EIRSAT-1 (Educational Irish Research Satellite 1)
EIRSAT-1 (Educational Irish Research Satellite 1)Spacecraft Launch Sensor Complement Ground Segment References
In 2017, ESA announced the selection of EIRSAT-1, Ireland's first ever satellite, which is being developed under the ESA Education Office "Fly Your Satellite!" 2017 Program. 1) 2)
EIRSAT-1 will include two different payloads on a 2 Unit CubeSat. Both payloads contain technology from the industrial partners that will be flown in space for the first time, marking an important step in their space heritage. The payloads for EIRSAT-1 have been made possible through funding by ESA technology programs including the Science Core Technology Program.
The project is primarily educational in nature and aims to: 3)
- develop the know-how of the Irish higher education sector in space science and engineering, by supporting student teams to build, test and operate the satellite;
- address skills shortages in the space sector by fostering collaboration between student teams and industry through the launch of three payloads that use innovative Irish technology;
- inspire the next generation of students towards the study of science, technology, engineering and mathematics (STEM) by launching the very first Irish satellite.
To achieve these educational aims, the EIRSAT-1 team have developed the following scientific and technical objectives for the spacecraft:
- study gamma-ray bursts (GRBs) using a bespoke gamma-ray detector to assess the capability of this technology for use on nextgeneration gamma-ray astrophysics missions;
- perform the first measurements on the performance of SolarWhite and SolarBlack novel surface treatments in a low Earth orbit environment;
- implement and test a wave-based control algorithm to determine its potential as a viable alternative to standard attitude determination and control methods.
The project was initiated by the Space Science and Materials Research group within the UCD School of Physics. The group has a long track record of space science and astrophysics research, especially GRBs and the development of instruments and technologies related to the detection of gamma-rays. EIRSAT-1 was proposed as a mission concept to ESA (European Space Agency) in response to their FYS (Fly Your Satellite!) announcement of opportunity.
EIRSAT-1 brings together several strands of the Space Science group's research and educational activities. An R&D program into the development of a gamma-ray detector using novel sensors demonstrated that the footprint of such a compact detector would be compatible with a CubeSat platform. In parallel, an educational CubeSat called ‘EduCube' had been developed to train students of UCD's MSc in Space Science & Technology in systems engineering.
Collaboration with the UCD School of Mechanical and Materials Engineering led to the inclusion of two additional payloads. The materials experiment (EMOD) is based on UCD patented technology, while the Wave-Based Control (WBC) payload implements a novel approach to motion control which has been developed by the UCD Dynamics and Control group.
After proposal submission, the EIRSAT-1 student team was invited by ESA Education to participate in a selection workshop at ESTEC in early May 2017 to pitch their idea to a selection panel of spacecraft experts from ESA. Shortly after the workshop, EIRSAT-1 was announced as one of six CubeSats selected for the FYS program. In 2018, the EIRSAT-1 team completed the CDR (Critical Design Review) process.
Team organization: EIRSAT-1 is primarily a student-driven mission. Students are supported to take responsibility, make decisions and own the relevant parts of the program. Students are given prominence in outreach and publicity. There is a management structure in place which is composed of a Management Board, a Mission Team, and an Academic Oversight Board. The Management Board is composed of academics from the Schools of Physics and Mechanical and Materials Engineering, student leaders, and a space-industry mentor. The Mission Team comprises graduate students working on the project and is responsible for the implementation of EIRSAT-1. Undergraduates, visiting students, and interns join the Mission Team as associate members. The Academic Oversight Board comprises the supervisors of the students who are full members of the Mission Team. The EIRSAT-1 team has adopted a policy which governs this management structure and which includes a code of practice regarding "Equality, Diversity & Inclusion" that outlines the team's ethos towards team interactions, diversity of opinion and gender balance.
The EIRSAT-1 Mission Team is currently composed of 14 students. A further 15 students were involved in the design of EIRSAT-1 up to Critical Design Review stage. The students come from Physics, Mathematics, and Engineering backgrounds and 36% of the current Mission Team is female.
EIRSAT-1 has two hardware payloads,GMOD (Gamma-ray Module), the EMOD (ENBIO Module); and a software payload, WBC (Wave Based Control). The EMOD payload includes an assembly which requires special accommodation on the exterior of the spacecraft, rendering most COTS CubeSat structures unsuitable. A Clyde Space 2U structure has been heavily modified to meet these requirements.
