|21 Dec 2011
ISS Utilization: NightPod - Motion compensation mechanism for ISS-based long-exposure imaging
ISS night photography: The Cupola on board the ISS (International Space Station) provides a unique vantage point for remote sensing of the Earth. There has been a steadily increasing awareness of the potential for high resolution global photography of the night side of the Earth ever since the Earth observation group from the defense meteorological satellite program started releasing coarse resolution photographs of the night side of the Earth. Because a dedicated satellite is not foreseen in the near future, the most likely candidate suppliers of high resolution, global, nocturnal imagery is the crew on board the ISS. 1) 2)
The NightPod has been commissioned as a crew-attended and operated device. The mission objective was to assist the crew in taking high-resolution (long exposure) picture tracking of Earth targets at night from the Cupola, using on-board available cameras and lenses.
The main design drivers identified were:
• Crew safety
• Easy-to-use interfaces to allow the astronaut to quickly mount/dismount the NightPod, accurately align it, point and shoot
• High accuracy on alignment and pointing
• Structural strength to withstand all load cases (including launch and crew "kick-loads") and double fault-tolerant mechanical interfaces
• Maximum 10 kg total launch mass (including launch packaging)
• Compact volume for stowage on orbit and to comply with the Soyuz cargo constrains
• Capability to interface cameras and lenses ranging from 10.5 mm to 800 mm focal lengths (and similar physical lengths).
The NightPod project is the result of an efficient collaboration between Cosine Research BV (The Netherlands) as prime contractor, Astro-und Feinwerktechnik GmbH (Berlin-Adlershof, Germany) as subcontractor, and the directorate of ESA's HSO (Human Space Flight and Operations. The project is managed by ESA under GSTP (General Support Technology Program). The NightPOD project was funded by the Netherlands Space Office and the German government.
The NightPod is a state-of-the-art electro-mechanical system, a tracking device, which accommodates commercial optical cameras and compensates for the orbital motion and attitude of the ISS. The compensation is achieved by a non-linear motorized rotation of the camera with arcsecond accuracy. The NightPod computer directly controls the camera and synchronizes the non-linear rotation of the pointing axis and the integration time of the camera. The NightPod allows rotation in 4 axes. Two axes are used to align the NightPod to the ISS local nadir direction. The third motorized axis rotates during operation keeping the desired target steady in the camera's FOV (Field of View) for the several seconds integration period. The fourth axis is used to manually point at off-track targets.
Earth applications: The imagery of the Earth at Night has a large number of potential applications, from the original motivation to build NightPod, i.e. taking images of cities at night to create a map of the distribution of the population, to mapping fishing activities, light pollution and fires. These are only some of the applications that can benefit from a systematic collection of night imagery of the Earth at Night. The NightPod concept is very basic and allows for the tracking mechanism to be used by a large number of sensors and instruments.
The NightPod instrumentation has been conceived as a combination of custom built parts and ISS compatible COTS items. The instrument is comprised of:
• the "head", providing 3 manually adjustable axes, single motorized rotary stage with micro-stepper motor controlled by a single board computer, electronics enclosure (the Control Box), camera interface, USB port and Crew interfaces
• two "legs", providing 4 seat track interfaces to Cupola, and the interface to the NightPod "head"
• a lens support (for the Nikkor 400 mm lens only).
The design is optimized to accommodate a Nikon D3s camera mounting a Nikkor 400mm lens. Shorter lenses can be used as well.
The NightPod is powered by two batteries connected in series and placed on the side of the control box (Figure 3) which contains the control electronics. The batteries have internal over-current protection and are qualified by NASA for flight on-board the ISS and the Space Shuttle. The custom battery I/F is verified against all the safety requirements for battery usage on board the ISS. Fully charged batteries allow continuous operation for more than 6 hours in worst case conditions.
The control box also accommodates the buttons to control the payload, the OLED (Organic Light Emitting Diode) display to show the information, and the USB data interface used to communicate with the camera and to upgrade the control software. The interface to the camera is located on top of the control box, and accommodates the captive tripod mounting screw to mount the camera itself.
The non-linear rotation of the camera to compensate for the ISS orbital motion is actuated by a rotary stage. The rotary stage is composed by a gear box, a micro-stepper motor, a motor controller and an optical encoder. The encoder is also used as stall warning system, in case any force is applied to the camera lens while rotating. 3)
Gear drives are used to set and lock the orientation of the NightPod axes as shown in Figure 4. The gears are adjusted via an enumerated dial and their gear ratio provides high adjustment accuracy and ensures the correct alignment during operations.
