petrie greece airborne laser scanning technologies · current developments in airborne laser...

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Current Developments in Airborne Laser Scanning Technologies

byProf. Gordon Petrie(Univ. of Glasgow)

IX International Scientific & Technical Conference“From Imagery to Map: Digital Photogrammetric Technologies”

October 5-8, 2009 – Attica, Greece

Airborne Laser Scanners1. Technology Developments(i) Increased data acquisition rates – Improved pulse repetition rates (PRFs);

Multiple-pulses-in-the-air; higher scan rates; etc. => so more elevation data is acquired => leading to higher data densities in the DTM!

(ii) Multiple returns – including waveform digitizing & recording.(iii) Improved imaging using integrated digital frame cameras. Sensor fusion!(iv) Improved accuracy – better rangefinders; GNSS/IMU systems.

2. Airborne Topographic Laser Scanners(i) Commercial System Suppliers – Optech: Leica Geosystems; Riegl[=> IGI; TopoSys; iMAR]

(ii) Custom-Built Systems – these are built in-house and are operated by service providers – TopEye; Fugro (FLI-MAP); TopoSys (Falcon).

(iii) Research Systems – NASA (ATM; RASCAL; SLICER: LVIS)

3. Airborne Bathymetric Laser Scanners(i) Commercial System Suppliers – Optech; Saab (now AHAB); Tenix(ii) Research Systems - NASA (AOL; EAARL)

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Airborne Laser ScannersOverall Concept(i) The position & flying height + attitude of the aircraft is measured using GPS/IMU technology.(ii) The range & the scan angle to the ground within the vertical cross-track plane are being measured by the laser scanner.

System Components(i) Laser rangefinder + optics + receiver + time counter.(ii) Optical Scan Mechanism –oscillating mirror or rotating polygon.(iii) Electronics Unit – controls the rangefinder & scan mechanism.(iv) Position & Orientation System –GPS/IMU for position and attitude.(v) Software – to control the system + the data recording & storage.(vi) Imaging Device – digital frame camera, video camera or line scanner.

Airborne Laser ScannersNew Developments - MPiA

(i) The time of flight (TOF) of between the emission and reception of the short laser pulse is measured very accurately.

(ii) For H = 1 km, elapsed time = 6.7us, before the next pulse can be sent, so the maximum pulse repetition rate (PRF) = 150 kHz. If still higher flying heights, then the PRF will be still lower. (iii) This limitation

has been overcome through the recent introduction of the multiple-pulses-in-the-air (MPiA)technology by Leica, Optech, Fugro. Allows greater densities & higher altitudes.

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Airborne Laser Scanners(i) Scan rates & patternsare also very important.

(ii) Bi-directional scanning using oscillating mirrorshave slower scan rates –since they have to slow down and stop before going on to the return sweep scan.

(iii) Very lightweight (but stiff) mirrors are required. Beryllium material is used.

(iv) Continually rotating polygons are also used –giving a raster pattern.

(v) Scan rates now 70 to 100 Hz => higher pt. density.

Airborne Laser Scanners

(i) PRF v. Flying Height (ii) PRF v. Slant Range (iii) Scan Rate v. FOV

LeicaALS60

with Control

Box

The gains in performance arising from these various improvements in thepulse repetition frequencies (PRFs) andscan rates can be seen in the series of graphs that are set out below.

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IGI AEROcontrol Applanix POS/AV Leica IPAS

Specification & Accuracy Values for the LeicaIPAS GNSS/IMU system using different IMUs

(i) Range resolutionis typically 1 cm.(ii) Range & elevation accuracy is related to flying height – 5 cm @ 500 m; 10 cm @ 1 km; 15 cm @ 2 km; 20 cm @ 3 km; etc.(iii) The position & attitude values given by the GNSS/IMU form the biggest part of the total error budget.

Airborne Laser ScannersInertial Measuring Units (IMUs)

(i) MEMS (Micro Electro-Mechanical Systems) Gyros that utilize tiny quartz tuning forks as sensors integrated on to silicon chips are coming into widespread use in the lower end imaging systems that are employed on less demanding applications. Not much used in laser scanners. Cost – $30,000 & going down rapidly!!

(ii) Fibre-Optic Gyros (FOG) are very much more expensive than MEMS gyros - but give a very acceptable performance that satisfies many laser scanning applications. Much used nowadays.