The EIRSAT-1 spacecraft is based on COTS (Commercial Off-The-Shelf) CubeSat hardware components supplied by Clyde Space, augmented with payloads, electronic subsystems, and mechanical components, which have been designed and will be manufactured at UCD with input from industry partners. The spacecraft consists of typical electronic CubeSat subsystems which will be supplied by Clyde Space: ADCS (Attitude Determination and Control Subsystem), EPS (Electrical Power System), OBDH (On-Board Data Handling), Communications (Comms).
The design of the spacecraft was initially driven by the requirements of the GMOD payload. In order to accommodate the payload size along with the required supporting subsystems, it was determined that a 2U CubeSat would be required. Analysis of the GMOD performance (Figure 5) indicated that a zenith pointing attitude would be optimal but that the mission would be feasible in any attitude configuration. Reaction wheels were considered and while there was sufficient mass budget and space to accommodate them, there would be insufficient power generated with body-mounted solar arrays. To use reaction wheels, the spacecraft would require deployable solar arrays. De-orbit simulations were performed for 2U configurations both with and without deployable arrays. As EIRSAT-1 will be launched from the ISS (International Space Station), atmospheric drag at this altitude, means that the lifetime of a mission with deployable arrays would be significantly reduced. These analyses demonstrated that reaction wheels are infeasible for a spacecraft of this size in a 400 km altitude LEO (Low Earth Orbit). An exploded view of the final EIRSAT-1 configuration is shown in Figure 1, while a labelled view of the internal components is shown in Figure 2.
ADCS (Attitude Determination and Control Subsystem) consists of a magnetorquer based control system. It uses a Clyde Space ADCS Motherboard with support for up to 6 magnetorquers. Magnetic coils are integrated into the solar array PCBs, hence providing 2 × X-axis magnetorquers and 2 x Y-axis magnetorquers. A Z-axis magnetorquer is included at the bottom of the main stack of PCBs within the spacecraft structure. The ADCS motherboard includes magnetometers and gyroscopes, utilizes several external sensors such as a +Z-facing fine sun sensor and coarse sun sensors built in to the solar arrays, and can incorporate information from the GPS module in the OBC. This hardware will also be used for the WBC experiment payload.
Although EIRSAT-1's primary objectives do not require attitude control and are achievable even when the spacecraft is tumbling, a number of different pointing modes were considered for the mission. Zenith-pointing gives the best possible performance for the GMOD payload while also exposing the solar arrays to sufficient sunlight to power the spacecraft. Zenith-pointing would however pose difficulties for the EMOD payload as it causes the illumination of the thermal coupons to be changing constantly which complicates analysis. Eventually an offset Sun-pointing mode was chosen. While this mode produces less power from the solar-arrays than zenith-pointing, it produces it more consistently and therefore reduces times of accumulated discharge. Additionally, this mode can be achieved using a spin-stabilized inertial frame, thus improving pointing reliability.
The baseline control algorithm for the ADCS is a custom algorithm from Clyde Space designed specifically for EIRSAT-1 as this will be the first mission in which Clyde Space have performed magnetic-only control for a 2U spacecraft or with an offset applied to Sun-pointing mode.
EPS (Electrical Power System). The EPS consists of a Clyde Space 3rd generation 3U EPS motherboard, a Clyde Space 30 Whr Standalone Battery, and 4 x 2U body-mounted solar cell arrays. The motherboard is designed for CubeSats larger than 1U but without deployable solar panels. The motherboard can provide power at battery voltage, 12 V, 5 V, and 3.3 V, with latching current limiting over-current protection. Power may be supplied either directly or via switch-able power distribution modules, which are used to control power to the hardware payloads. The 30 Whr battery has existing flight heritage with CubeSat deployers, such as NanoRacks, and is compatible with ISS manned flight requirements having been certified to NASA EP-Wi-032 standards. The battery has a 2s3p configuration and features additional built-in over-current and undervoltage protection independent of that functionality in the EPS module. The flight activation inhibits are implemented as MOSFETs in the battery. The solar arrays are placed on the X and Y faces of EIRSAT-1. Each array features 5 x Spectrolab UTJ (Ultra Triple Junction) cells in a 5s1p configuration for an optimal power generation of 5 W per array.
