight heritage.
2.1. Organization
The HYPSO-1 project team is organized in such a way that enables sub-teams to develop components and subsystems independently before satellite integration. The task division led to more lateral responsibility, common in Scandinavian project management [
39], rather than waiting on hierarchical approval. The project owner/principle investigator (professor) was responsible for securing funding and connections that would form the basis of the scientific advisory board. The project manager was responsible for facilitating communication across the team, planning reviews, and overseeing the day-to-day activities of the development teams. This role was filled by a Ph.D. student with additional hours allocated specifically for the tasks. These two higher management roles for the mission were conducted by NTNU members.
Internally, at NTNU, the team consisted of
Bachelor’s/Master’s students (per semester), 8 Ph.D. students, 2 postdocs, and professors and engineers. The size of the team remained approximately consistent, but each semester, the student team base changed and new students had to be trained. Members were divided into payload subsystem development teams. The teams are summarized in
Figure 3. Each team was tasked with the assembly of their own components, software, or infrastructure. The lab manager played a key role in ensuring that the lab was functional at all times, ordering and acquisition was efficient, safety measures were adhered to, and equipment repairs were made without significant delays. The NTNU team could, however, have benefited from closer contact and mentorship from experts in the field, either external or internal. Most students involved in the project had no prior experience in space sciences and/or s/c development.
Externally, the NanoAvionics team independently developed their standard bus and modified it to fit the payload, while observing mission objectives. The 6U bus was entirely assembled at NanoAvionics facilities. The responsibility for integration and quality control also fell primarily on the NanoAvionics team. This was not initially planned, but instead, developed from the limited in-person contact enforced by the strict Coronavirus Pandemic (COVID-19) regulations at the time.
Testing was completed iteratively over many models and in various environmental conditions. Preliminary testing for both the payload and bus was performed by the respective teams. After final assembly and integration, NanoAvionics was responsible for the FM test campaign. All documentation of the AIT activities is organized as shown in
Figure 4.
Project documentation is a critical resource for the team, especially since HYPSO-1 is the first satellite for the SmallSat Lab at NTNU. Each document followed the European Cooperation for Space Standardization (ECSS) guidelines, in as much as was applicable, and will serve as a template for proceeding CubeSats at the SmallSat Lab. It was also important that the outlines were kept consistent with larger space projects to facilitate student transition from studies to industry. These reports were written internally on a shared cloud storage drive, which made it possible to continuously and concurrently update them throughout the project. Furthermore, the final versions of each document were archived in a document configuration control system. Each semester, an informal meeting was held to review documentation and to keep an ongoing record of version control. The Review Committee, comprising internal and external experts in the field, was responsible for ensuring quality, raising concerns and discrepancies in the work, and reporting on these at twice-annual meetings.
2.3. System Architecture and Interface Control
Being a small university team with limited funding and experience, the team decided early on to select CubeSat form factor, rideshare/piggyback launch missions to reduce risk. Of the sizes available at that time, 1U, 2U, 3U, 6U, or 12U CubeSats could be selected. Funding for and availability of the optical design and components were also limited, so this meant relying on COTS components. The components that could be purchased for the optics determined the minimum size of the imager. With this optical design and the orbital constraints, the imager could not be made smaller than 3U. To complete the mission, the remainder of the satellite functions such as radios (for data transmission), batteries, attitude determination and control (pointing accuracy), etc., was needed. A 6U platform from NanoAvionics was selected during an open tender process. In its entirety, the HYPSO-1 satellite can be broken down into eight main subsystems: communication, power, ADCS, Payload Controller (PC), On-Board Computer (OBC; and also called a Flight Controller (FC)), thermal, 6U frame and structure, and payload. Each of these subsystem interfaces depends on one another, as illustrated in
Figure 6.
The NTNU payload interfaces with NanoAvionics components are as follows: the 6U frame (mechanical, thermal), the thermal subsystem (thermal), the payload controller (data), the ADCS (data, mechanical), and power (electrical). The payload itself has many internal interfaces as well.
