SX BLUE GPS iSXblue II+ GNSSTM, User Manual

Page: 92 / 107

SXblue Series User Manual 91
can lead to an incompletely corrected error source that could degrade positioning
accuracy with increased distance from the base station.
SBAS systems (WAAS, EGNOS, MSAS, GAGAN, etc) use a different approach,
using a network of reference stations in strategic locations to take measurements
and model the real-time ionosphere. Updates of the ionospheric map are sent on
a continual basis to ensure that as the activity of the ionosphere changes with time,
the user’s positioning accuracy will be maintained. Compared to using a DGPS
beacon, the effect of geographic proximity to a single reference station is
minimized resulting in more consistent system performance throughout all
locations within the network.
Correction Latency
The latency of differential corrections to a lesser extent affects the achievable
positioning accuracy at the remote receiver since the magnitude of SA was turned
to zero in year 2000. Latency is a function of the following:
•
The time it takes the base station to calculate corrections
•
The data rate of the radio link
•
The time it takes the signal to reach the user
•
The time required for the remote differential receiver to demodulate the signal and
communicate it to the GPS receiver.
•
Any data loss that occurs through reception problems
Most of these delays require less than a second, though in some instances,
depending upon the amount of information being transferred, overall delays of
three to five seconds may occur. The effect of latency is mitigated by the COAST
technology within the SXblue receiver. This technology is especially valuable in
conditions of DGPS signal loss where the age of corrections increases for each
second of signal loss. See Section 2.6 for further information on COAST.
Satellite Constellation Geometry
The number of satellites visible and their geometry in the sky influences positioning
accuracy. The Dilution of Precision (DOP) describes the strength of location and
number of satellites in view of the receiver. A low DOP indicates a strong potential
for better accuracy than a high DOP. Generally, more satellites visible to both the
reference and remote receivers will provide a lower DOP (any satellites seen by
one receiver and not the other, are not used in the position solution). Additionally,
if the satellites are evenly spread around the receiver, rather than grouped in a few
regions of the sky, a lower DOP (stronger solution) will result.
GPS Signal Multipath
Satellite signals received from reflective object nearby the working area by the
GPS receiver can decrease positioning accuracy. These multipath signals
increase the measured range to a satellite as the signal takes a longer route to the