Ground penetrating radar field evaluation in Angola
Richard Walls U.S. Army RDECOM CERDEC, ATTN: AMSRD-CER-NV-CM-HD 10221 Burbeck Road, Fort Belvoir, VA 22060-5806
Todd Brown, Fred Clodfelter, Jeff Cours, Stephen Laudato, Steve Lauziere, Ajay Patrikar, Michael Poole, and Mike Price NIITEK Inc., 43671 Trade Center Place, Suite 124, Sterling, VA 20166 ABSTRACT
Deminers around the globe are still using handheld metal detectors that lack the capability to distinguish mines from clutter, detect mines containing very little metal, or find mines buried at deeper depths. In the southern African country of Angola, many areas and roads are impassable due to the threat of anti-tank landmines. Some of these mines are undetectable using current metal detector technology. The US Army has funded the development of the NIITEK ground penetrating radar (GPR) for detection of anti-tank (AT) landmines. This radar detects metal and plastic mines as well as mines that are buried too deep for handheld metal detectors to find. The US Department of Defense Humanitarian Demining (HD) Research & Development Program focuses on developing, testing, demonstrating, and validating new technology for immediate use in humanitarian demining operations around the globe. The HD team provided funding and guidance to NIITEK Incorporated for development of a prototype system called Mine Stalker – a relatively light- weight, remote-controlled vehicle outfitted with the NIITEK GPR, detection algorithms, and a marking system. Individuals from the HD team, NIITEK Inc, and the non-governmental organization Meschen Gegen Minen (MgM) participated in a field evaluation of the Mine Stalker in Angola. The primary aim was to evaluate the effectiveness and reliability of the NIITEK GPR under field conditions. The Mine Stalker was extremely reliable during the evaluation with no significant maintenance issues. All AT mines used to verify GPR performance were detected, even when buried to depths as deep as 25-33cm.
Keywords: Landmine Detection, Humanitarian Demining, Mine Stalker, NIITEK, Ground Penetrating Radar, GPR, Field Evaluation, Countermine, Anti-Tank Mine
1. INTRODUCTION
1.1 Background
The U.S. Department of Defense Humanitarian Demining (HD) Research & Development Program focuses on developing, testing, demonstrating, and validating new technology for immediate use in humanitarian demining operations around the globe. The HD R&D program receives funding and guidance from the Office of the Assistant Secretary of Defense for Special Operations and Low-Intensity Conflict (OASD SO/LIC), and the program is executed by the Humanitarian Demining Branch of the Countermine Division within the U.S. Army’s Night Vision and Electronic Sensors Directorate (NVESD).
Beginning in the late 1990’s, the U.S. Army Countermine Division funded the development of the NIITEK ground penetrating radar (GPR) for detection of anti-tank (AT) landmines. The NIITEK GPR is a very wide bandwidth, impulse radar with low radar cross-section, capable of producing clear and precise radar imaging of targets. During Army testing, the NIITEK GPR demonstrated the ability to detect all of the test AT mines with an extremely low false alarm rate.
To better understand user needs, the HD R&D program sponsors an annual humanitarian demining requirements workshop that brings together representatives of various mine clearance non-governmental organizations (NGOs) and mine action centers from all over the world. During the July 2003 requirements workshop, representatives from Angola described a low-metal anti-tank mine that they were finding in munitions depots. This AT mine was undetectable using
Draft Version 6 Page 1 of 12 Saved on 3-May-2006 at 03:20 PM handheld metal detectors. The NIITEK GPR demonstrated its ability to detect this mine and several other difficult plastic mines at the 2004 requirements workshop. Representatives from Angola expressed great interest in the NIITEK GPR. After the workshop, the German NGO Menschen gegen Minen (MgM) requested that the GPR be deployed to Angola for an operational field evaluation.
