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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)

5/14/09


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



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