GNSS Service Performance Commitments ..itial thoughts for consideration

1. GNSS ServicePerformance Commitments...initial thoughts for consideration ICG Workshop on GNSS Interoperability, Munich, Germany March 2-3, 2009 Mr. David Steare Jacobs Technology Inc (Contractor Support, USAF Directorate of Space Acquisition)

2. Background and Purpose • Proposed ICG Principle: Every GNSS provider should publish Performance Commitments* • The publication of Performance Commitments by each GNSS provider will help to quantify and support interoperability amongst GNSS services • This briefing provides an introduction and suggests initial information to be considered by all GNSS service providers and the ICG community • This briefing is NOT intended to convey a US Government position; rather, it is provided to facilitate a working-level discussion and exchange of ideas to inform each GNSS service provider’s sovereign decision making process • *US WG-A Presentation on Compatibility and Interoperability, 3rd ICG meeting, December 2008

3. ICG3 Review: US WG-A Charts(Excerpt 1 of 2) • Service Assurance: user confidence or provider commitment that a system will provide a specified level of service • Each new system should add value and not just contribute to the noise floor • Compatibility and interoperability are only the first steps to establishing a new service • Like interoperability, service assurance is multidimensional: • In the case of the L1 and L5 multi-platform signals, service assurance should include the “open and free” provisions • Includes minimum performance levels for metrics like accuracy, availability, and integrity • Must address management and maintenance of the system • Some dimensions are more important than others • Just like interoperability, different receiver manufacturers and different user classes will accept different levels of service assurance

4. ICG3 Review: US WG-A Charts(Excerpt 2 of 2) • The GPS SPS Performance Standard could be a basis for establishing many of the parameters associated with service assurance • GPS standards could also be used as a starting point for establishing minimum performance levels desired or provided from other systems • Accuracy, availability, integrity, etc • Issuance of international NANUs prior to any scheduled maintenance, and after the onset of any unscheduled outages • Some dimensions of service assurance are qualitative—no widely adopted definitions or hard thresholds exist for them • Backwards compatibility • Mature maintenance practices • Commitment to maintain a complete constellation of satellites • Individual providers will have to assess the need, desirability, and commitment for each parameter Proposed new ICG principle: Every GNSS provider should establish documented civil performance commitments to inform users about minimum levels of service

5. Line of Demarcation SPACE SEGMENT Signal In Space INTERFACE CONTROL SEGMENT USER SEGMENT

6. Per Satellite Signal In Space (SIS) Basis • The Signal in Space (SIS) interface typically represents a transition point from GNSS service provider to user responsibilities and control • GPS SPS Performance Standard has evolved • 4th Edition published in Sep 2008 (previously in 2001, 1995, and 1993) • Majority of parameters are now defined as being applicable to “any SIS” (i.e., in such cases the performance commitment refers to the individual signal in space transmission from each satellite) • A “per satellite SIS” approach for performance commitments allows a multi-GNSS user to determine the contributions from each satellite used to compute his/her positioning, navigation, and timing solution • If a Service Provider merely publishes constellation-level commitments, then contributions from individual GNSS satellites are unclear • Constellation-level parameters can often be derived Lesson Learned: Using “per satellite SIS” as the basis for performance commitments fosters greater interoperability amongst GNSS services

7. Performance Commitment Categories • Constellation Definition • SIS* Coverage & Minimum Received Power • SIS Accuracy • SIS Integrity • SIS Continuity • SIS Availability Combinations of “essential parameters” and/or user equipment assumptions allow for derived standards *SIS: Signal In Space

8. Performance Commitment Categories I. Constellation Definition Define the reference orbit specifications and tolerances for each satellite slot (i.e., satellite locations) in the constellation Rationale: Slot-based parameters are necessary in a multi-GNSS era to determine the contributions from each GNSS

9. Performance Commitment Categories II. SIS Coverage and Minimum Received Power Define the minimum received power and the geometric volume (3-dimensional space) applicable for the subsequent performance commitments Rationale: Users need to know the location where the GNSS service is provided as a function of a minimum received power Depends on: • Satellite antenna design & pointing accuracy • I. Constellation Definition

10. Performance Commitment Categories III. SIS Accuracy Define the error budget commitments (i.e., inaccuracy) attributed to the space and control segments Rationale: Users need to know the accuracy of the service to determine whether it is sufficient to meet their needs Includes: • User Range Error (URE) (pseudorange data set accuracy) • URE derivatives (e.g. rate & acceleration errors) • Timing error (i.e. to ultimately characterize the offset between the GNSS system time and UTC)

11. Performance Commitment Categories IV. SIS Integrity Define the trust which can be placed in the correctness of the information provided by the SIS Rationale: Users need to know whether they can rely upon the GNSS service as a standalone means of navigation or whether they require augmentation to meet their own requirements (e.g., Safety of Life) Includes the ability of the SIS to provide timely alerts to receivers when the SIS should not be used Comprised of: • Probability of a service failure • Time to alert • SIS URE “Not to exceed” tolerance

