A new Kongsberg-Simrad EM120 multibeam echo-sounder has been installed aboard Scripps Institution of Oceanography\u27s Research Vessel Roger Revelle in January 2001. This system can map reliably a 20 km swath of seafloor in 4000 m water depth with 191 soundings per ping. Such a wide swath width demands highly accurate (\u3c0.05\u27 RMS) roll information from a motion sensor, and makes estimating sounding accuracy across the swath an interesting challenge. It is shown that good accuracy estimates can be obtained by collecting data on station under control of the GPS-aided dynamic positioning system usually available on most modern long-range oceanographic vessels. A number of motion sensors, with RMS roll accuracy specifications ranging from 0.05\u27 to 0.01\u27 ,were tested with the EM120 sonar on station in 3800 m to 4000 m water depths. Unexpectedly, they yielded roughly the same depth uncertainty as a function of receive beam angle. This result might be explained by synchronization errors between the attitude data and the sonar data leading to beam pointing errors, other types of beam pointing errors, a range of roll accuracy narrower than specified for the motion sensors, or a combination of these factor

de Moustier, Christian

English

University of New Hampshire

Field Evaluation of Sounding Accuracy in Deep Water Multibeam Swath Bathymetry

UNH Scholars' Repository

University of New HampshireUniversity of New Hampshire Scholars' RepositoryCenter for Coastal and Ocean Mapping Center for Coastal and Ocean Mapping11-2001Field Evaluation of Sounding Accuracy in DeepWater Multibeam Swath BathymetryChristian de MoustierUniversity of California - San DiegoFollow this and additional works at: https://scholars.unh.edu/ccomPart of the Oceanography and Atmospheric Sciences and Meteorology CommonsThis Conference Proceeding is brought to you for free and open access by the Center for Coastal and Ocean Mapping at University of New HampshireScholars' Repository. It has been accepted for inclusion in Center for Coastal and Ocean Mapping by an authorized administrator of University of NewHampshire Scholars' Repository. For more information, please contact nicole.hentz@unh.edu.Recommended CitationC. de Moustier, "Field evaluation of sounding accuracy in deep water multibeam swath bathymetry," MTS/IEEE Oceans 2001. AnOcean Odyssey. Conference Proceedings (IEEE Cat. No.01CH37295).Field Evaluation of Sounding Accuracy in Deep Water Multibeam Swath Bathymetry Christian de Moustier Scripps Institution of Oceanography 9500 Gilman Drive, La Jolla, CA 92093-0205, USA Abstract- A new Kongsberg-Simrad EM120 multibeam echo-sounder has been installed aboard Scripps Institution of Oceanography's Research Vessel Roger Revelle in January 2001. This system can map reliably a 20 km swath of seafloor in 4000 m water depth with 191 soundings per ping. Such a wide swath width demands highly accurate (<0.05' RMS) roll information from a motion sensor, and makes estimating sounding accuracy across the swath an interesting challenge. It is shown that good accuracy estimates can be obtained by collecting data on station under control of the GPS-aided dynamic positioning system usually available on most modern long-range oceanographic vessels. A number of motion sensors, with RMS roll accuracy specifications ranging from 0.05' to 0.01' ,were tested with the EM120 sonar on station in 3800 m to 4000 m water depths. Unexpectedly, they yielded roughly the same depth uncertainty as a function of receive beam angle. This result might be explained by synchronization errors between the attitude data and the sonar data leading to beam pointing errors, other types of beam pointing errors, a range of roll accuracy narrower than specified for the motion sensors, or a combination of these factors. I. INTRODUCTION In January 2001, the Scripps Institution of Oceanography installed a Kongsberg-Simrad Inc EM120 multibeam echo- sounder aboard the newest shp in its fleet, the Research Vessel Roger Revelle owned by the US Navy and commissioned in 1996 (AGOR 24) . This sonar system operates at a nominal fkequency of 12 kHz, with a l"x150" overall transmit sector (fore-& x athwartships) and up to 191 receive beams steered athwartships at regular angular steps across the swath, or at gradually narrower angular steps to achieve uniform horizontal offset between soundings athwartships, or a combination of both. Its flat hydrophone array configuration yields nominal receive beam widths of 2"/cos(0) from broadside (H") to the outer steering angles ((3475"). Most importantly, the sonar achieves broad swath widths in deep water (e.g. 20 km at 4km depth ) by steering the transmit beam in 9 discrete sectors athwarthships, while compensating for the shp's yaw, pitch, and roll. However, it is necessary to know the ship's roll and pitch to better than 0.05" RMS to achieve G I ' s  specification for sounding accuracy of 0.2% of water depth across the swath. In fact, since the sea trials at the end of January 2001, an apparent roll artifact has ruffled along-track the outer edges of the bathymetric swath collected aboard RN R. Revelle. Several tests have been conducted with various motion sensors