Global Positioning System Reference
In-Depth Information
M( , )
33 3
λφ
U( , )
λφ
M( , )
11 1
M( , )
22 2
λφ
λφ
Longitude, λ
Figure 8.9
Calculating the correction weights.
deviation of the total error due to the monitors is decreased by a factor of
n from
that of one monitor.
8.3.3 Wide-Area DGPS
Wide-area DGPS (WADGPS) attempts to attain meter-level accuracy over a large
region while using a fraction of the number of reference stations that LADGPS would
require to attain the same accuracy within the same coverage region. The general
approach (e.g., see [11-13])—in contrast to that of LADGPS—is to break out the
total pseudorange error into its components and to estimate the variation of each
component over the entire region, rather than just at the station positions. The accu-
racy, then, does not depend on the proximity of the user to a single reference station.
The WADGPS concept, illustrated in Figure 8.10, includes a network of reference
stations, one or more central processing sites, and a data link to provide corrections to
users. Each reference station includes one or more GPS receivers that measure
pseudorange and carrier phase for the broadcast signals from all visible satellites. This
data is provided to the central processing site(s), which process the raw data to
develop estimates of the broadcast ephemeris and broadcast clock errors for each sat-
ellite. Single-frequency WADGPS systems also estimate ionospheric errors through-
out the service volume. Tropospheric delays are typically addressed through the use
of models employed by the reference stations and by the user.
8.3.3.1 Satellite Ephemeris and Clock Errors
Using pseudorange and carrier-phase data from the entire network of reference sta-
tions, each central processing site can develop precise estimates of the true locations
 
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