ISS Rendezvous, Proximity Operations, Docking

1
ISS Rendezvous, Proximity Operations,Docking
Berthing Considerations

• April 25, 2005
• Presented and edited by Al DuPont
• NASA/JSC/Aeroscience Flight Mechanics Division
• Data Information content provided
• by David Strack and Brian Rishikof
• Odyssey Space Research

2
Topics

• Background
• Introductory Charts Regions Around ISS, Sample
  Trajectory, Safe Free Drift Examples, Safe Free
  Drift Drag Effects
• ISS Safety Considerations
• Trajectory Considerations
• Navigation Considerations
• Control Considerations
• Docking/Capture Considerations
• Monitoring and Commanding
• Crewed Vehicle
• Demonstration Flight Considerations

3
Background

• This presentation was assembled by people working
  to support the integration of vehicles into the
  ISS (ATV, HTV, and SLI, OSP)
• Areas not included are launch, far rendezvous,
  berthing, deorbitalthough some aspects of
  monitoring are included, the broader aspects of
  ground and flight operations is not covered
• It was put together to provide a guide to basic
  constraints and considerations as opposed to a
  comprehensive set of requirements
• The presentation does not capture all
  requirements and considerations
• Despite the distinct groupings in this
  presentation, the requirements and
  considerations are often interrelated
• The bottom linedesign for safety of the ISSand
  be able to prove it.

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Regions Around ISS
Approach Ellipsoid
Keep-out Sphere (200m radius)
4km
V-Bar
2km
R-Bar
3 Sigma Dispersion
3 km radius spherical comm coverage
Out of plane minor axis of AE is 2km
Chart taken from a presentation prepared by Paul
Lane/USA in support of Mission Operations Director
ate and modified.
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Sample Trajectory
Spherical Space-to-Space Comm Range (3km)
Directional Space-to-Space Comm Range (30km)
KOS
AE
V-Bar
KOS
R-Bar
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ISS Safety Considerations

• The ISS Safety Requirements Document (SSP 50021)
  provides safety requirements that can effect a
  vehicles design. A few are paraphrased below
• Two Fault Tolerance for ISS Catastrophic Hazard
• Single Fault Tolerance for Critical Hazard
• Design for minimum risk where fault tolerance is
  not practical
• Two inhibits on functions whose inadvertent
  operation may cause a hazard
• Three inhibits on functions whose inadvertent
  operation could result in a catastrophic hazard.
  Two of the three inhibits shall be monitored.
• Monitor and control any function whose loss could
  result in a critical hazard
• Reporting of hazard, loss of function, change of
  inhibit status and change of monitoring status in
  time to control the hazard or compensate for the
  change
• SSP 50021 was not designed for free-flight phase
  so additional requirements may also apply

7
ISS Safety Considerations (2)

• The most likely hazard for free flying vehicles
  is collision. To mitigate this hazard the vehicle
  designers could
• Meet the Fault tolerance requirement for all
  systems (or design to minimum risk where
  appropriate)
• Pair being less than two fault tolerant with the
  ability to safely abort the operation and leave
  the vicinity of the ISS
• Show that collision does not create a
  catastrophic failure on the ISS
• The vehicles designers must receive concurrence
  on all safety related issues from ISSP and
  appropriate review panels

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ISS Safety Considerations (3)

• There are still other Safety related requirements
  that have been imposed on vehicles flying near
  the ISS
• Requirements implemented through Segment
  Specification Documents Interface Requirements
  Documents as opposed to standard SSP documents
• Fail Safe The system must be automatically (for
  uncrewed vehicles) fail safe or initiate a
  collision avoidance maneuver while in free flight
• Safe Trajectory Targeting and Trajectories must
  be designed such that the safety of the ISS is
  preserved
• No ISSP fail safe requirements exist for vehicles
• Baseline rules exist and may become requirements
  in the incoming vehicles Segment Specification
  or Interface Requirements Document
• The vehicle shall not complete rendezvous to the
  vicinity of the ISS if the vehicle is zero fault
  tolerant to catastrophic hazard
• The system shall automatically initiate a
  Collision Avoidance Maneuver (CAM) if a failure
  occurs that leaves the vehicle zero fault
  tolerant while in the vicinity of the ISS
• Design must consider how to handle failure cases
  that lead to a zero fault tolerant vehicle while
  in the docking/capture process

