Scale Factor

Scale factors are used to correct the received data for anode degradation. They vary with time, MCP level and anode. Scale factors are provided as a table, with a single value for each anode at every possible operating level (40 to 64) on each day of the mission.

Scale factors should be applied to the counts/sec data before any other calculations are made.

A gain test provides a curve of counts received versus MCP operating level for each anode at a particular point in time. The inflection point of each gain test curve marks the threshold value, in operating levels, of the anode. The increase in threshold value over time is a proxy for anode degradation.

It is important to note that the anodes only degrade when the instrument is switched on, so account must be taken of periods when it is not. Gain tests occur every 15 days whenever possible, but are sometimes missed due to operational concerns or data loss. Line-fitting to a plot of threshold by time allows extrapolation to values yet to come. Since the launch of Cassini, four anode degradation regimes have been identified. Rapid degradation occurred between launch and the beginning of the cruise phase. Very little degradation occurred during cruise phase, as electron activity was low and the MCPs were not operating at optimal level. Degradation rose as Cassini neared Saturn and began orbit, but the rate of degradation slowed after two years in orbit. Each of the four regimes is represented by a straight-line best fit to the thresholds, with the last line extrapolated to the end of mission. This schema may be changed if a new degradation regime is entered.

To counter the degradation in flight, the MCP operating level is raised approximately once per year. This produces a saw-tooth effect on a plot of effective degradation. Note that since the anodes degrade at different rates, but only one MCP setting is applied to all of them, that setting has to be a compromise to get the best response across all the anodes.

The scale factor is the correction needed to calculate the number of counts that would have been seen during on-ground pre-flight calibration from the number seen in flight, from which an absolute number of counts can be found. The scale factor is given as a divisor to the value seen.

corrected data (count/sec) = received data (count/sec) / scale factor