2. THE OMNISTAR™ SYSTEM 
2.1. General characteristics and design objectives of 
the system 
The OMNISTAR™ DGPS system is a commercial service 
which has been designed with the following objectives: 
(a) continental coverage; 
(b) sub-meter accuracy over the entire coverage area; 
(c) light and portable equipment; 
(d) ease of use. 
The first objective, for the reasons exposed in the 
introduction, has been reached adopting a geostationary 
satellite for the correction data transmission. The company 
initially purchased a transponder onboard a satellite 
covering all northern America. Later, a coverage over 
Europe and other continents was realised by means of 
further satellites. Figures 1 and 2 show respectively the 
coverage "footprints" over North America and Europe. 
An appropriate computation methodology, taking 
advantage from multiple GPS base stations and correctly 
modelling the biases due to the atmospheric delay of the 
GPS signal, has been implemented in the OMNISTAR 
software in order to meet the second objective. 
The equipment units are sufficiently portable to be carried 
in a rather small backpack. There are essentially two types 
of equipment, to be better described later: an add-on kit 
working together with an external GPS receiver, and an 
all-in-one equipment. 
Once effected the preliminary system set-up and 
connections, the users can take the equipment anywhere 
within the coverage area and get consistent results in real 
time, without any intervention or intimate knowledge of 
GPS or DGPS. The great ease of use permits the 
adoption of the system by generic technical users, not 
having a specific formation in survey sciences and 
techniques. 
Fig.1 - OMNISTAR™ coverage over North America 
Fig.2 - OMNISTAR™ coverage over Europe 
(and North Africa) 
2.2. System organisation and computation technique 
The OMNI STAR Network consists of some widely-spaced 
permanent base stations for each continental area (see 
fig. 1 and 2). Each station tracks all visible GPS Satellites 
above 5 degrees elevation; pseudorange corrections 
every 600 milliseconds are computed. The corrections are 
in the form of an industry standard message format called 
RTCM-104, Version II. 
The corrections are sent to the OMNISTAR continental 
Network Control Centre (NCC) via lease lines, with a dial 
back-up. At the NCC all messages are checked, 
compressed, and formed into data packets for 
transmission up to the satellite transponder. This occurs 
approximately every 2 to 3 seconds. A data packet will 
contain the latest data from each of the continental base 
stations. 
All user sets receive these packets of data from the 
satellite transponder. The messages are first decoded 
from the spread-spectrum transmission format and then 
uncompressed. At that point, the message is an exact 
duplicate of the data as it was generated at each base 
station. 
The atmospheric errors correction is one of the crucial 
phase of the OMNISTAR DGPS solution. Every base 
station automatically corrects for atmospheric errors at it’s 
location. Using the approximate rover position given by the 
GPS, the user set software computes a correction for his 
own location. This operation, totally automatic, is essential 
for getting a sub-meter positioning from DGPS with such a 
widely spaced fixed stations network. If the atmospheric 
biases are totally ignored, errors of up to ten meters can 
result. 
After the software has taken care of the atmospheric 
corrections, it then uses it’s location versus the base 
station locations, in an inverse distance-weighted least- 
squares solution. 
This computation methodology, together with the 
atmospheric biases processing, has been called by the 
Company "Virtual Base Station" technique, because from