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GNSS ServicePerformance Commitments...initial thoughts for consideration
ICG Workshop on GNSS Interoperability, Munich, Germany
March 2-3, 2009
Mr. David SteareJacobs Technology Inc
(Contractor Support, USAF Directorate of Space Acquisition)
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 20082
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
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• 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
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ICG3 Review: US WG-A Charts(Excerpt 2 of 2)
Line of Demarcation
USER SEGMENTCONTROL SEGMENT
SPACE SEGMENT
Signal In Space
INTERFACE
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• 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
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Per Satellite Signal In Space (SIS) Basis
Lesson Learned: Using “per satellite SIS” as the basis for performance commitments fosters greater interoperability amongst GNSS services
Performance Commitment Categories
I. Constellation Definition
II. SIS* Coverage & Minimum Received Power
III. SIS Accuracy
IV. SIS Integrity
V. SIS Continuity
VI. SIS Availability
Combinations of “essential parameters” and/or user equipment assumptions allow for derived standards
*SIS: Signal In Space 7
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
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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
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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)
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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
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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
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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)
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Performance Commitment Categories
*Trackable- refers to a SIS which can be preprocessed by a receiver sufficiently to be categorized as healthy or not
• 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
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Consistency with Aviation and Maritime Communities
*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)
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
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Position Accuracy Allocation (Cont)
DOP Allocation:
• Constellation Slots
• Slot Occupancies
UERE Allocation:
• GPS SIS URE
DOP Variations:
• Number of Channels
• Satellite Selection
• Mask Angle
• Vertical Aiding
UEE Variations:
• Dual-/Single-Frequency
• Troposphere Algorithm
• Multipath Environment
• Receiver Technology
Position Accuracy
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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
Your feedback & suggestions are requested
1.Proposed New ICG Principle
Every GNSS provider should establish documented civil performance commitments to inform users about minimum levels of service
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Request for Feedback
2. 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]
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Request for Feedback (Cont)
3. Traceability to IFMEA* & System SpecificationsGPS 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.
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Request for Feedback (Cont)
*IFMEA: Integrity Failure Modes and Effects Analysis
4. 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!
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Request for Feedback (Cont)
4. Performance Commitment ParametersI. 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
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Request for Feedback (Cont)
4. Performance Commitment ParametersIV. 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*
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Request for Feedback (Cont)
*Desired/Beneficial Parameter
• 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
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Way Ahead
Send feedback & suggestions (items 1-4) to:
Mr. David Steare
c/o GPS Cell
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Way Ahead (Cont)
