PRISMA Hyperspectral Imager

PRISMA is an Italian hyperspectral satellite aiming to qualify the technology, contribute to develop applications, and provide products to institutional and scientific users for environmental observation and risk management. It was launched on March 22, 2019, on a Vega launch vehicle in Kourou into a sun synchronous orbit.

PRISMA payload is composed of a hyperspectral imager and panchromatic (PAN) camera. The hyperspectral imager is made up of a VNIR prism based spectrometer and a SWIR prism based spectrometer to cover spectral bands ranging from 400 nm to 1010 nm and from 920 nm to 2505 nm. It provides hyperspectral images of the Earth with 30-m ground sampling distance (GSD), 30-km swath width, and spectral bands at a SSI of 12 nm.

The thermal and mechanical design of the satellite was driven by thermal requirements. The use of prism-based spectrometers requires a tight control of temperature variations to ensure the stability of all instrument features on orbit. Particular thermal design efforts w'ere devoted to the cooling of detector arrays, which is achieved without the use of mechanical coolers. The use of a mechanical cooler, due to intrinsic micro-vibrations, is a potential cause of modulation transfer function degradation, especially for very narrow IFOVs in addition the risk of life cycle reliability (Meini et al. 2016).

The mechanical design was guided by the constraints related to the satellite accommodation and the environmental conditions. The satellite accommodation includes the positions on the top of the spacecraft by taking into account the orbit, the mission phases, and maneuvers. The environmental conditions include launch, deployment, and in-orbit phase.

To deal with the constraints, the mechanical design was based on a closely integrated optical bench structural for providing support to the mirrors of the three mirrors anastigmat telescope on the upper side and support to the spectrometers optical elements on the low'er side.

The thermal design of the payload has the following features:

The SWIR and VNIR detector arrays are cooled in a range between 140 К and 185 К using two-stage passive radiators facing cold space.

The cold chain of each detector array uses a thermal strap and a high performance ethane heat pipe.

Specific parts of the spectrometers, such as the detector arrays and their cold shield, are positioned close to the radiator using a short thermal strap to improve the system efficiency.

The heat fluxes from the w'arm electronic part to the cold zones of the instruments are very low.

The prisms subsystems are maintained thermally insulated from the supporting structure.

The optical elements are mounted in an insulated enclosure on the optical bench and are maintained in a relatively small range of temperature by a dedicated thermal control so to reach a high thermomechanical stability.

The opto-mechanical design of the Optical Head utilizes reflective optical components and structural elements all use aluminum alloy to obtain a quasi-athermalized design. The optical bench is closed by the upper and lower covers equipped with patch heaters to stabilize the entire instrument at 20°C. The insulation of the Optical Head from the spacecraft conductive and radiative heat loads is achieved by means of insulating, mounting its feet (titanium alloy) positioned on the baseplate of the payload interface and wrapping with MLI blankets.

The SWIR and VNIR detector arrays are mounted on the lower side of the Optical Head. They are cooled between 140 К and 185 К by a cold chain connected to two-stage passive radiators as shown in Figure 8.10. The main element of the cold chain is a high efficiency ethane axial grooved heat pipe connected with a thermal strap that allows the structural decoupling of the detector arrays from the radiator. The passive cooling system consists of two-stage radiators, detector housings, thermal straps, heat pipes, heat pipe jackets, insulating supports, and mechanical support.

The heat sources inside the Optical Head are the proximity electronics boxes; the detector arrays of SWIR, VNIR, and PAN spectrometers; and the motors and actuators for the covers and the heaters. The proximity electronics boxes are located far from the cold zones. They are mounted on the baseplate outside (externally to optical bench) and thermally sunk to the baseplate to minimize parasitic heat flux.

There are three kinds of heaters that are used distributed on various parts of the optical bench structure for different purposes. The operational heaters enable accurate thermal control during operation. The survival heaters are for thermal conditioning before operation and in-between operations. The third kind heaters are for decontamination purposes.

FIGURE 8.10 Schematic layout of the passive cooling system for VNIR and SWIR detector arrays. (Courtesy ofSELEXES.)