OHDB (On-Board Data Handling) system. The OHDB is a Clyde Space Nanosatellite On-Board Computer (OBC). The OBC is based on a MicroSemi Smart Fusion 2 System on Chip. As the Smart Fusion 2 is flash-based, it is inherently SEU tolerant. The OBC includes other protections for radiation effects such as magnetoresistive RAM and a hardware watchdog. The OBC features a 150 MHz ARM Cortex M3 processor, 8 MB of EDAC protected MRAM, 4 GB of NAND flash, a Micro SD card slot, and a GPS receiver. The FPGA fabric of the Smart Fusion 2 is used to create the various interfaces between the OBC and the other subsystems. EIRSAT-1 uses I2C for all inter-subsystem communication with 3 separate I2C buses used for system, comms, and payloads.
On-board Communications: The on-board CMC (Common Mode Current) transceiver is the space-qualified CPUT VUTRX transceiver supplied by ClydeSpace. The communications system uses UHF downlink (430-440 MHz) and VHF uplink (140-150 MHz) bands. The transceiver provides 9600 baud downlink and 1200 baud uplink, and implements a GMSK downlink and AFSK uplink configuration. The AX.25 protocol is used for uplink packets, while a CCSDS convolutional encoder may be used for downlink. The CMC transceiver is a CubeSat compatible subsystem which interfaces to the OBC via I2C. The transceiver will be configured to use a reserved "comms" I2C channel to ensure no interference due to housekeeping and payload traffic.
ADM (Antenna Deployment Module): EIRSAT-1 will use a custom ADM designed and built at UCD which will be mounted on the -Z end of the satellite. The module is shown in Figure 3 and deploys two dipole antennas, one for UHF downlink and one for VHF uplink. Both
dipoles are composed of two tape spring antenna elements, deployed from opposite sides of the module, as seen in many previous and COTS antenna designs. The elements are 5 mm wide, made from a Copper Beryllium alloy and attached to spring loaded doors at each side of the module. They are coiled inside the ADM before deployment, within the 7 x 100 x 100 mm overall dimensions of the module. When EIRSAT-1 is clear of the CubeSat deployer the ADM will activate a burn wire release mechanism allowing the module doors to open and the elements to uncoil into their operational positions and stay in that configuration for the remainder of the mission.
The module has many attractive features. The main structure of the module is anodized aluminum and consists of three parts, a base, a central cover, and an outer cover. This makes the module very rigid and robust. For resetting melt lines the outer cover may be removed and for connecting/disconnecting harnessing the central cover may be removed while in both cases the remaining cover holds the coiled antennas in position. The central lid may also be used to mount an additional coarse sun sensor on the top of the module. The X-shaped main PCB provides clearance around the stowed elements allowing their width to be maximized at 5 mm. This in turn maximizes the rigidity of the elements when deployed. Furthermore each element is curved along its long axis to increase rigidity and straightness in the deployed configuration. To minimize the risk of early deployment there are two melt lines attached to each door. Similarly, for each door two melt resistors are used to cut the melt lines. To ensure good contact between the resistors and melt lines, the lines are threaded over one resistor and under the other. A lever-type limit switch is used to detect the successful deployment of each individual element.
On-board Software: EIRSAT-1's main on-board software will run on the satellite's Clyde Space OBC, with FreeRTOS as the operating system. The software, which is being written in C following the MISRA-C coding standards, is being developed using Bright Ascension's GenerationOne FSDK (Flight Software Development Kit). This kit, which provides a software framework, development tools and multiple readymade software components, was chosen for EIRSAT-1 to facilitate rapid software development. With FSDK, the functionality of the satellite is divided and developed within stand-alone software components, that all share a common interface. To produce a software image, hundreds of these components are then linked together to meet the requirements of the mission. This modular development strategy largely follows the methods used for OOP (Object-Oriented Programming) and is suited for iterative, team development.
The main software, running on the OBC, will access the data, functions and hardware of the individual subsystems via I2C. COTS subsystems are delivered with firmware installed by the manufacturer. However, for the science payloads, GMOD and EMOD, custom written software is being developed.
Driven by the design of FSDK, a three tier software architecture has been defined for EIRSAT-1, which consists of; 1) The Hardware Layer – the base layer, which contains components that interact directly with EIRSAT-1's system hardware; 2) The Capabilities Layer – a layer of abstraction above the hardware layer, which contains the main components needed to operate EIRSAT-1's subsystems; and 3) The Management Layer – the layer which provides the top level functionality of the satellite, that the user will see upon interacting with the satellite.
