Transcript New Challenges to Scheduling JWST

Scheduling Challenges for JWST
JIM Feb. 19, 2004
Peter Stockman
Major planning constraints
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
1.
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Simple model based upon 2001 TRW
sunshield design by Dennis Skelton
1.) Sun Avoidance
•
The sunshade
provides:
•
•
-5° to 45° pitch from
the ecliptic poles
~± 5° of operational
roll
•
•
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Sunshield shadows
Primary & Secondary
Mirrors
Required for 10 day
fixed-roll NIRSpec
observations.
Primary shadows
Secondary Mirror
Stayout Zone
5° safety band in both
pitch and roll
JIM
56°
2.) Simple L2, Earth, Moon Geometry
in X-Y plane shows how Earth and Moon
light can strike OTE
Sun
projection
±10° roll shadow band, ±5°in MRD
•
Earth
Moon
37°
27°
•
L2
JWST
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JIM
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.
Earthshine typical example
OTE components
overhanging
sunshield coverage
will be illuminated by
Earth crescent
•Northern hemisphere
•Pitch = 0
•Sunshield Roll = 0
•Yaw = 45
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Scattered Earthshine can exceed the
zodiacal background at l> 3 mm.
Earthshine
Zodi
Worst case
assumes:
• 100% of 1 mirror
(SM or PM)
• 1% dust
• Nominal BRDF
Moonlight is less
important (1-3%
Zodi)
From Larry Petro
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JIM
Beckman analyzed one DRM for
Earth/Moon Impacts
•
For analysis, he used:
•
•
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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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Results
•
•
70% of observations were “dark”
Earth and Moon each affected 25% of observations:
•
•
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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.
JIM
Earth seen in L2 XY plane
Earth
1.5Mkm
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.
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A rough idea of the constraint
and how it changes per year
Pattern repeats
• 90 L2/2
•180 L2
Increased
scattered
light
regions
NEP FOR in JWST frame
NEP
•~1 year
Can create
shorter
observing
seasons and
impact 180 day
repeats
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Z
JWST
L2
JIM
JWST
3 weeks
Y
Later
L2
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
•
•
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6 wheels = 40 n-m-s
4 wheels = 22 n-m-s
JIM
Schematic Maneuver Sequence
Possible
Additional
Momentum
Unload
21-day
Tracking
Arc
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Momentum
Unloads
(~ 1 day prior
to SK maneuver)
Station-Keeping
Maneuvers
(8 per rev,
~ 22 days apart)
Momentum accumulation is dominated
by roll offsets in current design
1/5th of 22 day
total accumulated
in one day!
dJ/dt
Pitch
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Roll
JIM
Comparison of momentum accumulation for
both new sunshields designs
Current design
(negative dihedral)
Positive dihedral
alternative
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Note
significant
angular
momentum
due to pitch
alone
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.
•
•
•
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Actively Manage momentum by balancing angular
momentum build-up over each 22 day period (and
potentially beyond) in the LRP.
•
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Constrain roll orientation and start-dates
Significantly decreases scheduling flexibility.
Failed observations will necessitate replan since all observations
would be shifted
Increases science return, but may create a a very brittle schedule.
JIM
Monitor Study:
DRM shows 30-40% of dump intervals less
than 22 days
Cumulative Distribution of Dump Intervals
40 Nms Limit, JMS v1.0_wmk135
100%
.
80%
of Dump Intervals
Cumulative Fraction
.
90%
70%
60%
50%
10 Day Visit Limit
40%
1 Day Visit Limit
30%
20%
10%
0%
0
20
40
60
80
100
120
140
Time Since Previous Dump [Days]
Monitor method will not work. Fails in 30-40% of
cases even with all reaction wheels working.
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160
Restricting to an average momentum :
10 day observations need special planning to
avoid excessive momentum build-up
Available Start Time per Year with Early Visit Start for 10 Day Visit and
Mean Momentum Limit of 40 Nms / 22 day
0°
Ecliptic Latitude
30°
15°
45°
60°
75° 90°
360
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
0.0 Day
0.5 Day
Availability per Year [Days]
300
1.0 Day
1.5 Day
240
2.0 Day
180
120
60
0
0%
10%
20%
30%
40%
50%
60%
Cumulative Fractional Sky Area
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70%
80%
90%
100%
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¡
Ecliptic Latitude
30¡
15¡
45¡
60¡
75¡ 90¡
360
0.0 Day
0.5 Day
Availability per Year [Days]
300
Note drop in available
Start-time at high
ecliptic latitudes even
for a 1-day early visit
1.0 Day
1.5 Day
2.0 Day
240
180
120
60
0
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Cumulative Fractional Sky Area
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Restricting average momentum method will be very
constraining…worth going to total momentum
JIM
management
Restricting average momentum : Loss of
reaction wheel leads to drastic constraints
Available Start Time per Year with Early Visit Start for 1 Day Visit and
Mean Momentum Limit of 24 Nms / 22 day
0¡
Ecliptic Latitude
30¡
15¡
45¡
60¡
75¡ 90¡
360
0.0 Day
0.5 Day
Availability per Year [Days]
300
1.0 Day
1.5 Day
2.0 Day
240
Note loss of all flexibility
above 45° even for 1 day
observations.
180
120
60
0
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Cumulative Fractional Sky Area
Restricting average momentum method is not viable.
Must go to total momentum
management or change
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vehicle
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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Summary
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Overall scheduling of JWST has become more complicated
and may significantly impact JWST science:
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•
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2/19/04
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.
JIM