Cassini Update - November 23, 2005

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

The most recent spacecraft telemetry was acquired Monday, November 21,
from
the Goldstone tracking stations. The Cassini spacecraft is in an
excellent
state of health and is operating normally. Information on the present
position and speed of the Cassini spacecraft may be found on the
"Present
Position" web page located at
http://saturn.jpl.nasa.gov/operation...t-position.cfm .

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:

Check out the Cassini web site at http://saturn.jpl.nasa.gov for the
latest
press releases and images.

The Cassini-Huygens mission is a cooperative project of NASA, the
European
Space Agency and the Italian Space Agency. The Jet Propulsion
Laboratory, a
division of the California Institute of Technology in Pasadena, manages
the
Cassini-Huygens mission for NASA's Science Mission Directorate,
Washington,
D.C. JPL designed, developed and assembled the Cassini orbiter.