Acquisition anomaly in Obsid 4977

Summary

Obsid 4977 is a 79 ksec ACIS-S imaging observation of Abell 2029 that began at 2004:008:12:57:38 (start of KALMAN interval). In this observation there appears to be an anomaly in the star acquisition sequence: the ACA acquired a star in slot 7 which is inconsistent with the commanded star catalog parameters. This is the first such case to be identified. The incorrect star led to the ACA tracking two ACQ stars for 8 seconds in the KALMAN interval, at which point the OBC intervened and sent commands to acquire the correct guide stars. The 8 seconds of unexpected data prevent proper completion of automatic aspect pipeline processing and force manual processing.

Details

The starcheck output for this catalog is shown below (but note that the entries for slots 4 and 7 have been highlighted in red for visual clarity). The key information is that for slot 7 the acquisition box for the BOT star is centered at (Yag,Zag) = -541,478 arcsec, with a box half-width of 120 arcsec. However, from PCAD engineering telemetry, the actual acquired star in slot 7 is at (Yag,Zag) = -1045,-1554. This coincides with the commanded position for slot 4. The other star acquisition slots show no unusual behavior.

In slot 4 the ACA acquired a spoiler star of comparable magnitude which is very near the edge of the search box. This star is clearly seen as the point on the lower-left side of the box labeled with index 5 on the star field plot below. This corroborates the picture from engineering telemetry that the ACA was tracking in slot 7 the ACQ star which was intended for slot 4.

The position of the acquired star was verified in a way independent of the OBC telemetry by inspecting the ACA image telemetry. This confirmed that the ACQ star in slot 7 is centered near (-1045,-1554) arcsec, and so the problem is not in the OBC telemetry.

We directly confirmed that the starcheck output which gives the search box angles matches the values in the Backstop output.

No previous instances have been identified where the ACA acquired a star at a position outside of the nominal commanded search box. As part of the study on acquisition star success rates, we have compared the expected position with the actual position for all acquisition stars since the year 2001. No cases of a large offset were seen.

The anomaly occurred at a time approximately 24 hours before the radiation shutdown by SCS107 at 2004:009:14:29:11. However, inpsection of the ACE rates at the time of the acquisition anomaly shows no sign of excessive rates.

Explanation

At this time we have no meaningful explanation for the anomaly. Examining the chain of events, possibilities include:
  • Corrupted/incorrect command load sent to spacecraft: This is highly implausible given that the OBC did eventually command the correct star coordinates when doing a directed search for the guide star in slot 7. The command load specifies a single BOT entry for slot 7, so there is no obvious mechanism for the ACQ coordinates to differ from the GUI coordinates.
  • Error in OBC command to ACA during ACQ phase or an Error in ACA after receiving correct command from OBC: either of these possibilities could produce the observed telemetry, although we have not identified any single bit error that could be the cause. In any case there is likely no way to distinguish these two possibilities given the available telemetry.


Star catalog
	  
OBSID: 4977   Abell 2029              ACIS-S  SIM Z offset: 0      Grating: NONE     
RA, Dec, Roll (deg):   227.746478     5.734288    62.487652
Dither: ON  Y_amp= 8.0  Z_amp= 8.0  Y_period=1000.0  Z_period= 707.1
BACKSTOP GUIDE_SUMM OR MANVR DOT MAKE_STARS 


MP_TARGQUAT at 2004:008:12:45:34.974 (VCDU count = 9176235)
  Q1,Q2,Q3,Q4: 0.17054824  -0.49101853  -0.77034055  0.36930954
  MANVR: Angle=  10.45 deg  Duration= 641 sec  Slew err= 25.8 arcsec