to try and idenm its cause . With swath widths in excess of 20 k m  it is difficult to find a seafloor area, with suitably little relief along and across track, on which to conduct sounding accuracy tests. Options include survey techniques developed to resolve biases in swath bathymetry data, such as running a patch test over a known seafloor area [l], or creating a reference surface h m  a highly redundant set of soundings obtained by running tightly spaced parallel tracks with up to 90% swath overlap between adjacent tracks. Soundmg accuracy is then estimated by comparing individual soundmgs to the reference surface [3]. In all cases, a deep water reference surface is very costly in data acquisition and processing time. Provided the ship has good dynamic positioning capabilities, a simpler Sind much cheaper alternative consists in maintaining the ship on station at a constant headmg over a relatively flat seafloor area. Ping after ping, the same patch of seaflloor is sampled in a given beam dmction and changes in bottom relief along and across track become nearly negligible. The purpose of h s  paper is to highlight the effectiveness and potential pitfdls of estimating soundmg accuracy fkom multibeam swath bathymetry data gathered while the ship holds station. EM120 swath bathymetry and associated navigation data collected aboard RN Roger Revelle are used to illustrate the ship's station keeping requirements in Section II, and the sounding statistics in Section III. In Section IV, a comparison is made between results obtained on station, in 38OOm to 4OOO m of water depth, with four different motion sensors providing attitude data to the EM120 sonar. Their unexpected similarity is discussed and potential causes are analyzed. 11. SHIP STATION KEEPING REQUIREMENTS A. Position At average oceanic depths (4 km), the along and across track extents of the footprint of a l"x2" specular beam are roughly 70m and 140m, respectively. Adjacent beams on either side athwartships are within lo, but the angular beam spacing becomes progressively narrower from nadir out when using the sonar's mode that provides equidistant soundings across-track, which is true for all the data presented here. Therefore one needs to maintain the ship's position within a watch circle 10 m in diameter for a given beam direction to sound the same patch of seafloor repeatedly. Aboard RN Roger Revelle, the dynamic positioning system controls two stem Z-drive azimuthal thrusters and a bow thruster. It can maintain the ship's position in a P-Code GPS reference frame in a circle less than 10 m in diameter for the 40 min required to collect 100 pings in 3800 m of water depth, as shown in Fig. 1. B. Heading The 1" fore-aft beamwidth of the transmit beams imposes restrictions on the ship's heading variability during a test, before relief variations along and across track can no longer MTS 0-93 3 95 7-28-9 1761 be neglected. As illustrated in Figs. 1-2, experience with R N  R.Revelle shows that the ship can hold station and heading to 0.6" RMS (Fig. 3) in sea states 4 or below. In the foregoing analysis, data with heading variations up to 0.75" RMS have been used, but they start showing the limitations of the negligible relief assumptions. I , . . . . .  I -  0 5 -  P L c 0 -  B 4 4 1 -  -0 - 0 5 -  -1 5 -  7 1 - 2 4  ' 1 Fig. 1. Variations in the ship's position while on station. -3 -25  -2 -15  -1 4 5  0 0 5  1 1.5 2 Longitude dsv&mn (m) I Fig.2 Variation in the ship's heading while on station. Likewise, the noise in the heading data supplied to the EM120 sonar should remain a small fraction of the fore-aft beam width. As a first order verification, Fig. 3 shows a noise histogram drawn from the residuals of detrending and low-pass filtering performed on the heading data of Fig. 2. Although not strictly speaking a noise sequence, the residuals have a mean of zero and a standard deviation of 0.12", which is within 20% of the 0.1" specified RMS accuracy of the Meridian Gyro used in these tests, and of the accuracy required by the EM120 sonar. Control of heading variations during a test is achieved by setting a maximum heading deviation in the dynamic positioning system. However, local weather conditions might make such settings moot and it becomes necessary to collect enough pings to be able to select a subset of pings that fall within the desired heading bounds. Fig. 3. Histogrks of variation in the ship's heading (left) and heading noise (right) while on station. 111. SOUNDING STATISTICS A. Depth Profiles Given proper control of the ship's heading and position during data collection on station, it is straightforward to compile statistics of the soundings as a h c t i o n  of receive beam angles referenced to nadir, hence corrected for the ship's roll and for refraction effects at the face of the array. Here, beam angles are considered in 0.1" increments, but only beam directions reporting data for more than half the total number of pings in the set are used in the statistics. Stacked profiles of depth vs. received beam angle are shown in Fig. 4, with details in Fig. 5 showing the mean depth (solid line) and the scatter of soundings about the mean. The scatter increases with steering angle, and tighter angular spacing of beams at increasing athwartships angles to achieve equidistant soundings can be seen also in these plots. I Fig. 4. Stacked instantaneous bottom profiles of depth vs. receive beam angles (port <O, starboard >O) for about 100 pings recorded with the ship on station. 