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ISS Safety Considerations (4)

• Computer Based Control Safety Systems
  Requirements (SSP 50038) will likely have a
  strong influence on vehicle design. A few are
  paraphrased below
• Overrides shall require at least two independent
  actions by the operator
• Need two independent commands to deactivate
  critical capabilities
• Separate control path for each inhibit used as a
  control
• Alternate functional paths shall be separated for
  critical functions
• A processor shall not independently control
  multiple inhibits to a hazard
• Safety requirements may also have a strong impact
  in other systems
• Payload handling
• Laser safety
• Battery safety
• Etc.

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Trajectory Considerations

• Trajectories must be designed such that ISS
  safety is preserved
• There are no ISSP requirements documents that
  dictate trajectory requirements, however there
  are concept documents and precedence
• Refers to all potential trajectories a vehicle
  may take when all dispersions (typically up to 3
  sigma) are taken into account (including GNC,
  environment, failures, etc.)
• Cases for failure to dock or to be captured by
  the SSRMS may have complex trajectory issues due
  to interaction with the ISSmay need special
  systems to ensure a safe trajectory
• Safe trajectories must be defined for each region
  near the ISS
• Baselined regions defined in concept documents
• Approach Ellipsoid (AE) 4x2x2 km (SSP 50011)
• Keep out Sphere (KOS) 200 m radius (SSP 50011)
• Omni directional communications disk 3x1½ km
  (SSP 50235)
• Safe free drift trajectories should be employed
  when ever possible.
• 24 hour safe free drift trajectories prior to the
  maneuver that takes the vehicle inside the AE
• 24 hour safe free drift trajectories prior to
  entering the KOS when practical attempt to
  maximize safe free drift region
• Maximize safe free drift trajectory when
  practical inside KOS

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Safe Free Drift Examples
Far Field Approach
ISS
ISS
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Safe Free Drift - Drag Effects
With Aero Drag
No Drag
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Trajectory Considerations (3)

• Other baseline goals and considerations related
  to safe trajectory
• In the vicinity of the ISS, the vehicle must
  follow a predefined trajectory with predefined
  collision avoidance maneuvers planned for any
  point on the trajectory
• The vehicle must not be targeted through the ISS
  except for final approach
• Trajectories within Keep Out Sphere (200m) of the
  ISS must stay within defined corridors (a survey
  flight may have exceptions)
• The vehicle shall not get closer than 6 ft to any
  ISS structure - specific requirements for capture
  mechanisms and attachment points may have
  exceptions

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Trajectory Considerations (4)

• A vehicle must be able to execute a Collision
  Avoidance Maneuver (CAM) at all times for all
  mission phases
• Where applicable, a safe free drift trajectory
  may be used
• An active CAM (thrust to maneuver the vehicle
  away from the ISS) must be used when free drift
  would be unsafe, too lengthy, or difficult to
  monitor
• A CAM must put the vehicle on a 24 hour safe free
  drift trajectory
• Planned post CAM actions must keep the vehicle on
  a permanently safe trajectory
• CAM must take the vehicle outside the AE within
  90 minutes and must remain outside the AE
• During a CAM the vehicle must stop closing and
  establish an opening rate within half the
  distance from the ISS
• CAM in close to the ISS must begin with an
  opening rate

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Trajectory Considerations (5)