Figure 7 shows the division of work within the satellite based on color-coding and illustrates the interface points mentioned above.
The electrical subsystem is integrated as shown in
Figure 8. The colored boxes again represent the split development responsibilities between NanoAvionics and NTNU. The HSI payload receives power from the electronic power supply on the M6P 6U platform bus, and data are passed between both. The onboard processing unit, referred to as picoBoB, is the combination of the break-out board and the PicoZed System-On-Module (SOM). The SOM used was the AES-Z7PZ-7Z030-SOM-I-G, and the mounted processing System-on-Chip (SoC) is Xilinx’s SBG485. More details can be found in the references listed in
Table 1.
From here on, the focus of this paper is on the payload exclusively, since the 6U bus is an available industrial platform that can be purchased through NanoAvionics. It has been fully tested and has flight heritage. Integration with the bus will be covered in the context of adding the payload to the established bus.
2.4. Payload Assembly Sequence and Methods
The payload was assembled in-house at NTNU. This was both an advantage and disadvantage, but the only option amidst the COVID-19 pandemic. The advantage was that logistics and travel were greatly simplified, which reduced wasted time; the disadvantage was that all tools, resources, and facilities needed to be accessible on campus at all times throughout the assembly procedure. This led to the basic assembly sequence shown in
Figure 9.
The steps are explained in the sections that follow. It is important to note that some steps were not followed exactly sequentially due to the nature of the assembly. For example, one side of the thermal strap needed to be secured on the electronics stack, PicoZed + Break-out-Board (picoBoB), during the assembly, and the other side was done after (as shown in frame F in the diagram) to allow for the thermal paste to dry in between. An estimate of the time used for each step is also included for reference—this estimate is based on the FM procedures and does not include the time needed to develop the methods. Some activities occurred concurrently. Together, the QM and FM payload model assemblies took roughly 10 months (from September 2019 until June 2020) to complete. Development activities took nearly a year.
A. Acquisition: Orders, acquisition, and machining were the largest unknowns when it came to scheduling and caused the greatest unanticipated delays. Duplicates of every COTS component were ordered from the start to ensure backup if anything broke, was lost, went out of stock, or had significant shipping delays (common during the pandemic, especially across international borders). This proved helpful, but sometimes not sufficient enough.
All of the custom parts were machined from aluminum 6082, in-house. In-house fabrication made communication more straightforward, but also caused many issues with priority amongst other projects within the department and the limited availability of machinists during vacation periods. Only trained machinists were allowed access to the machines, not students directly, causing unexpected delays:
Order parts: 2 weeks–3 months; |
Machine parts: 2–6 months. |
B. Modify parts: Many of the ordered COTS components included parts with high outgassing materials, such as stickers, plastics, and rubber seals. These materials had to be removed, modified, or replaced. Stickers were removed, and the glue residue was cleaned off with ethanol. Critical components, such as rubber gaskets, were replaced with space-proof plastics such as Polytetrafluoroethylene (PTFE).
Some COTS components also contained moving parts, particularly the adjustable aperture in the HSI objectives. The fragile, interwoven leaves were replaced with a fixed, circular baffle ring since the aperture diameter is fixed for imaging purposes in space. The slit tube assembly was designed with a fixed baffling plate so that the slit was automatically constrained and to mitigate the risk of using movable COTS-retaining rings.
All machined components in the optical train were anodized with a black, lusterless, MIL-A-8625 Type II coating (Alcoa Alumilite System Finishes, USA). This coating helped to minimize stray light and reflection inside the optics. Anodized components included the slit tube, aperture, detector housing, and grating assembly. Coatings can also help prevent corrosion or cold welding and should be added to the remaining machined parts.
