The Visual Monitoring Camera on Mars Express: calibrating a new science instrument made from an old webcam orbiting Mars

Hernández Bernal, Jorge; Cardesín Moinelo, Alejandro; Hueso Alonso, Ricardo; Ravanis, Eleni; Burgos Sierra, Abel; Wood, Simon; Marín Yaseli de la Parra, Julia; Merrit, Donald; Costa Sitja, Marc; Escalante, Alfredo; Grotheer, Emmanuel; Esquej, Pilar; Dias Almeida, Miguel; Martin, Patrick; Titov, Dima; Wilson, Colin; del Río Gaztelurrutia, Teresa; Sánchez Lavega, Agustín; Sierra, Mar
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European Planetary Science Congress

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The Visual Monitoring Camera (VMC) is a small camera onboard Mars Express, initially intended to provide visual confirmation of the separation of the Beagle 2 lander. In 2007, a few years after the end of its original mission, VMC was turned on again to obtain full-disk images of Mars for outreach purposes (Ormston et al., 2011). As VMC obtained more images, the scientific capabilities of the camera became evident (Sánchez-Lavega et al., 2018), and finally the small camera was upgraded to be a new scientific instrument, with an agreement between the European Space Agency (ESA) and the University of the Basque Country (Spain; UPV/EHU). In this work we describe the calibration and technical efforts that are allowing us to maximize the scientific output from this small camera.Figure 1. Image of VMC before launch (left) and scheme from the Flight User Manual (right) VMC is also called the Mars webcam, as it is similar to a typical webcam of the 2000s. The sensor has a a 640x480 pixel array, and a Field of View (FOV) of 30ºx40º. This wide FOV, together with the elliptical orbit of Mars Express, enables full-disk observations from apocenters, which are the most common product of VMC. It is also possible to use this wide FOV to image large sections of the limb, and therefore monitor the occurrence of high altitude aerosols, as shown by Sánchez-Lavega et al. (2018).Figure 2. Full disk of Mars as seen by VMC. OperationsSince 2018 VMC operations follow a similar routine as those used for other science instruments. The Science Ground Segment takes care of the Medium Term Planning (MTP) following the inputs from the science team. The science team performs the Short Term Planning (STP). Fig. 3 shows a typical VMC observation, which consists of a default image, followed by one to several loops of 6 images that use a set of predefined exposures. The exposure times are set to maximize the dynamic range of the final science products obtained by combining the individual images.Figure 3. Scheme of a typical VMC observation. Calibration of imagesThe images are calibrated following the standard scheme of subtracting a dark current and dividing by a flatfield image. The flat field correction is much more relevant than the dark correction in the quality of the final images after calibration. No onground calibration is known for VMC, therefore the dark current and flat-field corrections used are based only on in-flight observations. The dark was obtained by pointing VMC to the sky, specifically the area of Eridanus, where few bright stars are present.The flat-field was created using dark-corrected images of flat portions of Mars that were well and uniformly illuminated, as free as possible from large structures, and as flat as possible.Calibrated images are routinely archived at ESA's Planetary Science Archive (PSA), as described by Ravanis et al. (2020)Figure 4. VMC dark (left) and flat (right). GeometryThe original documents indicate the design parameters for the orientation of VMC in the reference frame of MEX, and for the pixel resolution (iFOV). However, the accurate parameters once VMC was mounted on MEX were never measured on the ground. In addition to this, we find that the timestamp of images suffers a random shift of a few seconds. As a result, we have 5 free parameters: 3 Euler angles for the attitude relative to the MEX reference frame; the pixel resolution (iFOV); and the shift in time from the actual timestamp to the labeled timestamp.In order to determine the attitude and iFOV of VMC relative to Mars Express, we used images showing stars. Many of these observations covered the stars of the constellation of Orion, because several suitable stars are present in that region of the sky. During these observations the spacecraft maintains a fixed attitude, therefore, the time related uncertainty is not present and only 4 free parameters remain: 3 Euler angles, and the pixel size. These parameters are shown in table 1.Table 1. VMC geometric parameters as given by the Flight User Manual (FUM) and calibrated values.The shift in time was estimated from observations showing Phobos. The relative speed of Phobos as seen from Mars Express is high, and therefore it is possible to use its position as an accurate clock. We find that our images are usually obtained between 6 and 13 seconds before the labeled time, but we find random variations. Subtracting 10 seconds is considered a good strategy in most cases, but this uncertainty remains as a limitation.Figure 5. VMC image showing the stars of Orion (left), and Phobos in front of Mars (right). Red circles represent the expected position before calibration. Green circles are the expected positions according to the new calibration. ConclusionsWithin the expectable limitations, the performance of this new instrument is very good and VMC is enabling novel science results and techniques (e.g. Hernández-Bernal et al. 2021). This is in part because VMC provides some capabilities that are not common among instruments in orbital planetary missions. Even with no on-ground calibration available, it has been possible to calibrate the camera, both photometrically and geometrically. Some hardware limitations remain, and others have been partially overcome with specially developed operational strategies. ReferencesHernández‑Bernal et al. (2021). A Long‑Term Study of Mars Mesospheric Clouds Seen at Twilight Based on Mars Express VMC Images.Ormston et al. (2011) An ordinary camera in an extraordinary location: Outreach with the Mars Webcam.Ravanis et al. (2020). From engineering to science: Mars Express Visual Monitoring Camera's first science data release.Sánchez-Lavega et al. (2018). Limb clouds and dust on Mars from images obtained by the Visual Monitoring Camera (VMC) onboard Mars Express.