• Vehicle Sensors
• Trajectory can be effected by sensor range and
  field of view
• GPS blockage/multipath may impact vehicle
  trajectory
• Blockage of vehicle attitude sensors may impact
  trajectory
• Loss of lock/re-acquire capabilities may affect
  vehicles accelerations
• Structural Clearance
• Clearance may impact trajectory and will
  partially define approach corridor
• Plume Contamination and Thermal Restrictions
• Plume impingement (from vehicle or ISS) may
  impact trajectory
• ISS antenna blockage
• Trajectory must not block ISS antennas
• Lighting
• Lighting for adequate visual monitoring and
  sensor conditions may restrict the trajectory
  profile, the timing for trajectories, and even
  the time of year that maneuvers take place
• The goal may be for lighting to not limit
  activities

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Trajectory Considerations (6)

• Communication Requirements
• May require timing maneuvers to take place over
  ground stations or within range of a
  communications satellite
• no fly regions due to vehicle-to-vehicle
  communication blockages

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Navigation Considerations

• There are no ISSP requirements documents that
  dictate vehicle navigation requirements only
  concept documents
• The navigation requirements may go in the Segment
  Spec.
• The vehicle should not rely on ISS for
  determining, maintaining, and monitoring the
  vehicles absolute state (except perhaps while
  attached) (minimize impact to ISS)
• Crewed vehicles should not rely on the ISS for
  determining the vehicles state prior to
  departure
• Crewed vehicles may need to separate from dead,
  uncontrolled ISS and therefore should not rely on
  the ISS for navigation
• For safety, the vehicle should always know its
  navigation state and should monitor it with
  respect to defined limits
• Assess protection from common mode failure
• Non-identical systems provides most reliable
  solution
• Assess the impact of using sensors not originally
  designed for use near a large structures - Earth
  Sensors, Star Trackers, Sun Sensors, GPS

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Navigation Considerations (2)Using ISS Resources

• Using ISS resources for relative navigation
  presents certain challenges
• ISS inertial navigation not specified for high
  performance
• Large structure introduces rotational errors
  between navigation base and capture point for
  relative attitude determination
• US GPS systems on the ISS have visibility and
  pointing issues
• Use of Russian segment ISS GPS system and KURS
  based range/range rate capability will require
  coordination with Russians
• JAXAs ISS GPS system and their RF communication
  based range/range rate capability are not yet
  available and use of these system will require
  coordination with JAXA
• Russias KURS system has performance and location
  limitations
• Navigation systems that require RF communication
  with the ISS needs to be assessed (US, Japan,
  Europe, and Russia all have space-to-space
  communication systems)
• Existing laser reflectors can effect new laser
  systems
• Existing laser reflectors may not match required
  locations or design for new systems

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Control Considerations

• There are no ISSP requirements documents that
  dictate vehicle control requirements however ISS
  constraints may drive control specification
• Vehicle control performance requirements can
  depend on several factors
• ISS control characteristics
• ISS attitude hold mode a major factor (e.g., ISS
  at LVLH TEA)
• Different ISS attitude control modes and options
  result in different ISS control motion
  characteristics
• Russian control system vs US system with Russian
  RCS
• TEA hold vs fixed attitude hold
• Design for degraded ISS can strongly impact
  control constraints
• Moment arms between c.g. and capture point can
  drive vehicle design
• ISS loads constraints for allowable contact
  conditions

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Control Considerations (2)

• Examples of vehicle control system functional
  requirements
• Must de-activate upon docking contact and/or
  capture
• Ability to inhibit jet firing (following safety
  inhibit requirements)
• Ability to re-activate in time to ensure safe
  trajectory after separation (docking and
  berthing)
• Ability to re-activate in time to safe vehicle
  after failed capture
• The control performance requirements may define
  several vehicle items
• Type of controller, size, placement and number of
  jets, control cycle frequency, guidance and
  navigational capabilities
• The control functional requirements may define
  several vehicle items
• Avionics architecture, communication design, CDH
  architecture, command and control design, etc.

