CurrentSurveyor Principles of Operation
1. Introduction
When performing water-current surveys covering large
areas, or when monitoring river discharge, it is often convenient to use a boat-mounted
CurrentSurveyor Acoustic Doppler Profiler (ADP) system. When operating from a moving
platform, an ADP measures relative currents (Figure 1). As such, it is important to
measure independently the speed of the platform so it can be subtracted from the raw
current measurements. This allows you to obtain the residual water currents relative to
the fixed Earth. It is generally desirable to perform these calculations in real-time.
Figure 1. Measuring currents from a moving boat using an ADP
Though there are a several ways to measure a vessel's speed and direction,
the two most-practical methods commonly used with ADPs are:
- The use of GPS equipment
- The use of the SonTek bottom-tracking algorithm
2. Using an ADP with GPS
The majority of vessels used for research have GPS
systems as part of their navigation equipment. SonTek ADPs manufactured since 1996 have a
built-in interface to receive GPS information, and are able to accept the NMEA 0183 data
formats (GPGGA/GPGXP/GPGGK) available from most commercial GPS receivers. A good-quality
GPS receiver with accurate differential corrections is recommended for robust real-time
operations. The GPS-to-ADP interface requires two serial ports (one for the ADP and one
for the GPS) on the PC running SonTek's real-time ADP software. The synchronization of
data is done in the PC (Figure 2). To
compute the vessel's velocity vector, the ADP records GPS positions at the beginning and
end of a user-selected averaging interval. The resulting boat velocity is then computed
from the total vessel displacement and is subtracted from the relative water currents
measured by the ADP. The GPS method for removal of the boat track from ADP measurements is
best suited for areas where the bottom is out of ADP tracking range or where currents
change slowly in time/space so that longer averaging intervals can be used.
Figure 2. Operating ADP from a boat: connecting necessary equipment
3. SonTek Bottom-Tracking Algorithm
Using the ADP's bottom-tracking algorithm greatly
enhances the instrument's capabilities and versatility when used from a moving boat in
shallow waters. Bottom-tracking enables you to obtain real-time vessel-speed-over-ground
data simultaneously with water current measurements without using any additional
equipment. While bottom-tracking, the ADP measures the Doppler shift of reflected acoustic
energy (from the bottom of a river, harbor, etc.) to infer the vessel speed. In contrast,
when an ADP is current-profiling, it is measuring acoustic reflections from suspended
material in the water column to determine the velocity of the water with respect to the
ADP. In bottom-tracking mode, the ADP determines bottom velocity once every second. At the
end of the averaging interval, all these bottom-velocity estimates are averaged and stored
with the profile data. In addition to the vessel-speed-over-ground data, the
bottom-tracking ADP also reports an averaged depth in real-time, which is often required
for surveys or river discharge applications.
The ADP can only track the bottom when it is within
the acoustic range of the ADP’s transducers. Generally, it is good practice to
understand the bottom characteristics of the area being surveyed, as some soft bottoms
with large amounts of plant growth can imitate "moving-bottom" characteristics
that might influence the ADP's bottom-track algorithm. Seabeds with strong acoustic
reflections (rocky bottoms provide the best results) can typically be tracked at ranges
that are 20-30% greater than the water-current profiling range.
When operating from a moving platform such as a boat, erratic course
changes during the averaging interval may yield erroneous vessel velocity vectors.
Therefore, when using ADP with GPS or bottom-track, you should take into consideration the
vessel's proposed track-line when choosing the averaging interval. The best results will
be obtained when steering a vessel in a straight line at a steady speed.
4. GPS vs. Bottom-Tracking
4.1. Performance Comparison
What are the advantages of bottom-tracking over GPS
(and vice versa), and what factors affect the inherent accuracy of both approaches? The
purpose of both bottom-tracking and GPS is to calculate vessel speed in order to correct
the relative water currents measured by the ADP. Therefore, the accuracy of the technique
used for vessel speed estimation directly determines the accuracy of the absolute
current-velocity profile.
First, consider the pros and cons of the GPS approach. Regular GPS is
accurate to within ~100 m, which would require extremely long averaging times (~2.5 h) to
achieve a 1 cm/s accuracy. Such measurements are useful only in the open ocean.