Initially, plans were discussed to operate the NIITEK GPR in concert with MgM’s Voodoo System – an operational methodology where an armored grader creates a path through a mine suspect area followed by mine detection dogs and manual deminers. The grader would push anti-personnel (AP) mines and most AT mines to the side while clearing vegetation and leaving a smooth path in its wake. This operational concept drove the design of the system that would be used to carry the NIITEK GPR. In the end, the host NGO decided it would be best to trial the system in former mine suspect areas to build confidence in the system instead of starting on a mined road immediately.
1.2 Objectives
The NIITEK GPR has successfully completed numerous evaluations at U.S. Army tests sites. The primary aim of this development trial was to evaluate the effectiveness and reliability of the NIITEK GPR under field conditions while operating in former minefields in Africa. Data collection in realistic minefield conditions was the second objective. Learning the limitations of the system, gaining a better understanding of real-world requirements and conditions, and determining the potential that may exist for this system to benefit deminers in its current configuration were additional goals of this field evaluation.
1.3 System Requirements and Design
One of the most important requirements during the operational field evaluation of the NIITEK GPR was that the carriage platform and overall system were reliable so that the performance of the radar was not affected. The prototype system has been named the Mine Stalker. The Mine Stalker’s original intended purpose was to detect and mark the position of anti-tank landmines in real-time while under remote control along a graded roadway. A scale drawing of the Mine Stalker system is given in Figure 1.
The following list contains the key performance parameters that drove the overall design of the system:
• The ground pressure of the system should be less than 5 PSI to minimize the chance of detonating an AT mine • 100 meter line of sight remote control operation from within a mine protected vehicle • Remote vehicle start and computer reboot • Vehicle automatically stops and marks the down-track position of every detected target • GPR data and imagery from alarms transmitted back to a remote operator station • Simplified, software interface for reviewing data • Inexpensive test platform that is able to operate on a graded roadway • System development complete within 8 months
Figure 1. Mine Stalker system – NIITEK GPR mounted on remote platform
The Mine Stalker is designed to carry the 1.2 meter-wide NIITEK GPR. The Mine Stalker consists of a modified riding-lawnmower chassis, commercial remote control hardware, an onboard computer, a path and target marking
Draft Version 6 Page 2 of 12 Saved on 3-May-2006 at 03:20 PM subsystem, and wireless Ethernet for data transmission to a remote base station. The base station is made up of an Ethernet access point, high gain antenna, and ruggedized, portable computer.
The Mine Stalker’s onboard computer acquires the data from the GPR and runs the detection algorithm. The detection algorithm used during this field evaluation was a pre-screener algorithm that alerts on mine-like objects without discriminating mines from clutter. The algorithm uses 30 scans of data to make its decision before generating an alarm. (Each scan is about 3cm long in the down-track direction.) The 1.2m wide GPR antenna is evenly divided into 24 channels in the cross-track direction. When an alarm is generated, the algorithm reports a confidence value and the alarm location with a down-track position and a cross-track channel number. The down-track position is relative to the marked start point of the system for each lane run.
The onboard computer also provides the output commands to the vehicle for status lights and vehicle stop commands, depending on GPR data analysis. The system stops at each alarm and a small window of GPR imagery data and alarm information is transmitted back to the base station. Once the system reaches the end of its predetermined lane (maximum length of about 40 meters based on the amount of onboard system memory), all of the GPR data acquired over the lane is transmitted to the base station for visual review.
The Mine Stalker has a water-based marking system with three jets that can be used to leave visible marks on the ground. During the scanning process, the system marks the scanned pathway at regular intervals via the two outside marking jets. This clearly identifies the area of ground that has been covered by the Mine Stalker. Every detected target is marked in the down-track position by the center marking jet as the system comes to a stop. To prevent the system from stopping on top of a detected landmine, there is a 30cm offset between each detected target and its spray mark.
