Cassini Significant Event Report

For Week Ending 05/29/98

Spacecraft Status:


The Cassini spacecraft is presently traveling at a speed of approximately 135,000 kilometers/hour (~83,000
mph) relative to the sun and has traveled approximately 650 million kilometers (~403 million miles) since
launch on October 15, 1997.


The most recent Spacecraft status is from the DSN tracking pass on Thursday, 05/28, over Canberra. The
Cassini spacecraft is in an excellent state of health and is executing the C8 sequence 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. See the Huygens Probe
AGC Test, below, for a scheduled 12 degrees off normal HGA-to-sun line attitude period on 5/28.


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, 5/22, a memory readout (MRO) was performed of some data stored when the backup CDS
computer underwent a reset at the end of April. The MRO showed that the reset occurred at exactly the same
time as a switch in data modes from SAF142 (a high-rate data collection mode) and RTE40 (the standard
40bps realtime engineering mode). Analysis will indicate whether any additional MROs will be needed to
complete the investigation of the event.


From Saturday, 05/23 through Tuesday 05/26, there were no changes to spacecraft configuration.


On Wednesday, 05/27, 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.


On Thursday, 05/28, 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.


Also on Thursday, a Huygens Probe AGC Test mini-sequence was executed and the first of three data
playbacks occurred during the DSN pass over Canberra. This test turned the Orbiter 12 degrees off its
normal HGA-to-Sun line for 33 minutes hour and powered on the Probe electronics for 30 minutes to
investigate unexpectedly low AGC values measured across the Probe-to-Orbiter umbilical link during the
second inflight Probe checkout in 3/98.


The present hypothesis is that the lower AGC value was caused by higher solar noise during the second
inflight checkout relative to the first, perihelion occurring the same day as the second checkout. The injection
of noise thru the HGA feed has been demonstrated to cause a reduced Probe AGC value using the
Engineering model for test at the Huygens Probe Operations Center (HPOC) in Darmstadt, Germany.


Thursday's test will allow the Solar effect to be demonstrated or ruled out. Huygens Probe reports that the
first 7 minutes of the Probe AGC test data were transmitted to Earth during the DSN pass available for the
test. The preliminary analysis of this first data set indicates that the Huygens radio receivers perform as
expected in a radio noise free environment. The rest of the AGC test data will be transmitted to earth on May
30 and May 31. A full test report will be published when the complete data set will have been analyzed.


Upcoming events:


Activities scheduled for the week of 5/29 - 6/04 include Huygens Probe AGC Test playbacks #2 and #3 (of
3) (05/29 and 05/30), an AACS Attitude Estimator MRO (05/30), and an SSR Pointer Reset (06/03)


The 17th meeting of the Cassini Project Science Group will occur in Pasadena from 6/01 through 6/05. The
primary topics will be cruise operations of the science instruments and plans for the future, including plans for
Science Cruise (7/01/2002 to 7/01/2004) and Saturn Tour (7/01/2004 to 7/01/2008).


DSN Coverage:


Over the past week Cassini had 3 scheduled DSN tracks, occurring on 5/22, 5/24 (ranging only) and 5/28. In
the coming week there will be 4 DSN track periods.



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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