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