1762 -4 r Fig. 5 .  Details of the port and starboard beam soundings (+) from Fig. 4 with the mean profile drawn as a solid line. Closer inspection of the outer beams from Figs. 4-5 reveals two interesting clues illustrated in Fig. 6,  where soundings at f65" from nadir are plotted as sequences of depth vs. consecutive ping numbers (equivalent to time at -20 s/ ping). These two sequences contain frequent spikes that are for the most part "180" out of phase" between port and starboard, indicating that the athwartships profile rolls with the ship. Second, there is a long term oscillation with a period of about 60 pings (-20 min) that does not seem to be correlated with anything obvious at this point. Fig. 6.  Evolution of soundings in time for two beams at f65" from vertical for the data shown in Figs. 4-5. Solid line port, dashed line starboard. B. Depth Uncertainty The depth accuracy for each sounding is estimated from the data in Figs 4-5, by forming the ratio of the standard deviation of the soundings in each angular bin to their mean. This yields a depth uncertainty in percent of mean water depth. As shown in Fig. 7-8, uncertainties remain below 0.2% from nadir to about +60" and climb rapidly thereafter to values in excess of 2% at f70". Fig. 7 Depth uncertainty (standard deviatiodmean) of soundings in Figs 4-5 for each beam direction referenced to vertical. Fig. 8. Details of the depth uncertainty (Fig. 7) measured on the outer beams for soundings in Figs. 4-5. C. Angular Variations To first order, the depth uncertainties AD vs. receive beam angles 8 can be converted to an apparent angular error in beam pointing A€? This is done by differentiating the conversion of straight path slant-range R to depth D (D=Rcos e) , yielding: A D /  D =  AR/ R - A0 tan0. (1) The range uncertainty AR of the EM120 is on the order of 37 cm in the deep water mode, hence the ratio of ranges on the right side of (1) is of order 1 O4 and is negligible relative to the angular term. The apparent angular error is then: A O = - D / ( D t a n O ) .  (2) The apparent angular error associated with the data in Figs. 5-8 is plotted in Fig. 9, along with its mean (zero) and standard deviation (solid line). The standard deviation line remains roughly constant and below 0.08" until f60" and increases to over 0.2" at +70". All else being equal, one would expect the apparent angular error to remain essentially constant across the swath, and the fact that it increases beyond +60" indicates that beams in the outermost sectors of the 9 sector transmit pattern behave differently than the rest. Their higher sensitivity to roll error could be one factor, so could beam pointing errors due 1763 to insufficiently accurate sound speed information at the face of the array to correct for refraction effects. The latter is less likely because the ship was on station and sound speed continuously measured at the depth of the array agreed to within I d s  with the corresponding sound speed in the measured sound speed vs. depth profile entered in the EM120. DMSO5 MRUS SEAPATH200 POS-MV320 Fig. 9. Apparent angular error (2) associated with the depth uncertainties in Fig. 7-8, showing the scatter of individual points, their standard deviation (solid line), and their mean (zero center line). Bottom Heading Position Standard Variations Slope Deviation (deg) (m x m) (d%) 0.64 3x4.5 0.1 1 0.57 5x5 0.1 1 0.750 8x6 0.13 0.71 9x4 0.4 IV. COMPARISON OF MOTION SENSORS SENSORS DMSO5 MRUS Seapath200 POS-MV320 RMS RoWPitch Heave Accuracy (deg) accuracy (cm) 0.05 5 0.03 5 0.03 5 0.01 5 All the tests reported here were conducted in sea state 3. Tests with the DMSO5 and the MRUS were conducted at the same location in 3800 m of water depth within one hour of each other, hence conditions can be deemed identical. Tests with the Seapath200 were conducted in 4000 m of water depth, and tests with the POS-MV320 were conducted on a gentle slope (3750 m to 3900 m over 21 km) with the swath parallel to the slope. A summary of the test conditions is given in Table 2. Results of the four tests are compared by plotting the respective depth uncertainties on the same graph (Figs. 10- 11). Fig. 10 provides the comparison results, and Fig 11 illustrates the limitation of the method as will be explained shortly. In spite of a factor of 5 in specified RMS roll accuracy between the POS-MV320 and the DMSO5, there are surprisingly small differences in Fig. 10 between the depth uncertainties obtained with the four motion sensors from nadir to f60". As expected, data gathered with the POS-MV320 has a somewhat lower depth uncertainty overall, but the improvement is not commensurate with the specified Rh4S roll accuracy. Results with the Seapath200 were obtained after the ship's roll compensation tank had been emptied to provide a nearly sinusoidal roll motion. With the roll tank in operation, the ship's roll