YSI SonTek RiverSurveyor, Системное руководство

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SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
35
Differential GPS (DGPS) receivers are capable of providing higher positioning accuracy (sub-
meter in the open ocean or even better when in the vicinity of the reference station on land).
Even the most advanced DGPS systems determine vessel position with some uncertainty
σ
R
,
which introduces an error
σ
V
into speed calculations:
t
R
V
Δ
=
σ
σ
2
Where t
Δ
is the time between successive GPS fixes (averaging interval).
If the positioning data has an uncertainty of ±2 m, and an averaging interval of 60 seconds is
chosen, the uncertainty of the vessel speed for each profile is about ±4.7 cm/s. This uncertainty
puts a lower limit on the accuracy of the absolute current measurements. If a current-profiling
precision of 1 cm/s is needed, the use of the GPS would require longer averaging intervals and
thus limit the time resolution of profiling surveys. Although this may not pose a problem in the
open ocean (where currents usually do not change over several minutes), coastal, frontal, and
convergent areas require shorter averages. Sub-meter accuracy can be achieved by the latest
DGPS systems; however, these systems are usually limited to operations close to a reference sta-
tion (
≈
100 km), and are not available for remote inland or remote coastal areas.
Multipath Error
Multipath error is caused by reflected signals from surfaces near the receiver that can either in-
terfere with, or be mistaken for, the signal that follows the straight-line path from the satellite.
Multipath error is difficult to detect and sometimes hard to avoid, especially in situations where
there are several potential sources of reflection.
Multipath error is more commonly experienced in applications such as:
•
Canyons, where the canyon walls can cause signal shadowing and reflections.
•
Heavily forested areas, where the trees reflect GPS signals.
•
Near large structures or major bridges.
Note : Some high-end DGPS receivers include multi-path filter functions. If you think
you will be operating under these conditions, we recommend you use the filtering fea-
tures to avoid the potential collection of erroneous discharge data.
Invalid GPS Strings
Some GPS receivers will continue to transmit the last valid GPS string that was received. In this
situation, the receiver time will not be updated, and RiverSurveyor will display the GPS informa-
tion as invalid.
GPS Filters
GPS filters are available on some GPS receiver units and can bias the reported vessel velocity
particularly when there are significant changes in acceleration.