Two-Gyro All-skyScheduling Study

TIPS

April 15, 2004

http://www.stsci.edu/org/ess/projects/two_gyro_science

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Personnel at Large

Special Projects Branch (SPB) Merle Reinhart George Chapman

Operations Planning Branch (OPB) Alison Vick Ian Jordan

Planning Systems Development Branch (PSDB) Mark Giuliano Bob Boyer

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Outline

Two Gyro background

Two Gyro Modes

Why they impact scheduling

All-sky scheduling study

Plots galore

Targets-by-week The Movie

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TGS Modes Three operational modes

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

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

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? Elaborate process to get on guide stars after a slew

Less schedulability (big surprise)

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3 gyro vs. 2 gyroGetting to guided science

3-gyro (current ops) Type 2 Slew FHST full update FGS guide star acquisition

2-gyro 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

Largest impact is the requirement to have overlapping visibility between the FHSTs and FGSs Limits the portion of sky available at any one time

Additionally, several constraints / restrictions are generically affected by the large pointing uncertainty (LAU) in M2G mode 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 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

Assess the availability of targets across the entire sky under the FHST driven scheduling restrictions imposed by two gyro mode

Examine how that schedulability is affected by FHST duration requirements and initial error estimates

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All-Sky Study Approach

5-degree-by-5-degree grid of targets covering 360 degrees in RA and +85 degrees to -85 degrees in DEC

Very schedulable Visit (one 30-minute exposure)

Used updated (reduced) FGS earth avoidance angles

Guide star availability not considered

Used Prototype Long Rang Planning tool (SPIKE) to determine constraint windows

Summed the window durations by target

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

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Scheduling under Two Gyro

FHST / FGS overlapping visibility requirement creates the most notable impact

60 – 172 degrees usable sun angle Maximum off-nominal roll is 22 degrees Visit duration will likely be ~30 minutes On average an individual target is only observable ~120

days per year Low-dec targets become observable after the sun passes them Hi-dec targets have periodic availability tied to orbital precession

Can absorb small changes to the two gyro activities without drastically affecting schedulability