Cassini CIRS post launch operations support

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Driving the Cassini Spacecraft. In July 2004 the Cassini spacecraft arrived at Saturn and began a four year detailed study of the planet, its spectacular rings and extensive system of moons. The sub-department of Atmospheric, Oceanic and Planetary Physics at the University of Oxford helped to build one of the instruments mounted on Cassini, the Composite Infrared Spectrometer (CIRS). CIRS records the characteristic heat signature emitted by objects in the form of an infrared spectrum. Using this data, we are able to determine the composition and temperature of the atmospheres of Saturn and its giant moon Titan (the only known moon in the Solar system to have a substantial atmosphere) and the surface properties of the rings and other satellites (which are mainly mixtures of rock and ice). How do we actually make these measurements? How do you control a spacecraft from a typical distance of 1.4 billion kilometres? Firstly, you have to get commands to the spacecraft ('Uplink') and then you have to get the results of your measurements as well as information about the spacecraft back to Earth ('Downlink'). Since the beginning of the mission, the group at Oxford have been involved in both uplink and downlink. The uplink process starts with turning observations designed by the science community in to commands that are used to turn Cassini to point at a target. For example, a typical observation request may say 'Point CIRS at Titan and scan up and down'. There are twelve instruments on Cassini, so these observations are made in co-ordination with all the other instrument teams. To help with this process, Cassini's orbital tour is divided into sections with each instrument allocated a block of time, based on the observations requirements, there is no point being allocated time to look at Titan if it is hidden behind Saturn! NASA's Jet Propulsion Laboratory (JPL) in Pasadena handles this part of the planning process, as they have overall responsibility for the Cassini mission. The instrument teams then have to use their allocated time to make the best possible observations. Observation designs are turned in to spacecraft commands and tested using a software tool provided by JPL. This simulates Cassini making the measurement to check that it does not do things that may harm the spacecraft. This includes such things as turning too quickly or pointing sensitive parts (e.g. the radiator to cool down CIRS) at hot objects such as the Sun. Once the design has been completed and passed these checks, it is sent to JPL for further testing. Finally, it is sent to Cassini using NASA's Deep Space Network (DSN). The DSN is a network of three ground-tracking sites in Spain, America and Australia. As you might expect, to communicate with a spacecraft as far away as Cassini requires a dish quite a bit bigger than a digital TV mini-dish / for example the main dish at the site in America is 70m in diameter! Our role at Oxford has been working with the instrument operations team for CIRS (based at NASA's Goddard Spaceflight Center) to provide designs mainly for Titan. So we actually drive Cassini a bit! With the observation design turned into real spacecraft commands and loaded on to Cassini, we now need to get the data returned (downlinked) to Earth. Cassini has a large dish mounted on one end to send the data back to Earth. However, because of the very large distances involved it needs the very sensitive dishes of the DSN to be able to receive the transmissions. Once the data is picked up by the DSN it is then distributed by JPL to each of the instrument science teams to be unpacked and then analysed. The group at Oxford provide software that takes the raw information received by the DSN and then formats it so that it can be calibrated (the raw numbers turned in to meaningful spectra) by the team at Goddard. The data is then ready to be analysed by the rest of the CIRS team, and the wider scientific community.