Advanced Land Imager ALI

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Section 10 Advanced Land Imager (ALI)
. . . Constantine J. Digenis MIT/LL ALI Program
Manager
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Advanced Land Imager (ALI)

• ALI is an instrument incorporating several new
  technologies that promise better performance, at
  a lower cost, of future Landsat missions.
• The relatively warm operating temperature of the
  HgCdTe detectors makes possible passive cooling
  of the focal plane which greatly simplifies
  instrument operation.
• ALI was designed, assembled, environmentally
  tested and calibrated by the MIT Lincoln
  Laboratory
• The Focal Plane System was supplied by Raytheon/
  SBRS
• The telescope was supplied by SSG Inc.

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Advanced Land Imager (ALI)
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ALI Block Diagram
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ALI Instrument Requirements
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ALI Verification Matrix
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ALI Calibration Matrix
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Internal Reference Lamps

• Three lamps in an integrating sphere, fed from a
  constant current source at reduced voltage,
  turned on sequentially.
• Lamp Custom version of Welch Allyn 997418-6,
  Krypton gas, color temperature 3085K, life 300
  hrs (700 hrs as used).
• Life testing 8 lamps, 12,000 cycles (30 sec on,
  30 sec off) followed by continuous on time until
  lamp burn-out.
• Minimum life 390 hrs (total on time),
• Average life 613 hrs,
• Maximum life 795 hrs.
• Total flight lamp on time to-date 32 hrs.
• Measured focal plane response change from Dec. 98
  (ALI T/V) to Jul. 00 (S/C T/VII), all 3 lamps on,
  focal plane at 220K.
• 2 , all bands,
• Repeatability of lamp readings 1 .

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Telescope Cover Cycles
Total Operating HoursExcluding Subsystem Testing
ALI FPE From ALI Assembly to S/C
Integration (17 Mar 99) 1,697 585 From S/C
Integration to 28 Mar 00 (Red Team Rev) 519
38 From 28 Mar 00 through 31 Jul 00 (Launch Sim)
270 10 Total 2,486 633
Mechanism Life Test (STM) 3,900 Ground Tests
(Flight ALI) 282 350-day Mission (4
DCEs/day) 1,400
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Problem Reports
PR Problem Resolution 12

• Condensable material deposited on the focal plane
  filters when the focal plane was at 220K in
  vacuum.
• ALICE did not respond to restart command.

• Focal plane was baked out at 273K. Condensable
  material effects were reduced by an order of
  magnitude. Plan in place to conduct additional
  bake-outs on orbit, as needed.
• Procedure error. Restart command was issued
  before execution of previous command was
  completed.

Note All issues resolved except source of
contamination (PR 12). The subject will be
discussed further by Mike Woronowicz.
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Summary of Open Items fromMarch Red Team Review

• Verification matrix (p. 10-18). No update
  necessary.
• Operating hours (p. 10-35). See p. 10-9 for
  current number.
• Technology effort to improve mirror stray light
  performance (p. 10-45). Work still in progress
  at SSG under direct NASA supervision (Dr. Ritva
  Keski-Kuha).
• Focal plane contamination (pp. 10-59 to 10-67).
  See following Special Topic.

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Statement of Readiness

• ALI demonstrates breakthrough technologies
• Large MS-PAN array operating warm,
• Wide-angle telescope,
• Silicon carbide mirrors,
• In-flight calibration scheme.
• Calibration tests have thoroughly characterized
  ALI.
• ALI operated nominally during T/V II.
• Flight test will validate Landsat data
  continuity.
• At least 200 scenes for comparison with Landsat
  7.
• ALI is ready for flight.
• No residual issues or Red Book candidates.

