Bright Earth "Fly-by" Test for the ACA, 2002-October-23

On 2002-October-23, a test was performed to operate the ACA to within 10 degrees of the bright Earth, near orbital perigee while operating PCAD in Normal Point Mode using the ACA data. This test was performed to measure the minimum angle between the Earth limb and the ACA line of sight, above which the ACA can be used for normal pointing control on Chandra. The objective of the test is to re-define the Observatory constraint which states that the ACA cannot be used within 20 degrees of the Bright Earth limb.

Test Description

Chandra was maneuvered to an attitude near which the Earth would transit, so that the limb of the bright Earth would tangentially pass within 10 degrees of the ACA boresight. The actual angle of closest approach to the ACA boresight was 9.9 degrees. Chandra was operated in Normal Pointing Mode during the test, with the OBC using ACA data for PCAD control. At about 20 degrees from the Earth limb, the ACA was commanded to acquire a typical starcat of 6 guide stars, and track these stars until the Earth limb angle had gone through the nearest approach and was again at 20 degrees.

Test details

Date: 23 October 2002
Obsid: 60985

Attitude (J2000): RA = 308.3 deg, Dec = -32.7 deg, Roll = 285.363794 deg
Test start (UTC): 2002:296:02:45:00
Test end   (UTC): 2002:296:05:30:00
Chandra geocentric altitude: 32100 km (start), 50000 km (end)
Earth angular radius: 11.5 deg (start), 7.3 deg (end)

--------------------------------------------------------------------------
 SLOT        ID  TYPE   SZ      RA            DEC      MAG    YANG   ZANG
--------------------------------------------------------------------------
  0   977798624   BOT  8x8  308.4438273  -32.7090136  10.019    147    411
  1   977801368   BOT  8x8  308.7891095  -32.3029370   9.661   -980   1813
  2   977802032   BOT  8x8  308.4282352  -32.0515458   9.883  -2147    995
  3   977808192   BOT  8x8  308.8123154  -33.2349483   9.968   2269    976
  4   977678032   GUI  8x8  307.6203313  -33.1555230   8.459   1045  -2411
  5   977801512   GUI  8x8  308.5453157  -32.3040804   9.917  -1175   1097

Analysis Assumptions

Earth Radius = 6371 km.

Individual ACA Image Slot Analysis Details

Separate data for each ACA image slot are shown below. The plots show the time histories of the median pixel value, Earth limb angle and Earth Angular Radius for each image slot. The ACA high background global status flag was never set during this test.

The blue lines show when the ACA reports that it is tracking the image.
The green lines show when the ACA set any of the image status bits for that image slot, indicating saturated pixels etc.

In addition, the time histories of the Y and Z centroid angles and the star magnitude calculated by the ACA are plotted. The start of the maneuver at the end of the test can be seen in the ACA Y and Z angle plots. On the magnitude plot:

The red data show the number of stars being used in the Kalman filter in the OBC PCAD software (msid = AOKALSTR).
The black data show the number of stars in the OBC star catalog (msid = AONSTARS).

Conclusions

There is no apparent perturbation of ACA tracking of stars during the test. As time progresses, the data become less noisy, the setting of image status flags on individual image slots become less frequent, and the number of stars used in the Kalman filter shows fewer dropouts. This correlates to the changing angular size of the Earth, but is most likely due to the increasing altitude of Chandra, so that the ACA is less affected by high background radiation as the test progresses. The results of this test and previous tests indicates that the ACA can successfully track faint stars to within 10 degrees of the bright Earth limb.

Thus the CARD constraint on ACA operation near the bright Earth could be reduced from 20 degrees to 10 degrees.

Rob Cameron

Last modified: 2002-October-29