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