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Special Topic Decontamination of the ALI Focal
Plane
. . . Jeff Mendenhall MIT/LL ALI Systems Engineer
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History of ALI Contamination

• First detected during thermal cycling of ALI at
  Lincoln Laboratory - October 1998
• Focal plane contamination eliminated after FPA
  warmed to gt-5ºC (268K)
• ALI baked out under vacuum
• 1 week at 303K - early January 1999
• 2 days at 303 K - late January 1999
• 3 hours at 273 K - October 1999 (T/V1)
• 1 day at 273 K - July 2000 (T/VII)
• Virtual elimination of contamination effects
  observed after each bakeout
• Focal plane radiator heaters added to instrument
  to provide on orbit decontamination if needed -
  February 1999
• Contamination observed during thermal vacuum
  testing of spacecraft at GSFC - October 1999 and
  July 2000
• Visual inspection of focal plane after boil-off
  at the end of thermal vacuum testing indicated no
  contamination present on focal plane - November
  1999

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Detection of Contamination
Illuminated by Internal Source
F1, Fold Mirror
Telescope Axis
Illuminated by External Source
Cal Lamp Assembly
Main Focal Plane Array

• Comparison of internal reference lamp data with
  external source simulating a ground scene shows a
  reduction in the pixel to pixel variability in
  the latter case by a factor of 4.
• The difference is due to the different f-number
  of the telescope (7.5) from the internal
  reference lamp system (40).

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Summary of T/V II Contamination Monitoring

• 5 Pixel to Pixel (P2P) variations observed
  within 1 day of cooldown for all bands
  (corresponding to 1 for ground scene)
• No additional growth of P2P variations after
  first day
• No mean level changes observed
• Decontamination of FPA successful
• P2P variation again observed after second cool
  down

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History of Band 3 During 1st Half of T/V II
S/C from 293 K to 303 K, FPA at 255K for 12 hours
S/C at 273 K, FPA at 220 K for additional 19 hours
S/C at 303 K, FPA at 220 K for additional 9
hours,
S/C at 273 K, FPA at 220 K for additional 72 hours
S/C at 273 K, FPA at 220 K for additional 29 hours
S/C at 303 K, Following FPA transition to 273 K
and 220 K (20 hours)
S/C at 273 K, FPA at 220 K for additional 30 hours
Baseline data Start of T/VII at ambient
temperature and pressure
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Band 3 Decontamination During T/V II
FPA at 230 K
FPA at 260 K
FPA at 240 K
FPA at 265 K
FPA at 250 K
FPA at 270 K
FPA at 255 K
Baseline data Start of T/VII at ambient
temperature and pressure S/C at 303 K
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Comparison of T/V I II Contamination Monitoring

• Pixel to Pixel Variations
• 10 Pixel to Pixel variations observed during
  T/V1
• corresponding to 2.5 for a ground scene
• 5 Pixel to Pixel variations observed during
  T/VII
• corresponding to 1 for a ground scene
• Mean level variations
• No mean level variations observed during T/V1 at
  220 K
• No mean level variations observed during T/VII at
  220 K
• Lamp and focal plane stability
• 1 change in FPA response to reference lamp
  stimulus between T/V1 and T/VII (290 K and 220
  K)

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Decontamination Plan

• Continuous purging of the telescope cavity with
  Nitrogen removes any outgassing products from the
  telescope cavity during IT.
• Outgas ALI focal plane for 5 days following Day
  5 data acquisition.
• Monitor focal plane contamination using on-board
  reference lamp source.
• Conduct on orbit bake-out as needed to remove
  contaminants.
• Turn on bake-out heater on FPA radiator and all
  telescope heaters.
• Telescope door partially open.
• Duration warm-up, from 220 K to 275 K, 11
  hrs cool down 6 hrs dwell at warm plateau
  7hrs.
• Initial bake-out frequency is projected to be
  every 2 weeks.
• Preserves goal of 5 radiometric accuracy.

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Summary

• Contamination of ALI focal plane identified at
  Lincoln Laboratory, cleared through boil-off, and
  later observed at GSFC at reduced level
• On-board reference lamp sensitive and effective
  tracker of contamination
• Heating of focal plane to gt265K will drive off
  contaminant
• Focal plane radiator heater ability to outgas
  focal plane demonstrated during thermal vacuum
  testing at GSFC
• On-orbit decontamination will require 1 day
• Acquisition of VNIR data still possible during
  this period