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Docking/Capture Considerations

• Vehicles must design their GNC to meet
  pre-specified docking or capture conditions (ISS
  specific)
• Docking mechanism capture performance may limit
• Lateral and rotational misalignment, Lateral and
  rotational rates, Minimum closing velocity
• ISS structural docking load allowance may limit
• Maximum closing velocity, Lateral and rotational
  rates
• Capture mechanism may limit
• Relative position, Relative rates
• ISS flex motion can play an important role in
  both capture performance and loads
• Motion during post-capture/pre-berthed phase may
  impact structure, avionics, and sensors (ISS
  specific)

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Docking/Capture Considerations (2)

• Vehicle must be able to recognize a failed
  capture and be able to recover (complete,
  back-out/retry, abort)
• To recover without abort the vehicle may need to
  understand its inertial and relative state and
  status of the ISS
• Vehicle must be able to recognize a capture with
  failed completion and be able to recover
  (complete, back-out/retry, abort)
• ISS may be in free drift for a significant time
  under this scenario
• ISS/vehicle relative attitude may have
  significant offset
• Vehicle should limit requirements on ISS to
  support separation from a docked/captured
  condition
• ISS attitude, control modes, Array orientation
• De-docking mechanism preparation
• Monitoring and commanding systems, Navigation
  system, communication systems
• If separation mechanism fails, vehicle must have
  an alternate method of separating that meets
  nominal separation requirements

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Monitoring and Commanding

• There are no ISSP requirements documents that
  dictate ISS crew monitoring or control
  requirements, except
• There are some CBCS requirements (SSP 50038) that
  are related to this especially related to
  placing, removing, and monitoring inhibits
• There are Human Rating requirements that are
  applicable to an uncrewed vehicle flying to the
  ISS
• There are concept documents and precedents for
  monitoring and control considerations
• Many of the monitoring and control requirements
  may be placed on the vehicles crew instead of
  the ISSs crew for a crewed vehicle
• When in the vicinity of the ISS, the vehicle will
  be monitored by the ISS crew and by the ground
  personnel when possible

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Monitoring and Commanding (2)

• Visual Monitoring (onboard crew)
• May limit the regions and durations that the
  vehicle can fly
• May require specific trajectories
• Likely to impact approach/departure corridor
  definition
• Examples of Visual monitoring considerations
• Identify vehicle at 1 km (be able to see it)
• Determine approximate attitude at 500 m
• Evaluate trajectory inside 200 m
• Evaluate docking/capture conditions prior to
  docking/capture
• Visual monitoring requirements must be met for
  any lighting conditions
• May affect launch dates, rendezvous timing,
  target positions, etc.
• Vehicle data must be provided to the ISS during
  all nominal proximity operations and any time the
  vehicle is within 3 km of the ISS
• This information is used to monitor vehicle
  health, status and trajectory

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Monitoring and Commanding (3)

• The vehicle may need to have ISS-to-vehicle
  commanding capabilities
• For uncrewed vehicles, the Station crew must have
  independent command capability to abort, inhibit
  thrust and enable thrust (SSP 50011)
• For SSRMS capture this must also include a
  command to inhibit vehicle jets and a command to
  activate an alternate separation mechanism
• Vehicle/mission specific design may cause
  addition of other required commands
• For operational flexibility this may include such
  things as Hold/resume, Retreat to hold
  point/continue, go to Free drift, reconfigure,
  separate, remote piloting, etc.
• Time critical, safety critical ISS-to-vehicle
  commands must be through hardware command (as
  opposed to ISS laptop)

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Crewed Vehicle

• Crewed vehicles have a few additional
  considerations (based on experience with Shuttle,
  Soyuz, CRV)
• Ability to escape from a dead station
• Ability to escape within a given time limit
• Ability to allow for safe multiple vehicle
  separation
• Monitoring role may be done on the vehicle
  instead of the ISS
• Space-to-space voice communication is likely a
  requirement

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Demonstration Flight Considerations

