The National Aeronautics and Space Administration (NASA) at Goddard Space Flight Center (GSFC) has developed a laser ranging device (LIDAR) which provides accurate and timely data of earth features. NASA/GSFC recently modified the sensor to include a scanning capability to produce LIDAR swaths. They have also integrated a Global Positioning System (GPS) and an Inertial Navigation System (INS) to accurately determine the absolute aircraft location and aircraft attitude (pitch, yaw, and roll), respectively. The sensor has been flown in research mode by NASA for many years. The LIDAR has been used in different configurations or modes to acquire such data as altimetry (topography), bathymetry (water depth), laser-induced fluorosensing (tracer dye movements, oil spills and oil thickness, chlorophyll and plant stress identification), forestry, and wetland discrimination studies. NASA and HARC are developing a commercial version of the instrument for topographic mapping applications. The next phase of the commercialization project will be to investigate other applications such as wetlands mapping and coastal bathymetry. In this paper we report on preliminary laboratory measurements to determine the feasibility of making accurate depth measurements in relatively shallow water (approximately 2 to 6 feet deep) using a LIDAR system. The LIDAR bathymetry measurements are relatively simple in theory. The water depth is determined by measuring the time interval between the water surface reflection and the bottom surface reflection signals. Depth is then calculated by dividing by the index of refraction of water. However, the measurements are somewhat complicated due to the convolution of the water surface return signal with the bottom surface return signal. Therefore in addition to the laboratory experiments, computer simulations of the data were made to show these convolution effects in the return pulse waveform due to: (1) water depth, and (2) changes in bottom surface reflectivity

Hill, John M.

Krabill, William

Krenek, Brendan D.

Kunz, Terry D.

