Cassini Significant Events 11/17/05 - 11/21/05

November 25, 2005

(Source: Cassini Project)


The Significant Events reporting period has been shortened due to the Thanksgiving Holiday. This week the report will cover from Thursday, November 17, through Monday, November 21. Next week will then pick up on Tuesday, November 22 and continue through Wednesday, November 30. Similar adjustments will be made to the reporting period around the Christmas Holidays.

Thursday, November 17 (DOY 321):

Flight software (FSW) normalization was performed for both the Imaging Science Subsystem (ISS) and Cosmic Dust Analyzer (CDA). The process of normalization involves placing identical copies of the software on all four partitions on the SSR, then performing memory readouts to verify all copies are correct. When an instrument team has a new version of FSW, it is uploaded into two of the partitions. The SSR then contains two copies of the new version and two copies of the old version. The old version is retained until the instrument team performs a checkout and is satisfied with the performance of the new software. Then the new version replaces the old version in the normalization process.

A Public Outreach image of Tethys and the rings was taken by ISS today. The Cassini program maintains a list of candidate opportunities for "aesthetically pleasing" images. It is expected that on average, about one image per sequence may be implemented, totaling a few dozen over the length of the tour.

Cassini has been notified that DSS-15, currently down for upgrades and modifications, will not be returning to service as scheduled. A new active date of December 4 is currently planned. There are two passes between now and the 4th where Cassini was scheduled to utilize DSS-15. The first is Monday, November 21, and the second Wednesday, November 30. DSS-24 has been provided by the DSN as an additional antenna, but for a much shorter period of time on both days. DSS-15 remains allocated to Cassini but as non-committed performance. Since DSS-24 is not a High-Efficiency DSN Station, it is unable to support the same data rates as DSS-15. Commands will be uplinked on Saturday morning to modify the telemetry rates as necessary. The current plan is that on these two days, we have DSS-24 prime for approximately the first 5 hours, with DSS-15 in parallel and continuing tracking approximately 4 hours longer than DSS-24.

The purpose of the changes in telemetry is to lower the downlink data rates to a level that is supportable by DSS-24, and then raising the rate to 47kbps to play back enough data so as not to overfill the SSR once DSS-24 has ended track. If the SSR were overfilled it would record over a following observation period. Telecom team predicts show that 47kbps is aggressive but should be within the capability of DSS-15 provided conditions are good. Therefore, the program has asked that DSS-15 track at 47kbps on a best efforts basis.

In addition to the adjustments to be made next week and the week after, the Stardust project has requested additional 70M coverage on DOY 014, 2006 in support of Level 1 Critical activities for their sample return, and backup 70M support for their final targeting maneuver. This unfortunately conflicts with the front end of the Cassini Goldstone 70M pass on DOY 014 in S17. Because of this, Science Planning has requested that the instruments cut 125 MB from the sponge bits allocated to the Cassini Plasma Spectrometer (CAPS), CDA, Magnetometer Subsystem (MAG), and Radio and Plasma Wave Science (RPWS). Those teams are currently in the process of redoing their data allocation.

Friday, November 18 (DOY 322):

An all-day Titan Orbiter Science Team (TOST) workshop was held today. Members considered the "Safe Atmospheric Densities for Low-Altitude Titan Flybys" work completed by the Spacecraft Operations Office ACS team. They are considering ways to retain flexibility in the tour design, using the scenario-specific results from ACS. Recommendations from TOST will be passed to the program prior to the start of the redesign activity scheduled for early December.

The Rhea flyby occurring on Saturday, November 26, is the closest with Rhea in the tour. As such, Science has requested that a contingency playback plan be implemented just in case there is a hitch in nominal playback. Most of the Rhea data is downlinked during the prime OTM-44 pass which begins at 3:27PM on Sunday Nov 27th and runs until 12:37AM Monday. If there is a playback gap during this pass, the Satellite Orbiter Science Team (SOST) is willing to zero out data policing tables of the following observation period in order to preserve Rhea data on the SSR. The Goldstone 70m pass on the next day will then fully empty this data from SSRs. All commands will be built in advance, and the ACE will monitor the Sunday pass for any outages in excess of 10 minutes that occur no later than three hours before end of track. If such an outage occurs, the contingency plan will be implemented. If the contingency commands are uplinked, no science data is collected on following observation period. SSRs are marked full and are fully played back over next downlink pass. The sequence then continues as planned.

Saturday, November 19 (DOY 323):

Spacecraft periodic engineering maintenance was performed today consisting of Backup Assisted Load Format Injection Loader (BAIL) maintenance, an Engine Gimbals Actuator (EGA) exercise, and a backup Reaction Wheel Assembly (RWA) exercise. This activity scrubs the memory in the BAIL and "exercises" mechanical parts for the EGAs and RWA.

Commands were uplinked to modify the telemetry rates for DSS-24 and DSS-15 on DOY 325.

Monday, November 21 (DOY 325):

Very good news! Last week, DSS-15's pass for today was declared a "Return to Service Demo" instead of committed support. DSS-24 was brought in to the schedule to provide committed support, and spacecraft data rates were commanded to lower values to accommodate DSS-24's lower capability, for the period of its support. After DSS-24's end of track, the commanded rate went up to 47 kbps, which was marginally outside the predicted link capability, meanwhile preserving SSR playback pointer progress per original S16 plans. As it turned out, DSS-15 was able to achieve and maintain lock on telemetry throughout the pass at the 47 kbps rate. There was little to no loss to science data.

Wrap up:

The most recent spacecraft telemetry was acquired Monday, November 21, from the Goldstone tracking complex. The Cassini spacecraft is in an excellent state of health and all subsystems are operating normally. Check out the Cassini web site at http://saturn.jpl.nasa.gov for the latest press releases and images.