GALA on JUICE Part 2 – From the first idea to the finished instrument – a development story
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GALA on JUICE Part 2 – From the first idea to the finished instrument – a development story
Jupiter’s Galilean moons – the target of the JUICE mission
Credit:
NASA/JPL/DLR
In the first part of this blog series on the Ganymede Laser Altimeter (GALA), we introduced the instrument and its scientific goals. In this article, we will describe the long development history that a complex instrument such as GALA must go through until it can be launched into space.
In 2007, more than 15 years ago, ESA selected a proposal for a Jupiter mission for an ‘Assessment Phase Study’. The idea was to fly to the unexplored icy moons of Jupiter and study their atmosphere, magnetic fields and radiation belts. The mission was named Laplace.
First block diagram of GALA. The basic structure with a Transceiver Unit for the laser and receiving telescope as well as the Electronics Units can be seen.
The scientific objectives were drawn up in the international scientific community including suggestions on the type of instruments that could be used to obtain the scientific data with the corresponding measurements. A laser altimeter was amongst the many candidates.
In September 2008, at the first payload meeting, we presented the first GALA block diagram, still very elementary and showing only the basic concept.
Here is a first prediction of what the surface coverage could look like, again with very preliminary assumptions regarding the orbital parameters in Ganymede orbit:
Early simulation of the ground tracks on the surface of Ganymede after 12 days in a 200-kilometre circular orbit
In 2009 and 2010, there was an interesting development from a programmatic point of view: NASA had been planning its own mission to Jupiter's moon Europa in parallel to ESA's Laplace mission. The two programmes were combined: there was a Jupiter Europa Orbiter from NASA and a Jupiter Ganymede Orbiter from ESA, formerly Laplace.
The joint programme was called the Europa Jupiter System Mission (EJSM) and the simultaneous presence of the two space probes in the Jupiter system was supposed to complement and enrich the scientific output. A good idea, but unfortunately NASA withdrew from the project in the years that followed for financial reasons. But the story doesn't end here: around 2015, a slimmed-down version of the original NASA mission was born under the name Europa Clipper. It is scheduled to launch in October 2024, arriving in the Jupiter system in 2030. Thus, two probes will travel to the Jupiter system at the same time and will complement each other perfectly. There are even plans for the European JUICE spacecraft to record the US Europa Clipper spacecraft when it impacts the surface of Ganymede at the end of its lifetime, hurling ice and dust into the sky. The particle measurement instruments and spectrometers on JUICE will be delighted, and the laser altimeter and camera will be able to explore a new crater.
But back to GALA in 2009: the calculations for the required performance, especially the link budget, have been constantly refined. Countless technical parameters on optical and electrical assemblies had to be considered, including the ageing of the components during the years-long mission, the characteristics of the detector (a silicon photodiode), the algorithms of pulse detection and so on.
All this was simulated with patchy, often inaccurate data from past Jupiter missions, such as the Galileo mission (mid-1990s), to see if GALA's targeted performance parameters would be sufficient. Of course, this step involves a great deal of uncertainty, since we are flying to unexplored icy moons.
The calculations led to an initial estimation of the required laser pulse energy of 15 to 26 millijoules (later it will be 17 millijoules) and a diameter of the telescope of 10 to 25 centimetres (it will eventually be 25 centimetres). The hand sketches were finally cast into a first CAD design, taking into account many experiences from our previous laser altimeter project BELA.
For example, the transceiver unit with laser and receiving telescope is built very compactly, which enormously improves the stability of the optical alignment between the laser beam axis and the axis of the receiving telescope. This also reduces the volume of the assembly, which reduces the mass, although the exterior walls have to be comparatively thick for the sake of radiation protection. More on the subject of radiation in the Jupiter system in the third part of the blog series!
We are able to shorten the electrical transmission path for the weak electrical signal from the detector to the digitising electronics to a few centimetres. Significant improvements were made in the electronics design, and interference from the pump current of 200 amperes for the laser diodes was almost completely suppressed.
The electronics unit contains the current and voltage supply, the rangefinder module for pulse analysis and calculation of the pulse propagation time, the main computer of the instrument (here called ICM) and the laser control module, which was later moved to a separate unit. Here, too, you can clearly see the thick outer walls for shielding and the complete cold redundancy that was still planned at the time.
In the years leading up to 2012, not only were the international GALA consortium (Japan, Switzerland, Spain) and the respective work packages defined. The technical development also progressed rapidly.
The assemblies of the Transceiver Unit were arranged on a stable optical bench: At the very top is the telescope, below it the detector with its detector-related electronics and at the very bottom the laser, whose laser beam was to be emitted towards the lunar surface via a deflecting mirror.
