1. Full Waveform LiDAR Understanding full waveform and how it works
Jamie Young Senior Manager-LiDAR Solutions - AeroMetric

2. History and background What is Full Waveform How does it work
Specifications
Hardware
Process and Software
Application comparison
Topics
Benefits
Questions?
Full Waveform

3. History of full waveform digitization almost as old as LIDAR
Capability exists in bathymetric systems
2d visualization concepts for pseudo-waveform data
Waveform digitizing adapted to terrestrial use
Gaussian waveform decomposition used in Bathymetric systems
Capability available from all major manufacturers
LAS 1.3 file format released
TerraScan processing of waveform data

4. Glossary -frequently used terms
FWD - Full Waveform Digitization
MRI - Multiple Returns with Intensity
Minimum return separation - minimum range difference for which independent range/intensity measurements can be made
Sample depth - resolution of the intensity measurement made at each digitizing interval
Sample interval- time interval (usually in nanoseconds) between intensity samples
Waveform length -number of samples, or total distance, digitized within the capture waveform

5. What is Full waveform?
The laser pulse is emitted and all the return information of that pulse is received back to the receiver and stored.
The system needs to be set up to store the amount of information desired
64 samples
128 samples
256 samples
The return information needs to be converted to usable data
Basically, the full waveform data is converted to discrete return information

6. What technology is used?
The systems used are the same as what is used for traditional LiDAR sensors with one exception.
A full waveform digitizer
GPS
IMU
Laser
Scanner

7. LIDAR waveform how is it created?
Multiple return pulses are generated as the laser pulse hits various levels in the forest canopy, creating in total a complete return waveform
Waveform measurement is a natural extension of the conventional “discrete-return + intensity” measurement process
Footprint
Return waveform is generated by all reflective surfaces within the laser footprint

8. Full Waveform Digitization (FWD) basic concept
Laser Footprint
Full Waveform Digitization (FWD) basic concept
Start Pulse
Detector Signal
T1 , I1
Tn , In

9. LIDAR waveform visualization
Each output laser pulse will hit a unique combination of surfaces on the terrain below:
Different elevation
Different percent of footprint intercepted at each foliage level
Different reflectivity of intercepted surfaces
Each output laser pulse will result in a unique waveform shape
PULSE 5
PULSE 4
Pulse 3
PULSE 1
PULSE 2

10. What is Full Waveform Digitization
What is Full Waveform Digitization? capturing the complete return, not just the peaks
Conventional discrete return electronics capture only the exact time of the peaks of independently-recognized return pulses
Peak intensity is also measured
In FWD systems, the entire return signal is measured, allowing capture of subtle deviations in the shape of the reflected pulse as compared to the shape of the outbound laser pulse
Discrete Returns
Waveform

11. Exploiting individual waveforms Gaussian decomposition for finding “buried” data
Return signal from ground
Return signal digitized at user-selected interval (typically 1 ns; equivalent to ~15 cm height)
Fitting of first return Gaussian component
Fitting of second Gaussian component
Fitting of third Gaussian component
Note: Each Gaussian component must be fitted for:
Time of occurrence
Peak amplitude
Pulse width
Benefit: any “stretching” of pulse detected can be used to indicate vegetation height on ground (and automatic adjustment of range) or inclined surfaces

12. Equivalent tree height
AeroMetric (Leica ALS-70) Specifications WDM65 Waveform Digitizer Module
Specification
Value
Equivalent tree height
Maximum waveform rate
(waveforms captured for every other laser shot if pulse rate>120 kHz)
120 kHz
Sample depth,
Sample interval ns ns ns ns ns ns
38.4 m
19.2 m
76.8 m
153.6 m
Integration
New internal DLM
Weight
1.0 kg
Power
77 W
Typical Altitude
2400 AGL
Storage
512 GB SSD

13. Operating envelope max waveform rate versus slant range
At pulse rates below 120 kHz, waveforms captured at laser pulse rate
At pulse rates above 120 kHz, waveforms capture for every other pulse, up to 200 kHz (150 kHz for ALS50-II)