Stetina, Fran

English

NASA Technical Reports Server

"Prellmlnary Studies Leading Toward the Develop~nenl 
of a LIDAR Bathymetry Mapping Instrurnent" 
U - 
f 
/ Dr. John M. Hill, Mr. Brendan D. Krenek, and Dr. Terry D. Kunz; 
Space Technology And Research (STAR) Center, Houston Advanced Research Center 
(HARC), The Woodlands, Texas 
Mr. William Krabill and Mr. Fran Stetina; 
NASA/Goddard Space Flight Center, Greenbelt, MD 
ABSTRACT; 
The National Aeronautics and Space Administration (NASA) at Goddard Space Flight Center (GSFC) 
has developed a laser ranging device (LIDAR) which provides accurate and timely data of earth 
features. NASA/GSFC recently modified the sensor to include a scanning capability to produce 
LIDAR swaths. They have also integrated a Global Positioning System (GPS) and an ilnerdd 
Navigation System (INS) to accurately determine the absolute aircraft location and aircraft attitude 
(pitch, yaw, and roll), respectively. The sensor has been flown in research mode by NASA for 
many years. The LIDAR has been used in different configurations or modes to acquire such data 
as altimetry (topography), bathymetry (water depth), laser-induced fluorosensing (tracer dye 
movements, oil spills and oil thickness, chlorophyll and plant stress identification), forestry, and 
wetland discrimination studies. 
NASA and HARC are developing a commercial version of the instrument for topographic mapping 
applications. The next phase of the commercialization project will be to investigate other applications 
such as wetlands mapping and coastal bathymetry. In this paper we report on preliminary laboratory 
measurements to determine the feasibility of making accurate depth measurements in relatively 
shallow water (approximately 2 to 6 feet deep) using a LIDAR system. The LIDAR batkymetry 
measurements are relatively simple in theory. The water depth is determined by mewuriag the 
time interval between the water surface reflection and the bottom surface reflection signals. Depth 
is then calculated by dividing by the index of refraction of water. However, the measuremen& 
are somewhat complicated due to the convolution of the water surface return signal with the bottom 
surface return signal. Therefore in addition to the laboratory experiments, computer simulations 
of the data were made to show these convolution effects in the return pulse waveform due to: a) 
water depth, and b) changes in bottom surface reflectivity. 
Biographical Information 
J. Hill: Jack Hill is the Head of the Remote Sensing/Geographic Information Systems (RS/GI'S) 
Laboratory at the Houston Advanced Research Center (HARC). He is the Principal 
Investigator at HARC on the NASA LIDAR Commercialization Program. 
B. Krenek: Brendan Krenek is a Laser Systems Specialist at HARC where he assists in laboratory 
feasibility demonstration experiments and system prototyping. 
T. Kunz: Terry Kunz is Head of Laser Applications in the Space Technology And Research 
(STAR) Center at HARC. He is the Principal Scientist on the NASA LIDAR 
Commercialization Program. 
W, Krabill: Bill Krabill is the Project Scientist for the NASA LIDAR Comrnercialirntion Project 
and is responsible for mission planning, instrument development, data analysis, and 
data interpretation. He is stationed at NASA Wallops Space Flight Center. 
Fran Stetina, stationed at the International Data Systems Office, Goddard Space Flight 
Center, is the Project Manager for NASA's LIDAR Commercialization Program. 
https://ntrs.nasa.gov/search.jsp?R=19930016425 2020-03-24T07:26:55+00:00Z
PWELIbIINARY STUDIES LEADING TOWARD THE DEVELOPMENT 
OF A LIDAR BATWYMETRY h1APPING INSTRUMENT 
P ,O INTRODUCTION 
%"he National Aeronautics and Space Administration (NASA) at Goddard Space Flight Center 
(GSF(3 has developed a laser ranging device (LIDAR) which provides accurate and timely data 
of earth features. NASA/GSFC recently modified the sensor to include a scanning capability to 
produce LIDAR swaths. They have also integrated a Global Positioning System (GPS) and an 
Inertid Navigation System (INS) to accurately determine the absolute aircraft location and aircraft 
attituale (pitch, yaw, and roll), respectively. The sensor has been flown in a research mode by 
NASA fof nhany years. It has been used in different configurations or modes of the LIDAR to 
acquire such data as altimetry (topography) [I], bathymetry [2], laser-induced fluorosensing (tracer 
dye movements [3], oil spills and oil thickness [4 and 51, chlorophyll and plant stress identification 
[dl), forestry [7], and wetland discrimination studies [8 through 111. 
PjASA and HARC are developing a commercial version of the instrument for topographic 
m-aappi~ng applications. The next phase of the commercialization project will be to investigate other 
applications such as wetlands mapping and coastal bathymetry. 
In this paper we report on preliminary laboratory measurements to determine the feasibility 
