GPS World, March 2011

Road TRANSPORTATION FIGURE 1 illustrates the general concept of combined V2V and V2I or V2X In a fully operational system all vehicles and other road users carry short range communication and positioning technology At present these technologies are expected to be based on dedicated short range communication DSRC and GNSS respectively This enables each user to be location aware and capable of sharing their location with others Vehicles may use built in systems the occupants personal mobile device Infrastructure elements and other road users such as pedestrians also form part of the V2X user community V2X Relative Positioning Relative positioning of all communicating entities with respect to a given user is a required functional capability of a V2X system To enable this functionality positioning information from all communicating entities must be exchanged For automotive V2X applications Society of Automotive Engineers SAE J2735 DSRC Message Set Dictionary serves as the primary standard for message messages consists of a basic safety message BSM an optional variable rate message VRM and an option for including proprietary messages With BSM and VRM vehicle position speed heading and GNSS measurements can be communicated to others GNSS relative positioning techniques such as real time kinematic RTK code based differential or individual position differing that is distance between the positions reported by individual vehicles can be used for relative positioning The latter method also known as DPOS is a particular focus of this article Given the above a system developer may develop a V2X relative positioning system that can operate based on techniques that can be techniques which include DPOS and measurement based differential techniques including RTK and others The Simpler Approach The SAE J2735 BSM accommodates the simpler approach of using the DPOS method as it enables the sharing of critical state parameters This approach is very attractive as it requires minimal OTA data volume compared to sending GNSS measurements Secondly DPOS relative position estimation process requires only a fraction of the processing resources required compared to measurement based differential processing Thirdly any GNSS receiver in the market today is capable of outputting a position solution and most of the critical GNSS state parameters required for the V2X BSM In contrast most low cost devices do not output measurements required for other methods However there are quite a few challenges associated with DPOS A vehicle or any other road user entity such as a location enabled handheld device will share its location information via BSM only A relative positioning engine in each entity will use this information to provide lanelevel and road level data relative distance speed and orientation for its V2X applications The challenges associated with DPOS method arise from multiple stages in this process The presence of many road user types brings in the possibility of thousands of GNSS receiver types models hardware and software in the user group Thus the system must be interoperable with devices with a wide range of performance characteristics Secondly each entity will transmit BSM only This OTA information offers no information about the constellation the GNSS device sees or how the solution was derived in terms Thirdly the position accuracy reported by each entity is a GNSS device dependent variable an estimate of the actual error as derived by a user device FIGURE 2 Vehicle test set up Finally and most importantly V2X applications expect relative positioning information for each of three possible accuracy categories Which Road Which Lane or Wherein Lane see Is GNSS up to the V2X Challenge GPS World October 2010 The V2X system must be able to reliably identify this accuracy entity with the limited information provided via the BSM Study Goals To illustrate the impact of these challenges several GNSS receiver scenarios were investigated Between multiple receiver types In a V2X environment vehicles and other road user entities may have different GNSS receiver types and makes dual frequency singlefrequency and so on Same receivers using different parts of visible constellation In an urban canyon it is possible for two adjacent vehicles to see two different parts of the GNSS constellation due to obstructions WAAS enabled and non WAAS receivers Data Collection collections and a series of RF record test setup included two GPS receivers a GNSS L1 RF data recording device and a high quality GPS INS reference system FIGURE 2 Type A receiver is a high sensitivity enabled automotivegrade GPS L1 receiver using a patch antenna WAAS capable although WAAS usage was disabled in the realtime data collection Type B receiver www gpsworld com March 2011 GPS World 37