# Presentation: 116789

## 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

## 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

## 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, deorbit...although 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 line...design for safety of the ISS...and be able to prove it.

## Regions Around ISS*

- V-Bar

- R-Bar

- 4km

- Keep-out Sphere
- (200m radius)

- 2km

- 3 Sigma Dispersion

- Out of plane minor axis of AE is 2km

- 3 km radius spherical comm coverage

***Chart taken from a presentation prepared by Paul Lane/USA in support of Mission Operations**
**Directorate and modified.**

- Approach
- Ellipsoid

## Sample Trajectory

**Sample Trajectory**

- V-Bar

- R-Bar

- Directional Space-to-Space
- Comm Range (~30km)

**AE**

**KOS**

- Spherical Space-to-Space
- Comm Range (~3km)

**KOS**

## ISS Safety Considerations

- The  ISS Safety Requirements Document (SSP 50021) provides safety requirements that can effect a vehicle’s 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

## 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 vehicle’s designers must receive concurrence on all safety related issues from ISSP and appropriate review panels

## 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 vehicle’s 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

## 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.

## 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 ISS...may 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: 3x11⁄2 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

## Safe Free Drift Examples

**Far Field Approach**

**ISS**

**ISS**

## Safe Free Drift - Drag Effects

**No Drag**

**With Aero Drag**

## 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

- 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

**Trajectory Considerations (3)**

## 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

## 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 vehicle’s 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

## 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

## 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 vehicle’s absolute state (except perhaps while attached) (minimize impact to ISS)
- Crewed vehicles should not rely on the ISS for determining the vehicle’s 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

## 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
- JAXA’s 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
- Russia’s 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

## 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

## 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, C&DH architecture, command and control design, etc.

## 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)

## 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

## 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 vehicle’s crew instead of the ISS’s 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

## 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

## 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)

## 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

## 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

## Back-up Charts

## 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)

## 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 AR&C)
- Japanese space-to-space for data (HTV AR&C)
- European space-to-space for data (ATV AR&C)
- 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)

## 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

## ISS EnvironmentOrbit

- Orbit ranges
- 278 – 460 km
- 51.3 to 51.9 degree inclination
- < 0.01 degree eccentricity (expect < 0.003)
- 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
- < 1e-5 m/s2 (TBC) orbit average for non-emergency conditions
- < 2.2e-5 m/s2 (TBC) instantaneous for non-emergency conditions

## ISS EnvironmentAttitude 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, <2 peak to peak for shuttle docking (SSP 41000)
- <0.02/s undisturbed, <0.04/s with Shuttle plume (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**

## ISS EnvironmentPointing

- 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)

## 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

## 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

|  | Fwd | Aft | Port | Stbd | Zenith | Nadir |
| --- | --- | --- | --- | --- | --- | --- |
| C.G. X | 0.62 | -8.35 |  |  |  |  |
| C.G. Y |  |  | -3.27 | 0.33 |  |  |
| C.G. Z |  |  |  |  | 2.83 | 5.35 |

## 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

## 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

## 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