When launched, EIRSAT-1 will be equipped with 2 primary images, which will be stored in non-volatile MRAM, as well as a failsafe image, which will be stored in non-volatile flash memory. While the 2 primary images will contain all the functionality required for the EIRSAT-1 mission, the failsafe image will only perform mission critical functions. Given the failsafe and redundant primary images, full software updates will be possible while in-orbit, with minimal risk to the mission.
• July 12, 2022: EIRSAT-1 is an ambitious 2U CubeSat designed and built by students from University College Dublin. Part of the Fly Your Satellite! programme, it is hoped that one day soon it will fly into space – but not before all the necessary tests have been undertaken. The team recently completed an intense environmental qualification campaign, and have now taken another important step forward by finishing their Flight Model's assembly and integration.
- "The Flight Model integration activities took place over a few days, but they represent years of preparation," explained a participating student. "After many prototypes and the qualification model, it is incredibly satisfying to be able to work with the Flight Model, the version of our satellite that will actually be in space!"
- Assembly and integration proved to be a particularly challenging part of the students' journey. EIRSAT-1's payloads and an Antenna Deployment Module were manufactured in-house by the students, and had to be rigorously tested at the ESA CubeSat Support Facility in Belgium to ensure confidence in the design. In addition, all COTS (commercial, off-the-shelf) subsystems have been inspected and undergone testing by the EIRSAT-1 team.
- All tests were conducted with the utmost care because despite its diminutive size, EIRSAT-1 is packed with three advanced experiments:
a) A Gamma-Ray detector (GMOD) to study the most luminous explosions in the Universe, which occur when a massive star dies, or when two stars collide;
b) A thermal coating experiment to study the performance of novel surface treatments (EMOD);
c) An attitude control algorithm experiment (wave-based control / WBC) to determine its potential as a viable alternative to standard Attitude Determination and Control methods.
- Once assembly was completed, the dimensions, mass, and physical requirements of the satellite were double-checked, along with a careful inspection to reveal any damage or anomalies. So far, so good!
- "Being part of the assembly of Ireland's first satellite, EIRSAT-1, was an amazing experience," enthused one of the students. "Every member of the team took part in the build. I helped build one of the payloads, GMOD, and installed some solar panel screws. A huge amount of work has gone into the project over the past few years, and culminated in the successful assembly of the Flight Model. It's incredible that the spacecraft we are now testing in Dublin will be launched into space and we will get to operate it."
- The pace will continue over the coming weeks as the team is scheduled to conduct a functional test campaign to demonstrate that EIRSAT-1 can successfully communicate and be operated by its ground segment.
- "It was really special to see the satellite come together after all the hard work it took to get to this point," added another EIRSAT-1 student. "This is the first time we've seen the spacecraft fully assembled, with real solar cells, and it looks great. It's hard to believe we're now going to send it to space!"
• February 21, 2022: According to Lorraine Hanlon, lead Prof. of the mission at UCD (University College Dublin), the EIRSAT-1 mission was originally proposed in response to the ESA FYS2 (Fly Your Satellite System 2) call, led by UCD in collaboration with QUB (Queen's University Belfast). The mission passed PDR (Preliminary Design Review) with both institutions involved. 4)
- Since CDR (Critical Design Review) in 2018, however, the mission has been solely developed and implemented by UCD.
- Regarding the launch, we hope to have an update in the near future, most likely going with a European launch vehicle rather than ISS.
• October 27, 2021: The EIRSAT-1 team, participating in the second round of ESA's Fly Your Satellite! Program, has successfully concluded the environmental test campaign on their Engineering Qualification Model (EQM). 5)
- This milestone ran from 13 September to 15 October 2021 at the CubeSat Support Facility in ESEC-Galaxia (Belgium), and was a vital step on the road to deployment in space.
- EIRSAT-1 is a 2-unit CubeSat from University College Dublin. It houses three experiments:
a) A Gamma-Ray detector (GMOD) to study the most luminous explosions in the Universe, which occur when a massive star dies, or when two stars collide;
b) A thermal coating experiment to study the performance of novel surface treatments (EMOD);
c) An attitude control algorithm experiment (wave-based control / WBC) to determine its potential as a viable alternative to standard Attitude Determination and Control methods.
