scheduling challenges for jwst jim feb. 19, 2004
DESCRIPTION
Scheduling Challenges for JWST JIM Feb. 19, 2004. Peter Stockman. Major planning constraints. Sun avoidance: well known Field of Regard Earth-Moon scattered light: Will constrain some orientations (still not completely understood) - PowerPoint PPT PresentationTRANSCRIPT
Scheduling Challenges for JWSTJIM Feb. 19, 2004
Peter Stockman
2/19/04 JIM
Major planning constraints
1. Sun avoidance: well known Field of Regard
2. Earth-Moon scattered light: Will constrain some orientations (still not completely understood)
3. Orbit maintenance (11 days between angular momentum dumps for FDF): would constrain roll choice/orientation for long observations
4. 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
1.) Sun Avoidance• 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
Simple model based upon 2001 TRW sunshield design by Dennis Skelton
Sunshield shadows Primary & Secondary
Mirrors
Primary shadows Secondary Mirror
Stayout Zone
56°
2/19/04 JIM
2.) Simple L2, Earth, Moon Geometry in X-Y plane shows how Earth and Moon
light can strike OTE
27°
37°
±10° roll shadow band, ±5°in MRD
• 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
JWSTL2
Sun projection
2/19/04 JIM
•Northern hemisphere
•Pitch = 0
•Sunshield Roll = 0
•Yaw = 45
Earthshine typical example
OTE components overhanging
sunshield coverage will be illuminated by
Earth crescent
2/19/04 JIM
Scattered Earthshine can exceed the zodiacal background at > 3 m.
NGST OTA Heated by SunshieldSunshield T=90K, ε=0.05; 20% Bandpass
1. -05E
1. -04E
1. -03E
1. -02E
1. -01E
1. +00E
1. +01E
1. +02E
1. +03E
1. +04E
1. +05E
0 5 10 15 20 25 30
[Wavelength ]m
Dust Scattered Sunshield Thermal
ZodiacalLight
MirrorThermal
Detector
Earthshine
Moonlight
From Larry Petro
Worst case assumes:• 100% of 1 mirror (SM or PM)• 1% dust• Nominal BRDF
Moonlight is less important (1-3% Zodi)
EarthshineZodi
2/19/04 JIM
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
2/19/04 JIM
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
• Very little correlation with time, but both Moon and Earth most easily seen at X-Y-Z extremes of the orbit.
Earth seen in L2 XY plane
1.5MkmEarth
2/19/04 JIM
The new sunshield (June 2003) is 43% smaller than previous design to reduce angular momentum buildup and mass
•67% area (based on inner layer)
•57% area (based on outer layer)
New design
Design in proposal
The smaller sunshade will increase the impact of scattered light from the Earth and Moon.
2/19/04 JIM
A rough idea of the constraint and how it changes per year
Pattern repeats
• 90 L2/2
•180 L2
•~1 year
Can create shorter observing seasons and impact 180 day repeats
L2 L2JWSTJWST
3 weeksLater
Increased scattered light regions
FOR in JWST frameNEP NEP
Y
Z
2/19/04 JIM
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
2/19/04 JIM
Schematic Maneuver Sequence
Station-KeepingManeuvers(8 per rev,
~ 22 days apart)
MomentumUnloads
(~ 1 day priorto SK maneuver)
21-dayTracking
Arc
PossibleAdditionalMomentum
Unload
2/19/04 JIM
Momentum accumulation is dominated
by roll offsets in current design
RollPitch
1/5th of 22 day total accumulated in one day!
dJ/dt
2/19/04 JIM
Comparison of momentum accumulation for both new sunshields designs
Current design (negative dihedral)
Positive dihedral alternative
Note significant angular momentum due to pitch alone
2/19/04 JIM
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
Cumulative Distribution of Dump Intervals
40 Nms Limit, JMS v1.0_wmk135
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 20 40 60 80 100 120 140 160
Time Since Previous Dump [Days]
10 Day Visit Limit
1 Day Visit Limit
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
Available Start Time per Year with Early Visit Start for 10 Day Visit and Mean Momentum Limit of 40 Nms / 22 day
0
60
120
180
240
300
360
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Cumulative Fractional Sky Area
Availability per Year [Days]
0.0 Day
0.5 Day
1.0 Day
1.5 Day
2.0 Day
Ecliptic Latitude0° 30° 45° 60°15° 75° 90°
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
2/19/04 JIM
Restricting to an average momentum : One day visits are also constrained
Available Start Time per Year with Early Visit Start for 1 Day Visit and
Mean Momentum Limit of 40 Nms / 22 day
0
60
120
180
240
300
360
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Cumulative Fractional Sky Area
0.0 Day
0.5 Day
1.0 Day
1.5 Day
2.0 Day
Ecliptic Latitude
0° 30° 45° 60°15° 75° 90°
Note drop in availableStart-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
2/19/04 JIM
Available Start Time per Year with Early Visit Start for 1 Day Visit and
Mean Momentum Limit of 24 Nms / 22 day
0
60
120
180
240
300
360
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Cumulative Fractional Sky Area
0.0 Day
0.5 Day
1.0 Day
1.5 Day
2.0 Day
Ecliptic Latitude
0° 30° 45° 60°15° 75° 90°
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
2/19/04 JIM
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
2/19/04 JIM
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.