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Federal Highway Administration > Publications > Research > Support of the System Test and Analysis Program for the Nationwide Differential Global Positioning System Modernization Program
Publication Number: FHWA-RD-02-110
Date: July 2002

Support of the System Test and Analysis Program for the Nationwide Differential Global Positioning System Modernization Program

FHWA Contact: Jim Arnold, HRDI-06,
202-493-3265, james.a.arnold@dot.gov

Figure 12. Real-Time Navigation Solution Compared to Post-Processed Solution
Chart. This chart compares the real time "navigation" or kinematic solution at the GPS antenna on the boat with the reference truth solution at 250 kilometers from Hagerstown. The plot includes north, east, and height envelopes, which represent the standard deviations of the three position solutions. The vertical scale is in meters (negative 0.5 to positive 0.5 meters), and the horizontal scale is in seconds (470,000 to 510,000 seconds). After several minutes, all horizontal navigation information is within the 10-centimeter solution range, and the vertical solution is within the 20-centimeter range.

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Figure 13. Dynamic Results on a Stationary Platform
Chart. This chart compares the real time "navigation" or kinematic solution at the GPS antenna on a stationary platform approximately 50 kilometers from Hagerstown. The plot includes north, east, and height envelopes, which represent the standard deviations of the three position solutions. The vertical scale is in meters (negative 0.3 to positive 0.35 meters), and the horizontal scale is in seconds (0 to 30,000 seconds). All horizontal navigation information is within the 10-centimeter solution range, and the vertical solution is within 20 centimeters more than 95 percent of the time.

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Figure 14. Error Due to Data Compression
Chart. This chart compares Hagerstown raw data-to-boat versus Hagerstown compressed data-to-boat at 250 kilometers. The X axis represents north-east-height differences from a range of negative .04 to positive .05 meters, and the Y axis represents GPS seconds of week within a range of 470,000 to 510,000 seconds. Distance is not a factor in this chart, but data compression is. The chart demonstrates that the error introduced by compression typically is less than 1 centimeter in the east and north directions, while the height error typically is less than 2 centimeters. The standard deviation of the error introduced by the height, which is the worst case, is less than 7.5 millimeters over the time period.

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Figure 15. Effects of Missing Epochs
Chart. This chart depicts the effects of missing epochs or data. The X axis represents north-east-height errors from a range of negative .035 to positive .025 meters, and the Y axis represents GPS seconds of week within a range of 470,000 to 510,000 seconds. Here, the compressed data at Hagerstown before broadcasting (no missing packets) is compared to what the user receives (same data exactly, except for missing packets). While figure 14 presents the effects of compression alone, figure 15 shows the effects of missing epochs, alone. North and east data stay very close to zero, and the height stays within .005 meters, except for a short time period when the boat is in the dock at Tangier Island. The number of missing epochs increases dramatically due to a loss of the signal around the buildings on the pier.

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