The Optical Head is accommodated in the spacecraft and has three thermal interfaces with the spacecraft platform at the top panel, with solar array panel on the backside, and with external environment. Thermal shields are installed to thermally decouple the Optical Head from outer thermal environment. The thermal shields are aluminum sandwich panels bolted together and covered externally by MLI. The optical bench is protected from the outer environment by the double-level MLI: one inner MLI between the optical bench covers and the payload panels, and an external MLI mounted on the lateral panels.

MAJIS for Exploring Galilean Moons of Jupiter

The MAJIS is a hyperspectral imager covering VNIR and IR regions. It was selected by ESA in February 2013 for JUICE mission to survey the Jovian system with a special focus on the three Galilean Moons: Callisto, Ganymede, and Europa. The JUICE mission plans to flyby Callisto, Ganymede, and Europa, then a 1-year orbital fly around Ganymede, the largest moon of Jupiter. It is scheduled to be launched in 2022 (Langevin et al. 2014).

MAJIS consists of two grating-based spectrometers with the VNIR spectrometer covering wavelengths from 0.5 pm to 2.35 pm and the IR spectrometer covering wavelength from 2.25 pm to 5.54 pm. The two spectrometers use two HgCdTe detector arrays of size 400 x 508 pixels.

FIGURE 8.11 MAJIS Optical Head structure. (Courtesy of Leonardo SpA. Airborne & Space Systems.)

The MAJIS will produce 2 x 508 spectral bands from 0.5 pm to 5.5 pm at SSI 3.6 nm and 6.4 nm with 400 spatial pixels. The SNR will exceed 100 over most of the spectral range. The instantaneous FOV of 150 prad of the instrument corresponds to a spatial sampling distance of 75 m from a 500-km circular orbit over Ganymede and to 150 km for observations of the atmosphere of Jupiter.

The MAJIS is made of many different subsystems. The main subsystem is the Optical Head, which includes a scanning mechanism, a telescope, two spectrometers, and a calibration unit. The thermal design of MAJIS is based completely on passive radiators. It provides operational temperatures <90 К for the infrared detector array, and <140 К for VNIR detector array, and the optical bench. These low temperatures are required to reduce detector noise and background radiation disturbances on the measured signals. Figure 8.11 shows the structure of the Optical Head. It can be seen that two-stage radiators are mounted on the top of the Optical Head.

MAJIS Thermal Design

In the Optical Head there are two different thermal requirements:

• 1. The optical bench, optical elements of the VNIR and IR spectrometers as well as the VNIR detector array need to be maintained at around 140 К or lower, which is achieved by thermally connected to the relatively warm radiator.
• 2. The IR detector array needs to be cooled below 90 K. which is achieved by thermally connecting to a second passive cold radiator. This cold radiator is surrounded by the relatively warm radiator, which acts as a shielding element against external sources of heat as shown in Figure 8.11.

MAJIS payload will be mounted on the external panel of the JUICE mission spacecraft. The temperature of the spacecraft during operation is between 263 К and 293 К (-10°C to 20°C). It is required to maintain the temperature of the Optical Head below 140 K. The payload supporting structure that connects the Optical Head and the spacecraft has to withstand a maximum temperature difference of 153 K, in addition to providing the strength and stiffness required to hold the Optical Head and radiators. Therefore, one of the thermo-structural design drivers is to minimize the thermal conductance between the payload supporting structure and the spacecraft. It is critical to select the supporting structural material that has sufficient thermal insulation between the instrument and the spacecraft. In order to provide the requested thermal insulation, a configuration based on composite material three bipods that connect to titanium heads and feet has been designed and adopted as shown in Figure 8.11. Selection of bipods material and geometry has been done following accurate trade-off between their thermal and structural properties.

In order to reduce the radiative heat load on the Optical Head, a double thermal barrier is designed for it by applying MLI to the outer surfaces and an single-layer insulation to the inner surfaces. This barrier has also been applied to enclose the volume below the warm radiator to avoid sun trapping.