1.4 Field Evaluation Structure
The NIITEK GPR field evaluation was split into three phases. The first phase was a blind test in Namibia during early October 2005. The second phase followed immediately after the blind test and consisted of training an Angolan deminer to use the Mine Stalker system. The third and final phase of the evaluation was a series of data collections in several previously cleared mine suspect areas and one in an area where active mine clearance operations were occurring at the time of the evaluation. Both the second and third phases occurred in Angola.
2. SITE DESCRIPTIONS
2.1 General Climate Descriptions
The NIITEK GPR field evaluation occurred in October to early November of 2005 in the southern African countries of Namibia and Angola. In this area of Africa, the dry winter months are from June until October, and the more rainy summer months are from November to May. Central Namibia, near the capital city of Windhoek, is a semi-desert climate with about 35cm of rainfall annually. Non-coastal southern Angola, along the northern border of Namibia, is also considered a semi-arid climate with an average rainfall of 70cm. There was no rain during the testing in Namibia and very minimal evening rain on a couple of nights during the 3 week deployment in Angola. The daytime temperatures during the evaluation ranged from approximately 24°C - 32°C (75-90°F). Intense sun exposure was an issue for both man and machine throughout the evaluation.
2.2 Blind Test Site
The NIITEK GPR was tested at a temporary test site near Windhoek, Namibia during the first phase of the evaluation. This site was the location of the HSTAMIDS test1 in March 2005 and was the most convenient international airport to the evaluation locations in Angola, which were about a ten-hour overland trip north of Windhoek.
Draft Version 6 Page 3 of 12 Saved on 3-May-2006 at 03:20 PM The Namibia test site was flat and covered with dry grass. The soil was composed of two distinct layers. The first 30cm (12in) was predominantly a clay loam soil layer that contained a low concentration of small to medium pebbles. Below that layer, the pebbles became larger and more concentrated.
2.2 Training and Initial Data Collection Site
The second phase of the field evaluation was the Mine Stalker operator training. This occurred at the host NGO’s base camp in Ondjiva, Angola. Ondjiva is the administrative capital of the Cunene Province in southern Angola.
The soil in the training area was a very fine, sandy clay layer with hardpan starting at about 5-20cm (2-8in). The sandy layer had many pieces of metal clutter and trash buried in it.
2.3 Field Data Collection Sites
All five field data collection sites were in the Angolan province of Cunene. Four of the sites were former mine suspect areas that had been cleared, and the last one was located at an active mine clearance operation. The five sites were designated MF1 - MF5 for record keeping purposes.
2.3.1 Xangongo Mine Belt Site 1 (MF1)
According to the host demining organization, the town of Xangongo, Angola was surrounded by three, mechanically laid, pattern mine belts of metal anti-tank mines sometime around the early 1980’s. These mine belts were subsequently removed in the early to mid 1990’s by MgM and other NGO’s. The first site that the Mine Stalker operated in during this field evaluation was along a section of this former mine belt. This area was mostly flat with a mix of bushes, small trees, open areas, and a few very large trees. Nearly all of the plant life had thorns which posed a threat to both feet and tires. The soil was a sandy clay mix and was very hard and dry during this time of the year. The spring rains were just beginning to gather, but no substantial rain had fallen yet. Water had to be used to soften the soil for digging. Cows, goats, and people frequently walked through this area.
2.3.2 Xangongo Mine Belt Site 2 (MF2)
The second site that the Mine Stalker collected data at during this field evaluation was along another section of the former Xangongo mine belt. This site was the location of a mine clearance accident in the mid 1990’s where the driver of a tank outfitted with rollers was killed when the tank tracks detonated an anti-tank mine. This site was a large, open, low area that floods during the rainy season. The surface of the ground around the tank was covered with crater-like cow footprints and dry grass. Again, the soil had to be softened with water before digging.