The printed circuit boards were conformally coated with primer and MAPSIL® 231-B (MAP Space Coatings, France). This clear coating can improve the structural integrity of the boards themselves and avoid short circuiting throughout the mission:
Modify objectives: 1–2 weeks; |
Modify detectors: 4 days; |
Modify slit: 2 weeks; |
Anodizing: 3 weeks; |
Conformal coating: 6 weeks. |
C. Clean parts: All components were thoroughly cleaned and bagged in new, metalized, ElectroStatic Discharge (ESD)-safe bags prior to assembly. In this way, contamination was limited to mitigate potential outgassing onto critical surfaces, such as the lenses and electronics boards. All machined components, the dissembled COTS objectives, and anodized parts were cleaned in an ultrasonic bath with a series of soapy distilled water and/or absolute ethanol. The glass lenses were removed from the objectives before cleaning and were polished with a cleanroom (low particle count) optical cloth and a solution of acetone and/or ethanol. The imager sensors, slit, and grating were wrapped in cleanroom cloths and bagged immediately without attempts at cleaning due to their sensitive, microscopic features:
Clean objectives: 2 weeks; |
Clean lenses: 3 days; |
Clean boards: 1 day; |
Clean machined parts: 1–2 weeks. |
D. Assemble and focus objectives: Once cleaned and modified, the HSI objectives were re-assembled. COTS objectives have slightly different dimensions due to production tolerances, so focus needed to be set individually before assembling the full imager. Since observation will be from LEO, all objectives needed infinite focus. The focus was set by using a collimator (backwards, to simulate infinite distance) and a 3D-printed reticle connected in series as the target. After setting, the focus was then verified by pointing toward a target at a hyperfocal distance of greater than 50 m. This was performed by focusing on man-made structures (buildings with straight lines) outside the window of the lab. When both methods agreed, the focus was fixed using set screws coated in space-approved epoxy, 3M™ Scotch-Weld™ DP2216 Gray (3M Company, Saint Paul, MN, USA):
Re-assemble objectives: 1 week; |
Focus setting: 1–3 days. |
E. Assemble HSI, RGB, and picoBoB: The payload consists of two separate physical components: the payload platform (a base for the HSI and the RGB) and the picoBoB (the payload electronics stack) connected via the payload harness. Both components were mounted rigidly to the 6U frame (see
Section 2.5 on integration) and connected electrically via cabling. Thus, they can be assembled independently.
Optical components of the HSI were assembled first. The grating was press fit into the cassette with PTFE gaskets and secured using set screws. All screws in the assembly were tightened using defined torques based on the screw dimensions. Additionally, a dot of space-approved epoxy was added to the head of each screw for extra security. The grating/cassette assembly was mounted directly to the payload platform with screws, similarly. The slit was mounted inside the slit tube, and the HSI detector was housed in its anodized shell. From here, the threaded COTS objectives connect the sub-assemblies in series.
The less complex RGB imager assembly consisted of securing the detector to the housing with internal screws and twisting the objective into the C-mount at the front of the housing. Again, torqued and epoxied screws fasten the sub-assemblies together. Both imagers were set in the payload platform groove and were secured with gasketed clamping brackets. The gaskets used in this case were simply a layer of Kapton® tape (DuPont de Nemours, Inc., Wilmington, DE, USA). Screws secure these brackets and must be tightened to the correct torque value.
The picoBoB assembly contains an electronics board, a break-out board, two shield plates, and two ring frames for attachment to the 6U frame. All have corresponding holes built-in and connect as a typical electronics stack with spacers in between. Before assembling the stack, cables and connectors were secured and checked, and thermal straps were added to the shield plates. It is not possible to connect these once the stack is complete due to tight clearances, and it is also best to let the thermal paste that attaches the straps cure on a flat surface. More details on the thermal straps follow in the next section:
Assemble HSI: 3 days; |
Assemble RGB: 1 day; |
Assemble picoBoB: 2 days. |
F. Add thermal straps: A Passive Thermal Control (PTC) system was implemented to dissipate excessive localized heat in active payload components. This was particularly chosen considering the limited power budget on-board. Our passive thermal solutions were basic, but effective, and included: anodizing, thermal straps, and thermal decoupling components. As mentioned in Step B, components along the optical train were anodized black. This changes the thermo-optical properties of the surface and improves the radiative heat transfer because of its high emittance. Thermal straps are thin, thermally conductive sheets that transport heat from a source to a sink. The three components on the payload that experience the highest temperatures are the picoBoB electronics and the two imager sensors. Thermal straps were attached to these components with thermal paste and linked to the large surface area aluminum heat “sinks”, namely the picoBoB shield plates and payload platform, respectively.