The leg assemblies are connected to the head by two interfaces (Figure 5) that lock into position when the head is correctly inserted by the crew members. In order to dismount the system, release levers shall be operated and, once unlocked, a body release lever shall be used to disengage the three components completely.
The interface to the Cupola is at the seat-tracks alongside the Cupola lateral windows (Figure 8). The NightPod seat-track interface consists of a custom designed mechanism equipped with a self-locking system as shown in Figure 6.
Maximum integration time
Best ground resolution
10 m/ pixel
Safety assessment: The NightPod has been designed and tested for compliance with the requirements for human spaceflight safety for pressurized payloads. These requirements cover a wide range of areas from interface definition to outgassing requirements.
The impact of different load cases, including launch, de-pressurization and crew-induced (kick) loads on structural integrity was analyzed with the FEM (Finite Element Method) model of the NightPod, and verified at later stage by qualification and acceptance testing. The mechanical and electrical interfaces are labelled, locking is visually verifiable and secured against inadvertent disengagement. The system has been designed to be operated with one hand, once installed, and is free from pinching points and sharp edges.
LEDs (Light Emitting Diodes) are used to indicate the power status of the main power line and of single battery low-voltage. Power is automatically cut-off when the voltage goes below the security margin set by the ISS safety policy.
Operational envelope limits are reported in Table 2. The motor speed and full swap angle has been analyzed (and mechanically constrained) to avoid impact and consequent injury to astronaut during operation.
Operational ranges for alignment / pointing
Ranges for non-main windows position
+40º/-20º (constrained by NightPod geometry)
Materials and components have been selected and verified in compliance with requirements for human spaceflight safety. Flammability and explosion assessments have also been supported by qualification test.
The control box is verified for structural integrity under the different load cases and provides containment for inadvertent flame (which would self-extinguish) and shatters (which would be contained). - A successful off-gassing test has been performed on the PFM (Proto-Flight Model).
Calibration: The NightPod (with the camera installed) must be calibrated in order to account for the current ISS attitude and altitude. The calibration is performed inserting the ISS orbit attitude information in terms of yaw pitch and roll in the NightPod computer, which then calculates how much each axis on the NightPod should be rotated so that the camera is pointing nadir. The crew member shall adjust the yaw and roll axis using the scales engraved on the axes. The motorized axis is then set by software. Once this calibration is complete the Nightpod is ready to be used and an operational mode can be selected. It is, however, possible to switch between all operational modes during a session.
Manual mode: The manual mode is intended to assist in manually pointing the camera to specific targets (either preselected by their latitude/longitude coordinates or hand-picked by the user), and making single-shot pictures of those targets. The motorised axis rotates to compensate the orbital motion of the ISS. The crew member decides when to take the picture.
Automatic mode: This mode is a semi-automatic mapping mode, the camera takes a number of pictures in a row, selecting the integration times and tracking axis reset times such that the entire swath covering the ISS ground track during one pass is imaged with minimum smear and distortion per image.
The idea behind the automatic mode is to use the mechanism to systematically take pictures of adjacent areas on the surface, so that combining these pictures results in large-scale but high resolution "maps" of the night side of the Earth. This principle is shown schematically in Figure 7. Given enough time and enough passes, equal areas can be imaged under a wide variety of angles and lighting conditions, increasing the quality of the resulting maps and increasing the number of areas of scientific research that can benefit from the data.
NightPOD is a one-of-a-kind product for mounting and accurately tracking a camcorder (or single-lens reflex camera) in the Node-3/Cupola of the ISS. NightPOD compensates for the motion of the Space Station by tracking single points on Earth automatically. The subject stays centered in frame during a long exposure time so the final image is in focus.
NightPod on the ISS
The challenging flight hardware design and development process, together with a full PFM (Proto-Flight Model) testing campaign, was successfully concluded in only five months in order to be on time for the launch of the Soyuz 29S to the ISS, on December 21, 2011. ESA astronaut astronaut André Kuipers took Cosine's NightPOD camera on board to the ISS and installed the instrumentation.
