Two-Gyro All-sky
Scheduling Study
TIPS
April 15, 2004
http://www.stsci.edu/org/ess/projects/two_gyro_science
Personnel at Large
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Special Projects Branch (SPB)
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Operations Planning Branch (OPB)
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Alison Vick
Ian Jordan
Planning Systems Development Branch (PSDB)
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Merle Reinhart
George Chapman
Mark Giuliano
Bob Boyer
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Outline
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Two Gyro background
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Two Gyro Modes
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Why they impact scheduling
All-sky scheduling study
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Plots galore
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Targets-by-week The Movie
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TGS Modes
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Three operational modes
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MSS and two gyros (M2G): Compares MSS output to magnetic field
model to control attitude and rates with errors required to be less than
10 degrees; supports vehicle maneuvers
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FHST and two gyros (T2G): Requires one tracker to be visible to use
FHST data and gyros to control rates; onboard attitude determination
(OAD) using FHST map data from two FHST units will bring attitude
error within FGS search radius
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FGS and two gyros (F2G): Requires FGS visibility to use FGS data
and gyros to control rates and attitude to allow for science
What does it mean?
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Elaborate process to get on guide stars after a slew
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Less schedulability (big surprise)
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3 gyro vs. 2 gyro
Getting to guided science
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3-gyro (current ops)
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Type 2 Slew
FHST full update
FGS guide star acquisition
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2-gyro
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Type 2 Slew
FHST rate control
First FHST map
First attitude correction
Second FHST map
Second attitude correction
FHST GOB (overlaps with
guide star acq)
FGS Guide star acquisition
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TGS Scheduling Scenario
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Scheduling Impacts & Constraints
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Largest impact is the requirement to have overlapping visibility
between the FHSTs and FGSs
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Additionally, several constraints / restrictions are generically
affected by the large pointing uncertainty (LAU) in M2G mode
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Thermal off-nominal roll angles
Avoidance angles (Sun, Moon, Earth)
Solar array incidence angles
Solar array shadowing table
Anti-Sun avoidance / roll angles
10 degree LAU means
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Limits the portion of sky available at any one time
Targets <60 degrees from sun will not be observable
An anti-sun avoidance zone of ~8 degrees radius develops
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The TGS All-sky Study
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Assess the availability of targets across the entire sky
under the FHST driven scheduling restrictions imposed
by two gyro mode
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Examine how that schedulability is affected by FHST
duration requirements and initial error estimates
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All-Sky Study Approach
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5-degree-by-5-degree grid of targets covering 360 degrees in
RA and +85 degrees to -85 degrees in DEC
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Very schedulable Visit (one 30-minute exposure)
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Used updated (reduced) FGS earth avoidance angles
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Guide star availability not considered
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Used Prototype Long Rang Planning tool (SPIKE) to
determine constraint windows
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Summed the window durations by target
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Vary parameters and rerun
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All-Sky Schedulability:
Three-Gyro 50°-180° Usable Sun Angle
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All-Sky Schedulability:
Two-Gyro 60°-172° Usable Sun Angle
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All-Sky Schedulability:
Two-Gyro 100°-172° Usable Sun Angle
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All-Sky Schedulability:
Sky Availability
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All-Sky Schedulability:
3-gyro vs. 2-gyro
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All-Sky Schedulability:
Sensitivity to LAU
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All-Sky Schedulability:
LAU difference distribution
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All-Sky Schedulability:
Visit duration sensitivity
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All-Sky Schedulability:
Visit difference distribution
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All-Sky Schedulability:
Activity duration sensitivity
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All-Sky Schedulability:
Onboard Attitude Determination difference
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Target Availability
QuickTime™ and a
Animation decompressor
are needed to see this picture.
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Scheduling under Two Gyro
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FHST / FGS overlapping visibility requirement creates the
most notable impact
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60 – 172 degrees usable sun angle
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Maximum off-nominal roll is 22 degrees
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Visit duration will likely be ~30 minutes
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On average an individual target is only observable ~120
days per year
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Low-dec targets become observable after the sun passes them
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Hi-dec targets have periodic availability tied to orbital precession
Can absorb small changes to the two gyro activities without
drastically affecting schedulability
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