2.3.3 Small Village near Humbe, Angola (MF3)
The third site was an open area beside a very poor village consisting of a dozen or so huts made from tree limbs with rusted pieces of corrugated roofing. The provincial governor lost his foot after an AT mine blew up under his Land Rover when he visited this village in the mid-1990’s. Two demining organizations, MgM and Mines Advisory Group (MAG), performed mine clearance operations at this site after the governor’s accident. The ground was littered with trash from the village, both on the surface and buried. The site also contained foundation remnants from huts/buildings that were no longer present.
2.3.4 Cunene River Crossing near Humbe, Angola (MF4)
The fourth site was a low, flat area beside the Cunene River not far from the MF3 site. The area was heavily vegetated with scrub brush that was cleared by local men on the day of the Mine Stalker data collection. The local men and the MgM deminers said that an AT mine blew up under an animal-drawn wagon in 1994. After that incident, the site was reportedly demined by Norwegian People’s Aid (NPA) and the Red Cross.
Draft Version 6 Page 4 of 12 Saved on 3-May-2006 at 03:20 PM 2.3.5 Active Demining Area – Malungo Wa Shikongo, Angola (MF5)
The last data collection site during the NIITEK GPR field evaluation was at a location where MgM was performing active mine clearance operations. The minefield, containing both AT and AP mines, surrounded the center of a village. The area was overgrown with vegetation and was being cleared mechanically using an armored chassis with a mulcher mounted on its excavator arm. It appeared that some controlled burning had also been used to clear vegetation. Manual deminers with metal detectors were searching the area for mines and removing all pieces of metal clutter. The soil was deep, powdery white sand.
3. METHODOLOGY
3.1 Phase 1 Blind Test
3.1.1 Test Lanes and Targets
The Namibia test site consisted of 10 test lanes with a total of 42 AT mines. Each test lane was 1m wide by 25m long with plastic pegs marking the four corners. All of the AT mines were buried 10cm (4in) from the surface of the ground to the top of the mine. The site was setup in January 2005. Originally the lanes had both AT and AP mines, but the AP mines were removed after the HSTAMIDS test in March and the AT mines were left in place. This means the AT mines had about 10 months to weather-in.
All AT mines, detonators, and fuses were free from explosive. The main charges were replaced with silicone rubber RTV 3110, which closely approximates the dielectric constant and loss tangent of explosives. This is the same fill used in the International Test Operations Procedure (ITOP) approved SIM test targets. The metal components and characteristics of the mines remained intact.
The AT mines were split into two categories based on metal content: low-metal anti-tank mines (AT-LM) and metal anti-tank mines (AT-M). Of all the AT mines encountered, 19% were AT-M and 81% were AT-LM. There were two types of AT-M with diameter/width of 30-35cm and four types of AT-LM with diameter/width of 20-35cm. During the test, the entire site was covered by the Mine Stalker system six times. All 10 lanes were covered once before each repetition. Table 1 contains a breakdown of the number of test targets and encounters.
Table 1. Anti-tank mines present in Namibia test lanes
Dia/Width # of Mines # of Encounters AT-M Type A 300-350mm 5 30 Type B 300-350mm 3 18 Total AT-M 8 48 AT-LM Type H 200-350mm 9 54 Type I 200-350mm 9 54 Type J 200-350mm 9 54 Type K 200-350mm 7 42 Total AT-LM 34 204 All AT Mines 42 252
3.1.2 Mine Stalker Test Procedures
The predetermined lane length for acquiring data was set at 29m in the Mine Stalker for the test. This provided 2m of run-up before and 2m of over-run at the end of the 25m test lane. As described in Section 1.3, the system transmitted small windows of data back to the base station operator at every alarm and all of the data at the end of the predetermined lane length. Once an alarm was generated by the detection algorithm, the system stopped and sprayed a mark on the ground. Plain water was used in the marking system during the test.
Draft Version 6 Page 5 of 12 Saved on 3-May-2006 at 03:20 PM Two data collectors followed the Mine Stalker down each lane. The first data collector placed a numbered plastic chip on each water-marked spot on the ground. The second data collector recorded the chip number and the down-track position of the chip relative to the beginning of the lane.
