Jet Propulsion Laboratory

Collection of Raw Images

Solid State Recorder (SSR) Description

Like many summer vacation travelers, Cassini spends the majority of its day observing its new environment and a shorter amount of time calling home to discuss it. Cassini spends about 15 hours each day actively observing and conducting science measurements, and it has only nine hours to return the information to Earth during downlink sessions. The spacecraft sends an average of 500 raw images during its daily conversation with team members here on Earth, but the short "call home" may not offer enough time to transmit all the information collected. Often times, the spacecraft will not complete the data transmission for a couple of days.

Cassini stores its data on the solid state recorder, turning towards Earth to send its information nearly a billion miles through space back to NASA's Deep Space Antenna Network during downlink sessions.

Regardless of time restraints, team members receive great amounts of data from Cassini each day and post the information on the public website as efficiently as possible.

Gathering the Information

Because Cassini's high-gain antenna must be fixed on Earth during downlink sessions, the spacecraft does not have the flexibility to point its cameras at targets of interest during a downlink. All remote sensing measurements (e.g. images of Saturn, its rings or moons) must be conducted during observation periods, where the spacecraft is free to point its cameras at the targets of interest.

Other measurements that do not require the spacecraft to point at a target can be conducted during both observation and downlink sessions. Measurements such as fields, particles and waves measurements only require the spacecraft to pass through the region of interest without actually pointing at a target. These measurements study the magnetic field of Saturn and "listen" for lightening in the atmosphere.

In the past, NASA spacecraft have had scan platforms (moving platforms) which allow remote sensing instruments such as cameras to take images while simultaneously transmitting that data to Earth. Faced with budgetary cuts before launch, Cassini team members discarded the scan platform option. Without the scan platform, Cassini engineers had to bolt the remote sensing scientific instruments to fixed positions on the spacecraft. As a result, the entire spacecraft must first point to its target, then point back to Earth to transmit data.

How Cassini Stores Information

Cassini stores the gathered information on two Solid State Recorders (SSR) with a combined capacity of 4 gigabits, about the volume of a compact disk. (While communication is often measured in bits, or binary digits, storage is commonly measured in bytes. One byte contains eight bits). Cassini does not always record 4 gigabits of observation data, and depending on antenna availability at the Deep Space Network, the downlink does not always allow the spacecraft to empty the recorders. The recorders often store data for one or more days until the team members can send it safely down to Earth.

For most high-priority observations, such as a close satellite flyby, the spacecraft typically gathers measurements for 15 hours. Some or all of those measurements will downlink during a nine-hour tracking pass, but Cassini may not begin transmitting the data to Earth until eight or more hours after the encounter. Therefore, the spacecraft may not be able to return all of the data to Earth on the first tracking pass. The complete, high-priority package of data may not reach the ground until after the downlinks on following days.