The installation and commissioning of NightPod had been successfully completed on February 24, 2012. The NightPod is part of the Crew standard training and will be operated by all Crew Members flying to the ISS in the coming years (Ref. 1). 4)
NightPOD is an intelligent tripod head that is used to accurately track a SLR (Single Lens Reflex) camera, potentially with a large SLR, to track objects. Extremely sturdy and user friendly, it can be used to take long exposure photographs or eliminate motion blur under demanding conditions. It can be used for accurate tracking of moving objects or track stationary objects from a moving platform, such as boats, airplanes and cars.
It has been confirmed that the NightPod helps in taking sharper high resolution images of the Earth at night. However, whereas Cupola is an outstanding location for optical observation, the windows (and in particular the protective panes) limit the use of high focal lengths. It has been observed that any picture taken using a lens with a focal length above 180 mm would result in "blurry" images.
The pictures produced during the first half of 2012 have already raised the attention of scientists from the Earth Observation community who expressed interest in further development of this project. 5) 6)
Legend to Figure 10: The largest cities of the Netherlands are clearly visible: Amsterdam, Utrecht, Rotterdam, and The Hague.
This is only the beginning for NightPod. The system has been designed to be adaptable, and astronauts on the Space Station are already thinking of using NightPod to look the other way, into space, taking images of stars.
Legend to Figure 13: Liège is the third most populous metro area in Belgium, after Brussels and Antwerp. It includes 52 municipalities and the nearby city of Seraing. It is also an important economic center for the country, home to a diverse array of industries including mechanical, information and biotechnology; beer and chocolate; light armaments; and steel-making. The metropolitan area also boasts a wide array of cultural, historical, and artistic attractions that make it a popular destination for residents of France, Germany, and the Netherlands. 8)
Night-time image with the Nikon D3S digital camera on the ISS
Astronaut photography of nighttime Earth is also acquired by other means than "NightPod-mounted" cameras on ISS. The image of Riyadh (Figure 15) was acquired on November 13, 2012, with a Nikon D3S digital camera using a 400 mm lens. It is provided by the ISS Crew Earth Observations experiment and Image Science & Analysis Laboratory, Johnson Space Center. The image has been cropped and enhanced to improve contrast, and lens artifacts have been removed. 9)
Legend to Figure 15: The population of Riyadh, the capital city of Saudi Arabia, has risen dramatically in the last half century—from 150,000 in 1960 to 5.4 million in 2012. The city appears as a brightly colored patchwork in this nighttime astronaut photograph. The brightest lights, apart from those on the old Riyadh Airbase, follow the commercial districts along King Abdullah Road and King Fahd Branch Road. Many of the darker patches within the built area are city parks.
University sectors stand out with different street and light patterns, including the King Saud University campus—which houses the Arabic Language Institute—and the Princess Nora Bint Abdul Rahman University—which is the largest all-female university in the world. Highways and various ring roads also stand out due to bright, regular lighting. Lighted developments beyond the ring roads mark the growth of the city (image lower left and lower right). Newer neighborhoods, set further from the city center, are recognizable by blue-gray lightning.
NASA is asking the help of the public to identify the wealth of imagery taken by the astronauts
A wealth of images of Earth at night taken by astronauts on the ISS could help save energy, contribute to better human health and safety and improve our understanding of atmospheric chemistry. But scientists need your help to make that happen. 10)
The images are available to the public through The Gateway to Astronaut Photography of Earth, the most complete online collection of images of Earth taken by astronauts. This database contains photographs beginning with those taken during Mercury missions in the early 1960s up to recent images from the station, with more added daily. As of August 2014, the collection included a total of nearly 1.8 million images, more than 1.3 million of them from the space station. Approximately 30 percent of those were taken at night.
The photographs taken by astronauts on the station are the highest-resolution night imagery available from orbit, according to William Stefanov, associate program scientist for Earth observations for the space station. Satellites collect data on a more regular basis, but those images typically have much lower resolution. This clarity is possible thanks to the ESA's (European Space Agency) NightPod, installed on the station in 2012.
Now the pictures are clear, but their location may not be, which limits their usefulness. That's where citizen science comes in. UCM (University of Madrid) is leading a project called Cities at Night to catalog the images. It includes three citizen science components: Dark Skies of ISS, Night Cities, and Lost at Night.
• Dark Skies asks people to sort images into those of cities, stars and other objects. The simplest of the three projects, it requires no specific expertise. "Anyone can help," says Alejandro Sanchez, a Ph.D. student at UCM. "In fact, without the help of citizens, it is almost impossible to use these images scientifically. Algorithms cannot distinguish between stars, cities, and other objects, such as the moon. Humans are much more efficient for complex image analysis."
