SCS 107 is the Main SI Safing SCS. It was created to replace the set
of safing actions that were in place at launch and through OAC. Early
in the mission it was discovered that the radiation belts were having
a deleterious affect on the ACIS instrument, specifically the FI
chips. It was found that moving the ACIS instrument out of the HRMA
FOV, into the Next In Line location, stopped the degradation to the FI
chips. The next step was to change the on-board safing actions to make
sure the ACIS was removed to the NIL position. Additional changes were
also made to consolidate all the safing actions into one main SCS.
One of the main issues to remember is that many of the steps we take
to safe ACIS are done through the OBC. For example, the procedures:
SOP_OPS_UNSAFE_ACIS_PHASE1
SOP_SYSTEM_UNSAFE_ACIS_PHASE2
assume the OBC is operative.
However it is possible that the OBC is temporarily inoperative. In
this case the system needs to be safed and restarted via Real Time
Commands.
Basic Steps
1. Was SCS-107 run? Is it running now?
2. Is ACIS safe?
3. Support OCC
Step 1: Has SCS-107 been run; is it running? Has it completed?
This can be checked on
the Chandra
Snapshot Browser page.
Command to run given and in the process of running: SCS-107 shows ACTIVE
SCS-107 completed or not run: INACTIVE
Step 2: Determine if ACIS is safe
From: SOP_SAFE_SAFING_ACTIONS
The following are telemetry values to verify that ACIS is moved to SAFE position
(HRC-S). These items can be looked at on our Real Time Web Pages:
ACIS
Real-Time Telemetry Webpage (acisway)
ACIS
Real-Time Telemetry Webpage (ishmael)
ACIS
Real-Time Telemetry Webpage (acis60-v)
ACIS
Real-Time Telemetry Webpage (aciscdp-v)
The SIM position and other important diagnostics may also be found at
the Chandra Snapshot Browser page. Note that 3 FEPs should be running, with the patch to SCS-106 as of June 2021.
SIM is at the HRC-S position: -99616
(SIM position MUST be < -25000 or ACIS is NOT safe)
1DPICACU (DPA Input Current A): ~ 0.71 - 0.77 A (Example Plot)
1DPICBCU (DPA Input Current B): ~ 0.86 - 0.95 A (Example Plot)
1CBAT (Camera Body Temperature A): -73 - -55 C
1CBBT (Camera Body Temperature B): > -72 C (if this is not the case
the detector housing heater needs to be turned on)
1STAT1ST (SW Bilevel Telemetry Bit 1): 1 (science inactive)
1DEICACU (DEA Input Current A): mean of ~0.6 - 0.8 A, but varying over wider range ~0.2 - 1.5 A (Example Plot)
NOTE: 1DEICACU is not stable and may not be within the specified
"expected value" during each update. Confer with the Lead Systems
Engineer during initial tag-up if the average value is not within the
expected range.
Main Steps in SCS-107
1. Disable Radmon Process
2. Call SCS #: 123 -- Terminate and Disable Observing SCSs 131 - 135
3. Disable Angular Momentum Unloading Logic (Subfunction 21)
4. Disable VDE A&B MUPS Hi-Side Valve Power
5. Disable S/C Momentum Monitor Safing Action (SCS #: 9 -- OO -
Spacecraft Momentum Monitor Fault Safing Action)
6. Disable RW Momentum Bias
Total time for steps 1 - 6: ~1.025 seconds
7. Activate SCS #: 82 -- OTG Emergency Halt, OTG Retraction
8. Call SCS #: 108 -- SIM Move to HRC-S (Total time: ~65.66 seconds)
9. Call SCS #: 106 -- ACIS Safing (Total time: ~31.78 seconds)
10. Call SCS #: 104 -- HRC Safing Part 1 (+ SCS #: 105 -- HRC Safing Part 2, Door
Partially Close)
11. Call SCSs 87, 88. Terminate SCSs 89, 90, and 91. (Total time:
~34.91 seconds)
432-Second Time Delay. This was originally included in the SCS to
allow time for the OTGs to Retract,
but this is no longer
done since SCS 82 has been modified to halt OTG motion. Now this time
delay is intended to give the SIM TSC
enough time to move before enabling
the momentum monitor safing
action. The longest move takes about
300 seconds. This time delay provides
adequate pad time for commands to be
issued by the SEA and for
overcurrents that could slow TSC
moves.
12. Enable Angular Momentum Unloading Logic (Subfunction 21)
13. Enable S/C Momentum Monitor Safing Action (SCS 9)
14. Enable RW Momentum Bias
15. Activate SCS #: 84 -- Command Normal Sun Mode (currently disabled)
16. Disable SCS 107
17. Turn off FID lights
Total Time: ~565 seconds (9.42 minutes)
List
of FOT Safe Mode Procedures
If you have questions pertaining to any information on this page,
please feel free to contact me.