- EIRSAT-1's EQM qualification campaign saw the CubeSat exposed to conditions representing space. For example, an electrodynamic shaker system mimicked the extreme vibrations along three axes that the satellite will experience during launch. Meanwhile, EIRSAT-1's Flight Model communications subsystem (CMC) and Antenna Deployment Module (ADM) were sealed in a thermal vacuum environment and then repeatedly heated and cooled to extreme levels. The EQM was then installed and tested in the thermal vacuum chamber over the course of several weeks.
- The test chamber's window was closed during these tests, meaning that the students were not able to see their spacecraft. Team members had to run functionality tests without direct visibility, checking that their satellite was in a good health. It was a great relief when the chamber was finally opened and the students could see with their own eyes that their CubeSat was still in one piece!
- The campaign was very challenging, but the students showed great adaptability and professionalism. They impressed their ESA Education mentors by acquiring new skills and knowledge, strengthening the pillars of their future careers as engineers and scientists.
- When back at their home university, the students will undertake further investigations to determine whether any additional design changes are necessary. They can then proceed to the assembly and integration of their Flight Model.
- "Over five weeks working with ESA Education staff at the CubeSat Support Facility we had excellent opportunities to learn about how our system performs in a representative space environment," explained EIRSAT-1's student Test Engineer. "This test campaign, which was conducted after months of preparation, was very rewarding and has given us great confidence in the robustness of our satellite. We had amazing support from the Fly Your Satellite! team every step of the way. Personally, I had a fantastic time and learned so much about environmental testing. I'm really looking forward to applying that knowledge to the development of our flight model of the spacecraft."
• June 10, 2020: Preparatory testing for Ireland's first space mission, EIRSAT-1, seen taking place at ESA's Hertz antenna test chamber. 6)
- EIRSAT-1 (Educational Irish Research Satellite-1) is being built by students and staff of University College Dublin, who are participating in ESA Education's Fly Your Satellite! program.
- At just 22 by 10 by 10 cm, the miniature EIRSAT-1 is smaller than a shoebox but is still equivalent in complexity to a standard space mission.
- Normally the EIRSAT-1 student team would have joined the test campaign in person, but current COVID-19 restrictions made this impossible. Instead the team delivered their self-made Antenna Deployment Module (ADM) plus a mock-up of the satellite body, along with detailed test preparation procedures.
- "Very often when antenna performance is assessed on a stand-alone basis, the results are not necessarily representative," explains Peter de Maagt, heading ESA's Antenna & Sub-mm Waves section. "The body of a satellite can make an enormous impact, so it's really important to verify the antenna performance in this way."
- ESA's metal-walled Hertz (Hybrid European Radio Frequency and Antenna Test Zone) at the Agency's ESTEC technical center in the Netherlands, shut off from all external influences for radio testing.
- Hertz's hybrid nature makes it unique: the facility can assess radio signals from antennas either on a local ‘near-field' basis or as if the signal has crossed thousands of kilometers of space, allowing it to serve all kinds of satellites and antenna systems.
- EIRSAT-1 carries a trio of Irish payloads, with platform subsystems coming from AAC Clyde Space in Scotland.
- "We thought it was important to have some ambitious science payloads, beyond simply achieving a working satellite," says David Murphy, systems engineer for the project. "So EIRSAT-1 carries an advanced gamma ray detector. Developed through an ESA-funded project, this GMOD (Gamma-ray Module) detects gamma ray bursts from deep space as well as ‘terrestrial gamma flashes' originating within the atmosphere, linked to lightning strikes.
- "Standard gamma ray detectors still rely on vacuum tubes; by comparison the compact, silicon-based GMOD is the equivalent of updating from a CRT to flatscreen TV."
- The mission will also carry protective oxide coatings developed by Irish company Enbio, already embarked on ESA's Sun-monitoring Solar Orbiter spacecraft.
- "These black and white coatings have already been tested for deep space," Murphy explains, "but EIRSAT-1 will gather the first data on their effectiveness in the low-Earth orbit environment, including exposure to highly-erosive atomic oxygen."
- EIRSAT-1's third payload is entirely software-based: a novel ‘wave based control' algorithm to operate the nanosatellite's magnetotorquers, used to control EIRSAT-1's attitude (or pointing direction) by reacting with Earth's magnetic field.
- EIRSAT-1 Chief Engineer Joe Thompson remarks: "This algorithm promises enhanced performance compared to traditional alternatives, which could subsequently be applied to manipulating large, flexible orbital structures."
