Support of the System Test and Analysis Program for the Nationwide Differential Global Positioning System Modernization Program
FHWA Contact: Jim Arnold, HRDI06,
2024933265, james.a.arnold@dot.gov
Figure 12. RealTime Navigation Solution Compared to PostProcessed 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 10centimeter solution range,
and the vertical solution is within the 20centimeter range.
back to the report
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 10centimeter solution range, and the vertical solution is within
20 centimeters more than 95 percent of the time.
back to the report
Figure 14. Error Due to Data Compression
Chart. This chart compares Hagerstown raw datatoboat versus Hagerstown
compressed datatoboat at 250 kilometers. The X axis represents northeastheight
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.
back to the report
Figure 15. Effects of Missing Epochs
Chart. This chart depicts the effects of missing epochs or data. The X axis
represents northeastheight 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.
back to the report