Figure 10 shows the assembly of the payload platform, with thermal straps installed, on the two imagers. These straps are fragile and can be easily damaged during handling, storage, or shipping:
Add thermal straps: 2 days. |
G. Add dampers: The final additions to the payload include the crosslinks and dampers. Through these structures, the payload platform is able to integrate with the rest of the 6U bus. Dampers were purchased commercially from SMAC [
41] and were selected based on their thermal isolating properties and high-frequency damping capability highlighted in the simulation. Here, 6 mm bolts pass through the payload platform, which is sandwiched between dampers and the crosslinks. In this way, the payload platform is thermally and mechanically decoupled from the rest of the satellite. Using the prescribed torque on these bolts is important to ensure correct damper functionality:
Add crosslinks + dampers: 3 h. |
2.6. Ground Support Equipment and Facilities
The Ground Support Equipment (GSE) used for HYPSO-1 has three varieties: Mechanical Ground Support Equipment (MGSE), Optical Ground Support Equipment (OGSE), and Electrical Ground Support Equipment (EGSE). Each of these items played a critical role in the payload development.
Table 3 gives a summary of the Ground Support Equipment (GSE) used in the HYPSO-1.
Mechanical GSE: includes all items used for storage, transportation, and test mounting. The payload was double-bagged in two ESD-safe, metalized bags and packed into Peli™ Protector cases (Pelican Products, Torrance, CA, USA) when stored or transported. All attached cables were secured with Kapton® tape (DuPont, Wilmington, DE, USA), and foam was used to fill voids in the packaging and secure the components.
Since the payload was assembled entirely in-house, transportation occurred only when visiting testing sites and shipping the payload FM to NanoAvionics for final integration. Most transportation was done by car to the testing sites. Obvious care was taken in these cases. The final shipment went via air freight. In this case, shock labels were attached to the outside of the packaging to ensure careful handling, and special export licenses had to be obtained.
Mounting plates were designed for each environmental test and each different chamber/table. Since NTNU focused only on the payload, standard dispenser test pods that fit the entire satellite were not used. The standard dispenser pod is a duplicate of actual dispenser pods from the launch vehicle that help simulate loading, as it will be experience at launch. Looking back, this would have been a safer approach, but no pods were available for use at the time. Instead, custom plates and brackets were designed to interface with the hole patterns of the chambers/tables and payload components. Each was designed to imitate the actual mounting structures in the satellite and launch vehicle.
Optical GSE: was used for optical calibration and verification activities. All calibration was performed in a dedicated optics laboratory on campus. This lab supplied an integrating sphere (coated in barium sulfate), calibration lamps (including a certified tungsten halogen lamp and spectral calibration lamps with known spectral emission), and mounting structures for the imagers. HSI calibration [
20] was completed using this method, while RGB calibration was performed with spatial targets, such as a checkerboard pattern, in the same lab.
Since testing took place at different facilities, a portable and robust setup was used to check optical performance before and after each test. A large, non-reflective (matte black) box that could be placed over the payload/satellite was used to simulate a dark room. A fluorescent light bulb was installed in the top as the light source has a known spectral signature. For the radiation optical tests, a stable tungsten halogen lamp was used. In addition, a solid target mount was built with mounting holes in its base. Targets were switched out depending on the goals of the test and the instruments involved. Since RGB images are spatial, checkerboard and standard pattern targets were used. For the HSI, matte white paper provided a near-uniform reflectance of whatever the chosen light source was. This setup enabled the HYPSO-1 team to easily transport and setup a repeatable optical environment despite the varied testing environments.
