ITOW-Based Ephemeris Identification in CNAV Assistance Data
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Solution Overview
Problem
Existing GPS systems lack a mechanism to unambiguously identify ephemeris data in modernized GPS signals, such as L2C, L5, and L1C, which hinders the ability of mobile stations and Serving Mobile Location Centers to provide accurate assistance for location fixes.
Innovation Solution
A mechanism using the ITOW (Interval Time of Week) value is introduced to unambiguously identify ephemeris data in CNAV-1 and CNAV-2 messages, allowing the mobile station and SMLC to determine the age of ephemeris data and correctly pair Differential GNSS corrections with the appropriate ephemeris information.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If modernized GPS signals (L2C, L5, L1C) are used, then signal accuracy and availability are improved, but the ability to unambiguously identify ephemeris data is lost
Solution Approach 1:
The patent introduces an intermediary mechanism (IOD-E field in assistance data messages) that mediates between the modernized GPS signals and the assistance server. This intermediary structure allows the system to maintain ephemeris identification capability by using the Issue-of-Data-Ephemeris field as a bridge to uniquely identify ephemeris versions transmitted in modernized signals, thereby resolving the information loss problem while preserving signal accuracy benefits.
2Device complexity
If assistance server is introduced in A-GPS network, then computational burden on GPS receiver is reduced, but additional cellular communication task is imposed on GPS receiver
Solution Approach 1:
The patent applies segmentation by dividing the ephemeris identification function into separate components: the assistance server handles the complex task of generating and managing IOD-E values for multiple satellites, while the GPS receiver simply extracts and uses these pre-computed identifiers. This segmentation reduces the computational burden on the receiver by offloading the complex ephemeris management tasks to the assistance server, while the receiver's communication task remains manageable as it only needs to exchange standardized assistance data messages.
3Reliability
If IOD-E mechanism is added to modernized signals, then ephemeris identification is improved, but message structure complexity increases
Solution Approach 1:
The patent applies universality by designing the IOD-E field to serve multiple functions within the assistance data message structure. The same IOD-E mechanism used for identifying ephemeris data in CNAV-1 and CNAV-2 signals also serves to identify DGPS correction data sets and to indicate the current ephemeris version. This multi-functionality approach improves reliability of ephemeris identification while minimizing the increase in message structure complexity by avoiding the need for separate identification fields for each purpose.
Data Source
AI summary
A module (software or ASIC) for use in a serving mobile Location Center (SMLC) or a mobile station having a GPS receiver (or an A-GPS receiver) for determining an ITOW for a CNAV-1 signal. Also provided is a module for a mobile with GPS (or A-GPS receiver) so that the mobile can use the ITOW (either the ITOW provided with the CNAV-2, or that calculated for the CNAV-1) to uniquely identify ephemeris information in a request for assistance message. Also provided is a module for a SMLC by which the SMLC can use the IOD fields in the assistance data messages in a way that uniquely identifies the associated ephemeris information.