MP_STARCAT at 2004:008:12:45:36.617 (VCDU count = 9176242)
----------------------------------------------------------------------------------------
 IDX SLOT        ID  TYPE   SZ  MINMAG    MAG   MAXMAG   YANG   ZANG DIM RES HALFW NOTES
----------------------------------------------------------------------------------------
[ 1]  0           1   FID  8x8   5.797   7.000   8.000    940  -1722   1   1   25       
[ 2]  1           4   FID  8x8   5.797   7.000   8.000   2158    181   1   1   25       
[ 3]  2           5   FID  8x8   5.797   7.000   8.000  -1808    175   1   1   25       
[ 4]  3    44959056   BOT  6x6   5.797   9.682  12.406  -1880    467  20   1  120  2    
[ 5]  4    44959728   BOT  6x6   5.797  10.242  13.000  -1037  -1553  20   1  120  2    
[ 6]  5    44961784   BOT  6x6   5.797  10.342  11.844   -594  -1834  20   1  120  2    
[ 7]  6    44966344   BOT  6x6   5.797   5.924   7.422  -2123   2241  20   1  120  2    
[ 8]  7    45482024   BOT  8x8   5.797  10.421  13.172   -541    478  20   1  120  2    
[ 9]  0    44961248   ACQ  6x6   5.797  10.589  12.641  -1565   1458  20   1  120  3    
[10]  1    44957744   ACQ  6x6   5.797  10.645  12.156  -1948   1120  20   1  120  2  cp
[11]  2    45490016   ACQ  8x8   5.797  10.027  12.781    376   -234  20   1  120  1    


>> WARNING: Magnitude. [ 7]:  5.924
>> WARNING: Magnitude. [ 5]: 10.242
>> WARNING: Search spoiler. [ 5]-   44960208: Y,Z,Radial,Mag seps: 117  71 137 -0.4
>> WARNING: Magnitude. [ 6]: 10.342
>> WARNING: Magnitude. [ 8]: 10.421
>> WARNING: Magnitude. [ 9]: 10.589
>> WARNING: Marginal star. [10]: B-V = 0.700, Mag_Err = 0.41, Pos_Err = 0.30
>> WARNING: Magnitude. [10]: 10.645


Figure of Merit : -5.62
Stars Expected  : 6.28



Engineering telemetry
Engineering telemetry reveals that during acquisition, slot 7 acquired the star which had been intended for slot 4, and that slot 4 acquired a spoiler star near the edge of its search box. The detailed data and timeline are shown below. The Time is in seconds, where 189953884 is 2004:008:12:57:00.
	  
                                   Slot 4                             Slot 7
                           Cmd =  -1037 -1553 (yag,zag)         Cmd =  -541  478         Comment
  Time      Slt 4  Slt 7       Yag           Zag            Yag         Zaz     AOACASEQ 
---------   ----   ----     ---------      ----------     ----------  ---------  -----    ------
189953883   SRCH    SRCH      -3276.80      -3276.80       -3276.80    -3276.80   AQXN
189953884   SRCH    SRCH      -3276.80      -3276.80       -3276.80    -3276.80   AQXN
189953885   SRCH    TRAK      -3276.80      -3276.80       -1044.60   -1553.650   AQXN      1
189953886   SRCH    TRAK      -3276.80      -3276.80       -1044.60   -1553.650   AQXN
189953887   TRAK    TRAK    -1163.2250    -1633.4250      -1044.750    -1553.80   AQXN      2
189953888   TRAK    TRAK    -1163.2250    -1633.4250      -1044.750    -1553.80   AQXN
  ...
189953927   TRAK    TRAK     -1163.350      -1634.30       -1045.20  -1556.5250   KALM
189953928   TRAK    TRAK     -1163.450    -1634.6250      -1045.550    -1556.70   KALM
189953929   TRAK    TRAK     -1163.450    -1634.6250      -1045.550    -1556.70   KALM
189953930   SRCH    SRCH      -3276.80      -3276.80       -3276.80    -3276.80   KALM      3
  ...
189953937   SRCH    SRCH      -3276.80      -3276.80       -3276.80    -3276.80   KALM
189953938   TRAK    TRAK     -1045.350    -1557.3750        -549.40      472.90   KALM      4
189953939   TRAK    TRAK     -1045.350    -1557.3750        -549.40      472.90   KALM
189953940   TRAK    TRAK     -1045.350    -1557.7250      -549.1250    472.7250   KALM
189953941   TRAK    TRAK     -1045.350    -1557.7250      -549.1250    472.7250   KALM

Comments:

1. Star which should have been acquired in slot 4 is instead acquired in slot 7.
   This is NOT compatible with commanded search box for slot 7.

2. A spoiler star near the edge of the slot 4 search box is acquired

3. OBC has set the KALMAN flag, but the star positions in slots 4 and 7 are
   wrong, so it commands a new guide search

4. Correct stars acquired




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Last modified:12/27/13