SonTek – a Xylem brand
FlowTracker2 User’s Manual (January 2019) 227
Figure 13:2 - Signal to Noise Ratio Profile
13.4. Pulse-Coherent Processing
This section does not attempt to provide a detailed description of pulse-coherent
processing. It presents a simple overview with a focus on how this affects
FlowTracker2 operation. SonTek can provide additional information on request.
The description of FlowTracker2 operation given in Bistatic Doppler Current Meters is
an oversimplification.
• Bistatic Doppler Current Meters describes incoherent Doppler processing in
which the transducer sends a single pulse of sound and measures the frequency
change of the return signal.
• The FlowTracker2 uses a technique called pulse-coherent processing.
Pulse-coherent processing provides the best possible performance of any Doppler
processing technique. In the simplest terms, pulse-coherent processing works as
follows.
a) The FlowTracker2 sends two pulses of sound separated by a time lag (τ).
b) Each receiver measures the phase (φ) of the return signal from each pulse.
c) The change in phase (φ2 - φ1) divided by the time lag (τ) is proportional to
velocity.
d) Because of the nature of the phase measurement (which can only be determined
from -180° to +180°), the system has an inherent maximum velocity limitation.
e) The FlowTracker2 velocity algorithms have been optimized to give the best
possible performance over a wide velocity range of ±4.0 m/s (±13 ft/s).
f) FlowTracker2 processing provides unmatched results for low flows (<1 cm/s;
<0.03 ft/s).
FlowTracker2 processing has been designed to give the best possible performance in
all environments. However, there is a situation where system performance may be
affected by operating conditions.
Time after transmit pulse
Received signal strength
Center of
Receiver Axis
Ambient
Noise Level
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