GPS World, September 2016

AUTOMATED VEHICLE FIGURE 7 Overview of the planned Austin area reference network Google Maps master The mm level scatter of these data points suggest that biases of the resolved reference station positions are also mm level Network Corrected Residuals FIGURE 6 displays similar data to Figure 5 except that the carrier phase residuals are those that remain after network corrections are applied Each data point corresponds to a particular subset of secondary stations together with the master and a particular rover selected at random from the remaining stations Both the size and specific selection of secondary stations comprising each subset were randomly selected In all 70 different network configurations and more than 367 million NRTK solutions were analyzed Figure 6 shows that carrier phase residuals after application of network corrections are considerably reduced compared to those original magnitudes seen in Figure 5 With increasing network density the DD residuals deviation asymptotically approaches a minimum value of about 4 mm which corresponds to an undifferenced deviation of 2 mm This floor is due to multipath at the rover Deviations in excess of this floor are caused by residual ionospheric errors and to a lesser extent neutral atmospheric errors Attributing the excess deviation entirely to residual ionospheric errors and assuming these are uncorrelated with multipath one can estimate from Figure 6 the undifferenced ionospheric uncertainty For example for a 50 km inter station distance σ ι 142 42 2 67mm To achieve the σ ι 2 mm recommended earlier for fast and reliable AR station separation should be no more than 22 km which we round down to a recommended value of 20 km to provide a margin of station redundancy NETWORK DEPLOYMENT We have developed and deployed a low cost reference network testbed in Austin Texas with site hosting courtesy of the Texas Department of Transportation The Longhorn Reference Network boasts a dozen stations with plans for 20 FIGURE 7 The networks average inter station spacing is far shorter than the 20 km spacing recommended earlier The tighter spacing provides redundancy 36 GPS WORLD WWW GPSWORLD COM SEPTEMBER 2016 FIGURE 11 GNSS antenna configuration A single baseline precise position solution between the primary antenna and the master reference station provides precise vehicle position A constrained baseline 2D attitude solution between the primary and secondary antennas provides heading and flexibility of experimentation The low cost reference stations are deployed in environments with greater multipath and signal blockage than those of the high quality stations studied earlier Such non ideal signal environments are to be expected in a dense low cost reference network for which choice of station siting is driven largely by opportunity The reference station design pictured in FIGURE 8 and diagrammed in FIGURE 9 is novel Each station is a self contained solar powered node supporting a software defined dual frequency dual antenna GNSS receiver with an always on cellular connection to university servers for data collection and software maintenance Live Vehicle Demonstration In partnership with Radiosense an FIGURE 8 Low cost reference station in the Longhorn Reference Network FIGURE 10 Demonstration route FIGURE 12 Low cost dual frequency rover system in the trunk of the vehicle