390 and met system requirements (with margin) . However, with the unexpected o n-orbit discovery of a thermal - change- induced sudden energy release causing  the PCS performanc e to fail  requirements , alternate  forensics were improvised to check  suspected source (s) and the nature of the phenomenon17. The only  onboard sensor s deemed sensitive enough to obtain  dynamic  data were gyr os, HST-designated Rate  Sensor Units  (RSUs). RSU telemetry , along with the correlated dynamics allowed the root cause to be wellcorrelated with other data ( e.g., via event timing and which known mode frequencies are excited by the  phenomenon plus what  disturbance locations are likely to excite particul ar modal responses)  and eventually   traced to locations within  the 2 solar arrays. Extremely flexible when deployed (due to weight -optimized  design) , solar arrays with extended bi -stem supports wer e expected to slowly  deform when reacting to  thermal changes transitioning through terminator crossings fro m orbital day to  night. However , it was   discovered  instead  that one solar array  would “lock-up” for some time then abruptly snap into a different   bowed position . This sudden impulse was sufficient to cause FGSs to  lose interferometric lock on  designated guide stars  revert ing to coarse- sensing mode and typically  resulting in a loss of science.  The  ensuing investigation found  a docum ented history of orbit al disturbances occurring during transitions from  orbital day -to-night (EOD) a nd night -to-day (EON), plus near -steady -state conditions during middle of orbit  day (MIDD)  and middle of orbit nig ht (MIDN)17. RSU telemetry data sensing vehicle rotational rate changes   were extensively measured under these conditions , and these data proved useful  in computing linear  accelerations experienced on the spacecraft at given distances from the HST center -of-gravity18. 

These established capabilities  allowed, for the first time,  an opportunity  to confirm ot her orbital dynamic  jitter-related predictions  thereby, also opening an applicability  to other large space platfo rm microgravity  predictive approaches4,12. Subsequently , a unique analytical effort was initiated to compute HST structural  dynamic responses ( accelerations)  due to prescribed input forces/ moments  substantiated with actual onorbit telemetry  measurements for  a specific  known input  disturber  propagating through proven HST  dynamics to a sufficiently sensitive and known  output  receiver. Although RWA IV input levels were well - established due to earlier  efforts  described, viable direct orbital response result s for RWAs was not practical   due to several pragmatic issues :  1) as researched, ground- to-orbit RWA IV variability could be substantial   and non- deterministic14 (i.e., although it had been bounded, i t could not be precisely known within that  range what long- term IV level was present on any given RWA  at a particular time ), 2) telemetry acquisition  data rates were not high enough to readily resolve key high-harmonic -ratio signal components  (one of the  factors supporting the earlier decision to not pursue on- orbit jitter verification) , 3) on- orbit RWA rotational  rates  where telemetry was most available during HST science acquisition tended to be near  the lowest  values of pre- launch predictions  (typically 0 -3 Hz)  which resulted in inherently very low  RWA IV  disturbance  levels , 4) effective vibration isolation highly suppressed RWA  IV9, and, lastly  (due large ly to such low IV  levels from 3 and 4), 5) HST telemetry data had never detected nor di stinctly identified a response clearly   due to RWA IV frequency inputs above general on- orbit operational noise environment s. However,  upon  further investigation,  an alternative  disturbance source was  found allowing a highly reliable /consistent  onorbit basis  for an analytical approach: Engineering/Science Tape Recorder s (ESTR s). HST’s ESTR s (2  onboar d for redundancy) countered RWA impediments  by having consistent  IV-to-response  knowledge:  1)  highly repeatable grou nd test IV data showing no basis for  significant  variabilit y transitioning to orbit or due  to changing condit ions (e.g., designed for similar performance independent of overall t ape  position/condition, i.e., start-of-reel/end- of-reel),  2) time-congruent  telemetry available (at 2 different  distu rbance frequencies matching 2 stable  ESTR recording rates), 3) telemetry  data uniformly exhibits a  dominant  and unambiguous  signal presence when an ESTR i s on, and 4) source and receiver have distinct  local orthogonal principal directions  (corresponding  to the ESTR input  IV known dataset directions and RSU   output  dynamic model along with sensed response directions  (i.e., no need for  a coupled coordinate  transformation of amplitude caused by difficulty in discerning  which response directions’ signal intensity   was from w hich individual disturbance DOF ). Specifically, the local primary spin axis of ESTR tape reels is  coincident with the vehicle V1 axis  which  is also accurately aligned with one of the principal  RSU sensing  axes  - a favorable situation.    As measured,  RSU angular acceleration telemetry signal s encompass ed all operating onboard sources  up  to 20 Hz , including  solar array residual motions , ESTRs, High Gain Antenna Gimbals, etc.  To isolate  ESTR - only contributions, a filter was  applied since the effective frequency bandwidth is reliably known (quite