departs noticeably from a simple harmonic modulation, and results with the Seapath200 were noisier than those shown here. The smaller than expected differences in depth uncertainties between motion sensors could be explained by a narrower range of RMS roll accuracy than specified in Table 1. Nonetheless, the apparent roll artifact is present at the edges of the swath with all four sensors, indicating that factors other than inaccuracies in roll are involved as well. TABLE 2. TEST CONDITIONS L1-218, U-218 Wffi-506 L16516 t DMSO5 Y- MRUS + SeapathPO0 A POS-MV320 i I 45 50 55 60 65 70 Receive beam angle (deg) Fig. 10. Depth uncertainties vs. starboard beam angles referenced to vertical for 4 different motion sensors. Except for data obtained with the POS-MV320, depth uncertainties exceed 0.2% of water depth beyond 60", and climb above 1% by 70" for the MRUS and the DMSO5. These much larger uncertainties are most likely due to bottom detection errors on the outerbeams causing a few outliers to skew the results. Ping by ping outlier removal will probably be necessary to obtain a picture of depth uncertainty vs. receive beam angle that remains consistent over several tests, and from which more definitive depth accuracy estimates can be derived. 1764 The requirement for careful data editing prior to assessing depth accuracies is illustrated in Fig. 11 where results obtained with the POS-MV320 have larger uncertainties than with the other sensors. Yet this plot corresponds to the port half of the data shown in Fig. 10. In this case, the higher depth uncertainties are due to larger bottom detection scatter upslope, which is most likely caused by local relief and the somewhat higher standard deviation of the ship’s heading (0.71’). Once again, careful data editing will be required to obtain a consistent picture because the uncertainties reported for POS-MV320 data are not representative of the actual depth accuracy capabilities of the sonar system. The other curves are more consistent and therefore closer to the actual accuracy. - t ’ DMSO5 MRU5 SeapathPO0 - POS-MV320 -65 -60 -55 -50 -45 -40 Receive beam angle (deg) Fig. 1 1. Depth uncertainties vs. port beam angles referenced to vertical for 4 different motion sensors. 111. CONCLUSIONS The sounding accuracy of a deep water multibeam swath bathymetry sonar can be assessed from data collected while the ship holds station, maintaining position and heading to tolerances set by the fore-aft beam width of the transmit beam, and by the nominal footprint of the intersection of the transmit beam and the narrowest receive beam. However examples provided in previous sections show that careful data editing is required to obtain reliable estimates. Comparisons of sounding accuracies obtained with 4 different motion sensors yielded smaller than expected differences given the factor of 5 in RMS roll accuracy among the sensors. Likely explanations include incorrect specification of RMS roll accuracy for the motion sensors, beam steering errors on the outermost sectors (beyond i 5 8 O )  of the EM120 sonar, and misregistration between the roll time series and the sonar data. The last two explanations are the most probable given the evidence of apparent roll errors found at the edges of the swath (Fig.6). ACKNOWLEDGMENTS This work would not have been possible without the generous contributions of Duane Fotheringham (KSI), Michael Mathewson (Seatex), Peter Stewart (Applanix) who loaned us the various vertical reference units used in the tests described here. Many thanks are due to the captain and the crew of RN Roger Revelle who provided the necessary precision ship handling, and to John Chatwood, Jim Charters and Dan Jacobson who helped with EM120 data acquisition. Purchase and installation of the EM120 system were funded in part by the US Navy under a DURIP grant, by the National Science Foundation and by UC ship funds. REFERENCES [ 11 Kongsberg-Simrad Inc, EM 120 product description, [2] D.R. Herlihy, B.F. Hillard, and T.D. Rolon, “ N O M  Sea Beam system patch test,” Int. Hydr. Rev., vol. 66, [3] J.A. Hammack, D.H. Fabre, J.E. Hughes Clarke, and B. Reed, “Hydrographic multibeam processing system (HMPS) swath alignment tool,” Proc. Canadian Hydrographic Conf., 157-167, 1998. 27-4-1 999 119-139, 1989. [4] TSS, www.tss-realworld.com [ 5 ]  Seatex, www.seatex.no [6] Applanix, www.applanix.com 1765 

University of New Hampshire
University of New Hampshire Scholars' Repository
Center for Coastal and Ocean Mapping Center for Coastal and Ocean Mapping
11-2001
Field Evaluation of Sounding Accuracy in Deep
Water Multibeam Swath Bathymetry
Christian de Moustier
University of California - San Diego
Follow this and additional works at: https://scholars.unh.edu/ccom
Part of the Oceanography and Atmospheric Sciences and Meteorology Commons
This Conference Proceeding is brought to you for free and open access by the Center for Coastal and Ocean Mapping at University of New Hampshire
Scholars' Repository. It has been accepted for inclusion in Center for Coastal and Ocean Mapping by an authorized administrator of University of New
Hampshire Scholars' Repository. For more information, please contact nicole.hentz@unh.edu.
