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CaSSIS Operations and Planning

Hello Planetary Pals,

Over the past few weeks I’ve had the unique opportunity to work intimately with one of the main instruments that my research is based on. The Colour and Stereo Surface Imaging System (CaSSIS) onboard the European Space Agency’s Trace Gas Orbiter. I use this instrument in my research to capture color images of the Martian surface in search of a clay-bearing unit southeast of Valles Marineris, and it was interesting to have experience with operations for the instrument. Working with Dr. Livio Tornabene’s team, I was involved in planning for this instrument, with the objective of identifying and verifying scientifically intriguing opportunities over a span of revolutions. These images will eventually be taken in November.

ESA’s CaSSIS team divides groups of revolutions in four-week spans called Medium Term Plans (MTP) and further subdivides each MTP into four individual weeks called Short Term Plans. ESA’s CaSSIS team then tasks individuals (or small groups in our case) to be invoved with planning for two STPs. For our planning stint, we were working with CaSSIS’s 47th MTP and and STPs 187 and 188. The orbiter revolves 83 times over the span of 1 week’s STP, which means we needed to find at most 166 targets for the spacecraft.

The first step to tackle was to figure out some engineering/instrument constraints. We needed to figure out which latitudes the spacecraft can operate in to succesfully capture images with the proper lighting conditions. The CaSSIS engineers provide a chart called a Beta Angle Evolution graph which essentially shows the lighting condiotions over an MTP. The lower the beta angle is, the better the lighting conditions are. As seen in the plot, in our STPs (2021-11-13 to 2021-11-27), the ligting conditions reach near perfect before slowly worsening. Overall, however, the lighting conditions for our orbits were superb.

The next chart to consider is the Aligned Motor Position graph. The charts show a color scale that depicts the incidence (for capturing the best images, low incidence is ideal). Since we desire to pick the best incidence conditions, we must set the instrument’s motor rotation angle to that perfect condition. To complicate matters, due to motor failure under angles of 100 degrees, we can only set a rotation angle above this, which can limit the capabilities of the instrument. For STP 187, the ideal incidence occurs if we set a motor angle of around 275 degrees. From this set motor position, there is roughly +- 30 degrees of from this angle that the instrument can image in. This means we are only able to image latitudes of roughly 0 to 50 N. In actuality, Livio made the executive decision to use a different central motor angle in each STP since he wanted to image southern hemisphere targets. So instead, we sacrifieced the ideal lighting conditions in favor of more flexibility for targets in the southern hemisphere – we set the motor angle for this STP to 295 degrees, which allowed us to image latitudes from -25 to 25 N.

Possible stereo coverage for each STP is seen in another chart. For specific motor angle positions, stereo images can only be taken for specific latitudes and longitudes.

After this is done, we reported our desired motor angle to the ESA engineer team who then provided us with state files for each STP which can be used to visualize the orbits. It also contains information about which orbits are to be considered “exclusion zones” where no images can be taken due to instrument constraints, tests, or maintenance.

To visualize all of this information, we use a program called PLAN-C, which is based off of JMARS. PLAN-C was very intensive to install, and required the use of a mac, which meant I needed to spend the planning weeks on campus using one of the lab’s macs. After a lot of trouble, Livio and I were able to finally get PLAN-C working. In PLAN-C, each STP orbit can be visualized, images that have already been aquired can be shown, HiRISE, CRISM, and other contextualizing instruments can be loaded, exclusion zones and other engineering constraints can be visualized, and possible targets can be shown.

Anyone on the CaSSIS team can add sites of interest that they want to have imaged to a database called CAST which is loaded into PLAN-C. An automated process will pick out images requests that are imageable for our specific STP orbits. While this sometimes does a good job, it was our main duty to verify the scientific value of these targets, and if necesssary choose better, more interesting targets. If a more interesting target along an orbit was found, the old suggestion would be replaced. If a target was especially intriguing, we could take a pair of images to create stereos, though we were limited to just two stereo opportunities per STP. We kept track of the targets we wanted to image in a google doc to keep things organized. We’d fill up each orbit to the maximum possible amount of images dictated by the data volume, though our STPs were in a particulary low data volume time; if the data volume were higher, we would be able to take many more images, but we were limited, and even had to leave some orbits completely empty.

After all targets were identified we needed to add a footprint on each target which gave the overall length (determined by number of exposures needed to fully capture the target) and width (based on the combination of PAN-RED-NIR-BLU filters) of the footprint. For morphologic targets, we used a color combination of PAN-NIR-BLU and for all other targets we used the full filter combination.

After a number of checks and a few emails back and forth with the ESA team, our targets were passed as complete, closing this first portion of planning. Now that the ESA team has our targets, the engineers will doubly verify them over the span of many weeks. By mid-October, the next phase of planning will commence.

Though the process was sometimes confusing, I feel like this was a great learning opportunity for understanding instrument operations. If I have enough confidence and time, in the future I should be able to sign up to do another planning stint all on my own. And hopefully once these images are taken and processed, I will be able to show you them in a future blog post!


Published by Anthony Dicecca

Hello and welcome to my blog. I am Anthony Dicecca, and I am currently pursuing a thesis-based Masters degree in Geology with a Specialization in Planetary Science and Exploration. I am a native of Rochester, New York but moved to London, Ontario to attend the University of Western Ontario. From 2016 to 2020 I worked to complete my undergraduate degree, finishing with a BSc in Physics and a BSc in Geology. During this time I developed a passion for geology, and in particular, planetary science. I've had the pleasure of working with Dr. Gordon Osinski and his team during this time aiding in research ranging from Arctic peri-glaciology to global impact cratering, and from Lunar spectroscopy to Martian mapping. In Autumn 2020 I continued my education at the U.W.O., working towards a MSc in Geology with a Specialization in Planetary Science and Exploration. My research will likely involve insights obtained from the Holuhraun Lava Field in Iceland and their applications to other bodies in the Solar System. This blog serves as an archive of my progression over the next few semesters.

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