The most important key figures for energy consumption are always the required power and the mass. At that time, with the preliminary design, the values were 47.3 watts and 24.5 kilograms, which hardly differed from the later values of the flight model. Good job!
Further development of the transceiver unit with arrangement of the assemblies
In 2013 and 2014, the GALA project was increasingly integrated into ESA's mission planning; the interfaces to the spacecraft were specifically considered. This was followed by the first reviews with the beautiful names Instrument Preliminary Requirement Review and Instrument Consolidation Review, which were the basis for ESA's invitation to tender for the construction of the spacecraft. In 2015, Airbus became the prime contractor and henceforth another important contact for all parties involved. On the part of GALA, the contractual basis for the coming years was laid with our German industrial partner Hensoldt Optronics (formerly Zeiss Optronics).
In 2015 and 2016, the technical development was pushed full steam ahead in an ever-growing team. In addition to CAD design, mathematical simulation models in the areas of mechanics, thermal design, optics and radiation resistance were created and constantly fine-tuned. The interfaces with the spacecraft were further improved and adjusted. Since radiation-resistant components, coatings and materials are vital for a mission into the Jupiter system, a large number of radiation tests were carried out. Powerful electron beams shot the high-energy charged particles at optics and circuits, which had to survive this ordeal undamaged at best.
Development of the design between 2015 and 2016
Credit:
Hensoldt Optronics
Then, in October 2016, it was time for the first major review by ESA. The Preliminary Design Review examined whether the instrument met all scientific and technical requirements, whether the schedule was coherent and whether preparations had been made for the coming phases of production of the individual parts, assembly and testing.
We successfully passed the review and were thus able to start production of the first GALA models. On the one hand, the Structural Thermal Model (STM) had to prove mechanical stability and required thermal properties. On the other hand, the Electrical Model (EM) was electrically representative and was going to be used for testing the GALA software.
At the same time, this meant that the design of the GALA flight model entered the critical phase, reaching its almost final stage at the end of 2018. This was a particularly intense time, as in addition to building and testing the STM and EM with us and the international partners, the design of the flight model also had to be finalised. The radiation tests were almost complete, and the interfaces with the spacecraft had to be defined down to the last detail.
Design of the GALA flight model at the end of 2018 (from left to right). The Transceiver Unit (with radiator), the Electronic Unit and the Laser Electronic Unit.
Highlights of this phase were three milestones in 2019:
In April, we passed the Critical Design Review with flying colours. This meant that we were given the green light to build the flight model. In June, we were able to complete the tests of the STM and deliver this model to ESA/Airbus, as well as the EM in August. Phew!
The year 2020 turned out to be completely different than we could have ever imagined. Normally, in this phase of assembly and testing of the flight model, the goal involves working intensively in the lab directly on the hardware, but Covid-19 shattered this planning.
Staff in the lab had to be reduced to the absolute minimum and work was done in shifts. We installed webcams in the lab so that we could carry out the work in a 'hybrid' way from the home office with colleagues in the lab. Almost every control computer was made remotely controllable – often at the pain of our IT department.
The schedule was already enormously tight at this point. We had planned through all the activities for the next 12 months to the day. ESA and Airbus did not allow any delay. The GALA team pulled together, was flexible and managed to complete both electronics units of the flight model in June 2020 and then the transceiver unit in October 2020.
Figure 1: Analogue electronics module from Japan. It amplifies and digitises the detector signal and is built into the transceiver unit. Credit: JAXA
Figure 3: Capacitor banks inside the Laser Electronic Unit. This is where the energy is stored that is used to generate the very short laser pulses of five nanoseconds. Credit: Hensoldt Optronics
Figure 4: Flight model of the Transceiver Unit shortly before assembly is completed. Credit: Hensoldt Optronics
Figure 6: Flight model of the Laser Electronic Unit. Credit: Hensoldt Optronics
Credit:
see image descriptions
It was time to rigorously test the complete flight model. This was done in the following ten months until August 2021. Still under Covid-19 restrictions, we performed EMC tests, including AC magnetic tests, vibration tests, thermal cycling and balance tests, DC magnetic tests, software tests and many others.
On 14 August 2021, the big moment arrived: the extensive test campaign was over. ESA and Airbus reviewed, accepted and approved the test results and agreed to deliver the GALA flight model towards Toulouse to Airbus. Again, phew! The next major stages were the integration of GALA onto the JUICE spacecraft and the test campaign that followed.
A summary can be found here. GALA also completed this 15-month phase successfully and proved its full technical performance. Now we are optimistic that we will gain excellent scientific data during the mission.