Finally, a laser-based surveying system was used to record the location of the chips. The surveyor crosschecked the number of chips that were placed with the number of alarms that were transmitted back to the base station operator by the Mine Stalker.
3.1.3 Mine Stalker Test Metrics
The detection probability (Pd) and false alarm rate (FAR) were the primary metrics used to evaluate the Mine Stalker performance in this blind test. The detection probability was defined as the fraction of the encountered mines that were detected:
# of mines detected P = . (1) d # of mines encountered
The false-alarm rate has been commonly defined in the test community as the number of false alarms per square meter of test lane. The NIITEK GPR array used in this evaluation was 1.2m wide while the test lane was only 1m wide. Without an accurate means of eliminating any false alarms that the GPR generated on the outside edges of the test lanes, the area used to calculate the FAR had to include the extra area covered by the NIITEK GPR. Thus the area and FAR were calculated using the following two equations:
Area Covered = (Width of GPR × Length of Lane)× Number of Lanes, (2)
# of false alarms FAR = . (3) Area Covered
As described in Section 1.3 and Section 3.1.2, the Mine Stalker sprayed a centrally located mark about 30cm in front of every detected object and a data collector placed a chip on that spray mark. Then, the chip location was surveyed by a relative positioning system. The Mine Stalker was credited with a detection if the spray mark was within a reasonable distance of a mine without going past it. Figure 2 illustrates what defined a detection and a false alarm during the Mine Stalker test.
Test Lane Reasonable Distance = False False 42cm Alarm Alarm Credited Detection Mine
Direction of Mine Stalker Travel
Figure 2. Pictorial definition of detections and false alarms during Mine Stalker test
A number of factors influenced the recorded distance between the spray mark and the detected objects, and thus gave rise to the distance used to score the test. The Mine Stalker marking system was designed for field use and not precision marking of targets during a test. The inherent distance from the detected object to the spray mark was 30cm. (This marking offset was caused by the requirement for the system to stop before running over a detected landmine.) The size of the spray mark was 15cm in diameter. The mark was created by plain water being sprayed onto grass, and the data collector did his best to find the center of this mark to place the plastic chip. The average distance that the mines were detected within was 33cm. The maximum distance that all mines were detected within was 42cm. Considering the 30cm offset and 15cm-diameter spray mark, 42cm was a reasonable distance to assume for scoring the test.
Draft Version 6 Page 6 of 12 Saved on 3-May-2006 at 03:20 PM The scoring of the Mine Stalker test was done using an Excel spreadsheet. In addition to measuring the distance between the chip position and the mine, Excel was setup to automatically compare the corresponding channel position of all alarms to the cross-track position of the mine. By doing this, all of the alarms that were credited as detections based on the distance calculation were confirmed as proper detections in the cross-track as well.
3.2 Data Collection during Training Phase
During the training phase at the MgM demining base camp, the Angolan deminer operated the Mine Stalker just as it was going to be operated during the field data collections in the third phase of the evaluation. A training lane with a few targets was laid out at the base camp to show the deminer how the system responded while detecting mines. This also gave the US personnel another opportunity to collect data. Table 2 contains a breakdown of the types and number of targets in this lane.
Table 2. Targets in Ondjiva data collection lane
Dia/Width # of Mines # of Encounters AT-M Type B 300-350mm 1 9 Type C 300-350mm 1 9 Total AT-M 2 18 AT-LM Type D 250-300mm 1 9 Type E 250-300mm 1 9 Total AT-LM 2 18 AP-M Type F 100-125mm 1 6 Type G 100-125mm 1 6 Total AP-M 2 12 UXO-M TYPE-N 270mm X 82mm 1 9 Total UXO-M 1 9
The content of the Ondjiva data collection lane was suggested by MgM. Initially, the four AT mines and one piece of UXO were the only targets provided by MgM for the lane. After seeing how well the GPR did with the AT mines, MgM then requested AP mines be added to the lane. The final suggestion was for additional runs to be made on all the targets buried at deeper depths. The difficult low-metal mine that initiated this whole field evaluation is designated AT- LM Type E in the table above and in the results section.
