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International cooperation and technology transfer
Mussio, Luigi

D. Dominici*, M.L. Pecetti**, F. Radicioni**, A.Stoppini**
* University of L'Aquila
** University of Perugia
ISPRS Commission VI, Working Group 3
KEY WORDS: GPS, DGPS, pseudorange, differential, real-time, OMNISTAR.
The OMNISTAR™ system is a differential GPS (DGPS) system based on a network of GPS permanent stations.
Pseudorange corrections are broadcasted to the users by means of a geostationary satellite having a continental
coverage. Users get real-time DGPS positions (about one-meter accuracy) by means of only one GPS receiver,
connected to or including an OMNISTAR™ receiver. Main advantages of the system are the ease of use and the
affordability of the data transmission.
The first part of this paper consists of a general overview of the OMNISTAR™ system and its possible applications. In
the second part, some field tests of the system are presented, both in static and kinematic mode. The experimental
results are analysed in order to evaluate the system performance in various conditions and survey applications.
The accuracy of the absolute positions obtained by means
of GPS code (pseudoranges) using only one receiver is
normally in the order of some tenths of meters.
The greatest part of that uncertainty derives from the so-
called Selective Availability (SA), intentionally introduced
by the U.S. Department of Defence in order to limit the
accuracy of the GPS absolute positioning in real time. The
indicated position varies in time in a near-random mode,
and the instantaneous errors with respect to the "true" co
ordinates may be as much as 100 meters.
The differential GPS (DGPS) technique is substantially a
relative positioning technique, using a GPS station of
known position to calculate pseudorange corrections for
each satellite and each epoch. The same corrections are
applied to the pseudoranges measured by the GPS
receiver in the unknown point. This way, the uncertainty of
the instantaneous positions can be generally reduced to a
few meters. Using more fixed stations and modelling the
atmospheric delay appropriately, a one-meter or even a
sub-meter accuracy can be reached. With such results,
the DGPS technique becomes interesting for many survey
The DGPS technique can be obviously used in post
processing, but the most interesting applications are those
in real time. This requires the transmission of the
pseudorange corrections from the "master" fixed station(s)
to the "rover" receiver, where an appropriate software
computes the corrected solution for every epoch, with a
minimal time delay.
The correction data transmission is generally effected by
radio-modem. The key point of the real-time DGPS
process is indeed the affordability of the radio
transmission. When the corrections are correctly received
by the rover, a good position (one-meter or few meters
accuracy) is generally obtained. If the transmission is
incorrect or disturbed, the forced use of bad corrected or
uncorrected pseudoranges can result in co-ordinates
having up to 100 meter errors. The data flow must be
regular, almost continuous, because the random errors
induced by SA change in a very rapid way.
A direct radio transmission from master to rover is only
possible at rather short distances. In Italy the power of
radio broadcasting for private applications like surveying is
limited by law to a few watts. The maximum distance
covered by such a small power is a few kilometres
(without obstructions).
Some attempts have been made with radio networks,
generally established for other purposes (like audio
broadcasting). In some cases the results are good, but the
network is rarely able to completely cover all the territory
of a nation or a region.
Other DGPS systems, including the OMNISTAR™, make
use of a geostationary satellite for the correction data
broadcasting. This way the radio coverage is generally
very good, and the results are consequently affordable
and homogeneous. This is the main advantage of a
system which has got other interesting and peculiar
characteristics to be described in the following chapter.