GPS World - May 2008 - (Page 52)

INNOVATION | Algorithms & Methods 50 Northing (meters) 0 -50 -100 -150 -300 -250 -200 -150 -100 -50 0 Easting (meters) Å FIGURE 3 Vehicle test: 2D trajectory in local topocentric coordinates. 3D offset (TD - RTK) 0.08 3D error estimate (TD) Å PHOTO Research aircraft of the Institute of Flight System Dynamics. 3D offset and error estimate (meters) 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 0 50 100 150 Å FIGURE 4 Vehicle test: 3D offset of the time-difference solution from the RTK solution with error estimation. Å FIGURE 5 Flight test: Overall trajectory (visualized using Google Earth). 2.25 only, we installed a second receiver nearby to serve as a base station, and a static initialization period of about 15 minutes was provided to enable RTK processing. Seven satellites above an elevation mask angle of 10 degrees with a PDOP of 2.1 were tracked during 170 seconds of driving. The time-difference solution was obtained using all of the above-mentioned corrections. Its offset from the reference RTK solution stays below 8 centimeters during the whole processing interval (see the black line in FIGURE 4). As with the static test, this result coincides well with the estimate of the 3D error (see the grey line in the figure). Flight test. We performed a flight test on July 17, 2007, starting from the airfield in Oberpfaffenhofen, Germany. The GPS antenna was mounted on the top of the cockpit of the Institute of Flight System Dynamics' research aircraft (see PHOTO). As for the vehicle test, a base receiver was mounted nearby (the one used for the static test previously described) and a static initialization period was provided for generating a reference trajectory. Several dynamic maneuvers with bank angles of up to 70 degrees were performed during the 47-minute flight (see FIGURE 5). Because of the high dynamics, the recorded phase measurements are afflicted with numerous outliers and cycle slips. These errors were detected, identified, and adapted at TU Delft through successful calculation of an RTK solution for the flight. Here, final precise orbits, Saastamoinen a priori tropospheric correc52 GPS World | May 2008 150 Up (meters) 2.20 100 50 0 2.15 2.10 2.05 0 Eas ting 500 (me ters ) 0 1000 -500 rth No ing (m 500 rs) ete 2.00 Å FIGURE 6 Flight test/take-off: 3D trajectory color-coded with PDOP. tions, and Kalman filtering with constant ambiguities (assuming no cycle slips occur) were used to process the data with an elevation cutoff angle of 15 degrees. (The 4-Hz sampled data were processed at 1 Hz for reasons beyond the scope of this article.) Correct ambiguities were obtained after 9 minutes of static initialization. A standard deviation of 1 centimeter was assumed for the undifferenced phase measurements (compared to 1 meter for the code ranges) taken by the low-cost receiver. Two representative sections of the flight path were processed with the time-difference method (marked in magenta in Figure 5). Again, high-rate clock corrections in conjunction with www.gpsworld.com http://www.gpsworld.com

Table of Contents Feed for the Digital Edition of GPS World - May 2008

GPS World - May 2008 Contents Out in Front Letters to the Editor Expert Advice GPS III Contract Award Now a Reality? Near-Space Location Boost RTK Crops Up in Precision Ag Safety Afoot 50+ Leaders to Watch Making a Difference with GPS Product Showcase Advertisers Index & Company Directory Marketplace Classifieds Seen & Heard

GPS World - May 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.