1. ISS COTS Interface Requirements Document SSP 50808
SSP is an ITAR controlled document that identifies the requirements for rendezvous, proximity operations, and physically meeting the ISS interface. It will not be provided at this time. However, this referenced document has been added for this RFI to provide non-ITAR controlled high-level information regarding the requirements in SSP

2. 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

3. 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

4. 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.

5. Out of plane minor axis of AE is 2km
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
Directorate and modified.

6. Sample Trajectory V-Bar R-Bar Spherical Space-to-Space
Comm Range (~3km)
Directional Space-to-Space
Comm Range (~30km)
KOS AE
V-Bar
KOS
R-Bar

7. 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 was not designed for free-flight phase so additional requirements may also apply

8. 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

9. 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

10. 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.

11. 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: 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

12. Safe Free Drift Examples
Flies around ISS
Passes by before
crossing ISS altitude
Altitude variation does
not cross ISS orbit
Closest altitude
is out-of-plane
IN PLANE
OUT OF PLANE
(Looking down)
Far Field Approach
ISS
ISS

13. Safe Free Drift - Drag Effects
With Aero Drag
No Drag

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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.

22. 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)

23. 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

24. 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

25. 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

26. 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)

27. 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

28. 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

29. Back-up Charts

30. 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)

31. 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)

32. 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

33. 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

34. 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

35. 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)

36. 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

37. 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

38. 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

39. 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

40. 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