1	BroadBand ADCP Basics 2 Discussion of Concepts re How It Works
2	Multi-pulse Transmit into Multiple Scatterers Setting Pulse Separation
3	Multiple Scatterers Echo time series is complex* superposition of echoes Ý Many returns from multiple scatterers & locations In practice, you can't measure delay between pulses by using time (per se) (*Individual pulses do not appear like Measure phase lag between pulses and use it as a proxy for elapsed time Elapsed time = phase lag / frequency t = p / Df In echo series, Doppler-shifted carrier frequency is time base = Df This gives a new equation to calculate water speed U_water= C/2 x p / (Df x T_apart)
4	Multi-pulse Transmit: Pulse Separation
5	Using Phase Lag Water speed: U_water= C/2 x p / (Df x T_apart) RECAP: p is a proxy for elapsed time => measures change in pulse separation But phase is periodic, repeating its value each 360 degrees So using Phase Lag introduces a potential source of ambiguity Ambiguity? Consider: Elapsed time = minutes + seconds For a stopwatch that has lost its minute digit, what is the elapsed time? REQUIRES: Another means for counting complete 60-s cycles
6	Controlling Ambiguity Rearrange: U_waterx T_apart= C/2 x p / Df Total phase lag, p = N x carrier cycles + fraction of cycle Larger U_waterproduces bigger p (change in pulse separation) When p crosses ± 180 degrees then N=1 p and U_water become ambiguous if N is unknown , Phase lags of -210 and +150 have the same value Controlling ambiguity: make a choice re determining N Avoid the ambiguity (ensure N=0 at all times) Resolve the ambiguity: Count 360 deg. phase wraps Key Point: Important Difference between Profiling Modes
7	Avoiding Ambiguity Rearrange: U_waterx T_apart= C/2 x p / Df p = N x carrier cycles + fraction of cycle (1) (2) At water velocity where p becomes ambiguous (± 180 degrees) then U_waterx T_apart= constant Choose small T_apartin transmitted pulse pair to set large velocity before U_waterwill be ambiguous Modes 1 and 12
8	Resolving Ambiguity p = N x carrier cycles + fraction of cycle (1) (2) Knowing N in (1) resolves any ambiguity in p N is found by measuring average velocity in a specific, auto-selected depth layer, called the ambiguity resolving bin (ARB) Modes 5 and 11 If N cannot be determined by the ARB, Modes 5 and 11 do not output data. **The ARB is set automatically. Changing it can ruin your day!
9	Trade Off for Avoiding Ambiguity RECAP: U_water= C/2 x t / T_apart sd (U_water) = C/2 x 1/ T_apartx sd ( t ) Smaller T_apart=> larger sd (U_water) Key point: Sending Pulses close together Avoids complexity of resolving ambiguity COST: Higher variance (random noise on velocity profile) Advantages of pulses being close together (Mode 1, 12) More resistant to Decorrelation causes Measure higher speeds More robust ADCP operation under dynamic conditions
10	Advantages for Resolving Ambiguity Key point: Sending pulses farther apart Requires complexity of resolving ambiguity Advantages (Mode 5, 11) Lower variance in velocity profile Enables much finer resolution in data set (profile, time) Permits profiling shallower depths COST of pulses being farther apart More susceptible to Decorrelation causes Limits maximum measurable speed, profiling range Less robust ADCP operation under dynamic conditions
11	Difficulties for Ambiguity Resolving Bin p = N x carrier cycles + fraction of cycle (1) (2) Problems for determining (1) => Resolving ambiguity High Shear: Velocity changes too much across the depth of the ambiguity resolving bin Turbulence or High boat speed: Back-scattering sources change too much during time between pulses in pair Ý(Details are described in Section titled Decorrelation Effects)
12	BroadBand ADCP Basics 2 Discussion of Concepts re How It Works