There were a total of 11 runs made over this lane. During the first nine runs, additional targets were added and then all of the targets were re-buried at deeper depths. For the last two runs, all of the targets were dug up and GPR data was collected over the re-filled holes.
During one of the runs over this data collection lane, all of the false alarms that were detected in that run were dug up and cataloged. False alarms of particular interest were dug up during some of the other runs as well.
3.3 Phase 3 Field Data Collections
The focus of the field data collections was to take the system to different sites (described in Section 2.3), gather GPR data in the real-world areas, and see how the system performed in general. The sites had varying terrain, varying types of vegetation, and varying types/quantities of clutter. The modes of operation in all of the field data collection sites were similar, but with some significant changes at the last site where there was an active demining operation being performed by MgM.
At the first four sites, the Mine Stalker operated without restriction. The MgM staff and U.S. personnel were able to freely walk around the system. Data could be collected in long or short lengths depending on the amount of open area
Draft Version 6 Page 7 of 12 Saved on 3-May-2006 at 03:20 PM available. The fifth site was a live minefield that contained small AP mines (less than 100cm in diameter) in addition to AT mines. Since the system was not intended to detect very small AP mines or to at least survive an AP blast, it could only reach about a meter into the mine suspect area before stopping.
GPR data was collected at all five sites. False alarms that produced interesting GPR signatures were dug up and cataloged. US personnel observed real-world conditions and deminers working with the prototype system. A verification mine (AT-M Type C) was used at several of the sites to demonstrate the GPR capability of detecting that mine in the given soil/terrain.
4. RESULTS
4.1 Blind Test Results
The Mine Stalker system and NIITEK GPR operated very reliably during the Namibia blind test. There were no maintenance issues and the system operated as designed. Some of the quantitative blind test results for the NIITEK GPR are provided below.
Table 3 contains the probability of detection, the number of detected mines out of mine encounters, and the false alarm rate. All of the results are based on the output of the pre-screener detection algorithm.
Table 3. Namibia blind test results
Detected Out of P FAR (m-2) d Total # of Mines AT-M Type A 0.97 29 out of 30 0.077 Type B 1.00 18 out of 18 0.079 Total AT-M 0.98 47 out of 48 0.079 AT-LM Type H 1.00 54 out of 54 0.006 Type I 1.00 54 out of 54 0.008 Type J 1.00 54 out of 54 0.018 Type K 1.00 42 out of 42 0.020 Total AT-LM 1.00 204 out of 204 0.020 All AT Mines 0.996 251 out of 252 0.079
The pre-screener algorithm did very well with the NIITEK GPR data. The algorithm was able to find 100% of the 204 low-metal AT mines. Of the 48 metal AT mines that were encountered, the algorithm only missed one. (The human reviewing the GPR data at the end of each lane spotted the mine, but for the purposes of the test, only mines detected by the algorithm and marked by the system were credited as detections.) The overall false alarm rate was 0.079 per square meter. When considering the false alarm rate with respect to only the low metal AT mines, all of these mines were detected by the algorithm with a FAR of 0.020m-2.
The above results lead to an intriguing question: Why did the algorithm miss a metal AT mine that gives the strongest GPR return? The simple explanation is that the pre-screener algorithm was designed to ignore the GPR signals that are returned from the surface of the ground, which are typically the largest return. Metal AT mines reflect most of the GPR signal and occasionally return a larger signal than the ground. In these rare circumstances, the existing pre-screener algorithm may ignore the signal from the metal AT mine. NIITEK has been continuing to improve the pre-screener algorithm and resolve this one issue seen in the Namibia testing.
