GPS World - May 2008 - (Page 51)

Algorithms & Methods | INNOVATION airfield of Oberpfaffenhofen, Germany. To attenuate the effect of drifting errors, we applied various corrections when processing the data: signal propagation delays caused by the troposphere were ameliorated using the UNB3 model developed at the University of New Brunswick. The model features three surface parameters (temperature, total pressure, and water vapor pressure) in conjunction with expressions to describe their change with altitude. No external meteorological data are required for modeling. We accounted for ionospheric phase advances by a thin layer model. Here, the total electron content (TEC) is extracted from ionospheric correction maps provided by the International GNSS Service (IGS) with a latency of approximately eight days. For obtaining improved satellite positions and clock corrections, we used final ephemeris products in conjunction with 30-second sampled satellite clocks Ð both provided by the Center for Orbit Determination in Europe (CODE), also provided with a latency of approximately eight days. More detailed information concerning the correction methods can be found in our research paper listed in ª Further Reading.º At the beginning of the test, eight satellites were used yielding a PDOP of 1.4. After 140 seconds, PRN27 drops below the elevation mask angle of 10 degrees causing the PDOP to increase to 1.6. FIGURE 2 illustrates the 3D offset of the resulting solution from the reference trajectory. Line (1) shows the best solution calculated using all corrections mentioned above. The drift of the remaining errors is slow and the error stays within 11 centimeters during 10 minutes of processed data. This result agrees well with the estimate of the 3D error shown by line (1.1). Line (2) represents the same solution with a base-epoch handover artificially triggered after 5 minutes. While the final result is virtually unaffected by this event, the estimate of the error, line (2.1), is. The estimate is reset to zero when changing the base epoch. Omitting the 30-second sampled clock corrections, line (3), or using broadcast instead of precise ephemerides, line (4), significantly degrades the solution. Here, the use of clock corrections is even more important than the application of precise orbits. Omitting ionospheric, line (5), or tropospheric, line (6), models while using all other corrections causes a strongly increased error drift which significantly reduces the time span with good precision. The dropout of PRN27 causes steps in all solutions not using high-rate clock solutions. Even though this effect demands further investigation, it is clear that using good satellite-clock corrections when performing time-relative data processing is very important. The evaluation of a single test can only provide a first impression. It is not sufficient for making general statements concerning the precision of the time-difference approach or the effect of error-correction models. A strict statistical evaluation is missing. However, the coincidence of the estimate of the error and the true offset of the solution helps us to judge the quality of future tests in the absence of a reference trajectory. Vehicle Test. The trajectory of a vehicle test performed through open fields on February 14, 2007, with a measurement rate of 10 Hz is illustrated in FIGURE 3. For validation purposes www.gpsworld.com GPS Antennas for GPS that matters Best Possible GPS Performance! Tightly tuned Best noise gure Best lters Professional housings Highest reliability Precision Ag Military COTS Security Fleet grade Ruggedized Embedded Wi-Sys Communications Inc. Antennas & RF Modules Tracking & Timing Receivers Complete LBS Solutions Quality Wireless Engineering www.wi-sys.com 613.254.7386 May 2008 | GPS World 51 http://www.wi-sys.com http://www.wi-sys.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.