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The position accuracy achievable with the OMNISTAR 
system depends from the GPS receiver quality. 
Every GPS receiver, indeed, is characterised by a certain 
level of "noise", which affects the quality of the results. In 
the case of DGPS, noise exhibits itself by variations in the 
final position, originated by errors in the range 
measurements to the GPS satellites. These variations can 
be caused by different factors, including the receiver 
hardware design, the internal clock quality, the antenna, 
and the internal software that calculates the position. 
Using a GPS receiver for pseudorange positioning in a 
non-differential way, the effects due to SA are so large 
that the noise cannot normally be seen. With DGPS 
corrections, most of the SA effects are removed and the 
internal noise becomes important. It appears as a nearly 
constant change in position that may be as much as 10 or 
15 meters for low cost code-only GPS receivers, and as 
low as some tenths of centimetres in geodetic or survey- 
grade receivers. Internal errors in a differentially corrected 
GPS receiver generally are the determining factor as to 
how much time will be necessary to achieve a desired 
accuracy level in a measurement. Obviously, a "noisy" 
receiver will require more time to average out the errors 
due to the internal noise. 
Concluding the discussion on the user's hardware, the 
best results (that is, an effective sub-meter accuracy) can 
be obtained using geodetic or survey-grade GPS 
receivers. 
2.5. Subscription options 
As referred above, OMNISTAR is a private, commercial 
service. It is achievable by time-limited subscription. It is 
possible to subscribe for a defined period of time (e.g. one 
year) or "purchase" a defined connection time (e.g. 100 
hours, to be used in a given period). 
Subscription options also include the use of a defined 
number of the network station. The most complete option 
is the already referred "Virtual Base Station" or VBS, 
which can be obtained with a coverage for one entire 
continent or a limited zone (e.g. inside a circle of 200 km 
radius). A less expensive alternative is the so-called 
"Virtual Reference Cell" (VRC) option, which creates one 
"virtual station" at a fixed location inside the user's working 
area. 
2.6. Advantages of the OMNISTAR system versus 
other DGPS systems 
On the basis of the system overview, one can conclude 
that the OMNISTAR system is characterised by two 
noteworthy advantages, with respect to other DGPS 
solutions (like, e.g., the direct connection to one base GPS 
station): 
a) the good coverage of the transmission of the correction 
data, obtained by the geostationary satellite broadcasting; 
b) the accuracy uniformity achievable in all the covered 
region, by means of the multiple base station technique 
and the correct modelling of the atmospheric delay, and of 
the good coverage itself. 
2.7. OMNISTAR applications 
The sub-meter orfew-meters accuracy achievable with the 
OMNISTAR system (the effective performance depends, 
as referred above, from the GPS receiver quality) is a 
result of a great interest for many applications, not only in 
the navigation field but also in the surveying one. 
The real-time availability of the results makes the system 
even more appealing. 
Some possible applications are the following: 
• Precision navigation (ships, aircraft, cars; aerial 
photogrammetry) 
• Vehicle location and positioning 
• GIS data acquisition, thematic cartography (e.g. for 
geological survey) 
• All low-precision surveys (e.g. small or medium scale 
cartography realisation or updating) 
• Agricultural specific applications: crop dusting, 
"precision farming" 
• Search and rescue guidance 
• Environmental monitoring 
• Military applications (not to mention in detail) 
3. FIELD TESTS ON THE OMNISTAR SYSTEM 
3.1. General test criteria 
In order to validate and confirm the information, data and 
specifications given by the Company, we have performed 
some field tests, from which the most significant are here 
presented. 
Different tests have been carried out, which can be divided 
in two main categories: 
a) tests in static mode; 
b) tests in kinematic mode. 
A particular thanks goes to the Forest Office of the 
Regione Umbria and to the GeoTop company of Ancona, 
who borrowed us most of the instrumentation used for the 
tests. 
3.2. Tests in static mode 
This series of tests was performed in the frame of the 
COFIN97 research project, inherent some survey aspects 
connected with the telecommunications. 
A particular activity involved with that project was the 
study of methodologies for the rapid determination of 
antenna installation sites. 
For each site, a GPS static sessions has been carried out 
to determine the position of a marker located on the