Recommended Citation
C. de Moustier, "Field evaluation of sounding accuracy in deep water multibeam swath bathymetry," MTS/IEEE Oceans 2001. An
Ocean Odyssey. Conference Proceedings (IEEE Cat. No.01CH37295).
Field Evaluation of Sounding Accuracy in 
Deep Water Multibeam Swath Bathymetry 
Christian de Moustier 
Scripps Institution of Oceanography 
9500 Gilman Drive, La Jolla, CA 92093-0205, USA 
Abstract- A new Kongsberg-Simrad EM120 multibeam 
echo-sounder has been installed aboard Scripps Institution of 
Oceanography's Research Vessel Roger Revelle in January 
2001. This system can map reliably a 20 km swath of seafloor 
in 4000 m water depth with 191 soundings per ping. Such a 
wide swath width demands highly accurate (<0.05' RMS) roll 
information from a motion sensor, and makes estimating 
sounding accuracy across the swath an interesting challenge. 
It is shown that good accuracy estimates can be obtained by 
collecting data on station under control of the GPS-aided 
dynamic positioning system usually available on most modern 
long-range oceanographic vessels. A number of motion 
sensors, with RMS roll accuracy specifications ranging from 
0.05' to 0.01' ,were tested with the EM120 sonar on station in 
3800 m to 4000 m water depths. Unexpectedly, they yielded 
roughly the same depth uncertainty as a function of receive 
beam angle. This result might be explained by synchronization 
errors between the attitude data and the sonar data leading to 
beam pointing errors, other types of beam pointing errors, a 
range of roll accuracy narrower than specified for the motion 
sensors, or a combination of these factors. 
I. INTRODUCTION 
In January 2001, the Scripps Institution of Oceanography 
installed a Kongsberg-Simrad Inc EM120 multibeam echo- 
sounder aboard the newest shp in its fleet, the Research Vessel 
Roger Revelle owned by the US Navy and commissioned 
in 1996 (AGOR 24) . 
This sonar system operates at a nominal fkequency of 12 kHz, 
with a l"x150" overall transmit sector (fore-& x athwartships) 
and up to 191 receive beams steered athwartships at regular 
angular steps across the swath, or at gradually narrower angular 
steps to achieve uniform horizontal offset between soundings 
athwartships, or a combination of both. Its flat hydrophone array 
configuration yields nominal receive beam widths of 2"/cos(0) 
from broadside (H") to the outer steering angles ((3475"). 
Most importantly, the sonar achieves broad swath widths in deep 
water (e.g. 20 km at 4km depth ) by steering the transmit beam in 
9 discrete sectors athwarthships, while compensating for the shp's 
yaw, pitch, and roll. However, it is necessary to know the ship's 
roll and pitch to better than 0.05" RMS to achieve G I ' s  
specification for sounding accuracy of 0.2% of water depth across 
the swath. In fact, since the sea trials at the end of January 2001, 
an apparent roll artifact has ruffled along-track the outer edges of 
the bathymetric swath collected aboard RN R. Revelle. Several 
tests have been conducted with various motion sensors to try and 
idenm its cause . 
With swath widths in excess of 20 k m  it is difficult to find a 
seafloor area, with suitably little relief along and across track, on 
which to conduct sounding accuracy tests. Options include survey 
techniques developed to resolve biases in swath bathymetry data, 
such as running a patch test over a known seafloor area [l], or 
creating a reference surface h m  a highly redundant set of 
soundings obtained by running tightly spaced parallel tracks with 
up to 90% swath overlap between adjacent tracks. Soundmg 
accuracy is then estimated by comparing individual soundmgs to 
the reference surface [3]. In all cases, a deep water reference 
surface is very costly in data acquisition and processing time. 
Provided the ship has good dynamic positioning capabilities, a 
simpler Sind much cheaper alternative consists in maintaining the 
ship on station at a constant headmg over a relatively flat seafloor 
area. Ping after ping, the same patch of seaflloor is sampled in a 
given beam dmction and changes in bottom relief along and 
across track become nearly negligible. 
The purpose of h s  paper is to highlight the effectiveness and 
potential pitfdls of estimating soundmg accuracy fkom multibeam 
swath bathymetry data gathered while the ship holds station. 