14. Hardware configuration full waveform digitizing for ALS
Upcoming release – announce 17 Nov 2009
Core is new “FWD-ready” Data Logger Module (DLM65) – installed in all new ALS60 systems
Existing HDD replaced by 160 GB SATA SSD (MM60) and allows missions up to 7620 m AMSL equivalent cabin pressure
3 variations
Option on new ALS60 (771706)
Upgrade on fielded ALS60 (771708)
Upgrade on fielded ALS50-II ( )
Waveform viewer software + ALSPP data output in LAS 1.3 format

15. Hardware details DLM65 / digitizer kit
DLM65 chassis
Waveform Interface PCB added to System Controller tray (signal splitter)
Double-wide CPU replaced with faster single-wide CPU with on-board SATA driver (releases 2 slots)
Slot Blocker
Existing 32-bit DIO PCB for System Controller data logging remains
FWD kit
2x waveform digitizer PCB (60 kHz max each)
1x time synchronizer PCB
Firmware license
DLM Power supply hard mounted to card cage (releases 2 slots)
Baffle to direct airflow

16. Some points about FWD
Intensities must be digitized at <2 ns intervals to minimize aliasing, though 1 ns more common
1 ns in time represents 0.15 m in range (i.e., elevation)
Signal amplitude at each interval typically digitized at 8-bit resolution (i.e., one byte)
Therefore, 256 additional bytes of waveform data needed to digitize the return waveform from a 38.4 meter-tall 1 ns intervals
Range data is still be measured independently to achieve typical 1.5 cm (i.e., 100 ps) range resolution

17. Using waveform data for classification concept
Assumes that waveform shape/content, as opposed to mere extraction of equivalent discrete returns, is used to classify object which reflected laser pulse
Assumes that waveform is compared to a “catalog” of “typical” waveforms for different target classes

18. Software FWD-equipped systems release summary
Release Number
Comments
TracGUI
2.73 #2
Datalogger
FCMS
3.20
Officially released beta to be used only for FWD-equipped; otherwise use 3.15 even for FWD-ready systems
Finished testing 10 November 2009
Intel SSD
2CV102HA
ALSPP
2.70 #7
Executable version only – works OK
Still need installable version
WaveViewer
TerraScan
9.15
Released by Terrasolid November 2009

19. FWD post processing Overview
05/04/2017
FWD post processing Overview
ALS Post Processor
Support is available. v2.73 #2 (or greater)
Outputs LAS 1.3 type 4 files
Wave Viewer Utility. v (or greater)
Simple LAS 1.3 Waveform file viewer
TerraScan (See other CSS workflow documentation for details)
Support for LAS 1.3 released Oct ’09. v9.14 (on
The following features are included:
View waveform data for a selected point in the point cloud
Scan the waveform for returns that were too close together for the discrete-ranging electronics to detect or because the returns were below the threshold discriminator (i.e., creates a new “discrete” return)
No other calibration needed 19

20. FWD processing directory structure
05/04/2017
FWD processing directory structure 20

21. 05/04/2017
FWD - flight planning
No support for waveform data collection in FPES initially (i.e., waveform capture settings must be manually entered in FCMS)
Four settings control system configuration for waveform capture
Pre-trigger samples (configured in FCMS hardware configuration)
Number of samples
Sample interval
Maximum pulse rate
Options can be preconfigured or changed during flight execution 21

22. FWD - flight execution using FCMS
05/04/2017
FWD - flight execution using FCMS 22

23. FWD -flight execution with FCMS
05/04/2017
FWD -flight execution with FCMS 23

24. FWD - TracGUI real-time waveform display
05/04/2017
FWD - TracGUI real-time waveform display
Waveforms are shown in this display, in flight, while on- line using F4 F4 W command 24

25. Classification flowchart
Load sample waveform
Sample waveform
Load next target waveform
Target waveform 1
Compare sample waveform to target waveform
Target waveform 2
Assign class
Match?
Yes No
Target waveform 3
Any target samples left?
No match
Yes No

26. Wave Viewer screen

27. Data output waveform viewing and access
Wave Viewer
Main statistics (sample rate, waveform depth, waveform sequence number, timing data)
Allows scrolling through entire captured series of waveforms
Displays waveform
Displays time/intensity indicator (2 black boxes near waveform peaks) from discrete-return data collection that operates in parallel with waveform capture
TerraScan
Waveform display associated with any given discrete-return point
Future expansion allows possible improvement by deriving additional parameter (pulse stretch) in pre-processing and passing these on to TerraScan for more efficient//accurate filtering

