GPS World, March 2011

TRANSPORATION Road Open Sky Dense Urban SIGNATURE 1 3 COTS Warm 28 136 COTS Hot 1 2 COTS Assisted 8 9 TABLE 1 Time to first fix results Mean TTFF s FIGURE 6 RAIM Impact red no RAIM yellow RAIM ing a signal generator to control the strength of the received signal to assess acquisition and tracking sensitivity At 145 dBm the SIGNATURE receiver takes an average of 11 sec seconds to acquire 12 satellites Positioning Algorithms ations have been investigated in the positioning algorithms including applying outlier detection routines at different stages of the FIGURE 6 surements contaminated by multipath By removing these meaurements and re computing the OBU location better position estimates are obtained FIGURE 7 shows the accuracy and integrity of the SIGNATURE solution assessed using a high grade GNSS INS reference in Nottingham city center In this case the horizontal accuracy is 44 meters 95 percent and the computed protection level is shown to bound the actual position error with In rural semi urban and urban Nottingham conditions a positioning solution has been demonstrated that supports all charging accuracy integrity and availability requirements FIGURE 7 Position error and protection level Nottingham city center FIGURE 8 HPL GPS vs GPS EGNOS Further tests were also conducted in the center of London in a worst case obstruction environment In this area the for this project In such cases better performance could be obtained using a hybrid solution making use of additional sensor inputs but this will increase equipment costs and potentially installation costs too A more practical approach may be to simplify charging schemes in the densest urban environments using zones and cordons rather than using more detailed approaches that require a continuous highperformance positioning solution to be maintained in all conditions Benefits of EGNOS Data The SIGNATURE solution has the ability to provide EGNOS data to positioning algorithms on the OBU and to vary the rate at which this information is updated and used Field tests have assessed the potential ben from the case in which EGNOS messages are continuously available for the positioning solution and then investigating algorithm This led to more consistent measurements and produced lower HPL values FIGURE 8 shows a comparison for a Nottingham test in which a GPS only solution is compared against an EGNOS solution in which a full set of corrections is provided This reduction in HPL values through the application of EGNOS corrections is clearer when the distribution of HPL values falling into discrete bins is assessed FIGURES 9 and GPS World March 2011 www gpsworld com 34