- The EIRSAT-1 team also developed their own highly compact ADM for antenna deployment. This deploys VHF and UHF antennas from a structure just 7 mm high, positioned on one end of the nanosatellite using a simple 'burn wire' release mechanism.
- "The module behaved well under experimental launch scenarios during its vibration and thermal vacuum testing," notes Rakhi Rajagopal, working on ADM testing.
- Lána Salmon, part of the EIRSAT-1 communications team adds: "Having previously studied the performance of the ADM, both outdoors and in small anechoic chambers, this test campaign is a crucial next step which allows the study of the ADM, with an attached representative structure, in a chamber built to conduct precise measurements of satellite antenna performance."
- ESA antenna engineer Alfredo Catalani was the person following the team's procedures: "The antennas needed tuning to very precise frequencies, by cutting them to their final lengths. Because the antennas are made to operate in weightlessness, they come with supports to safely extend in Earth gravity – made of plastic to avoid any potential radio interference, for the same reason a wooden support holds the mock-up satellite in place. Similarly, we also used an optical transducer to feed RF signals to the antennas, rather than a metal connector."
- EIRSAT-1's individual payloads have already been qualified for space at ESA Education's CubeSat Support Facility at the ESEC-Galaxia facility at Transinne in Belgium. As a next step a prototype ‘Qualification Model' will be tested there later this year, with a flight model set to follow at the end of the year, ahead of a planned delivery to ESA in the first half of 2021.
• 2017: "Ireland is about to become an independent spacefaring country. UCD (University College Dublin) is designing, building and testing a research satellite to detect gamma-ray bursts, evaluate advanced materials, and demonstrate a new method of spacecraft control. 7)
- EIRSAT-1 is the first spacecraft to be developed in Ireland. It will be delivered to the European Space Agency (ESA) in 2020. The EIRSAT-1 project is carried out with the support of the Education Office of the European Space Agency, under the educational Fly your Satellite! Program.
- EIRSAT-1 will position UCD and Ireland to take a leading role in international space missions, support growth of the Irish space industry, deliver highly skilled graduates into the workforce and continue to inspire school children in STEM subjects. The project has been presented to media outlets, to primary and secondary schools, to senior ESA and NASA management, and to government."
The EIRSAT-1 Flight Model with a mass of 2.2 kg will be delivered to ESA in 2022. A launch will be arranged.
GMOD (Gamma-ray Module)
GMOD is an experiment payload designed to detect cosmic gamma-ray phenomena such as GRBs which are short-lived intense flashes of gamma-rays associated with the collapse of very massive stars in the distant universe and with the merger of neutron stars. 8) It is based on the design of the UCD GRD (Gamma-Ray Detector) which was developed by the Space Science and Materials Research group under contract to ESA. The GRD design utilizes a 28 mm x 28 mm x 20 mm LaBr3 scintillator coupled to a 4 x 4 array of 36 mm2 SensL B-series silicon photomultipliers (SiPMs).A detailed description of the GRD can be found in [9)].
GMOD is therefore the latest detector design in a series which have been developed at UCD in order to address the technical challenges of building sufficiently advanced next-generation high-energy astrophysics missions to meet the scientific requirements while being of manageable mass and complexity. These detectors benefit from several novel enabling technologies which have recently been made available to the scientific community, e.g. modern high-efficiency scintillators, SiPMs which replace bulky, high-voltage PMTs (Photomultiplier Tubes) and for GMOD a dedicated SiPM readout ASIC. The SiPM Readout ASIC (SIPHRA) has been developed by the Norwegian company Integrated Detector Electronics AS (IDEAS) based on the requirements of operating the UCD GRD in space. 10) SIPHRA has been incorporated into the GMOD design.
Detector Hardware: The detector assembly consists primarily the scintillator, SiPMs, the SIPHRA ASIC used to process and digitize the analog signals from the SiPMs, and a light-tight detector enclosure. An exploded view of the detector assembly is shown in Figure 3. The scintillator is a 25 x 25 x 40 mm Cerium Bromide (CeBr3) crystal supplied by Scionix. The CeBr3 crystal is supplied by the manufacturer enclosed within a hermetically sealed unit. The housing includes a quartz window, exposing a 25 x 25 mm face of the crystal, allowing the scintillation light to exit.