28. FWD - ALS Post Processor/ing
05/04/2017
FWD - ALS Post Processor/ing
Select “Process Waveform Data” option.
Software looks for “RawWfd” folder with matching mission ID.
Automatically output LAS 1.3 file for each flight line 28

29. FWD - Wave Viewer X-axis: sample (number), time (ns) or range (m)
05/04/2017
FWD - Wave Viewer
X-axis: sample (number), time (ns) or range (m)
Y–axis: signal strength (volts or counts)
Used to review data and confirm that waveform data is correctly correlated with discrete-return data 29

30. FWD - post processing: Wave Viewer
05/04/2017
FWD - post processing: Wave Viewer
In this example, two returns were missed by the discrete-return ALS range electronics
The first missed pulse was too close to the first pulse to be seen
The second missed pulse was too small to be seen. 30

31. FWD - post processing: intensity scale
Note: Y scale factor of the raw intensity values and the waveform digitized values are not the same.
Intensity PCB in the SCM is designed to use the full 8-bits for the typical signal amplitudes
The waveform digitizer PCB has limited configuration options so the the option closest to the Intensity PCB scale factor is used
The intensity board uses the following scale (latest revision only, earlier versions may have different scale factors):
0.110V signal = 10 counts
3.7V signal = 250 counts
The waveform digitizers use the following scale:
0 V = 0 counts
4.1V = 255 counts

32. FWD - post processing: sample files
05/04/2017
FWD - post processing: sample files
Sample LAS 1.3 files will be located on FTP site
Contents
Wave Viewer
LAS 1.3 file format specification (can be released to public)
Data set
Note: Contact PM for FTP information 32

33. FWD –post processing TerraScan displays

34. FWD - post processing TerraScan capabilities
Viewing of waveform profile when clicking on discrete points in the point cloud
Mensurating more discrete returns from WF data (“extract echos” feature)

35. FWD -TerraScan V10.17+ “Extract Echoes” feature
Allows extraction of small signals via user-adjustable “ambient noise” threshold
Allows extraction of returns at less than the minimum separation dictated by discrete-return electronics (see cyan-colored points below)

36. MRI versus FWD: Applications
Application or surface type
MRI LIDAR
FWD LIDAR
Tree height
Yes
Yes, but overkill
Forest canopy structure
Maybe, species dependent
Yes, but may be overkill
Forest floor
Yes, depending on floor cover
Tall dense grass No
Power line profiling
Maybe, if lines separated by less than minimum separation distance
Tree species identification
Sloped surface detection
Yes, but height difference over laser footprint must be ~1 sample interval
Bathymetry

37. Using waveform data for classification caveats
Each return waveform highly dependent on
Specific geometry (i.e., portion of laser footprint intercepted at different heights above ground)
Reflectivity of each surface intercepting a portion of the laser footprint
Therefore
Waveforms returned from a group of nearby laser shots may have to be averaged to arrive at a more consistent “typical” waveform for comparison to the target waveform library
Waveforms to be averaged must be referenced to some consistent key point (e.g., ground level) before averaging, in order to generate a meaningful comparison

38. MRI and FWD provide some similar functionality
FWD exploitation techniques can extract returns that are “buried”
FWD waveforms from a group of adjacent laser shots may need to be averaged to make automated classification feasible
Averaging MRI “pseudo-waveforms” from a group of adjacent laser shots may be an effective alternative to FWD, especially if
LIDAR system can measure intensity for a large enough number of returns for each outbound laser pulse (e.g., 3+)
Inter-return minimum separation distance is small enough (target dependent)
High point density in MRI systems helps to overcome the additional per-pulse information supplied by FWD systems
Some applications may be accomplished by both methods, but may be better suited to one or the other (see table)

39. Benefits of FWD getting more from a single flight
Present
Extraction of points below the discrimination threshold of discrete-return electronics (weak returns)
Extraction of points with smaller vertical separation than detectable by discrete-return electronics (close, but not overlapping pulses)
Future
Detection of pulse stretching (return pulse wider than laser pulse) indicating
Potentially sloped surfaces
Low vegetation on ground, indicating need to adjust point elevation downward
Improved classification by using combination of return pulse width and spatial context
Indication of biomass by evaluating area contained under the pulse shape

40. WILDER LiDAR Blog

41. Thank You
Thank You to Leica for Contributions to this presentation