of ma.king accurate depth measurements in relatively shallow water (approximately 2 to 6 feet 
deep) using a LIDAR system. The LIDAR bathymetry measurements are relatively simple in 
theorq~. The water depth is determined by measuring the time interval between the water surface 
reflection and the bottom surface reflection signals. Depth is then calculated by dividing by the 
index of refraction of water. However, the measurements are somewhat complicated due to the 
conv6rok'~iasn of the water surface return signal with the bottom surface return signal. Therefore, 
in addition to the laboratory experiments, computer simulations of the data were made to show 
these con+olution effects in the return pulse waveform due to: a) water depth, and b) changes in 
bottom surface reflectivity. 
The experiment was simplified by eliminating the water column and substituting graphite 
plates for the top and bottom surfaces. Depth and signal return intensities were adjusted by simply 
moving the plates relative to each other and intercepting different amounts of the laser beam, 
respectively. Section 2 describes the laboratory apparatus used in these measurements. Section 3 
presena the experimental results and the computer simulations. Section 4 makes general conclusions 
regarding the feasibility of performing LIDAR bathymetry measurements in shallow water. 
2.0 LABORATORY LIDAR APPARATUS DESCRIPTION 
These measurements were performed at HARC's Laser Applications Laboratory. Figure 1 
shows the apparatus used for performing the LIDAR experiments. This apparatus consisted of a 
pulsed laser, telescope, fast photodiode/amplifier, and digital oscilloscope. The X = 532 nrn output 
of the Nd:YAG laser (Spectra Physics, Model GCR-11-3) emitting E 2 155 mJ'/pulse in a 6-7 ns 
pulse width was directed approximately 50 feet down the laboratory and onto two carbon plates. 
The top carbon plate functioned as the water surface and the bottom carbon plate served as the 
bottom surface. The reflected light from the top and bottom surfaces wastollected by a 1" aperture 
telescope. A fast photodiode (Electro-Optics Technology, Model ET-2000, 200 ps rise time) and 
amplifier (Stanford Research Systems, Inc., Model SR440, DC-300MHz) and digital oscilloscope 
(LeCroy, Model 9400,125 MHz) detected and recorded the return laser pulse waveform, respectively. 
Adjusting the distance between the two carbon plates simulated different water depths; moving 
the plates in and out of the laser beam adjusted the strength of the top and bottom surface return 
signals. The distances between the two plates for these experiments were 1 foot, 3 feet, and 5 
feet. For each experiment, the digital scope averaged 50 waveforms before transferring the result 
to the plotter. 
Figure 2 shows the instrumental response of the LIDAR apparatus shown in Figure 1, in this 
experiment, the top carbon plate was removed and the Nd:YAG laser was in long pulse mode (6-7 
ns F W M  pulsewidth according to manufacturer's specifications). As seen in Figure 2, the LIDAR 
system is recording the laser FWHM as approximately 12.8 ns. This is attributed to WO factors: 
1. The bandwidth of the digital scope is 125 MHz; therefore, its rise time is approximately 
2.8 ns (i-e., .35/125 MHz), and 
2. The bandwidth of the amplifier is 300 H z ,  corresponding to a rise time of 1-2 ns. 
The Spectra Physics Nd:YAG laser can also be operated in a short pulse mode which reduces 
the temporal pulse width by approximately a factor of 3 to about 2.5 ns; this resulls in only a 10% 
loss of pulse energy. Figure 3 shows the instrumental response with the laser in b e  short pulse 
mode. After consulting with the manufacturer, the 2.5 ns specification (alignment sensitive) applies 
only to the base of the center peak. Therefore, in all subsequent measuremenls, the I ae r  was 
opemted in long pulse mode. 
3,0 RESULTS 
3.1 EXPERIMENTAL DATA and SIMULATIONS 
Figures 4, 5, and 6 show the experimental results (rough curves) when the plates were separated 
by 1 foot, 3 feet, and 5 feet, respectively. Also shown in these figures are simulated data (smooth 
curves) of each experiment. The simulations were generated by convoluting two Gaussian lineshapes, 
each defined by a 12.8 ns FWHM, and different line intensities (as noted in the figure captions). 
These simulated data were generated using a program called COCON1 .BAS (described below) and 
are simply best fits to the experimental data as determined by eye. 
3-2 ADDITIONAL COMPUTER SIMULATIONS 
After understanding the experimental results above, additional computer simulations were 
performed to reconstruct idealized bathymetry waveform data (i.e., no noise due to intermediate 
scattering layers located between the top and bottom surfaces) predicted in 2 to 15 feet water 
depths. These simulations are based on the model shown in Figure 7.  In this simple model, a 
Imer pulse of intensity I, is shot towards the water surface. At the air - water interface a portion 