EM120 swath bathymetry and associated navigation data 
collected aboard RN Roger Revelle are used to illustrate the 
ship's station keeping requirements in Section II, and the sounding 
statistics in Section III. In Section IV, a comparison is made 
between results obtained on station, in 38OOm to 4OOO m of water 
depth, with four different motion sensors providing attitude data to 
the EM120 sonar. Their unexpected similarity is discussed and 
potential causes are analyzed. 
11. SHIP STATION KEEPING REQUIREMENTS 
A. Position 
At average oceanic depths (4 km), the along and across 
track extents of the footprint of a l"x2" specular beam are 
roughly 70m and 140m, respectively. Adjacent beams on 
either side athwartships are within lo, but the angular beam 
spacing becomes progressively narrower from nadir out 
when using the sonar's mode that provides equidistant 
soundings across-track, which is true for all the data 
presented here. Therefore one needs to maintain the ship's 
position within a watch circle 10 m in diameter for a given 
beam direction to sound the same patch of seafloor 
repeatedly. 
Aboard RN Roger Revelle, the dynamic positioning 
system controls two stem Z-drive azimuthal thrusters and a 
bow thruster. It can maintain the ship's position in a P-Code 
GPS reference frame in a circle less than 10 m in diameter 
for the 40 min required to collect 100 pings in 3800 m of 
water depth, as shown in Fig. 1. 
B. Heading 
The 1" fore-aft beamwidth of the transmit beams imposes 
restrictions on the ship's heading variability during a test, 
before relief variations along and across track can no longer 
MTS 0-93 3 95 7-28-9 1761 
be neglected. As illustrated in Figs. 1-2, experience with 
R N  R.Revelle shows that the ship can hold station and 
heading to 0.6" RMS (Fig. 3) in sea states 4 or below. In 
the foregoing analysis, data with heading variations up to 
0.75" RMS have been used, but they start showing the 
limitations of the negligible relief assumptions. 
I 
, . . . . .  
I -  
0 5 -  
P 
L 
c 0 -  
B 
4 
4 1 -  
-0 - 0 5 -  
-1 5 -  
7 1 
- 2 4  ' 1 
Fig. 1. Variations in the ship's position while on station. 
-3 -25  -2 -15  -1 4 5  0 0 5  1 1.5 2 
Longitude dsv&mn (m) 
I 
Fig.2 Variation in the ship's heading while on station. 
Likewise, the noise in the heading data supplied to the 
EM120 sonar should remain a small fraction of the fore-aft 
beam width. As a first order verification, Fig. 3 shows a 
noise histogram drawn from the residuals of detrending and 
low-pass filtering performed on the heading data of Fig. 2. 
Although not strictly speaking a noise sequence, the 
residuals have a mean of zero and a standard deviation of 
0.12", which is within 20% of the 0.1" specified RMS 
accuracy of the Meridian Gyro used in these tests, and of 
the accuracy required by the EM120 sonar. 
Control of heading variations during a test is achieved by 
setting a maximum heading deviation in the dynamic 
positioning system. However, local weather conditions 
might make such settings moot and it becomes necessary to 
collect enough pings to be able to select a subset of pings 
that fall within the desired heading bounds. 
Fig. 3. Histogrks of variation in the ship's heading (left) 
and heading noise (right) while on station. 
111. SOUNDING STATISTICS 
A. Depth Profiles 
Given proper control of the ship's heading and position 
during data collection on station, it is straightforward to 
compile statistics of the soundings as a h c t i o n  of receive 
beam angles referenced to nadir, hence corrected for the 
ship's roll and for refraction effects at the face of the array. 
Here, beam angles are considered in 0.1" increments, but 
only beam directions reporting data for more than half the 
total number of pings in the set are used in the statistics. 
Stacked profiles of depth vs. received beam angle are 
shown in Fig. 4, with details in Fig. 5 showing the mean 
depth (solid line) and the scatter of soundings about the 
mean. The scatter increases with steering angle, and tighter 
angular spacing of beams at increasing athwartships angles 
to achieve equidistant soundings can be seen also in these 
plots. 
I 
Fig. 4. Stacked instantaneous bottom profiles of depth vs. 
receive beam angles (port <O, starboard >O) for about 100 
pings recorded with the ship on station. 
1762 
-4 r 
Fig. 5 .  Details of the port and starboard beam soundings (+) 
from Fig. 4 with the mean profile drawn as a solid line. 
Closer inspection of the outer beams from Figs. 4-5 
reveals two interesting clues illustrated in Fig. 6,  where 
soundings at f65" from nadir are plotted as sequences of 
depth vs. consecutive ping numbers (equivalent to time at 
-20 s/ ping). These two sequences contain frequent spikes 
that are for the most part "180" out of phase" between port 
and starboard, indicating that the athwartships profile rolls 
with the ship. Second, there is a long term oscillation with 
a period of about 60 pings (-20 min) that does not seem to 
be correlated with anything obvious at this point. 
