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DORISMail               15-Jun-2004 13:23:33                   Message No 0320
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Author: Granier Jean-Pierre <Jean-Pierre.Granier@cnes.fr> 
Subject: TOPEX last information 04/06/15 


DATE:  15 June 2004, 1600 PDT


PROJECT:  TOPEX/Poseidon (T/P)


DATE OF INCIDENT:  26 May 2004


LOCATION OF INCIDENT:   In Earth Orbit


DESCRIPTION OF INCIDENT:


The T/P satellite entered a sun-pointing safehold mode (SHM) on 26 
May 2004 at 18:58 UTC due to a roll reaction wheel (RW) failure. 
Although the three remaining RWs are functional, all have been 
subject to intermittent halts when passing through zero wheel-speed 
crossings.  A s/w patch developed a priori to detect and correct this 
problem was only partially successful.  On June 1 at 11:16 UTC, an 
erroneous hardware interrupt caused the On-Board Computer (OBC) to 
halt.  The computer memory was reloaded and reinitialized on June 3, 
and nominal OBC functions were subsequently restored.  SHM operations 
have continued, and options to return to a nominal science data mode 
were assessed.


CORRECTIVE ACTION:


Modifications to the OBC s/w restart patch were implemented based on 
characterization and analysis of the wheel halts.  On June 11, T/P 
was successfully transitioned from the Sun-Pointing SHM to an 
Earth-Pointing computer mode using the OBC controller.  The modified 
wheel s/w patch has functioned well since the transition point, and 
the NASA altimeter has been powered up and calibrated.  Analysis of 
the RW failure mechanism and an estimate of remaining wheel lifetime 
is currently in progress by teams from both JPL and the T/P Guidance 
and Navigation Control contractor Swales Aerospace, with additional 
support from Honeywell (wheel contractor).


IMPACT ON PROJECT AND SCHEDULE:


T/P continues to operate well in a computer-controlled Earth pointing 
transitional mode, with the skew RW substituted for the failed roll 
wheel.  A return to the normal mission science mode is planned for 
June 17-18, and it is anticipated that just over two (ten day) cycles 
of mission science data will be lost as a result of the wheel 
anomalies.



=========================================
Mark Fujishin
NASA TOPEX/Poseidon Project Manager
NASA Jason-1 Mission Manager
Jet Propulsion Laboratory


ph: (818) 393-0573
e-mail: Mark.D.Fujishin@jpl.nasa.gov ========================================


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