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G. Seeber, J. Campbell, L. Wanninger: Nuisance effects in precise GPS positioning 29
to <strong>the</strong> tropospheric effect on vertical positioning (DEPENTHAL
et al., 2000)
Ionospheric propagation delay
Like <strong>the</strong> troposphere also <strong>the</strong> ionosphere can be modeled
by <strong>the</strong> available wealth <strong>of</strong> data from ground tracking stations
and LEO occultations (KLEUSBERG, 1998). Severe ionospheric
conditions were experienced during <strong>the</strong> recent years
<strong>of</strong> solar maximum. Various studies have been performed
on how <strong>the</strong>se effects can be mitigated by <strong>the</strong> use <strong>of</strong> permanent
reference station networks (WANNINGER, 1999). New aspects
arise with <strong>the</strong> availability <strong>of</strong> additional frequencies using
<strong>the</strong> forthcoming Galileo system (PANY et al., 2002).
Ambiguity resolution techniques
Significant progress has been achieved in ambiguity resolution
techniques by a more complete modeling <strong>of</strong> GPS
observations (WÜBBENA, 2001). Particular investigation
improves <strong>the</strong> techniques (GRAFAREND, 2000). Emphasis
is given to near real-time solutions (KUTTERER, 2000). A
powerful procedure uses data from permanent reference
stations for an error modelling in real-time (WANNINGER,
2000). This technique reduces distance dependent errors
(orbit, troposphere, ionosphere) and at <strong>the</strong> same time it
reduces <strong>the</strong> time to fix ambiguities and increases <strong>the</strong> success
rate <strong>of</strong> ambiguity fixing. In Germany this concept is widely
used in <strong>the</strong> SAPOS service (AdV, 2002). A modern approach
is <strong>the</strong> modelling <strong>of</strong> <strong>the</strong> error state within a regional network
<strong>of</strong> reference stations (WÜBBENA et al., 2001) and <strong>the</strong> integration
with continental and global networks (WÜBBENA,
2002b). Fur<strong>the</strong>r advantages are expected by <strong>the</strong> inclusion
<strong>of</strong> additional GPS signals (VOLLATH et al., 1999) or Galileo
data (TIBERIUS et al., 2002).
Integrated modelling <strong>of</strong> effects
The domination <strong>of</strong> station dependent and distance dependent
nuisance effects is intensively treated in permanent reference
station networks. The procedures in use are station calibration,
virtual reference stations (VRS) and area correction
parameters (ACP). Comprehensive publications can be taken
from <strong>the</strong> proceedings <strong>of</strong> <strong>the</strong> annual SAPOS conferences,
for example AdV (2002). Some key publications in this area
are BÖDER et al. (1999), WANNINGER (2000), WÜBBENA
(2002a).
The capacities and limits <strong>of</strong> nuisance effect modeling are
demonstrated in a comprehensive intercomparison study.
The overall effect in a data analysis can be considered as
a process noise, including s<strong>of</strong>tware noise (selection <strong>of</strong> a
particular s<strong>of</strong>tware package), operator noise (selection <strong>of</strong>
particular options), reference frame realization noise (selection
<strong>of</strong> a set <strong>of</strong> fiducials). As a consequence, a given data
set will lead to slightly different results, when different
operators work with different s<strong>of</strong>tware packages. A 40-day
long data set from about 50 stations on <strong>the</strong> sou<strong>the</strong>rn hemisphere
have been analysed with 4 different s<strong>of</strong>tware packages
in 7 laboratories. The mean differences between solutions
are 1 cm in horizontal position and 2 cm in height (DIETRICH
et al., 2001) .
References
AdV: Vorträge des 4. SAPOS Symposiums, 21.-23. Mai 2002
in Hannover. Arbeitsgemeinschaft der Vermessungsverwaltungen
der Länder der Bundesrepublik Deutschland, 2002
AUGATH, W.: Signal and Station Validation <strong>of</strong> GNSS Permanent
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Navigation. 5th Setmana Geomatica, Cartography, Telematics
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AUGATH, W., BLUMENBACH, TH., WILDT, ST.: SAPOS 2002 Symposium:
Qualitätssicherung bei Referenzstationen und in
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