Fig. 6.  Evolution of soundings in time for two beams 
at f65" from vertical for the data shown in Figs. 4-5. 
Solid line port, dashed line starboard. 
B. Depth Uncertainty 
The depth accuracy for each sounding is estimated from 
the data in Figs 4-5, by forming the ratio of the standard 
deviation of the soundings in each angular bin to their 
mean. This yields a depth uncertainty in percent of mean 
water depth. 
As shown in Fig. 7-8, uncertainties remain below 0.2% 
from nadir to about +60" and climb rapidly thereafter to 
values in excess of 2% at f70". 
Fig. 7 Depth uncertainty (standard deviatiodmean) of 
soundings in Figs 4-5 for each beam direction 
referenced to vertical. 
Fig. 8. Details of the depth uncertainty (Fig. 7) measured on 
the outer beams for soundings in Figs. 4-5. 
C. Angular Variations 
To first order, the depth uncertainties AD vs. receive 
beam angles 8 can be converted to an apparent angular 
error in beam pointing A€? This is done by differentiating 
the conversion of straight path slant-range R to depth D 
(D=Rcos e) , yielding: 
A D /  D =  AR/ R - A0 tan0. (1) 
The range uncertainty AR of the EM120 is on the order of 
37 cm in the deep water mode, hence the ratio of ranges on 
the right side of (1) is of order 1 O4 and is negligible relative 
to the angular term. The apparent angular error is then: 
A O = - D / ( D t a n O ) .  (2) 
The apparent angular error associated with the data in 
Figs. 5-8 is plotted in Fig. 9, along with its mean (zero) and 
standard deviation (solid line). The standard deviation line 
remains roughly constant and below 0.08" until f60" and 
increases to over 0.2" at +70". All else being equal, one 
would expect the apparent angular error to remain 
essentially constant across the swath, and the fact that it 
increases beyond +60" indicates that beams in the 
outermost sectors of the 9 sector transmit pattern behave 
differently than the rest. Their higher sensitivity to roll 
error could be one factor, so could beam pointing errors due 
1763 
to insufficiently accurate sound speed information at the 
face of the array to correct for refraction effects. The latter 
is less likely because the ship was on station and sound 
speed continuously measured at the depth of the array 
agreed to within I d s  with the corresponding sound speed 
in the measured sound speed vs. depth profile entered in the 
EM120. 
DMSO5 
MRUS 
SEAPATH200 
POS-MV320 
Fig. 9. Apparent angular error (2) associated with the depth 
uncertainties in Fig. 7-8, showing the scatter of individual 
points, their standard deviation (solid line), 
and their mean (zero center line). 
Bottom Heading Position 
Standard Variations Slope 
Deviation 
(deg) (m x m) (d%) 
0.64 3x4.5 0.1 1 
0.57 5x5 0.1 1 
0.750 8x6 0.13 
0.71 9x4 0.4 
IV. COMPARISON OF MOTION SENSORS 
SENSORS 
DMSO5 
MRUS 
Seapath200 
POS-MV320 
RMS RoWPitch Heave 
Accuracy (deg) accuracy (cm) 
0.05 5 
0.03 5 
0.03 5 
0.01 5 
All the tests reported here were conducted in sea state 3. 
Tests with the DMSO5 and the MRUS were conducted at 
the same location in 3800 m of water depth within one hour 
of each other, hence conditions can be deemed identical. 
Tests with the Seapath200 were conducted in 4000 m of 
water depth, and tests with the POS-MV320 were 
conducted on a gentle slope (3750 m to 3900 m over 21 
km) with the swath parallel to the slope. A summary of the 
test conditions is given in Table 2. 
Results of the four tests are compared by plotting the 
respective depth uncertainties on the same graph (Figs. 10- 
11). Fig. 10 provides the comparison results, and Fig 11 
illustrates the limitation of the method as will be explained 
shortly. In spite of a factor of 5 in specified RMS roll 
accuracy between the POS-MV320 and the DMSO5, there 
are surprisingly small differences in Fig. 10 between the 
depth uncertainties obtained with the four motion sensors 
from nadir to f60". As expected, data gathered with the 
POS-MV320 has a somewhat lower depth uncertainty 
overall, but the improvement is not commensurate with the 
specified Rh4S roll accuracy. 
Results with the Seapath200 were obtained after the 
ship's roll compensation tank had been emptied to provide 
a nearly sinusoidal roll motion. With the roll tank in 
operation, the ship's roll departs noticeably from a simple 
harmonic modulation, and results with the Seapath200 were 
noisier than those shown here. 