(Il) of the incident beam is reflected back towards the detector based on the water index of 
refraction [i.e., I1 = I,(n-1)2/(n+1)2]. The rest of the laser pulse, I2 (= 1-I,), is transmitted down 
through the water column to the bottom surface. In this process, the laser is attenuated according 
to the Beer-Lambert law [log (Ifinal/IhCident) = -at, where a is the attenuation coefficient and l is 
the path length] and is represented as Is. The beam is then reflected off the bottom surface (RBS) 
resulting in I, and attenuated again travelling up the water column to the surface. A portion of 
E5 is again lost at the water - air interface. I6 is reflected back in the water, and I7 is transmitted 
back towards the detector. 
As shown in Figure 7, the two most important quantities for the generating reflected waveform 
simu'8ation data are I1 and 17. Using n = 1.33 as the water index of refraction at 20°C, I1 is 2% of 
I, In g e n e d ,  I7 can be calculated by: 
Using this model, two families of simulations were generated. The first simulation used a 
water attenuation coefficient (a) of O.l/foot [12] and bottom surface reflectivity (RBS) of 40%. 
Reflected wsavefom data were simulated for water depths of 1.5, 3.0, 4.5, 6.0, 7.5, and 11.3 feet 
corresponding to 4, 8, 12, 16, 20, and 30 ns top and bottom surface peak separations, respectively 
(i.e,, the speed of light in water was taken to be 1.33 times less than in air). This family of curves 
is shown in Figure 8. An analogous family of curves was generated using a bottom surface 
reflectivity (Wss) of 10%. These data are shown in Figure 9. As noted from Reference 12, these 
are resonable values for a and RBS. Each of the panels in Figures 8 and 9 list the water depth, 
separation time between the water surface and bottom surface return pulses, and the relative line 
intensities for the water surface return signal (Inti) and the bottom surface return signal (Int,). 
The data were simulated by convoluting two 6 ns FWHM Gaussian lineshapes defined by intensities 
Inti and Int*. This simple convolution software, named "COCONI", was developed and run on an 
IBM compatible computer and written in basic. The software will convolute any number of lines 
using a Lorentzian or Gaussian lineshape (defined by the lineshape FWWM). Spectral resolution 
can be controlled by a combination of the total number of points in the file and the wavelength 
range of convolution. The convoluted spectrum can be written in wavenumbers (energy) or 
angstroms (wavelength). The results were plotted on a HP7475A using software developed in-house 
and named "TRANSPLT", also written in basic. 
In the simulations shown in Figure 8 (a = O.l/foot and RBS = 40%), the bottom return pulse 
is stronger than the surface return pulse for the 3 feet (8 ns peak separation) and 4.5 feet (12 ns 
peak separation) water depths. At water depths deeper than approximately 5 feet, the laser will 
be attenuated in a sufficient length of water column so that the surface return pulse will be the 
larger of the signals. In contrast, in the simulations shown in Figure 9 (a = O.l/foot and RBS = 
lo%), the surface return signal is always stronger than the bottom return signal. 
4.0 CONCLUSIONS 
The following conclusions are made based on the results of these experiments and computer 
simulations. 
1. The 125 MHz digital oscilloscope (2.8 ns rise time) was not fast enough to resolve the 
6 ns FWNM pulse width of the Nd:YAG laser. Therefore, the experimental data in 
Figures 2, 4, 5, and 6 are not in true physical agreement with the measured block 
separation distances. 
2. The simulated data can easily be made to agree quantitatively with the experimental 
data. 
3. Even with the slow digital oscilloscope, top and bottom surface signals coressponding 
to approximate water depths of 3 feet and 5 feet were clearly discernable. 
4. When the water depth is of the order of 1 to 2 feet, the recorded lineshape will simply 
look like a single broadened line as shown in the experimental data of Figure 4, and 
the top panels of the simulated data in Figures 8 and 9. 
5. The simulated data in Figures 8 and 9 show the effects of the water depth and bottom 
surface reflectivity on the return waveform. Clearly, a trade-off exists between signal 
strength and signal separation between the top and bottom signal reflections. In shallow 
water, both top and bottom return signals should be strong, but close together in time, 
Beading to spectrum resembling a single broadened line at 2 feet depths. As the water 
gets deeper, the bottom surface return pulse becomes weaker due to attenuation, but is 
separated farther in time from the top surface return pulse. 
6. A Marquardt algorithmlfitting technique 1131, or similar data reduction routine(s), would 
be useful for deconvoluting these lineshapes in a real bathymetry application. 
7. Two (2) new digital scopes will soon be available that would nicely fill the waveform 
