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It's day 74 of 1996 at 1800 hours Greenwich Mean Time (GMT). The Hubble Space Telescope (HST) is passing over central India headed northeast at an altitude of 320 nautical miles. The HST has already completed 12 of the 15 orbits it makes each day. The satellite is in orbit night. It is drawing power stored in its six batteries. The telescopes current target is occulted by the earth. The Faint Object Spectrograph (FOS) instrument is activated waiting to resume its work once the target is back in view. The other instruments (Faint Object Camera, Goddard High Rate Spectrograph, and the Wide Field Planetary Camera II ) are not active. The FOC and GHRS are in HOLD. The WFPC-II is in standby mode. The satellite is not in radio contact with the ground. There is a small part of HSTs orbit where it is not in view of the Tracking and Data Relay Satellite (TDRS) it uses. This area is called the Zone of Exclusion (ZOE).
18:04 The satellite is now over southern China headed northeast. It points its high gain antenna (HGA) (number 2) to the TDRS-WEST satellite and turns on is transmitter. The signal travels up to the TDRS and is received and retransmitted to the TDRS Ground Terminal located in White Sands, New Mexico. From there the signal is relayed to the Space Telescope Operations Control Center (STOCC) at the Goddard Space Flight Center (GSFC) located in Greenbelt, Maryland.
The satellite sends back measurements from all of its subsystems. The computers onboard take measurements of voltage, current, temperature, and position using sensors located throughout the satellite. The computer packs the measurements together and transmits them. The control center unpacks the measurements and checks each one for abnormal behavior.
18:09 The STOCC establishes forward link communication with the satellite via the TDRS. The forward link allows the STOCC to uplink real-time or stored commands and new flight software.
18:09 The HST has been tracking a target (stars that the rings of Uranus will occult). The Pointing Control Subsystem performs a small attitude adjustment to align the target with the FOS aperture. It uses its reaction wheel assembly (RWA) and gyros.
18:10 The satellite has now moved further in its orbit. Its now over southeast China, headed southeast over the Philippine Sea. The Pointing Control System (PCS) begins to prepare its Fine Guidance Sensors (FGS) to find the guide stars near its target.
18:11 The earth no longer blocks the target. The occultation is complete. During the next few minutes the FGS slowly moves around to find its guide star.
18:16 The satellite begins tracking the FOS target. This is called a type 3 slew.
18:18 The FOS begins to take measurements of its target. The data from the instruments is recorder by a tape recorder onboard.
18:19 The satellite passes from orbit night into orbit day. The solar arrays begin to generate electricity. The Electrical Power Subsystem uses this power to recharge the batteries and run the spacecraft.
18:23 The Fixed Head Star Tracker #3 view is blocked (occulted) by the earth. It is not needed at this part of the schedule. As the satellite continues in its orbit, the other 2 FHST views will be blocked as well.
18:24 The satellite loses forward link communications. No commanding from the STOCC is possible.
18:25 HST is heading southeast over the Pacific near the Marshall Islands.
18:26 The FOS begins to take its second measurements of its target. The data from the instruments is recorder by a tape recorder onboard.
18:28 The Fixed Head Star Tracker #2 view is blocked (occulted) by the earth.
1:30:00 HST is heading southeast, southwest of the Hawaiian Islands.
18:31 The satellite establishes forward link communications. Commanding from the STOCC is possible.
NOTE: (The timeline depicts a Corrective Optics Space Telescope Axial Replacement (COSTAR) Deployable Optical Bench (DOB) movement. This activity has been replaced by a movement of the Secondary Mirror.)
18:31 During the next 22 minutes the HST Secondary Mirror will be re-adjusted by sending real-time commands to its actuators. There are 6 actuators on the back of the secondary mirror, and each are moved one at a time the same amount. Once all actuators are commanded, the mirror would have been moved by only 6 micro-meters further apart from the primary mirror. This is needed to correct for continuous but slow shrinkage of the structure on which the mirrors are mounted, due to water loss (on Earth, components absorb a certain amount of water, which then escapes to the vacuum of space once in orbit).
18:35 The Fixed Head Star Tracker #1 view is blocked (occulted) by the earth.
18:40 HST is heading southeast over the south Pacific, east of French Polynesia.
18:50 HST is heading southeast over South Pacific approaching coast of South America/Chile.
18:52 The satellite loses forward link communications. No commanding from the STOCC is possible.
18:53 The satellite loses return link communications with TDRS (WEST). The STOCC cannot monitor the engineering telemetry data until communications is re-established.
18:54 The satellite establishes forward and return link communications. Commanding from the STOCC is possible.
18:56 The satellite enters the South Atlantic Anomaly (SAA). This will last for the next 19 minutes.
18:55 One of the onboard tape recorders (number 1) is filled with science data. The data is played back and transmitted via TDRS to the ground and back to GSFC. It is processed by the Packet Processor II (Pacor II) Data Capture Facility, transferred to the Data Distribution Facility (DDF) , then transferred to the Space Telescope Science Institute in Baltimore, Maryland.
18:59 Using the PCS, the satellite begins to slew to a new target.
18:59 The satellites view is occulted for the next 15 minutes.
19:00 The satellite is now passing over South America/Chile. In 22 minutes the HST will begin its 32,118 orbit of earth.