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.