Geneq iSXBlue II GNSS Series Technical Reference Manual page 171

layer. This charged activity affects the GPS signals that penetrate this layer, affecting the measured
ranges. The difficulty in removing the effect of the ionosphere is that it varies from day to day, and even
hour to hour due to the sun's 11-year solar cycle and the rotation of the earth, respectively. During the
summer of 2001, the sun's solar cycle reached an 11-year high and going forward we saw a general
cooling trend of the ionosphere over the few years that followed, thus with reduced ionospheric activity.
Removing the effect of the ionosphere depends on the architecture of the differential network. DGPS
radiobeacons, for example, use a more conventional approach than WAAS or SBAS in general. DGPS
beacons make use of a single reference station, which provides real-time GPS error corrections based
upon measurements that it makes at its location. It is possible that the state of the ionosphere differs
between the remote user and the single reference station. This 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 iSXBlue/SXBlue II GNSS. 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 2.3 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.
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