CSI Wireless Vector Sensor Reference Manual page 131

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 new COAST technology within the SLX. This technology is
especially valuable in conditions of DGPS signal loss where the age of corrections increases for
each second of signal loss. Consult Section 1.7 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 either 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 by the GPS receiver by a reflection from an object can decrease
positioning accuracy. These multipath signals increase the measured range to a satellite as the
signal takes a longer route to the GPS antenna. Certain precautions will minimize GPS antenna
sensitivity to these reflected signals. Operating away from large reflective structures such as
buildings or using special antennas and GPS equipment can help to reduce the impact of multipath.
For most consumer-level applications, a small amount of multipath is tolerable.
GPS Receiver Quality
The quality of a GPS receiver has a dramatic influence on positioning accuracy. Consumer-based
GPS products, such as many affordable handheld and fixed-mount receivers, typically operate with
an accuracy of 3 to 5 meters horizontally 95% of the time. The accuracy of a particular product
depends on the specific receiver's performance characteristics. Higher accuracy GPS receivers
are able to achieve sub-1 meter horizontal accuracy 95% of the time using real-time DGPS
transmissions. The Vector Sensor system falls in to this latter category.
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