206 
FIELD TESTS ON DGPS USING OMNISTAR™ CORRECTION 
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. 
ABSTRACT: 
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. 
1. INTRODUCTION 
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 
applications. 
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.