Emergency Call Delivery Standards Impair Cellular Positioning Accuracy

Cellular location based services need to use a variety of positioning methods in order to fulfill regulatory requirements in terms of availability, response time and accuracy. Each positioning method generates results in its own format. In the wideband code division multiple access (WCDMA) standard, 7 geographical formats are therefore available for reporting from the radio network (RAN) to the core network (CN). Unfortunately, the standards used in the US for emergency call delivery support only the ellipsoid point with uncertainty circle format when delivering the calculated position to the public safety answer point (PSAP). This limitation therefore requires geographical shape conversion algorithms to be applied in the cellular network. Three new such conversions are derived in the paper. The paper also shows by several examples that the shape conversions that are needed due to the reporting restriction can increase the inaccuracy by more than a factor of 30 for A-GPS positioning and by more than a factor of 200 for RTT positioning, as measured by the area of the reported format. This makes the current location reporting standards a major bottleneck for fulfilling E911 Phase 2 requirements. The obvious solution is to enhance the number of reporting formats for emergency positioning reporting to the PSAP, and to provision supporting software systems for distribution of graphical position information to field personnel.