12. Performance Commitment Categories V. SIS Continuity Define the probability that the SIS will continue to be healthy without unscheduled interruption over a specified time interval Rationale: This information is required for users that plan their operations based on the likelihood of uninterrupted GNSS service Also address the timeliness of issuing an appropriate “Notice Advisory” both: • Prior to a scheduled event affecting service • After an unscheduled event affecting service

13. Performance Commitment Categories VI. SIS Availability Define the probability constellation slots will be occupied by satellites transmitting a trackable* and healthy SIS Rationale: Users need to know the likelihood that the GNSS service will be provided in accordance with the complete set of performance commitments in order to determine whether the service is sufficient to meet their needs • Address “per-slot” availability • Desirable/Beneficial to also address “constellation-level” availability (i.e. ‘X’ of ‘Y’ defined slots with ‘Z’ probability) *Trackable- refers to a SIS which can be preprocessed by a receiver sufficiently to be categorized as healthy or not

14. Consistency with Aviation and Maritime Communities • Desire for consistency of parameters between Performance Commitments and the ICAO SARPS & IMO SOLAS* • Aviation and maritime users both define their needs in terms of: • Coverage; • Accuracy; • Integrity; • Continuity; and • Availability *International Civil Aviation Organization (ICAO) International Standards and Recommended Practices (SARPS); International Maritime Organization (IMO) International Convention for the Safety of Life at Sea (SOLAS)

15. Example of a GPS Derived/Desired Performance Commitment: Position Accuracy • Position Accuracy depends on two factors: • Satellite-to-user geometry (i.e., the dilution of precision (DOPs)) • User Equivalent Range Error (UERE) • DOPs allocated between GPS SIS and Receivers • GPS SIS: constellation slots, number of healthy satellites • GPS Receivers: number of channels, mask angle, etc. • UERE allocated between GPS SIS and Receivers • GPS SIS: User Range Error (URE) • GPS Receivers: User Equipment Error (UEE) • GPS Performance Commitments cover GPS SIS performance allocations

16. Position Accuracy Allocation (Cont) • DOP Allocation: • Constellation Slots • Slot Occupancies • DOP Variations: • Number of Channels • Satellite Selection • Mask Angle • Vertical Aiding • UERE Allocation: • GPS SIS URE • UEE Variations: • Dual-/Single-Frequency • Troposphere Algorithm • Multipath Environment • Receiver Technology Position Accuracy

17. GPS SPS Performance Standard • The GPS Standard Positioning Service (SPS) Performance Standard was updated in September 2008 • Reflects GPS lessons learned over 15 years of experience • An example & potential basis for other GNSS service providers to consider when establishing many of the parameters associated with performance commitments • Freely available from the internet http://pnt.gov/public/docs/2008-SPSPS.pdf

18. Request for Feedback Your feedback & suggestions are requested • Proposed New ICG Principle Every GNSS provider should establish documented civil performance commitments to inform users about minimum levels of service

19. Request for Feedback (Cont) • GNSS Providers’ Template for Performance Commitments Create a template (as a cooperative ICG WG-A effort) that GNSS Providers could use on a voluntary basis when writing their own performance commitments [intended to increase standardization & interoperability]

20. Request for Feedback (Cont) • Traceability to IFMEA* & System Specifications GPS Lesson Learned: IFMEA & System Specifications provide the foundation for writing successful performance commitments. If interest exists, GPS could brief this topic in more detail at a future meeting. *IFMEA: Integrity Failure Modes and Effects Analysis

21. Request for Feedback (Cont) • Performance Commitment Parameters Provide your comments and suggestions regarding which parameters are “essential” or “desired” performance commitments. [Refer to the following charts for a listing] Are there new parameters applicable to a multi-GNSS world? Note: Suggestions to improve the content of the GPS SPS Performance Standard are also welcomed!

22. Request for Feedback (Cont) • Performance Commitment Parameters I. Constellation Definition • Reference Orbital Slot Parameters II. SIS Coverage and Minimum Received Power • Minimum Received Power • 3-Dimensional Service Volume III. SIS Accuracy • URE • URE Derivatives (i.e. rate and acceleration error) • Timing Error

23. Request for Feedback (Cont) • Performance Commitment Parameters IV. SIS Integrity • Instantaneous URE Integrity (i.e., probability of SIS URE exceeding a specified Not to Exceed) • Instantaneous Timing Error Integrity V. SIS Continuity • Probability of an Unscheduled Failure Interruption • Timeliness of Notice Advisories for both Scheduled and Unscheduled Interruptions VI. SIS Availability • Per-Slot Availability • Constellation-level Availability* *Desired/Beneficial Parameter

24. Way Ahead • A follow-up email to be provided to all interested workshop participants and ICG WG-A representatives • Timeline & Opportunities: • June 2009 (TBD): Next ICG WG-A meeting • Continue discussions and presentations on items 1-4 based on initial feedback • Others’ views & contributions highly encouraged • Sep 2009: 4th ICG meeting • Adopt ICG Principle on Performance Commitments • Draft Template for GNSS Providers’ Performance Commitments available for review

25. Way Ahead (Cont) Send feedback & suggestions (items 1-4) to: Mr. David Steare c/o GPS Cell david.steare@pentagon.af.mil