Figure 3 on the next page contains a Receiver Operating Characteristics (ROC) curve showing the probability of detection versus the false alarm rate for the two categories of AT mines that were present in the Namibia blind test. These curves were generated using the confidence values that were output by the pre-screener algorithm for each alarm. A higher confidence value means that the detected object is more mine-like according to the algorithm parameters.
Draft Version 6 Page 8 of 12 Saved on 3-May-2006 at 03:20 PM 1 0.9 0.8 0.7 0.6 0.5 Pd 0.4 0.3 0.2 0.1 0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 FAR
AT-Metal AT-Low Metal
Figure 3. Mine Stalker ROC curve for metal and low-metal AT mines in Namibia test. (Note the scale on the x-axis.)
The performance shown in the ROC curve is very good for both metal and low-metal AT mines. The ROC curve does illustrate that the algorithm produced lower confidence values for metal AT mines than for low-metal AT mines. The reason for this is that some of the parameters that are evaluated in the GPR data are more evident for low-metal AT mines than for metal AT mines. One such parameter is related to the signal reverberations that are produced within the mine. Even though the confidence values were lower for metal AT mines than for low-metal AT mines, the algorithm still detected all but one of the metal AT mines in the Namibia blind test while producing a very low false alarm rate of less than 8 per 100m2.
4.2 Data Collection Results from Ondjiva, Angola
The Mine Stalker system and NIITEK GPR operated very reliably during the training and initial data collection phase at the MgM base camp. Other than one fuse and one relay being replaced, there were no significant maintenance issues and the system operated as designed during training. Table 5 on the next page contains the results of the algorithm performance demonstrated on the data collection lane.
Table 5. Ondjiva data collection results
# of Detections # of Detections on # of Detections on Shallow Targets Deep Encounter Depth Deep Targets on Refilled Holes (Flush to 5cm) AT-M Type B 5 out of 5 4 out of 4 30cm to top 2 out of 2 AT-M Type C 4 out of 5 4 out of 4 27cm to top 2 out of 2 AT-LM Type D 5 out of 5 4 out of 4 25cm to top 2 out of 2 AT-LM Type E 5 out of 5 3 out of 4 33cm to top 0 out of 2 AP-M Type F 2 out of 2 4 out of 4 13cm to top 0 out of 2 AP-M Type G 2 out of 2 4 out of 4 15cm to top 0 out of 2 UXO-M Type N 5 out of 5 3 out of 4 15cm to top 0 out of 2
These results demonstrate that the difficult low-metal AT mine (Type E) – described at the HD annual requirements workshop – could be detected by the NIITEK GPR and its pre-screener algorithm. Burying the AT mines at very deep depths (25-33cm) did not have a significant effect on detection performance. Finally, most of the re-filled holes were not detected by the algorithm, indicating that the mines were not detected simply by the algorithm picking up the edges of the holes.
Draft Version 6 Page 9 of 12 Saved on 3-May-2006 at 03:20 PM 4.3 Phase 3 Field Data Collection Results
The Mine Stalker system and NIITEK GPR were very reliable over a wide variety of conditions during the final phase of the field evaluation. There were no maintenance issues and the system operated as designed. Thus, the US personnel were able to collect valuable data with the GPR for future improvements. The only issue observed was that the test platform was not suited for some of the harsher off-road conditions. The platform was intended for on-road testing and not the off-road conditions that were encountered during this deployment.
4.4 Select GPR Imagery
The NIITEK GPR data is most commonly viewed in the format shown in the subsequent images. The top half of each of the images is the down-track view from one of the 24 GPR channels. The bottom half of the image is the cross-track view showing all of the GPR channels at one particular down-track position.
The following images in Figure 4 were displayed on the base station operator’s laptop during the Namibia test.