Survival and anticontamination heaters are implemented and are controlled by the spacecraft. The survival heaters are aimed at maintaining the temperature of the Optical Head within the nonoperative temperature range of each subsystem, while the anticontamination heaters are used during the first part of the cruise after launch and in case of need to prevent water ice formation on the optics and the detectors (Tommasi et al. 2018).

MAJIS Structure Design

Thermomechanical design of the MAJIS is driven by the following requirements (Saggin et al. 2014):

• • Allocated mass for the instrument support interface is 2.7 kg and the mass for the radiators is 2.9 kg.
• • A quasi-static design load is 500 m/s².
• • First natural frequency is above 130 Hz to avoid coupling with lower frequency satellite modes.
• • Sine vibration amplitude is 240 m/s² between 20 and 100 Hz.
• • Random vibration is between 20 and 2000 Hz with amplitude of power spectral density up to 0.102 g²/Hz.

Optical elements of the Optical Head are mounted and aligned on the optical bench that constitutes the main structure of the instrument and provides support to all the other subsystems. The optical bench surface is stiffened by a series of ribs that provide the necessary bending stiffness with a limited cost of mass. It is highly beneficial to machine such components from a single piece of material (or to minimize the number). The entire external border of the optical bench would typically be used, providing reinforcement by a robust continuous rib that increases the stiffening of the structure, designed to provide adequate strength and stiffness for the connection of supports.

The strict thermal requirements and the passive cooling strategy have a strong impact on the structural design. The Optical Head supports should provide the required resistance under mechanical loading and a proper dynamic behavior, but at the same time, insulating the Optical Head from the warm spacecraft. As described in Section 8.8.3.1, three bipods of composite material connected to titanium heads and feet are adopted. These bipods are placed on three reinforced zones of the optical bench located at about 120° among them, which are in turn connected to the network of inner ribs. This configuration of the bipods ensures that the direction of minimal flexural stiffness of each bipod is directed toward a common point located in the proximity of the center of mass of the instrument, resulting in an isostatic design. The thermal elongation of the optical bench is thus compensated by the bipod transversal flexibility minimizing the induced stress in the optical bench. Preliminary structural analysis focused at bipods design verification showed that the first Eigen- frequency is well above the 100 Hz requirement.

The two-stage passive radiators are on top of the Optical Head. To meet the allocation of the radiators requires an accurate control of all the thermal exchanges in the minimization of the dimensions and mass of the radiators.

The Optical Head includes two mechanisms, a scan unit and a shutter:

• 1. The scan unit is equipped with a lightweight mirror supported by two frictionless flexural pivots. The rotation of the mirror is pre-programmable in order to synchronize its position with scientific data acquisitions. Moreover, the scan unit can direct the telescope beam toward the line of sight of the internal calibration unit for calibration purpose. The scan mirror and other a few components in the scan unit are made of beryllium. The combination of the coefficient of thermal expansion (CTE) of beryllium and the CTE of the scan axis ensure that the mirror remains blocked at room temperature and is free to rotate at cryogenic temperatures. This temperature difference driven CTE ensures maximum stability of the mechanism during the launch.
• 2. The shutter mechanism is placed at the entrance slit of instrument. The closure of the shutter allows for acquisition of dark current and thermal background data from the spectrometers, which are particularly relevant for an IR spectrometer.

Since the Optical Head has to operate in the harsh radiation environment of the Jovian system, radiation hardiness has been taken into account for the design of the structure in order to protect most sensitive items, especially detector arrays and optical coatings, from ionizing and nonionizing radiation. Dedicated analyses were iteratively performed during the design phase to assess the implemented architecture against radiation doses and dedicated radiation shielding has been added when required. This was coupled with a qualification plan that includes radiation testing to the foreseen levels of the most sensitive parts, especially optical coatings and calibration sources. Whenever possible, testing will be carried on at cryogenic temperature with an electron source, for the best representative radiation environment experienced by the JUICE mission. A maximum of 50 Mrad total ionizing dose is foreseen on the scan mirror, which is the optical element most exposed to spare radiation.