• Demonstration is required prior to flight to ISS
• Precedents and concept document (SSP 50235)
• Demonstration flight can be to ISS under certain
  conditions
• Failure of any demonstration will not lead to
  hazard
• Functions/capabilities are proven in
  demonstration prior to being relied upon for
  safety
• Demonstrations must have pass/fail criteria that
  clearly demonstrates the function/capability
• There must be a reliable method to measure the
  pass/fail criteria
• ISS may require on-orbit test for key functions
  as part of verification

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Back-up Charts
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ISS Resources

• Attachment Mechanisms
• APAS Docking Mechanisms (used by the Shuttle)
• Probe and Drogue Docking Mechanisms (used by
  Russian vehicles and the European ATV)
• Common Berthing Mechanism (CBM) variety of
  utilities
• Payload Attachment Systems for unpressurized
  attachment
• Canadian Mobile Services System (MSS) used for
  capture, for manipulating payloads, for
  attachment at CBMs
• Japanese JEM Robotic Manipulator System for
  payload manipulation
• Navigation Support Equipment
• Laser Reflectors (Shuttle, ATV and HTV)
• Video Targets (SSRMS, ATV)
• Visual Targets (US, Russian)
• Range/range rate capability on JAXA communication
  system
• Kurs Radar system for Russian vehicles and ATV
• GPS Receivers/antennas (US, Russian, Japanese)

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ISS Resources (2)

• Communication Systems
• US space-to-ground (including TDRSS) for data,
  voice, and video
• Russian space-to-ground for data, voice and video
• US space-to-space for data and voice
• Russian space-to-space for data, voice, video
  (Russian ARC)
• Japanese space-to-space for data (HTV ARC)
• European space-to-space for data (ATV ARC)
• ISS command and data handling equipment (US and
  Russian)
• Voice communication equipment
• Monitoring equipment (cameras, lights, targets,
  windows, monitors, laptops, displays on hardware
  command panels)
• Command support equipment (laptops, hardware
  command panels, hand controllers)

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ISS Resources (3)

• ISS attitude control system
• ISS navigation system
• Rotational and Translational state information
• Raw GPS data for relative GPS navigation
• Crew members for ISS preparation, vehicle
  monitoring and commanding
• Links to ground control for ISS preparation,
  vehicle monitoring and commanding
• ISS command and monitor capability for ISS
  preparation and contingency trouble-shooting
• Utilities for attached phase support

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ISS EnvironmentOrbit

• Orbit ranges
• 278 460 km
• 51.3 to 51.9 degree inclination
• Orbit knowledge
• Spec from SSP 41000
• Position 3000 feet (RSS)
• Velocity Undefined
• Actual knowledge TBD (much better than 3000
  feet)
• Change in Orbit due to drag
• conditions
• non-emergency conditions

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ISS EnvironmentAttitude Control

• Different Attitude Control Modes are available
• TEA momentum management - low angular rates but
  non-fixed attitude and slow response to
  disturbance
• Assembly complete design range (SSP 41000, SSP
  50261)
• Range without Orbiter attached ?15? Yaw, -20? to
  15? Pitch, ?15? Roll
• Range with Orbiter attached ?15? Yaw, 0? to 25?
  Pitch, ?15? Roll
• Less than 3.5? variation per orbit (SSP 41000)
• LVLH hold - fixed attitude, faster response,
  higher angular rates
• ? 5? undisturbed, docking (SSP 41000)
• (SSP 41000)
• Controls to defined attitude such as 0,0,0
  average TEA DTEA (average TEA pitch with no
  out-of-plane component along the approach axis)
• Adjustable response to CMG saturation levels (set
  at low momentum levels for quick response and
  minimized rate changes for desaturation, set at
  high momentum levels for maximized time between
  jet firings)

Simplified description - many caveats and
details not included
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ISS EnvironmentPointing

• ISS specifications from SSP 41000
• Angular Alignment of US docking port 3.4?/axis
• Angular alignment of SM aft port 3.4?/axis
• Any non-articulatable point on ISS 5?/axis
  (under LVLH attitude hold)
• Angular alignment of Alpha Joint 4?
• Angular alignment of Beta Joint 6?
• Attitude knowledge 3?/axis
• Attitude rate knowledge 0.01?/s/axis
• Camera pointing accuracy (TBD)
• SSRMS pointing accuracy (TBD)

