Transcript Document 7271439

GNSS Service
Performance 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)
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
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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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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
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Line of Demarcation
SPACE SEGMENT
Signal In
Space
INTERFACE
CONTROL SEGMENT
USER SEGMENT
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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
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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
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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
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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)
*Trackable- refers to a SIS which can be preprocessed by a receiver sufficiently to
be categorized as healthy or not
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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)
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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:
DOP Variations:
• Constellation Slots
• Number of Channels
• Slot Occupancies
• Satellite Selection
• Mask Angle
• Vertical Aiding
UERE Allocation:
UEE Variations:
• GPS SIS URE
• Dual-/Single-Frequency
Position
Accuracy
• Troposphere Algorithm
• Multipath Environment
• Receiver Technology
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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
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Request for Feedback
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 (Cont)
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 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
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Request for Feedback (Cont)
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 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
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Request for Feedback (Cont)
4. 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
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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
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Way Ahead (Cont)
Send feedback & suggestions (items 1-4) to:
Mr. David Steare
c/o GPS Cell
[email protected]
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