ASCA ACS reconfiguration

["Fumiaki NAGASE" <nagase@astro.isas.ac.jp>]

Dear ASCA users,

Since the attitude control system trouble due to degradation of a gyro
that happened on April 23, we have been operating ASCA with a somehow
unusual mode, i.e., two step manual maneuvers of coarse and fine tuning with
attitude determination in between. During the time we analyzed the ACS HK
data and reached to the conclusion that the degradation was happened on the 
IRU SKEW-2 gyro, by an increase of the scale factor (pulse weight) of about
0.5 %. We have decided to replace this S2 gyro with the Z-gyro, thus new
congiguration of ASCA ACS becomes available using X-, Y-, and Z- gyros,
i.e., originally a standard combination of gyros for the attitude control
(demerit of this configuration is that the Z-gyro, which is now  a member 
of ACS, must also work as a gyro for safe-hold control in case of emergent
SH mode).

Today (May 7, 23:12 (UT), 1st AOS of ASCA at KSC), we have started the
operation to replace S2 gyro to Z-gyro. In the following I show a brief
schedule of the operation.

May 7, 23:12  		Transfer to a Safe Hold Mode
	- May 8, 6:08	Replace the S2 Gyro with the Z Gyro
			Finding and Holding to the current attitude
			Transfer to a Normal Mode

May 8, 23:07		Test of the Z-gyro
	- May 9, 6:04	A few times Slew maneuvers along Z-axis
			(Confirmation of Z-gyro properties)
			
May 9, 23:03		Overall test of ACS system
	- May 10, 5:59	Trial maneuvers
			
May 10, 22:58		Upload of the new Attitude conrol parameters
	- May 11, 5:55	Maneuver to a scheduled target

When these operations are performed as scheduled, we will be able to
transfer to the normal scheduled observations next week. I wish to pray 
with you that we can smoothly move to the new configuration of ASCA ACS.

F. Nagase