1. Scheduling Challenges for JWST JIM Feb. 19, 2004
Peter Stockman

2. Major planning constraints
Sun avoidance: well known Field of Regard
Earth-Moon scattered light: Will constrain some orientations (still not completely understood)
Orbit maintenance (11 days between angular momentum dumps for FDF): would constrain roll choice/orientation for long observations
Fuel conservation (22 days between angular momentum dumps): would constrain the roll constraints of all observations and potentially the mix of observations in a 22 day period.
Red = New and Exciting
2/19/04
JIM

3. 1.) Sun Avoidance The sunshade provides:
Simple model based upon 2001 TRW sunshield design by Dennis Skelton
The sunshade provides:
-5° to 45° pitch from the ecliptic poles
~± 5° of operational roll
Required for 10 day fixed-roll NIRSpec observations.
5° safety band in both pitch and roll
Sunshield shadows Primary & Secondary Mirrors
Primary shadows Secondary Mirror
Stayout Zone
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JIM
56°

4. 2.) Simple L2, Earth, Moon Geometry in X-Y plane shows how Earth and Moon light can strike OTE
±10° roll shadow band, ±5°in MRD
Sun projection
The Earth and/or Moon can illuminate the optical surfaces, particularly at L2 orbit (Y and Z) extremes
Could be improved by tighter L2 orbit or larger sunshade.
Earth
Moon
37°
27° L2
JWST
2/19/04
JIM

5. Earthshine typical example
OTE components overhanging sunshield coverage will be illuminated by Earth crescent
Northern hemisphere
Pitch = 0
Sunshield Roll = 0
Yaw = 45
2/19/04
JIM

6. Scattered Earthshine can exceed the zodiacal background at l> 3 mm.
Worst case assumes:
100% of 1 mirror (SM or PM)
1% dust
Nominal BRDF
Moonlight is less important (1-3% Zodi)
From Larry Petro
2/19/04
JIM

7. Beckman analyzed one DRM for Earth/Moon Impacts
For analysis, he used:
Skelton’s stay out zones from 2001 TRW Phase 1 design
15 yr ephemeris and DRM v3.6b
Periods exist when either the Earthlight or Moonlight would strike the primary or secondary mirror
15 yr JWST orbit seen from the Sun
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JIM

8. Results 70% of observations were “dark”
Earth and Moon each affected 25% of observations:
Earth intruded as much as 22° into keep-out zone
Moon intruded as much as 30° into keep-out zone
Earth seen in L2 XY plane
Very little correlation with time, but both Moon and Earth most easily seen at X-Y-Z extremes of the orbit.
1.5Mkm
Earth
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JIM

9. The new sunshield (June 2003) is 43% smaller than previous design to reduce angular momentum buildup and mass
Design in proposal
New design
67% area (based on inner layer)
57% area (based on outer layer)
The smaller sunshade will increase the impact of scattered light from the Earth and Moon.
2/19/04
JIM

10. A rough idea of the constraint and how it changes per year
NEP
NEP
Increased scattered light regions
Pattern repeats
90 L2/2
180 L2
~1 year
Can create shorter observing seasons and impact 180 day repeats
FOR in JWST frame Z
JWST
3 weeks
Later
JWST L2 L2 Y
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JIM

11. 3.& 4.) Angular Momentum and Orbit Maintenance:
To determine orbit, FDF is allowing at most 2 momentum dumps per 22 day period (e-folding time for orbit errors).
Limited propellant mass for orbit maintenance and momentum dumps has led to concept of 1 dump/22 days (24 hrs before orbit burn)
Flexibility for scheduling depends on wheel momentum storage capability
6 wheels = 40 n-m-s
4 wheels = 22 n-m-s
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JIM

12. Schematic Maneuver Sequence
Possible
Additional
Momentum
Unload
21-day
Tracking
Arc
Momentum
Unloads
(~ 1 day prior
to SK maneuver)
Station-Keeping
Maneuvers
(8 per rev,
~ 22 days apart)
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JIM

13. Momentum accumulation is dominated by roll offsets in current design
1/5th of 22 day total accumulated in one day!
dJ/dt
Pitch
Roll
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14. Comparison of momentum accumulation for both new sunshields designs
Current design (negative dihedral)
Note significant angular momentum due to pitch alone
Positive dihedral alternative
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JIM

15. Possible ways to manage angular momentum in the scheduling system
Baseline today: Monitor: Check long range plan to see if there is a potential for exceeding the momentum between 22 day dumps. Feasible if problems are rare
Restrict average momentum buildup per observation to less than 2 n-m-s average during development of LRP.
Constrain roll orientation and start-dates
Significantly decreases scheduling flexibility.
Failed observations will necessitate replan since all observations would be shifted
Actively Manage momentum by balancing angular momentum build-up over each 22 day period (and potentially beyond) in the LRP.
Increases science return, but may create a a very brittle schedule.
2/19/04
JIM

16. DRM shows 30-40% of dump intervals less than 22 days
Monitor Study:
DRM shows 30-40% of dump intervals less than 22 days
Monitor method will not work. Fails in 30-40% of cases even with all reaction wheels working.
2/19/04
JIM

17. Restricting to an average momentum :
10 day observations need special planning to avoid excessive momentum build-up
At high ecliptic latitudes, the visits must be centered within one day: either fixed start times or intervention needed if started early by failure of previous observation
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JIM

18. Restricting to an average momentum : One day visits are also constrained
Note drop in available
Start-time at high ecliptic latitudes even for a 1-day early visit
Restricting average momentum method will be very constraining…worth going to total momentum management
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JIM

19. Restricting average momentum : Loss of reaction wheel leads to drastic constraints
Note loss of all flexibility above 45° even for 1 day observations.
Restricting average momentum method is not viable. Must go to total momentum management or change vehicle
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JIM

20. A typical 22 day managed schedule
Possible 22 day rules
Only one 8-10 day obs
Only one 4-7 day obs
Fill in with 1 day obs
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JIM

21. Summary
Overall scheduling of JWST has become more complicated and may significantly impact JWST science:
Long observations are almost time critical
Full roll (± 5°) is not routinely available
Even observations with varying roll but in the same part of the sky will be limited to ≤ 10 day stretches.
Thermal radiation from the Earth can produce significant scattered light and preferred observing seasons (potentially impacting NGP & SGP depending on launch date)
Angular momentum issue could be mitigated with positive dihedral design, increased momentum wheel capability or added fuel (~ 70 kg).
Scattered light issue needs to be confirmed by Ball, STScI, and GSFC (Beckman/Skelton). Larger sunshield makes angular momentum problem worse.
2/19/04
JIM