(iii) Ring Laser Gyros (RLG) are the most accurate type, but they are the most expensive to produce – which limits their use to high-end imaging systems & laser scanners and to only the most demanding applications in terms of accuracy. Cost – Still very high – $200,000

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Airborne Laser Scanners(i) A number of laser scanners can now record multiple reflections from the tree tops and branches & from ground.

(ii) A very few laser scanners can carry out full waveform recording - but it is only really useful for forestry projects [??]

(iii) The use of waveform digitizing needs enormous data storage. Riegl, Optech & Leica all now offer a full waveform digitizer & recorder as an option.

Airborne Laser Scanners(i) The intensity values returned from the reflected laser pulses hitting the ground objects [along with the ranges] produce a poor quality image.

(ii) So nowadays most airborne laser scanner systems have a medium-format digital frame camera that is integrated to form an essential part of the overall system and produce higher-quality images of the terrain.

Colour-coded elevation values (DSM)

Intensity values Fused elevation + intensity data

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Leica ALS50 scanners with NexVue & Applanix DSS cameras

IGI LiteMapper + DigiCAM Optech ALTM + Applanix DSS

(i) The airborne laser scanner & the frame camera are rigidly mounted & calibrated together.

(ii) The frame camera sizes have steadily increased from 16 Mpix(before) to 39 Mpix(now). Will increase to 60 Mpix soon.

(iii) Using the GNSS/ IMU georeferencing data + scanner elevation data + camera image data, orthophotos can readily be generated

Airborne Laser ScannersSummary – Technology Developments

1. There has been an increase in the power of the lasers that are used in the rangefinders employed in airborne laser scanners; yet they are smaller and more compact in size. The increased power means higher altitudes can be used.

2. Higher PRF values, together with the introduction of Multiple-Pulse-in-the-Air (MPiA) technology and higher scan rates, has led to higher data acquisition rates and a higher density of elevation values over the terrain.

3. On the receiver side, most rangefinders can record multiple (typically four) discrete returns/ reflections . However the principal system suppliers – Optech, Leica & Riegl – now offer waveform recording as an additional (optional) capability. It is not clear that this capability will be useful outside forestry.

4. The intensity values that are measured by the detectors/ receivers give rise to images of a rather poor quality. In general terms, the imaging of the objects that are being scanned by the laser scanner has to be carried out using supplementary medium-format digital frame cameras.

5. The resolution of the measured range data is very high (circa 1 cm), but the accuracy of the elevation data is much less, due to the limitations of the position and attitude data generated in-flight by the GNSS/IMU systems.

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Airborne Laser Scanners(a) Topographic Scanners; (i) Commercial Suppliers

Numerous models – 1020 (1993);1225 (1999); 2033/3033 (2001-3);3070/3100 (2005); Gemini (2006)--------------------------------------------------------The ALTM laser scanners all use a very

powerful (Class IV) laser; + an Applanix POS/AV system (GPS/IMU)

Most ALTM systems are supplied with an integrated digital frame camera from Applanix (DSS) or Rollei (AIC)

ALTM 1020

OptechAirborne Laser Terrain Mapper (ALTM) – well over 100 systems (? 140) have been delivered!

ALTM 3070 with

AIC Camera

Airborne Laser Scanners(a) Topographic Scanners; (i) Commercial SuppliersAll Optech ALTM laser scanners use a bi-directional oscillating

plane mirror to scan the ground – giving rise to a sawtooth patternof measured points

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OptechThe original ALTM scanners comprised a number of large and heavy units, especially the electronics control boxes. The newer models (Orion) are much smaller and lighter.

ALTMwith IMU

& AIC Camera


--------------------------------------------------(iv) In 2008, the very compact ALTM Orion was released – the M200 model is intended for wide area mappingfrom medium altitudes (up to H = 2.5 km); the C200 model is for corridor mapping from lower altitudes (up to H = 1 km).

Optech - Newest Developments(i) The latest model in the traditional ALTM series is the Gemini.(ii) Compared with the previous models, the Gemini offers a PRF of up to 167 kHz utilizing a multi-pulse (MPiA) technique.(iii) The Gemini can be operated at altitudes (H) up to 4.5 km.

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Leica Geosystems(i) Leica bought Azimuth Corp. in 2001 – its

Aeroscan laser scanner was re-badged by Leica as the ALS40. Very large & heavy.