Differential GPS (DGPS) receivers are capable of providing higher positioning accuracy --
2-m 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 
, which introduces
an error 
into speed
calculations:
where 
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. A sub-meter accuracy can be achieved by the
latest DGPS systems, however it is usually limited to operations close to a reference
station (~100 km), and is not available for remote inland or coastal areas. Also,
operating in deep canyons or in mountainous area can cause "shadow," where the
direct path to a GPS satellite is obscured.
If DGPS mode is not available during the deployment, necessary corrections
can be done in post-processing. ADP software stores the raw GPS positions used for the
vessel speed calculations, and differential corrections can be downloaded later from
appropriate sources. These corrected positions can be used to obtain more-accurate
estimates of the vessel speed, and hence, the true water currents.
Now let us consider the performance of the bottom-tracking algorithm. As
mentioned above, a bottom-tracking ADP determines the bottom velocity once each second,
and then averages the raw estimates over the user-selected averaging interval. Because the
bottom velocity is derived from solid-object reflections, natural variability (standard
deviation) of the bottom-track velocity measurements is lower by an order of magnitude
when compared to the current-profiling data. Hence, the precision of bottom-velocity
measurements is always better than that of water currents. Because of this,
bottom-tracking can be considered to introduce no additional errors to water current
measurements.
A basic limitation of bottom-tracking is that it can operate in waters
with depth no more than approximately (1.3 x Maximum ADP Profiling Range). In addition,
significant plant growth, heavy wave conditions, and a moving near-bottom layer can
degrade bottom-track performance.
4.2. Compass Alignment
Another important consideration when operating from a
moving platform is the accuracy and alignment of the compasses used for obtaining vessel
speed and the ADP compass. When using GPS, the ship’s gyro is normally used to
provide vessel direction. If a small misalignment (offset) exists between the gyro and the
ADP, an error proportional to the speed of the vessel is
introduced. That is, for a 5° offset, a vessel moving at 2.5 m/s (5 knots) will introduce
a 22 cm/s additional velocity component into ADP measurements.
In the case of bottom-tracking, compass offset will still produce an error
in water-current measurements, but this error is proportional to the speed of
the current. In most applications, water-current velocities are much smaller
than vessel speed, so the introduced errors are smaller. For the same compass offset shown
in the above GPS example, and a current speed of 0.5 m/s, the corresponding error is only
4 cm/s.
4.3. Using GPS while Bottom-Tracking with an ADP
To avoid compass misalignment errors, the most robust
solution is to use bottom-tracking data together with GPS data. At the beginning of the
transect, you can simply compare the ship’s direction derived from bottom-tracking
with the gyro output, and then correct for possible misalignment.
The ADP is capable of recording GPS data while simultaneously
bottom-tracking. This not only provides a check between the two navigational methods, but
it allows the use of GPS when the bottom is out of range. All the advantages of both
methods can be used, and the ADP software easily works with both data sets. If both the
GPS and bottom-track velocities are collected, you are able to extract water currents
relative to either bottom track or to GPS simply by using the software provided with the ADP.
5. Field Test of Bottom-Track Performance vs. GPS
Test data were collected in Mission Bay, San Diego
with a 0.25-MHz ADP in January, 1999 (Figure 3). Bottom depth ranged from 80 m at the beginning to less than 10 m at the end of the
transect. Ocean swell was less than 0.5 m, and pitch and roll varied within 6°. Although
transecting over a bottom of different composition and slope, 99% of the profiles produced
percent-good pings greater than 80 (percent-good pings are that portion of the
bottom-tracking pings within the averaging interval when the bottom velocity was measured
successfully). This confirms the robustness of the SonTek bottom-tracking algorithms.
Overall, test results demonstrate a one-percent or better agreement between the GPS and
the bottom-track estimates.
The capability of using an ADP from a moving platform greatly extends the
versatility of this instrument. Whether you use DGPS, bottom-tracking, or both, you can be
assured that the SonTek ADP maintains high standards for quality, value, and reliability.
Figure 3. Field test of bottom-track vs. GPS
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