Cassini Significant Event Report

For Week Ending 05/15/98

Spacecraft Status:


The Cassini spacecraft is presently traveling at a speed of approximately 142,000 kilometers/hour (~88,000
mph) relative to the sun and has traveled approximately 607 million kilometers (~377 million miles) since
launch on October 15, 1997.


The most recent Spacecraft status is from the DSN tracking pass on Wednesday, 05/13, over Goldstone.
The Cassini spacecraft is in an excellent state of health and has successfully completed execution of the C7
sequence. The C8 sequence is now on board and is executing nominally. The speed of the spacecraft can be viewed on the "Where is Cassini Now?" web page (http://saturn.jpl.nasa.gov/operations/present-position.cfm)


Inertial attitude control is being maintained using the spacecraft's hydrazine thrusters (RCS system). The
spacecraft continues to fly in a High Gain Antenna-to-Sun attitude. It will maintain the HGA-to-Sun attitude,
except for planned trajectory correction maneuvers, for the first 14 months of flight.


Communication with Earth during early cruise is via one of the spacecraft's two low-gain antennas; the antenna
selected depends on the relative geometry of the Sun, Earth and the spacecraft. The downlink telemetry rate is
presently 40 bps.


For the next several months, due to increasing range from Earth and relatively high angles with respect to the
Low Gain Antenna boresight, the DSN tracking passes for Cassini will be dedicated either to command and
telemetry (for spacecraft activities and health monitoring) or to Navigation ranging data (for orbit
determination). This approach manages the available telecommunications signal strength, directing it to either
command/telemetry or Navigation, according to plan. Telecommunications performance will improve again
this Fall.


Spacecraft Activity Summary:


On Friday, 05/08, Stellar Reference Unit A (SRU-A) Decontamination activities were successfully concluded.
Decontamination results were very similar to those observed on SRU-B when its radiator was
decontaminated in mid-March. SRU-A was powered off at the end of the activity; SRU-B remains on as the
prime unit.


Also on Friday, an SSR double bit error (DBE) was repaired in a non-software portion of one of the Flight
Software (FSW) partitions using the standard "partition copy" procedure. This procedure is scheduled
whenever the sequenced DBE repair, which only repairs DBEs in the FSW-containing portions, identifies a
DBE in the unused portion of the FSW partition. Such a DBE had been identified during the SSR partition
maintenance activity which occurred on May 1.


Finally on Friday, the Radio & Plasma Wave Science (RPWS) Instrument was turned off following
confirmation of the successful playback of the RPWS Venus-1 flyby data. Data had been preserved in
instrument memory, until its receipt on Earth had been assured, by keeping the RPWS in the "sleep" state from
April 26 till May 1. Results of the Venus-1 observations will be reported separately.


On Saturday, 05/09, there were no changes to spacecraft configuration.


On Sunday, 05/10, the Cruise 8 sequence began executing on board the spacecraft.


On Monday, 05/11, the Solid State Recorder (SSR) record and playback pointers were reset, according to
plan. This housekeeping activity, done approximately weekly, maximizes the amount of time that recorded
engineering data is available for playback to the ground should an anomaly occur on the spacecraft.


Also on Monday, a housekeeping activity was performed which reads out a set of AACS Attitude Estimator
(ATE) measurements not available in regular engineering telemetry. This ATE telemetry allows ground
controllers to track the normal functioning of the attitude estimator software on the spacecraft. The readout is
scheduled approximately every 2-4 weeks, over an available DSN telemetry pass.


On Tuesday, 05/12, there were no changes to spacecraft configuration.


On Wednesday, 05/13, the Solid State Recorder (SSR) record and playback pointers were reset, according
to plan.


Also on Wednesday, a maintenance activity was performed on the SSR Flight Software Partitions. This
activity, performed approximately every 2 weeks, repairs any SSR double bit errors (DBEs) which have
occurred in the code- containing portions of the Flight Software partitions during the preceding period.


On Thursday, 05/14, there were no changes to spacecraft configuration.


Upcoming events:


Activities scheduled for the week of 5/15 - 5/21 include an SSR Pointer Reset (05/20).


DSN Coverage:


Over the past week Cassini had 4 scheduled DSN tracks, occurring from 05/08 through 5/13. In the coming
week there will be 2 DSN passes.



Additional information about Cassini-Huygens is online at http://saturn.jpl.nasa.gov.


Cassini will begin orbiting Saturn on July 1, 2004, and release its piggybacked Huygens probe about six months later for descent through the thick atmosphere of the moon Titan. Cassini-Huygens is a cooperative mission of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Office of Space Science, Washington, D.C.


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