Positioning in satellites with micrometer-level high precision
OHB Systems is building EnMAP, a new hyperspectral satellite for surface monitoring. The satellite contains five reflectors, six prisms, three cameras, and electronic components, and is being precisely installed in a confined space, with the space between components typically only one-tenth of a millimeter. During installation, contact between these components must be prevented, otherwise, these sensitive parts could be damaged. Heidenhain's LIP 581 linear encoder and POSITIP 880 digital display device have played a crucial role in this process.
As the opening of the Star Trek movie says, "Space, the last frontier." Gavin Staton, the EnMAP satellite project manager at OHB, and lead engineer Dave Mussett certainly wanted more space within the satellite, even though "outer space is vast, but when building a satellite, we simply don't have ample room. Moreover, our task is to minimize space and weight." Therefore, we had to cleverly utilize every cubic millimeter of the installation space to minimize the satellite's mass and size, ensuring its successful operation in space and in its Earth orbit. Maintaining extremely high stability during manufacturing was also a major challenge, given the labyrinthine layout within the EnMAP satellite.
The IOU-PIT installation tool, developed and manufactured by Airworks, sits on a multi-ton marble platform in OHB Systems' ISO Class 5 cleanroom, the largest in Europe. The IOU-PIT installation tool uses a Heidenhain LIP 581 linear encoder.
Rods, struts, and hammering—the formidable challenges of assembly
OHB Systems is manufacturing the EnMAP satellite at its Wessling factory, west of Munich, Germany. Most drivers on the A96 motorway passing by the unassuming office building would never guess the high-tech sophistication inside. The building's exterior gives no indication that it houses Europe's largest ISO 5 cleanroom, where the EnMAP satellite is being assembled. More precisely, it assembles parts, not the entire satellite, initially only the project's core component – the optics.
The core components are located within its housing, with its base plate milled from a block of aluminum. Its base, outer walls, and internal struts form a highly rigid unit within which all other components of the complex optoelectronic device are installed and connected. Installing the first piece requires extremely high precision, as access to the location is limited to one direction despite the ample space. The large angle of the struts severely restricts the degree of freedom of movement.
With each additional component, the space in the work area shrinks, until the range of motion is reduced to only a few hundredths of a millimeter. Not only are the optical components—mirrors, prisms, and cameras—not allowed to bump into each other, but the housings and tools are also strictly prohibited from any contact, as these surfaces are extremely sensitive. Even a minor scratch can affect the imaging of the optical system and may even render it unusable. “The incident light cannot be diffracted correctly, cannot be focused accurately, and the mirror cannot properly guide light to the camera. This means the camera cannot receive the full illumination and spectrum, and in severe cases, no light may reach the camera at all. This will cause image distortion or unusability, or even no image at all—meaning the failure of the entire EnMAP mission,” Gavin Staton explained the importance of error-free installation of the optical components.
The high precision requirements and applications in cleanrooms are the decisive reasons why IOU-PIT installation tools are difficult to manufacture and use with Heidenhain linear scales.
Installation tools for micron-level high-precision assembly
The Italian company Airworks provided assistance to OHB engineers by supplying them with a special mounting tool, the IOU-PIT (High-Precision Mounting Tool for Optical Instruments). When this high-performance instrument was first seen in the cleanroom, the focus was not on the heavy loads being lifted by the crane, but on the tool itself. It's a tool for precisely positioning extremely small and lightweight components, with each component weighing no more than 20 kg. The IOU-PIT is quite large, measuring 3040 mm x 2510 mm x 2300 mm and weighing 2.5 tons. The difficulty in manufacturing it stems from the extremely stringent requirements of the IOU-PIT, demanding exceptionally high precision for installation and use in a cleanroom. This requires both extremely high mechanical rigidity and the prohibition of using many materials forbidden in cleanrooms.