The scintillation light is measured using 16 J-series 60035 SiPMs from SensL. The SiPMs are arranged in a 4 x 4 array which gives a very good match to the scintillator size. The array is a custom design implementing a common-anode configuration with all SiPM anodes being connected to a common negative bias supply via independent low-pass filters. The cathode of each SiPM is connected directly to the ASIC inputs via board-to-board connectors.
The analog signals from the SiPMs are digitized using the SIPHRA ASIC. SIPHRA is a 16 channel SiPM read-out IC which is used as a pulse height spectrometer in GMOD. Each of the 16 SiPM inputs have a current integrator, pulse shaper, and track & hold circuit. Additionally, a 17th channel provides the sum of the 16 inputs. Readout can be triggered by thresholds on any of the 17 channels. When triggered, the heights of all 17 channels are digitized by a 12-bit ADC. The 17 pulse values and trigger information are output via a high-speed serial output. SIPHRA is configured by programming its configuration registers via SPI. Configuration options include enabling of individual channels, enabling triggering and thresholds on individual channels, input offsets, pulse shaping parameters, and readout options. SIPHRA has specifically been designed for use in space applications with latch-up immunity, single event upset mitigation, and error correction, and low-power considerations. It is not expected that SEU will occur even in high radiation encountered in the South Atlantic Anomaly.
Expected Performance: GMOD's sensitivity has been simulated using the MEGAlib toolkit. 11) A simplified mass model of the EIRSAT-1 spacecraft was created and the response of the GMOD detector to a GRB spectrum with a slope of -1.1 was simulated. Figure 11 shows the effective area of the GMOD detector as a function off-axis angle and azimuth of the source GRB. The effective area is calculated at the number of detected counts in the 50 - 300 keV range divided by the GRB flux. For each GRB in the BATSE 4B catalogue, the detection significance has been calculated. The cumulative detection significance distribution is shown in Figure 12 at a range of spacecraft attitudes from zenith (0 degrees) to nadir (180 degrees).
Assuming the optimal zenith pointing attitude strategy and that for a single detector a signal threshold of 10 sigma would be used in order to avoid false positives, it is expected that GMOD will detect approximately 20 GRBs per year. Coincident GRB detections with other high-energy missions would allow for a lower detection threshold and therefore significantly increase the number of observed GRBs.
EMOD (ENBIO Module)
EMOD is an experimental payload which is designed to demonstrate and test the performance of SolarBlack and SolarWhite spacecraft surface treatments developed by ENBIO Ltd. of Dublin, Ireland. SolarBlack and SolarWhite have been developed by ENBIO for use on ESA's Solar Orbiter mission. EMOD will measure the performance of these coatings using four ‘thermal coupons' which are attached to the +Z face of the spacecraft. As EIRSAT-1 orbits the Earth, the thermal coupons will be exposed to periods of solar illumination followed by eclipse which will thermally cycle the coupons. Due to the extreme temperature fluctuations that the test coupons will experience, comprehensive adhesion testing is required to ensure the temperature sensors do not de-laminate from the surface during flight. Using input thermal data from an orbital simulation software developed in-house, extensive thermal modelling of the coupons using ANSYS and MatLab will allow the EMOD team to predict the expected temperatures of the test panels during flight and modify the design accordingly. Through continuous monitoring of the temperature of the coupons as they are thermally cycled, it will be possible to characterize the coating performance and degradation over the duration of the mission.
Thermal Coupon Assembly: The coupons are made from Al2024-T3 aluminum and measure 35 x 35 x 1 mm with two coated in SolarBlack and two coated in SolarWhite. Each coupon has an RTD adhesively attached to its underside to monitor the temperature. It is important that the coupons are as thermally isolated as much as possible to prevent thermal energy from the spacecraft itself from influencing the temperature of the coupons. The coupons are therefore supported in a TCA (Thermal Coupon Assembly) which is designed to support the coupons while insulating them from the spacecraft. The TCA is shown in Figure 13. The coupons are suspended above a MLI (Multi-Layer Insulation) blanket using PEEK support struts. The MLI blanket and the support struts themselves are in turn supported on a titanium baseplate. The PEEK support struts have been designed to minimize thermal conduction from the test panels to the baseplate and a number of different designs were thermally modelled as above before determining the most thermally efficient design. The baseplate is adhesively bonded to a custom Al6082-T6 aluminum structural end-cap, demonstrating another ENBIO product, an adhesive primer, known as CoBlast Prime, which is based on the same process used to apply the SolarBlack and SolarWhite coatings.