Electrical GSE: was used to power and communicate with the payload before it was completely integrated into the satellite. An external power supply unit that could be plugged into a wall outlet was built into a Peli™case to simply and safely provide power to the payload at various test sites. It included different and well-labeled connectors and fuses to protect the payload against surges. The testing umbilical could be paired with this and a laptop to provide connection and control of the payload.
The chosen PicoLock (Molex LLC, Lisle, IL, USA) connectors have a maximum recommended number of mate/de-mate cycles of 30. Connector savers were made to reduce the number of cycles during testing. A connector saver is a length of cable where one end is attached to the protected connector and the other end can be “sacrificed” and mated/de-mated more often. The connector saver can be replaced when necessary, greatly reducing the mating cycles on payload connectors. Several connectors were mounted on one common Printed Circuit Board (PCB), for example to mimic the interface of the Electronic Power Supply (EPS). The connector savers were connected to the payload cable harness on the payload/satellite interface.
To avoid damage from static electrical discharge at and around the sensitive equipment, ESD equipment was required whenever handling s/c components. The SmallSat lab is an ESD-safe environment, including: mats, coats, flooring, shoes, grounded work surfaces, gloves, etc. Precautions for ESD handling were also taken when removing the payload from the SmallSat lab, for example during environmental tests.
For the environmental tests (shock/vibration and thermal vacuum), sensors were needed to evaluate the componentwise results of testing. This included accelerometers and temperature sensors, placed at various locations on the payload assembly.
Figure 12 summarizes the validation setup used at an external testing site. The most common GSE components used throughout the testing campaign are highlighted here.
Facilities: Several facilities were required in addition to the equipment needed for building and testing the satellite. Most work was completed in the SmallSatellite Lab at NTNU, which included an ESD-safe working area, computers, storage space, soldering equipment, 3D printers, workshop tools, multimeters, oscilloscopes/spectrum analyzers, and spare parts.
Machined components were made at the Mechanical Workshop in-house. All optical components were configured in a flow bench to limit particle contamination. A dedicated optics laboratory on campus was used for calibration and characterization of the optical parameters. Electronics were tested in the ESD-safe section of the SmallSatellite lab. The FM was integrated in an ISO class 7-certified cleanroom at NanoAvionics. Preliminary vacuum and thermal testing was carried out at various chambers on campus, while qualification and acceptance testing required machines that were able to meet the specified levels. These testing facilities are detailed in the section that follows.
For end-to-end functional testing, a flexible Hardware-In-the-Loop (HIL)-setup was created [
22]. It is capable of connecting to the payload and FlatSat, either locally, through the NanoAvionics operations architecture, or through a UHF radio link between the local FlatSat and the local ground station (NTNU ground station,
https://www.ntnu.edu/web/smallsat/infrastructure, accessed on 5 July 2022). The ground station communicates with the satellite through both UHF (backup, 400 MHz-band, 1.2 kbps) and S-band (primary, 2200 GHz-band, up to 1.4 Mbps) radios.
Figure 13 shows a model of the communications infrastructure, including ground stations.
The main ground station, including antennas, radios, and control equipment, is located on the roof of the SmallSatellite Lab building at NTNU and can be remotely accessed. For on-site end-to-end tests, only the UHF chain was used. When operational, the satellite is commanded and supervised from a mission control center, also physically located at NTNU. While in orbit, the satellite will also be accessible from other ground stations through the KSATlite’s (KSATlite ground station,
https://www.ksat.no/no/ground-network-services/ksatlite/, accessed on 5 July 2022) network. Which specific station to use depends on specific mission needs, and the acquired license for operations and may change during the mission lifetime. Details on the communications and operational architectures for different mission types can be found in [
45].