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Using inertially-aided real-time kinematic technology as a land-based mapping tool
The technology - POS LV with IARTK The Position and Orientation System for Land Vehicles (POS LV) is designed to provide a full navigation (position and orientation) solution for land vehicles engaged in road surveying and mapping, road profiling, transportation mapping and GIS database feature acquisition. These applications require continuous position and orientation information while operating in areas where problematic GPS reception would be encountered. The system comprises four primary components, an inertial measurement unit (IMU), a two-receiver GPS azimuth measurement sub-system (GAMS), a distance measurement indicator (DMI), and a POS computer system (PCS). The vehicle-mounted POS LV system integrates inertial data from an IMU, containing accelerometers and gyros for each axis. The IMU is designed to measure the position and orientation differences and provide a true representation of motion in all axes. Velocity aiding and linear distance traveled data is derived from the DMI attached to one of the vehicle’s rear wheels. The embedded GPS receivers in the PCS computer provide additional position and velocity information with further heading assistance coming from the GAMS subsystem. The RTK corrections data are received from external sources. In its tightly coupled navigation mode (IARTK), the POS LV system does not use the position and velocity data provided by the GPS receiver, rather POS LV automatically integrates raw GPS satellite data directly into the system. This allows the POS LV to utilize position-aiding data from just one GPS satellite observable (CA code), when all others are experiencing lockout. This robust positioning solution in its simplest form is a combination of GPS data and dead reckoning navigation. In conjunction with the combined integration of GAMS heading aiding, and DMI integrated velocity aiding, the position-error growth during a GPS outage is thus severely restricted. In this configuration, the POS LV system is able to deliver decimeter-level positioning accuracy, which gracefully degrades over time with distance traveled, together with attitude accuracies of better than 0.05 degrees.  Figure 1. King Street A mobile mapping campaign to evaluate the POS LV system was carried out in Toronto, a major Canadian city with all the attributes associated with a modern metropolis, tall buildings, multi-level roadways and a busy downtown area with numerous road infrastructure features. Two areas were chosen that would present the mapping vehicle with real-world conditions regarding problematic GPS reception. Figure 1 shows the first area King Street, a typical downtown core street with both sides lined with highrise office buildings, apartment blocks and multi-storey commercial buildings. The road edge and sidewalks accommodate various utilities such as manholes, catchbasins, fire hydrants, lamp standards and hydro poles.  Figure 2. Lakeshore boulevard beneath the gardiner expressway. The second area chosen was Lakeshore Boulevard, which runs beneath the Gardiner Expressway on the south side of the city. The expressway is an elevated multi-lane highway that parallels the Toronto waterfront. Beneath the expressway is Lakeshore Boulevard. The view from the mapping vehicle (fig 2) shows it is crossed with intersections and traffic lights, various utility poles and assorted road signage, all of which would remain hidden on an aerial photograph. For the transportation planner and GIS professional this type of planimetric detail would be required information, geographically positioned and geocoded for input into their particular systems. To successfully map areas such as these using aerial photography alone, and attain a 100% positional accuracy rate for all features, particularly those obscured by the expressway, would be extremely difficult, if not impossible to achieve. As discussed earlier in this paper, urban canyons and the area beneath elevated highways are particularly difficult environments in which to attain continuous GPS reception. If time constraints are an issue then adopting a conventional survey approach to generate the data would take too long. A mobile land-based mapping alternative impervious to GPS dropout, such as the POS LV, would need to be considered in order to generate a complete and accurate data set in the shortest possible time to supplement or replace the information obtained from aerial photography. |