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LADEE Mission Lunar Atmosphere and Dust Environment Explorer LDEX

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LADEE Mission Lunar Atmosphere and Dust Environment Explorer LDEX Payload Accommodation Study (PAS) 5/14/09. Payload Accommodation Study. 1. Instrument Description 2. Driving Instrument Interfaces Electrical Power Interface Command and Data interfaces Software Interfaces

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LADEE MissionLunar Atmosphere and Dust

Environment ExplorerLDEX

Payload Accommodation Study (PAS)


payload accommodation study
Payload Accommodation Study

1. Instrument Description

2. Driving Instrument Interfaces

  • Electrical Power Interface
  • Command and Data interfaces
  • Software Interfaces
  • Mechanical Interfaces (mass & CG)
  • Thermal Interfaces

3. Instrument FOV Requirements

4. Driving Environmental Interfaces (inc. radiation, contamination, EMI)

5. Driving Operational Requirements

  • OPS Concept (including instrument modes, etc)
  • Operational Constraints

6. Programmatic Requirements

  • Schedule (including payload I&T and System I&T support)

7. Potential Trade Studies:

1 instrument description
1. Instrument Description
  • Electronics and detector packaged in the same ‘box’ with the c.g. approximately in the geometric center
1 instrument description1
1. Instrument Description
  • LDEX detects dust particle impacts over an aperture of approximately 100 cm2
  • LDEX determines the mass of individual particles with radii ~ 0.3 < r < 5 mm
  • LDEX sets an upper limit for the number density of particles with radii ~0.1 < r < ~0.3 mm
  • LDEX measures the charge generated by dust impacts
  • The aperture must point within 5° of ram direction for primary science
  • Measurements are not possible when sun is in the UV FOV (damage to instrument if HV is on)
2 driving instrument interfaces
2. Driving Instrument Interfaces
  • Electrical Power Interface
  • Command and Data Interfaces
  • Software Interfaces
  • Mechanical Interfaces (mass & CG)
  • Thermal Interfaces
2b command data interfaces
2b) Command & Data Interfaces
  • Asynchronous RS-422
    • One Hertz Pulse (separate lines to instrument) for time sync
    • 33.6 kbps will allow full-resolution science data during I&T and any other opportunities during mission
      • 57.6 kbps also acceptable
  • Telemetry rates
    • 1000 bits/sec (science mode)
    • 300 bits/sec (standby mode)
  • 5.2 million bits/orbit
    • Assuming 65% science and 35% standby for a 113 min orbit
2c software interfaces
2c) Software Interfaces
  • Will need time synchronization signal from S/C
  • Need to understand plan for spacecraft clock to design LDEX telemetry processing system – UTC, MET, other?
  • A “sun in FOV” warning message from S/C is desired to protect the instrument if stored commands run out
    • Instrument has built-in sun detection and autonomous switch to safe mode when sun in UV FOV as a backup
      • Return to science mode only via ground command
  • We have a strong preference to use CCSDS TC and Telemetry Packets to simplify the interface to the S/C
    • Allows reuse of existing ground and operations software
  • S/C will not have to do any manipulation (compression, etc) on the LDEX telemetry data
2e thermal interfaces
2e) Thermal Interfaces
  • LDEX will be thermally isolated from S/C bus using passive thermal design to stay within instrument operating temperature range under nominal (powered) conditions
  • A thermostatically controlled survival heater (separate power line from S/C) will be used to maintain the instrument within its survival temperature range
3 instrument fov requirements
3) Instrument FOV Requirements
  • Primary concern is to prevent, as much as possible, UV reflections from the lunar surface from entering LDEX aperture, which has a 60° half-cone UV FOV
  • Minimum required dust FOV for science is simply the aperture area extended to infinity
  • Required pointing: normal to ram +/- 5 deg
  • Pointing knowledge (from post-processing) +/- 1 deg
4 driving environmental interfaces
4. Driving Environmental Interfaces
  • Nominal Launch Environment
    • Vibration, shock, acoustics: need environment description to complete the design
  • Nominal Space Environment
    • Radiation, charging: need environment description to complete the design
  • I&T Environment
    • Cleanliness
      • Contaminants on impact target surface can degrade UV rejection
      • Aperture cover provides mitigation (also helps with purging, described below)
    • Humidity
      • MCP can be damaged by water vapor
      • Instrument maintained under N2 purge except for brief durations (tests, whenever precluded by S/C movement, etc)
      • Will be an issue during encapsulation in LV fairing
    • See Trade Studies slide for more details
5 driving operational requirements
5. Driving Operational Requirements
  • LDEX cannot be pointed at the sun while in science mode (HV ON)
    • Planning should schedule commands to revert to standby mode when the sun is predicted to be within 5° (TBR) of the UV FOV
    • Therefore, LDEX data rate will probably (depending on orbit b angle) be reduced when LADEE crosses the night-to-day terminator and for some time after that (again, depending on orbit properties)
  • Instrument detects when the sun is in the FOV and protects itself from accidental sun pointing
  • Request the S/C to alert instrument within 5 sec of a sun pointing event (if feasible)
  • Instrument would like to be powered and in science mode as much as possible, even when not pointed in ram direction (useful science data can be acquired in most orientations)
7 potential trade studies
7. Potential Trade Studies
  • Deployable aperture cover for contaminant and water vapor protection
    • Status: hinged, spring-driven, one-time actuation aperture cover is now included in mechanical design
    • Mass and cost included in instrument lien list
  • MCP and/or target heater for contamination mitigation
    • Status: under investigation
  • T-0 (flyaway) purge would aid contamination mitigation and risk of MCP damage during launch activities
    • Status: under investigation at mission level
  • Techniques to safe instrument when sun in aperture
    • Status: under investigation to determine if S/C can provide an HV-OFF command if Sun nears LDEX FOV