Software to plan Mars Express mission orbit by orbit
25 Jul 2000Scientists building instruments for Mars Express are taking delivery this month of a software package to help them plan how and when to operate their instruments during each of the 2293 orbits the spacecraft will make during its first 687 E arth-days of operation. "The mission has to be planned orbit by orbit: there's no other way. STAT (Science Timeline Analysis Tool) is a very comprehensive software package for planning science operations," says Agustin Chicarro, Mars Express project scientist.
STAT simulates the conditions that the spacecraft will encounter during each orbit. It has been developed by the Mars Express Ground Segment team at ESOC, ESA's Mission Operations Centre in Darmstadt, Germany. "The observing conditions of any one orbit are different from any other. When the spacecraft is close to Mars, sometimes it's morning, sometimes it's afternoon and sometimes it's night," explains Chicarro.
Each of the seven instruments on board the Mars Express orbiter has different ideal viewing conditions. Top priority for the radar (MARSIS), for example, is nighttime operation; for the camera (HRSC) it is mid-morning or mid-afternoon when the shadows are neither too long nor too short. Some instruments also require special pointing conditions. SPICAM and the radio science experiment, for example, want to spend some time looking across the limb of Mars's atmosphere and observing the Sun and stars during occultation; ASPERA wants to point towards the centre of the planet for some entire orbits; and the HRSC sometimes wants to rotate the spacecraft from side to side slightly to obtain stereo imaging at high resolution.
As well as simulating illumination conditions, the software also takes into account how much data can be returned to Earth from each orbit. Mars Express will normally spend the part of its orbit when it's closest to Mars observing the red planet. During the rest of the orbit, it will typically point towards Earth for data transmission back to an ESA ground station in Perth, Australia. STAT will allow the observation versus data transmission time to be optimised for each orbit.
It is not always possible, however, to return the same data volume back to Earth. The distance between Mars and Earth varies as the two planets proceed along their celestial paths. When they are relatively close together, Mars Express can return more data than when they are far apart.
At most Mars-Earth distances, the data rate will be enough to meet the needs of all the instruments. For the occasions when it is insufficient, the available data volume will be shared optimally between the instruments depending on their observation needs and the phase of the mission. On average, the HRSC will take the lion's share at 40% and the Beagle 2 lander will take just 2%. Beagle 2 will have data storage capacity on board allowing the lander to send its data to Mars Express just once every orbit as the spacecraft passes overhead. "Beagle's share sounds very little, but it's enough," says Chicarro.
"The combination of a variable data rate and changing illumination conditions means that we have to work out a detailed plan for each orbit in advance. So we developed STAT which allows the instrument scientists to ask what orbit 323 looks like, for example, and choose how they'd like to operate their instrument," says Chicarro. As well as simulating observing conditions and spacecraft-to-Earth data rate, STAT also simulates resources available on board such as battery energy and data storage capacity.
The instrument scientists now have the task of working through each orbit and sending their requests back to the Mars Express Science Team at ESTEC. "We will then reconcile the requests and propose a feasible scenario. The aim is to come up with a master plan which defines the science operations for each orbit," says Chicarro. Once the spacecraft is in orbit, the master plan will be converted weekly into operations that define when to slew the spacecraft and when to switch each instrument on and off. The operations will be passed to the Mission Control Centre at ESOC for inclusion in the final mission planning, which will be sent as commands to the spacecraft.