• For Night Cities, citizen scientists use their knowledge of local geography to identify points in night images and match them to positions on maps. As Sanchez explains, a resident of a city can likely identify its features more easily than someone who does not live there. This geo-referenced data will be used to generate light maps of cities.
• Lost at Night requires the most skill, seeking to identify cities in images encompassing a circle 310 miles around. "We don't know which direction the astronaut pointed the camera, only where the station was at the time the image was taken," explains Sanchez. "Some images are bright cities but others are small towns. It is like a puzzle with 300,000 pieces."
So far, hundreds of volunteers have classified nearly 20,000 images, but to ensure accuracy, each one should be classified by multiple individuals. One of the outcomes of the project will be determining the optimum number of people needed to inspect each image, but its primary goal is producing an open atlas of night time images available any time for use by the media, public, and scientists.
Scientists can, for example, use colors in images to estimate the types of light sources and, thus, the energy efficiency of a particular city. Researchers could use the data to compare the lighting and the economic health of a city as well. "A clear example is comparison of Madrid and Berlin," Sanchez says. "Madrid is the capital of Spain, a country facing a major economic crisis. It is much brighter in astronaut images than Berlin, the capital of Germany, the country with the healthiest economy in Europe. Perhaps that is an indication that Germany more efficiently manages its resources. The images can provide evidence and data to verify that."
Other potential applications include evaluating lighting for road and public safety and correlating light pollution with effects on human health and biodiversity.
The atlas is a collaboration of UCM, MediaLab-Prado, Spanish Light Pollution Research Network, European Cooperation in Science and Technology's Action Loss of the Night Network, Crowdcrafting, Celfosc and AstroMadrid.
NightPod also was used for Crew Earth Observations (CEO), an investigation for which astronauts photographed natural and human-made events on Earth, recording changes over time; atmospheric phenomena; and events such as storms, floods, fires and volcanic eruptions. Because astronauts can take images at a variety of resolutions and angles, their images provide atmospheric data, and because the station passes over different places on Earth at different times of day and night and with different a frequency than free-flying satellites, they also provide data on changes over time.
Astronauts on the station have made sharp images of Earth at night available to scientists. Now the public can help scientists put those images to better use.
1) Luigi Castiglione, Simon Silvio Conticello, Marco Esposito, Rody Oldenhuis, Scott G. Moon, Anja Nicolai, Stephan Stoltz, Jan Dettmann, "The NightPod – An orbital motion compensation mechanism for ISS based imaging," Proceedings of the 63rd IAC (International Astronautical Congress), Naples, Italy, Oct. 1-5, 2012, paper: IAC-12-B3.3.12
2) "ESA NightPod," NASA Fact Sheet, Nov. 01, 2012, URL: http://www.nasa.gov/mission_pages/station/research/experiments/ESA-NightPod.html#overview
3) "Nodding Mechanism für den NightPod," Astro-und Feinwerktechnik, URL: http://www.astrofein.com/astro-und-feinwerktechnik-adlershof/projekte/nightpod/
4) Massimo Sabbatini, Luigi Castiglione, "NightPOD - Capturing Earth by night from space," ESA Bulletin, No 155, August 2013, pp. 46-55
5) "Tracking cities at night from the Space Station," ESA, April 5, 2012, URL: http://www.esa.int/SPECIALS/PromISSe/SEM3HNEWF0H_0.html
6) "Tracking cities at night from the ISS," ESA, April 5, 2012, URL: http://blogs.esa.int/promisse/2012/04/05/nightpod/
7) "NightPod Images Bring Earth to Light From Space Station," NASA, Feb. 06, 2013, URL: http://www.nasa.gov/mission_pages/station/research/news/nightpod.html
8) "Liège at Night," NASA Earth Observatory, Jan. 14, 2013, URL: http://earthobservatory.nasa.gov/IOTD/view.php?id=80145
9) M. Justin Wilkinson, "Riyadh at Night," NASA Earth Observatory, Dec. 3, 2012, URL: http://earthobservatory.nasa.gov/IOTD/view.php?id=79828
10) Melissa Gaskill, "Space Station Sharper Images of Earth at Night Crowdsourced For Science," NASA, Aug. 14, 2014, URL: http://www.nasa.gov/mission_pages/station/research/news/crowdsourcin-night_images
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).