35
ISS Environment Blockage/Clearance

• Clearance and keep out zones
• ISS structural clearance (safety clearance
  envelopes, robotic arm/MSS pathways,
  articulating/deployable elements)
• Other vehicle approach/separation corridors
• Antenna visibility/radiation
• ISS crew and equipment viewing constraints
• Sun blockage/shadowing and thermal constraints
• ISS Solar and Thermal radiator orientation
• Should design to not impact ISS power/thermal
• May need to limit solar arrays motion to reduce
  RF signal multipath and/or blockage, plume
  impingement, sun reflection/blockage, etc.
• Can not stop motion of thermal radiators

36
ISS Environment Configuration

• Mass properties are not fixed or guaranteed by
  ISS
• Can design to range of properties for example
• Aerodynamic properties are not fixed or
  guaranteed by ISS
• Can use range of ISS configurations
• Aero properties change during the orbit
• Surface characteristics are not fixed or
  guaranteed by ISS
• Expect the ISS configuration to change during its
  life

37
ISS Environment Additional Environment
Considerations

• GPS Environment
• US antenna placements have significant blockage
  (array orientation has significant effect) and do
  not point zenith
• Japanese and Russian antennas have some blockage
• All have multi-path potential
• RF Environment
• Vehicle RF must not interfere with ISS RF or
  radiate sensitive ISS systems
• Vehicle must consider ISS RF for interference,
  and radiation of the vehicle
• ISS plume impingement on the vehicle vehicle
  plume impingement on ISS
• Contamination, thermal heating, structural loads,
  and torques
• Vehicle design should accommodate all lighting
  conditions
• Thermal and ESD constraints at attachment points
• Solar Beta impacts lighting and thermal conditions

38
SSRMS Free Flyer Capture

• Vehicles being captured by SSRMS are required to
  enter a Capture Box, station-keep for 5 minutes
  during which ISS prepares for capture and then,
  on command from the ISS crew, inhibit jet firing
• The size and shape of the Capture Box is defined
  by several factors
• SSRMS reach and stopping distance (partly a
  function of the vehicle mass)
• Relative state sensor position, orientation and
  field of view
• Residual relative velocity at free drift
• Attitudes and attitude rates of ISS and vehicle
  at free drift
• Position and orientation of grapple fixture
• Location of the center of mass with respect to
  capture point
• The structural envelope of both the vehicle and
  the ISS
• Attachment point of the SSRMS
• Crew direct field of view
• The vehicle and the SSRMS will likely have
  different electromagnetic charges so precautions
  must be taken to ensure proper electro-static
  discharge
• As a precaution against problems with the arm
  after capture but before berthing the vehicle it
  is recommended to plan for 24 hour contingency
  operations on the arm

39
SSRMS Capture Failure Recovery

• Vehicles must be able to recover from a failed
  SSRMS capture
• There are a number of different failures
• Vehicle drifts out of reach
• Vehicle bumped by the SSRMS Latching End Effector
  (LEE)
• Vehicle is hit by LEE snares during capture
  attempt, but no capture
• SSRMS goes to safe mode and cannot capture the
  vehicle
• Accommodations will need to be made for each
  scenario
• Vehicle can only be re-activated by ISS Crew
  command
• SSRMS may still be in the vicinity
• Both the ISS and the vehicle may be out of
  attitude
• Vehicles must be able to recover from a capture
  with failed rigidization
• This situation causes a series of problems
  because the vehicle can still rotate
• The grapple fixture will eventually contact the
  inside of the LEE and may damage the LEE
• The vehicle can rotate such that it can contact
  with the SSRMS booms and potentially set up a
  catastrophic hazard
• This can put both the ISS and vehicle out of
  attitude for separation
• The vehicle will need an alternate separation
  method that can be commanded by the ISS crew