recording requirements of this bathymetry application. LeCroy will soon market their 
3200/7200A digital scope equipped with a 2 Gsamplelsec (500 MHz analog system 
bandwidth, 0.7 ns rise time) for approximately $45,000. MP will also be marketing their 
54710154720 Mainframe digital scope equipped with a 4 Gsamplelsec (1.1 GHz analog 
system bandwidth, 0.3 ns rise time) for approximately $40,000. Both scopes will have 
a SCSI-2 (small computer systems interface) port option. 
8. Future Directions will investigate three main topics: 
a) range biasing as a function of scan angle, 
b) range biasing as a function of water turbidity, and 
c) maximum depth capability of the technique. 
5.0 REFERENCES 
1. W. B. Krabill, J. 6. Collins, L. E. Link, R. N. Swift, M. L. Butler, "Airborne L a e r  
Topographic Mapping Results from Joint NASA/U. S. Army Corps Of Engineers 
Experiments", Photogrammetric Engineering and Remote Sensing, $90, 685 (1984). 
2. F. E. Hoge, R. N. Swift, and E. B. Frederick, "Water Depth Measurement Using An 
Airborne Pulsed Neon Laser System", Appl. Opt., a, 871 (1980). 
3. W. B. Krabill and R. N. Swift, "Airborne LIDAR Experiments at the Savannah River 
Plant", NASA Technical Memorandum 4007, 1987. 
4. F. E. Hoge and R. N. Swift, "Experimental Feasibility of the Airborne Measurement of 
Absolute Oil Fluorescence Spectral Conversion Efficiency", Appl. Opt., 22, 37 (1983). 
5. F. E. Hoge and R. N. Swift, "Oil Thickness Measurements Using Airborne Laser-Induced 
Water Raman Backscatter", Appl. Opt., 19( 191, 3269 (1 980). 
6. F. E. Hoge, R. N. Swift, and J. K. Yungel, "Feasibility of Airborne Detection of 
Laser-Induced Fluorescence of Green Plants. 1: A Technique for the Remote Detection sf 
Plant Stress and Species Differentiationn, Appl. Opt., 23, 134 (1983). 
7. 6. A. McClean and 6. L. Martin, "Merchantable Timber Volume Estimation Using An 
Airborne LIDAR System", Canadian Journal of Remote Sensing, 121 11, 7 (1986). 
8. V. Carter, "Applications of Remote Sensing to Wetlands" in G. J. Johannsen and J, L. Sanders 
(Eds.) Remote Sensing for Resources Management, Ankeny, Iowa, Soil Conservation Sc i e ty  
of America, 284-300 (1982). 
9. M. K. Butera, "Remote Sensing of Wetlands", IEEE Transactions on Geoscience m d  Remote 
Sensing, GE-23, 383 (1983). 
10. J. R. Jensen, M. E. Wodgson, E. J. Christensen, H. E. Mackey, and L. Tinney, "Remote Sensing 
of Inland Wetlands: A Multispectral Approach", Photogrammetric Engineering and Remote 
Sensing, 52, 87 (1986). 
11. J. R. Jensen, E. W. Ramsey, H. E. Mackey, E. J. Christensen, and R. R. Sharitz, "iinland 
Wetland Change Detection Using Aircraft MSS Data", Photogrammetric Engineering m d  Remote 
Sensing, 52, 521 (1987). 
12. F. E. Hoge, C. Wayne Wright, William B. Krabill, Rodney R. Buntzen, Gary D. Gilbert, Robert 
N. Swift, James K. Yungel, and Richard E. Berry, Applied Optics, 27(19), 3969 (1988). 
13. W. Schreiner, M. Kramer, S. Krischer, and Y. Langsam, PC Tech. J., 2, 170 (1985). 
1" Diameter 
Telescope 
Pulsed Nd:YAG Laser 
Figure 1 .  Laboratory LIDAR apparatus for simulating bathymetry measurements. 
80 rnV Full Scale 
Simulated Data: 
Figure 2. Instrumental response of the LIDAR apparatus shown in Figure 1 with the Nd:YAG 
laser in long pulse mode. 
Figure 3. Instrumental response of the LIDAR apparatus shown in Figure 1 with the Nd:YAG 
laser in short pulse mode. Note the extm side lobes. 
511 
80 mV Full Scale 
50 scans averaged I 
Simulated Data: 
Figure 4. Experimental data (rough line) recorded with the carbon blocks spaced 1 foot apart. 
Simulated data (smooth line) consisting of two 12.8 ns FWHM pulses separated by 8 
ns (i-e., 4 feet due to round trip travel) with 1.0 and 0.7 relative intensities. 
80 mV Full Scale 
Simulated Data: 
12.8 ns FWHM I 
Figure 5. Experirnental data (rough line) recorded with the carbon blocks spaced 3 feet apart. 
Simulated data (smooth line) consisting of two 12.8 ns FWWM pulses separated by 13 
ns (i.e., 6.5 feet round trip travel) with 1.0 and 0.6 relative intensities. 
5 12 
50 scans averaged 
Simulated Data: 
12.8 ns WEIN1 
Figure 6. Experimental data (rough Pine) recorded with the carbon blocks spaced 5 feet apart 
Simulated data (smooth Pine) consisting of two 12.8 ns FWWM pulses separated by 14.6 
ns (i.e., 7.3 feet round trip travel) with 1.0 and 0.7 relative intensities. 
Water 
Surface 
Figure 7. Simple model for determining relative return signal intensities for LTDAR bathyrnetry 
measuremen&. 
5 13 
4 n s  Peak Separation 
12 ns Peak Separation 
16 ns  Peak Separation 
20 ns Peak Separatio 
Figure 9. Family of simulated return waveform traces using a water attenuation coefficient (a) 
- O.l/foot and bottom surface reflectivity (RBs) = 10%. TSud designates a reference 
time as the peak of the water surface return pulse. 


Preliminary Studies Leading Toward the Development of a LIDAR Bathymetry Mapping Instrument

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