The IOU-PIT was developed and manufactured by engineers at Airworks, a company located in Monfalcone, Italy. Airworks is an independent development agency that develops advanced high-tech projects, including aerospace projects. Stefano Picinich was in charge of the IOU-PIT device. “Actually, for the design of the main mechanical systems of the IOU-PIT, we contacted several high-end suppliers of positioning systems. Unfortunately, the project requirements were too high, quickly dampening their interest. In the end, we couldn't find a partner with the courage to turn our device into a practical tool,” Picinich recalled with a wry smile. “Therefore, we manufactured it ourselves, and fortunately, we found a measurement technology partner, Heidenhain, who, like us, was not afraid of the high precision requirements.”
Installing optical systems in satellites requires an ultra-clean and touch-sensitive environment.
Heidenhain linear encoders and digital displays—accurately determining the tip of the tool.
Heidenhain's incremental open linear encoder LIP 581 calculates the position values of each axis of the IOU-PIT with the required high resolution. This encoder provides position values with a baseline error of less than 0.175 µm and a subdivision error of only ±12 nm over a 5 mm pitch range. The Heidenhain POSITIP 880 digital display shows the position values.
The IOU-PIT moves the EnMAP satellite components in two stages: three basic axis pre-positioning components driven by motors, with no impact on the movement speed. Two movement speeds are available: 1 mm/s and 0.1 mm/s, or figuratively, a snail's pace and a tenth of a snail's pace. The IOU-PIT has a large maximum travel of 1250 mm x 1125 mm x 650 mm, with a maximum position error of only 50 µm over a 500 mm travel.
In the final millimeters and micrometers to the installation position, and during movement within the satellite's confined working area, OHB technicians operated entirely manually using installation tools. Micrometer by micrometer, the components were moved through a maze of installed struts, parts, and cables until the final position was reached. "During this time, we had to keep a close eye on the components being installed and the digital display," explains Dave Mussett, describing this delicate manual operation. "As a result, we had two safeguards—first, we visually observed the object ourselves, and second, the position value was displayed very accurately on the clearly visible POSITIP 880 display." The installation process was verified daily by OHB Systems, who trusted Airworks' IOU-PIT installation tools. So far, all components are installed accurately and without damage—thanks to Heidenhain's linear scales and digital displays.
The top view reveals the cramped working area inside the satellite shell and the high precision required for positioning and installing tools.
EnMAP project
EnMAP (Environmental Remote Sensing and Analysis Programme) is a hyperspectral satellite project for monitoring the Earth's surface in Germany. Its imaging spectrometer measures solar radiation reflected from the Earth's surface, spanning the spectral range from visible light to shortwave infrared, to accurately describe surface conditions and their changes. This allows for the reflection of the latest changes in the environment, agriculture, land use, water resource management, and geology globally.
The imaging spectrometer onboard the EnMAP satellite can map the Earth's surface using continuous spectra across 250 narrow bands. Therefore, EnMAP provides quantitative and diagnostic information on vegetation, land use, rock surfaces, and water resources. This information also includes the mineral composition of rocks, the impact of air pollution on vegetation lifespan, and the degree of soil pollution.
The satellite records surface scan data in an orbit of 650 kilometers with a resolution of 30 x 30 meters. It can tilt no more than ±30 degrees perpendicular to its flight direction, allowing for four days of comparative observations at the same location. This makes EnMAP ideal for recording spatial-temporal variations, such as vegetation encroachment processes or cycles. Spectroscopic surface observations provide a wealth of information about how diverse natural environments form and spread ecosystems—for example, human-influenced coastal zones and human-built landscapes, as well as grasslands, deserts, and forests.
The EnMAP project was scientifically guided by the German Research Centre for Geosciences (GFZ) in Potsdam, with the DLR Space Center responsible for overall project management. OHB Systems was the prime contractor, developing the dedicated hyperspectral remote sensing satellite, including instruments and the spacecraft bus. The DLR in Oberfafenhofen, Germany, was responsible for establishing and setting up the ground facilities. Satellite operation and data reception were jointly conducted by the German Space Control Center (GSOC), the German Data Center for Remote Sensing (DFD), and the Institute for Remote Sensing Technology (IMF).
Work completed for OHB Systems, the prime contractor for EnMAP and a party to the contract with DLR, funded by the German Federal Ministry for Economic Affairs and Energy, project reference number 50 EP 0801.