The smaller than expected differences in depth 
uncertainties between motion sensors could be explained by 
a narrower range of RMS roll accuracy than specified in 
Table 1. Nonetheless, the apparent roll artifact is present at 
the edges of the swath with all four sensors, indicating that 
factors other than inaccuracies in roll are involved as well. 
TABLE 2. TEST CONDITIONS 
L1-218, U-218 Wffi-506 L16516 
t DMSO5 
Y- MRUS 
+ SeapathPO0 
A POS-MV320 
i 
I 
45 50 55 60 65 70 
Receive beam angle (deg) 
Fig. 10. Depth uncertainties vs. starboard beam angles 
referenced to vertical for 4 different motion sensors. 
Except for data obtained with the POS-MV320, depth 
uncertainties exceed 0.2% of water depth beyond 60", and 
climb above 1% by 70" for the MRUS and the DMSO5. 
These much larger uncertainties are most likely due to 
bottom detection errors on the outerbeams causing a few 
outliers to skew the results. Ping by ping outlier removal 
will probably be necessary to obtain a picture of depth 
uncertainty vs. receive beam angle that remains consistent 
over several tests, and from which more definitive depth 
accuracy estimates can be derived. 
1764 
The requirement for careful data editing prior to 
assessing depth accuracies is illustrated in Fig. 11 where 
results obtained with the POS-MV320 have larger 
uncertainties than with the other sensors. Yet this plot 
corresponds to the port half of the data shown in Fig. 10. In 
this case, the higher depth uncertainties are due to larger 
bottom detection scatter upslope, which is most likely 
caused by local relief and the somewhat higher standard 
deviation of the ship’s heading (0.71’). Once again, careful 
data editing will be required to obtain a consistent picture 
because the uncertainties reported for POS-MV320 data are 
not representative of the actual depth accuracy capabilities 
of the sonar system. The other curves are more consistent 
and therefore closer to the actual accuracy. 
- t ’ DMSO5 
MRU5 
SeapathPO0 - 
POS-MV320 
-65 -60 -55 -50 -45 -40 
Receive beam angle (deg) 
Fig. 1 1. Depth uncertainties vs. port beam angles 
referenced to vertical for 4 different motion sensors. 
111. CONCLUSIONS 
The sounding accuracy of a deep water multibeam swath 
bathymetry sonar can be assessed from data collected while 
the ship holds station, maintaining position and heading to 
tolerances set by the fore-aft beam width of the transmit 
beam, and by the nominal footprint of the intersection of 
the transmit beam and the narrowest receive beam. 
However examples provided in previous sections show that 
careful data editing is required to obtain reliable estimates. 
Comparisons of sounding accuracies obtained with 4 
different motion sensors yielded smaller than expected 
differences given the factor of 5 in RMS roll accuracy 
among the sensors. Likely explanations include incorrect 
specification of RMS roll accuracy for the motion sensors, 
beam steering errors on the outermost sectors (beyond 
i 5 8 O )  of the EM120 sonar, and misregistration between the 
roll time series and the sonar data. The last two 
explanations are the most probable given the evidence of 
apparent roll errors found at the edges of the swath (Fig.6). 
ACKNOWLEDGMENTS 
This work would not have been possible without the 
generous contributions of Duane Fotheringham (KSI), 
Michael Mathewson (Seatex), Peter Stewart (Applanix) 
who loaned us the various vertical reference units used in 
the tests described here. Many thanks are due to the captain 
and the crew of RN Roger Revelle who provided the 
necessary precision ship handling, and to John Chatwood, 
Jim Charters and Dan Jacobson who helped with EM120 
data acquisition. Purchase and installation of the EM120 
system were funded in part by the US Navy under a DURIP 
grant, by the National Science Foundation and by UC ship 
funds. 
REFERENCES 
[ 11 Kongsberg-Simrad Inc, EM 120 product description, 
[2] D.R. Herlihy, B.F. Hillard, and T.D. Rolon, “ N O M  
Sea Beam system patch test,” Int. Hydr. Rev., vol. 66, 
[3] J.A. Hammack, D.H. Fabre, J.E. Hughes Clarke, and B. 
Reed, “Hydrographic multibeam processing system 
(HMPS) swath alignment tool,” Proc. Canadian 
Hydrographic Conf., 157-167, 1998. 
27-4-1 999 
119-139, 1989. 
[4] TSS, www.tss-realworld.com 
[ 5 ]  Seatex, www.seatex.no 
[6] Applanix, www.applanix.com 
1765 


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Field Evaluation of Sounding Accuracy in Deep Water Multibeam Swath Bathymetry

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