1	BroadBand ADCP Basics 1 Discussion of Concepts re How It Works
2	Overview Broader bandwidth => Better results Method Differences 2-pulse Signal Velocity Precision
3	Broadband Signaling RDI's ADCPs use patented broadband technology. Advantages of broadband signaling are everywhere in our daily lives: better quality, clearer reception, and faster results. Examples include high-speed internet access, cable TV, FM stereo radio, 3G cell telephones, GPS positioning, and CD music recordings. Improved performance is due to broadband signaling providing more information.
4	More Information How does broader bandwidth signal => more precise / repeatable results? Bandwidth: rate at which modulate carrier signal (e.g., by phase shift) Determines how many samples are taken by a single acoustic transmission (ping) More bandwidth => More independent samples => More information
5	Better Results How does broader bandwidth signal => more precise / repeatable results? More information produces better data set-- better basis, more confidence for making important decisions How is it better? ÝIf you wait the same time, you obtain better certainty of the measurement (signal is clearer) => More precise / repeatable results You don't have to wait as long to see the signal, so your time series or boat path have better definition => Better resolution in time or space (or both)
6	Method Differences
7	Method Differences (1) Bandwidth = sampling rate Broadband method Broad bandwidth signal => 100's samples per ping Noise NOT hide the signal Take only few pings NO WAITING to see signal Narrowband method Narrow bandwidth signal => few samples per ping Noise can hide the signal Must take many pings to reduce noise Wait to see signal
8	Method Differences (2) Broadband method = More information More information: top-to-bottom without sacrificing resolution in time, along boat path OR More information in time history, along boat path without sacrificing resolution top-to-bottom Narrowband method Trade Off for Improving Data Resolution ÝIf you want MORE information in profile, COST = LESS information in time history, along boat path
9	2-pulse signal
10	2-Pulse Signal (1) Transmit 2 pulses at known time separation: T_apart They travel along a beam, away from ADCP Suspended material is moving along beam, to ADCP At encounter with 1st pulse, an echo is scattered back to ADCP
11	2-Pulse Signal (2)
12	Time Dilation of 2-Pulse Echo Speed of scatterer = moving with the water = U_water In the time = T_apart, scatterer moves a distance = L So L = U_waterx T_apart The 2nd pulse has a round trip shorter by about Ý2 x L compared with the 1st pulse So 2nd pulse travels for less time than 1st pulse Key Result: in echoes, time between pulses is reduced
13	Water Velocity Speed of pulses = speed of sound = C Change in time between pulses in echoes = t t = 2 x L / C = 2 x U_waterx T_apart/ C This gives the water speed Ý U_water= C/2 x t / T_apart Key points By determining t in the echo data, you know water speed Velocity comes from displacement of scatterer during a known time
14	Velocity Precision
15	Velocity Precision: Controlling Random Noise Measurement: U_water= C/2 x t / T_apart Standard deviation of U_water= sd (U_water) sd (U_water) = C/2 x 1/ T_apartx sd ( t ) Key point Larger T_apart=>smaller sd (U_water) Modes 5, 11 are quieter than Mode 1
16	Velocity Precision: Example Estimating velocity from position data Compute the change in position with time Velocity = (X₂- X₁) / elapsed time Uncertainty of velocity estimate = sqrt(2) x (uncertainty in position) / TIME Longer TIME delay between two position fixes => more certainty of velocity
17	BroadBand ADCP Basics 1 Discussion of Concepts re How It Works