From: Jet Propulsion Laboratory
Posted: Tuesday, January 31, 2012
Capture of the telemetry data that is carried on Cassini's 1-way downlink signal, whose frequency is based on the Auxiliary Oscillator in the absence of an operable Ultrastable Oscillator (USO), continues to be normal. The most recent spacecraft tracking and telemetry data were acquired on Jan. 24 from the Deep Space Network 70 meter Deep Space Station 43 at Canberra, Australia. The Cassini spacecraft is in an excellent state of health with all subsystems operating normally except for the Cassini Plasma Spectrometer remaining powered off and the anomalous USO. Information on the present position and speed of the Cassini spacecraft may be found on the "Present Position" page at: http://saturn.jpl.nasa.gov/mission/presentposition/.
While the spacecraft started inward from apoapsis, Saturn and Titan were again observed with the Optical Remote Sensing (ORS) instruments. The Cassini Infrared Spectrometer (CIRS) measured oxygen compounds (H2O, CO2) in Saturn's stratosphere as a function of latitude, and the Imaging Science Subsystem (ISS) measured winds. The Ultraviolet Imaging Spectrograph imaged Titan and Saturn in the extreme- and far-ultraviolet parts of the spectrum, and ISS led a joint-ORS photometric measurement of Saturn known as an emission-angle scan. CIRS acquired a Saturn mid-infrared map to determine upper troposphere and tropopause temperatures. On Tuesday, ISS observed the small, irregular moon Siarnaq from approximately 26 million kilometers away. Preparations were made for the 56th Project Science Group meeting to be held next week at JPL.
Wednesday, Jan. 18 (DOY 018)
Today the Spacecraft Operations Office opened Incident-Surprise-Anomaly report (ISA) #51031, "Z-Sigma Spike on 2012-013" to document an incident that occurred on Jan. 13. In Cassini's Attitude & Articulation Control Subsystem, the Z-sigma ratio compares the measured brightness of a star in the Stellar Reference Unit (SRU) to that expected. It reached a high value of 11, and persisted for 41 realtime interrupts (1/8 second each) during a rotation of the spacecraft. There were no bright bodies in the SRU field of view at the time, which is usually the cause for Z-sigma excursions. The excursion was not sufficient to trigger a fault response, and analysis of the event is continuing.
The Encounter Strategy Meeting for Titan encounters T-81 and T-82 was held today.
Thursday, Jan. 19 (DOY 019)
This week, the Navigation team has been re-converging Cassini's trajectory solution based on tracking data acquired after execution of Orbit Trim Maneuver #306.
Friday, Jan. 20 (DOY 020)
The S72 Background Sequence Part 1 was uplinked today. Its 6,743 timed commands will execute over a period of 37 days. A Reaction Wheel Bias activity for DOY 024 was uplinked separately.
As part of the investigation of ISA #50901, "Loss of Cassini spacecraft 1-way Downlink Signal", the Ultrastable Oscillator (USO) was enabled today for about an hour (recall the USO was power cycled Jan. 9). Deep Space Station 43 in Canberra, Australia, was tracking Cassini, but observed no normal signal from the USO. The Radio Science team observed a wide swath of the radio spectrum at X-band and S-band frequencies using Station 43's Radio Science Receiver, and their results were as expected for a nonfunctioning USO. There was no loss of science telemetry data since the playback had been commanded to pause.
Sunday, Jan. 22 (DOY 022)
Cassini's X-band radio receiver was utilized today in a test of the Deep Space Network's uplink polarization isolation. This test was designed to troubleshoot problems experienced with the New Horizons Spacecraft Radio Science Experiment in 2011 (New Horizons encounters Pluto in 2015). The uplink from Station 14 at Goldstone, California, was configured four ways: right- and left-hand circular, each with and without the dichroic plate (a frequency-band separating device) extended into the microwave path. The test provided data that, along with design analysis of the dichroic plate, will be used to characterize DSN performance.
Monday, Jan. 23 (DOY 023)
The feature "Cassini Sees the Two Faces of Titan's Dunes" was published on the Cassini public website: http://saturn.jpl.nasa.gov/news/cassinifeatures/feature20120123/
Tuesday, Jan. 24 (DOY 024)
Background command sequence S71 completed execution today. The start of S72, expected at 4:14 p.m. PST Earth-receive time, could not be verified because of a shortened tracking station allocation.
Contingency S72-deactivate commands were approved and placed on the shelf, as standard preparation in case of an anomaly. The Attitude and Articulation Control Subsystem executed a Reaction Wheel Bias activity while Cassini's High-Gain Antenna was off Earth-point.
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