Figure 4. (a) AT-M Type A, only mine missed in the Namibia test, (b) AT-LM Type J properly detected by algorithm
The above images illustrate that when the pre-screener algorithm missed a single metal AT mine during the Namibia test, it was not missed because of GPR signal performance. In fact, this mine was spotted in the GPR imagery by the human operator reviewing the data at the end of the test lane.
Figures 5 – 7 contain images of GPR data that was collected over various pieces of clutter.
Figure 5. GPR data from a metal can lid during training in Ondjiva, Angola
Draft Version 6 Page 10 of 12 Saved on 3-May-2006 at 03:20 PM
Figure 6. GPR data from a foil wrapper during training in Ondjiva, Angola
Figure 7. GPR data from a crushed metal pot at field data collection site MF3
As described in Section 1.3, the real-time pre-screener algorithm alerts on mine-like objects without discriminating mines from clutter. A significant portion of the false alarms that occurred during the Namibia test and the Angola data collections were easily classified by the human-in-the-loop as clutter. Even a novice could probably see that some of the GPR imagery in Figures 5-7 does not match the mine signatures that are shown in Figure 4 on the previous page. NIITEK and other algorithm developers are continuing to work on creating and improving real-time detection algorithms for the NIITEK GPR that can truly discriminate mines from clutter. These algorithms would go beyond the impressive performance demonstrated by the pre-screener algorithm in the Namibia test.
Draft Version 6 Page 11 of 12 Saved on 3-May-2006 at 03:20 PM 5. CONCLUSIONS
The NIITEK GPR field evaluation was a success. Throughout the five week deployment, the system produced repeatable results in the Namibian blind test and Angolan data collections. Between the algorithm and the human-in- the-loop, it was demonstrated that the NIITEK GPR could detect all of the AT mines that it encountered in Namibia and Angola. The Mine Stalker’s ease of use allowed for the training of a local Angolan deminer who operated the system at the five data collection sites. Even with the harsh African environment, no significant repair or maintenance issues were encountered during the trial. Overall, the NIITEK GPR was demonstrated to be effective and reliable under these field conditions and showed great potential for use in humanitarian demining efforts.
While successful in the mission the Mine Stalker was designed to complete, the evaluation aided in identifying various improvements that could be made to the system. The current test platform has limited off-road capability because it was designed for use on roads. Anti-personnel mines create an additional liability with the present system. As the system was not designed to detect small AP mines or survive AP detonations, the Mine Stalker is limited to areas that do not have AP mines. If the GPR were mounted on a remote sensor arm, then the system could reach into mixed threat areas to detect AT mines without exposing the platform to AP blasts.
The research and development of the NIITEK GPR is an ongoing effort. System improvements include varying form factors for mounting flexibility, better performance against AP mines, and discriminating algorithms for accurate detection and reduced false alarms. Furthermore, NIITEK is combining technology improvements across all of its projects. Such improvements allow the system to travel at high rates of speed while detecting mines and other explosive hazards without sacrificing detection performance.
ACKNOWLEDGEMENTS
Thanks to MgM for hosting this field evaluation. They provided logistical support, interpreters, and access to their personnel and facilities. A tremendous thanks to all the NIITEK personnel who put in long hours preparing the system for a very successful deployment. Special recognition for Mike Price, the NIITEK Mine Stalker project lead. And, finally, a well-deserved and grateful acknowledgement to the team members who participated in the NIITEK GPR field evaluation: Greg Bullock (NVESD); David Eisenhauer (MTC Technologies); Fred Clodfelter, Stephen Laudato, Steve Lauziere (NIITEK); and Phil Straw (Fibertek).
REFERENCES
1. Doheny, R., S. Burke, R. Cresci, P. Ngan, R. Walls, J. Chernoff, “Handheld Standoff Mine Detection (HSTAMIDS) field evaluation in Namibia,” Proceedings of the SPIE Defense and Security Conference, 16-21 April 2006, Orlando, FL USA, in press..
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