The adhesive primer has been developed to replace the harsh chromate conversion coatings that are typically used in industry. 12) Extensive environmental and mechanical tests have been conducted to meet the ESA acceptance criteria and ensure that the adhesive bond will endure the thermal cycling the EMOD payload will experience during orbit. Throughout testing this adhesive primer has out-performed the current state-of-the-art surface treatment. It will play a vital role in ensuring the structural integrity of the EMOD payload is maintained during launch and in orbit.
WBC (Wave Based Control)
WBC is a novel motion control scheme that has been developed by the Dynamics and Control group at the School of Mechanical and Materials Engineering at UCD. 13) 14) The WBC approach is particularly effective in controlling flexible or under-actuated systems with poorly modelled dynamics. WBC has been applied to simulations of the International X-ray Observatory and the DELIAN robotic arm as part of an ESA study and has been tested experimentally in parabolic flight. EIRSAT-1 will be the first time that WBC has been used in space.
During a series of tests, WBC will take control of EIRSAT-1's attitude to perform a number of maneuvers designed to evaluate the performance of the control scheme. The WBC payload takes the form of a software component which runs on the OBC. The ADCS motherboard will be placed into a test mode which disables the COTS control algorithm and allows direct control of the magnetorquer actuators via I2C commands sent from the OBC.
While WBC is a motion control algorithm, as part of the WBC experiment on EIRSAT-1, students will also produce an attitude determination algorithm. This algorithm will also run as a software component on the OBC, interfacing with the ADCS motherboard to monitor sensor values. Before the WBC attitude control test maneuvers are performed, this attitude determination part of the WBC experiment will be operated in parallel to the COTS ADC algorithm to evaluate its performance. The control test maneuvers may then be performed using attitude solutions determined by the student-written algorithm or using solutions determined by the COTS algorithm (Figure 13).
Throughout the WBC control test maneuvers, a Control Authority Watchdog will monitor the spin rate and several other parameters such as elapsed testing time. Control will revert to the COTS ADCS algorithm if any of these parameters exceeds predetermined bounds. The Control Authority Watchdog is also responsible for preventing the spacecraft from starting a WBC experiment in certain situations, e.g. if the spacecraft is in safe mode, or if the battery depth of discharge is too high. WBC will be evaluated based on pointing accuracy, slew rate, settling time, and power consumption.
The EIRSAT-1 Ground Station will be situated at the UCD School of Physics (53°18'32.4"N, 6°13'28.2"W). A 70 cm crossed-Yagi UHF antenna will be used for downlink at 9.600 kbaud GMSK. Uplink to the spacecraft at 1.200 kbaud AFSK will be facilitated by a 2m crossed-Yagi VHF antenna. The ground station hardware consists of COTS amateur radio hardware.
This hardware is chosen to ensure optimal compatibility with the on-board communications system and to meet the link budget requirements.
An ICOM-9100 transceiver will be used to transmit commands and receive telemetry from EIRSAT-1. Tracking of the spacecraft will be possible via a Yeasu G-5500 rotator. Orbit prediction will be carried out using the open-source program Gpredict, interfaced to the radio and rotator control program Hamlib. Hamlib will receive pass parameters from Gpredict to control the rotator and implement Doppler correction on the transceiver during each pass autonomously.
The AX.25 protocol and AFSK modulation will be applied to the uplinked telecommands using the SCS Tracker TNC. GMSK demodulation will be performed using the DV-Mega GMSK Modem interfaced to an Arduino. CCSDS decoding will be performed on downlinked telemetry using custom software written as a state machine in Python 3.6.
The mission control software architecture is designed to be compatible with the on-board software. To this end, the ground station will initiate a ‘communications pass' on-board by automatically sending pass start commands when the satellite is above the horizon. Upon receipt of these commands the onboard software will send the formatted housekeeping and payload data. A python socket server will provide the link between the mission control software and the ground station hardware. All packets passed through the socket will be persisted in a relational database using MySQL. Packets will be processed further by a custom packet processor, which is a state machine in Python 3.6. A custom-built Django web application will be used for parameter visualization and parameter warnings for operators and for publication of public information.
Two members of the EIRSAT-1 team have amateur radio licenses (David Murphy (EI9HWB) and Lána Salmon (EI9HXB)). To enhance the educational goals of the mission, it is envisaged that the outreach plan will involve amateur radio education and outreach activities.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).
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