TRACE Eclipse Season Operations

The purpose here is to document the activities associated with TRACE spacecraft transition to eclipse season.

TRACE enters eclipse each year in early November, peaking about 12/20-23, and ending in early February. Future predicts are below.

Entry Exit

11/6/99 2/12/00

11/5/99 2/11/01

11/5/01 2/08/02

11/2/02 2/08/03

Since the initial design of TRACE did not call for eclipse season operations, approval from NASA management was sought for the 1998/99 eclipse season. It was agreed that TRACE has sufficient power throughout eclipse season to support the instrument however a verbal agreement was made with the FOT to re-examine operations should the battery state of charge (SOC) drop beneath 80%. TRACE safing functions will turn the instrument off at a battery voltage of 25.3 V or a state of charge (SOC) of 70%.

It was also agreed that the EOF would supply the FOT with timelines only three days/week to reduce the workload on the FOT.

Through the creative use of timeline, Command Management System (CMS), and spacecraft software, we are able to automate the observing process through eclipse transitions. There is some manipulation however at the beginning and end of eclipse season. All processes are described below.

The process begins when the timeline inserts the following syntax into the rego files:

IF (FULLSUN) THEN AT START DO

+000:00:07:00 ACT ECLIPSEEXIT;

IF (FULLSUN) THEN AT STOP DO

-000:00:06:00 ACT ECLIPSEENTRY;

It seems backwards but works. The Flight Dynamics predicts are listed in terms of "Full Sun" entry and exit rather than "Eclipse." The delta times were selected to cover the deepest Atmospheric Absorption periods which encompass the eclipse periods.

The CMS will insert the activities relative to the FULLSUN entry and exit times based on the FDF predicts. The activities consist of "Start RTS" commands which are executed on the spacecraft. The purpose here is to save space in the daily ATS (absolute time sequence) command load by defining spacecraft RTS (relative time sequence) command loads.

Activity ECLIPSEENTRY = "SCRTSSTART RTSNUM=56"

Activity ECLIPSEEXIT = "SCRTSSTART RTSNUM=57"

The purpose of the RTS command loads is to pause the sequence upon eclipse entry, allow two minutes for the sequence to finish any active exposures, and then point the telescope to sun center. By pointing to sun center, the diodes will have the best chance of being illuminated upon eclipse exit and allowing an easier return to Fine Sunpoint Mode. The sequence pauses are accomplished through the use of global sequence register 28 while re-targeting uses register 29. You will also notice a command to disable Action Point 44. This action point was defined in 1998 to pause the sequence indefinitely via hard command if the spacecraft fell from Fine (FSP) to Inertial Sunpoint Modes (ISP). During full sun operations, a failure to ISP would be an anomalous condition and the sequencer should be stopped. Since this will occur every orbit during eclipse season, it is disabled during the eclipse times.

RTS 56 - Eclipse Entry

00:00:00 LCAPSTATE AP=44, DISABLED (disable action point 44)

00:00:00 ICSQREG REG=28, VAL=1 (pauses sequence code via global register 28)

00:02:00 ICISOPNL (open ISS loop)

00:00:01 ICGTWCNFR WEDGE1=45, WEDGE2=135 (commands wedges to sun center)

RTS 57 - Eclipse Exit

00:00:00 ICSQREG REG=29, VAL=1 (uses global register 29 to retarget upon exit)

00:00:01 ICISCLSL (closes ISS loop

00:00:01 ICSQREG REG=28, VAL=0 (resumes sequence via global register 28)

00:00:01 LCAPSTATE AP=44, ACTIVE (enable action point 44)

Upon eclipse exit, the spacecraft will transition automatically to FSP when one or more of the diodes are illuminated and the digital sun sensor (DSS) has seen sun for 60 seconds. This is accomplished via Action Point #39 and RTS #24.

RTS24: Enter Fine Sunpoint Acquisition Mode

WATCHPOINTS sampled:

DSS Settle Flag

Guide Telescope Diode Mask

ACTIONPOINT (SUNP_TRANS) logic:

If ( DSS Settle Flag == 1 ) AND

(any of 4 bits in the Guide Telescope Diode Mask == 1) Execute ACSFSP24

file: ACSFSP24.atf

00:00:00 AFINESUNPT ENABLE

There exists a period of atmospheric absorption about 3 weeks on each side of eclipse season. The absorption is significantly noticeable in the EUV wavelengths about 2 weeks prior to eclipse entry and 2 weeks post season. During these events we pause the sequencer only. Similar to the above eclipse process and activity had been defined on the CMS and is called out via timeline in the rego files.

IF (AAZ) THEN AT START DO

+000:00:04:00 ACT AAZEENTRY;

IF (AAZ) THEN AT STOP DO

-000:00:04:00 ACT AAZEEXIT;

AAZENTRY = ICSQREG REG=28, VAL=1 (pauses sequence code via global register 28)

AAZEXIT = ICSQREG REG=28, VAL=0 (resumes sequence via global register 28)

The delta of 4 minutes allows for more observing time as the FDF predicts are a bit too conservative.

At this point, the guide telescope diodes and solar arrays begin to lose signal/power due to the atmospheric blockage. Pointing will be come less stable with the diodes blocked. Therefore it becomes necessary to call the full eclipse activity (opening the ISS, moving to sun center, etc.) rather than just the AAZ activity (pausing the sequence only).

IF (AAZ) THEN AT START DO

+000:00:04:00 ACT ECLIPSEENTRY;

IF (AAZ) THEN AT STOP DO

-000:00:04:00 ACT ECLIPSEEXIT;

The FDF predicts for AAZ times are disjointed around the eclipse times they encompass. Therefore on the first and last day of eclipse season, we must make a manual transition between methods. The standard eclipse statement should be inserted into the rego file:

IF (FULLSUN) THEN AT START DO

+000:00:07:00 ACT ECLIPSEEXIT;

IF (FULLSUN) THEN AT STOP DO

-000:00:06:00 ACT ECLIPSEENTRY;

However, the "ACT ECLIPSEENTRY" and "ACT ECLIPSEEXIT" statements must be inserted by hand into the rego file at the AAZ entry and exit times for the orbits preceding the first eclipsed orbit.

Discussions with Tom Spitzer, Power System Lead Engineer, revealed that with current battery charge configurations, the TRACE battery can handle an orbit average load of 155 watts, BOL, and 135 watts, EOL. On average, TRACE consumes ~114 watts. Some actuals from the 1998 season are below.

There has been no noticeable solar array degradation throughout the 1999 full-sun season. The battery performance will not be evident until it begins discharge and recharge cycles.

During the eclipse season of 1998, the telescope operational heaters consumed ~3.15 watts more than the full sun full sun period. In 1999, it is likely this number will be higher due to the increased/tighter deadbands (16.0-16.3 C) used in 1999. The CCD sees an increase in temperature and orbital delta due to increased Earthshine. The CCD rises about 4 degrees and swings about 10 degrees when in eclipse compared to 5-degree swings in full-sun.