Topic: NOAA-18 Attitude Control Mode Chang= e for Yaw, Roll, and Pitch

Date/Time Issued: =C2= =A0August 9, 13:35 UTC

Product(s) or Data Impacted= : =C2=A0All data and products from NOAA-18 (AVHRR, AMSU-A, HIRS, MHS, &= SBUV) might be impacted =C2=A0

Date/Time of Initi= al Impact: =C2=A0August 9, 2017 =C2=A015:07:50 UTC

Date/Time of Expected End: =C2=A0Permanent=C2=A0

= Length of Outage: =C2=A0Permanent=C2=A0

Details/Sp= ecifics of Change: =C2=A0The last laser gyro on NOAA-18 in MIMU 1 (Miniatur= e Inertial Measure Unit #1) for Yaw is expected to reach critical LIM (Lase= r Intensity Measure) threshold August 12-15, 2017.=C2=A0 Engineering has ch= osen to be proactive and change the attitude control mode to GYE (Gyroless)= before this date on Wednesday morning August 09, 2017 (jday 221) at 15:07:= 50 UTC Rev 62980. This action will avoid risk of autonomous spacecraft inte= rvention.

Engineering expects attitude pointing fo= r roll and pitch will remain in spec with nominal Yaw errors better than = =C2=B10.5 degrees (about 7.5 km) over 95% of the orbit.=C2=A0 AVHRR imagery= at scan edges will show geolocation errors but should be usable for fire a= nd snow/ice products and users.=C2=A0 Other instruments have a larger FOV (= Field of View) and hence this increase in Yaw pointing error above spec of = 0.2 degrees is nullified. =C2=A0

The performance o= f NOAA-18 instrument L1b data in GYE is expected to be better than that of = NOAA-15, which has been operated in the enhanced Reduced Gyro Mode since Ma= rch 17, 2010 in estimated Yaw, and equivalent to the operation of NOAA-18 i= n GYE from Jan 13, 2015 until Dec 15, 2015 when the s/c was in an attitude = control mode transition. =C2=A0

NOAA-18 MIMU 1 GYR= O became operational on June 7, 2009 after MIMU 2 X Ring Laser Gyro (RLG) a= xis failed due to neon depletion.=C2=A0 The MIMU 1 Z and Y axes reached the= ir critical threshold where reduced gyro software was necessary for stable = Attitude Control on October 9, 2013 and January 06, 2015 respectively.=C2= =A0 The MIMU 1 Z and Y axes failed on March 7, 2014 and July 5, 2015, five = to six months after reaching their respective critical thresholds.=C2=A0 Fi= nal failure of the X axis is expected to occur much sooner than five months= after reaching critical threshold.=C2=A0 The design lifetime of these MIMU= units was 4 years with the laser ring gyros (LRG) being the limiting compo= nent.=C2=A0 NOAA-18 went from 3 to 2 to 1 to 0 axes gyro control with the m= issing axes now being derived from the remaining good gyro axes and the ESA= (Earth Sensor Array) by flight software.=C2=A0 The enhanced ACS (Attitude = Control System) flight software developed by the POES Team extended the lif= e of NOAA-18 by about 4 years meeting attitude error spec of 0.2 degrees pe= r axis.

The GYE attitude control mode will now all= ow for several more years of operation of NOAA-18 with good attitude pointi= ng errors for Yaw, Roll, and Pitch. =C2=A0

An ESPC= reminder notice will also be sent out shortly before this attitude control= mode change to GYE on NOAA-18.

Web Site(s) for ap= plicable information: N/A

Contact Information for = Further Information:

ESPC Operations at ESPCOperations@noaa.gov at 301-817-3880

Car= l Gliniak at carl.gliniak@noaa.gov= at 301-817-4207

Dejiang Han at dejiang.han@noaa.gov at 301-817-4119

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