(ii) In 2003, new compact ALS50 introduced;(iii) In 2006 improved ALS50-II model; (iv) In 2007, new ALS Corridor Mapper;(v) In 2008, new ALS60 @ ISPRS Congress.As with Optech ALTM, huge reduction in size!

(a) ALS50 (b) Display (c) IPAS10


Leica ALS laser scanners also use a bi-directional scanning mirror to scan the ground - giving a sinusoidal pattern of measured points.

Multiple Pulses in Air (MPIA) –The next measurement cycle begins before reflected pulse from previous cycle has been received – allows 150 kHz on ALS50-II model; 200 kHz on newest ALS60 model.

Leica ALS pattern of measured ground points (sinusoidal)

Optech ALTM pattern of measured ground points (sawtooth)

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Part IV - Airborne Laser Scanners(a) Topographic Scanners; (i) Commercial Suppliers

Leica Geosystems – Current Models

(i) ALS60 – Very compact; large aperture optics; very high maximum altitude (H = 6 km); high pulse rate with use of MPiA technology (200 kHz); high scan rate (100 Hz); high density of measured elevation points on the ground.

(ii) ALS Corridor Mapper – Similar compact size to ALS60; lower maximum altitude (H = 1 km); much lower cost.

(iii) Formerly Leica used ApplanixPOS/AV GPS/IMU & DSS camera; now it uses its own IPAS10/20 GNSS/IMU (ex-Terramatics) and RCD105 39 MPixcamera (from Geospatial Systems).

(iv) Circa 140 ALS scanners sold to date.


Riegl (Horn, Austria)Its LMS laser scanning engines utilize a continuously rotating uni-directional four-faced reflecting polygon that is producing a parallel pattern of measured ground points.

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ReiglTwo main families of laser scan engines – for low and high altitude operation respectively.

(i) LMS-Q140 (low) & LMS-Q280 (high); [oldest]

(ii) LMS-Q240 (low) & LMS-Q560 (high); [older]

(iii) LMS-Q680 – new; higher alt. (H = 3 km)

(iv) VQ-480 – new; medium -altitude.

LMS-Q240

VQ-480

LMS-Q560

LMS-Q160 on Aeroscoutrobotic mini-helicopter

LMS-Q680


Diamond DA42

MPP with scanner

podLMS-S560 System

Parallel Pattern

RieglMainly an OEM supplier of laser scanning engines (rangefinder + scanning mechanism + electronics), but also supplies complete laser scanner systems.

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TopoSys (now Trimble) Harrier 56

IGILiteMapperSystem

Riegl-based 3D-ALSScanner System

from iMAR

RieglSeveral system suppliers in Germany have built their airborne laser scanning systems on the basis of the laser scanning engines that are supplied on an OEM basis by Riegl. These companies include IGI, TopoSys (Trimble) & iMAR.


IGI (Kreustal, Germany)(i) Well known supplier of the CCNS

airborne navigation and guidance system & AEROcontrol GPS/IMU.

(ii) Now offers its LiteMapper 2400 & 5600 series of scanner systems based on Riegl laser scanners & its CCNS & AEROcontrol systems.

(iii) Also the LiteMapper 4800 which is based on the new Riegl VQ-480.

LiteMapper4800

LiteMapper 5600

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(i) IGI offers a specially modified version of the Canadian-built Dart pod (made of kevlar) to accommodate the complete LiteMapper system externally on helicopters such as the Eurocopter AS350.

(ii) In other cases, the customer has had a specially-built case and supporting frame constructed to accommodate the LiteMapper.


IGI

(i) Here are examples of its LiteMapper systems being fitted (shoe-horned) into small helicopters – a Bell 206 (left) and a Eurocopter EC-120(right).

(ii) However there are no problems accommodating a LiteMappersystem in a Russian Mil-8helicopter!

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TopoSys, Biberach, Germany

(i) Formerly the company was best known for its unique Falcon laser scanners, based on fibre-optic technology. These had little commercial success and have now been discontinued by the new owners of the company (Trimble).


TopoSys (Trimble)

(ii) The company now concentrates on its Trimble Harrier 24 & 56 systems. These are based on the Riegl LMS-Q240 & LMS-Q560 laser engines, together with the POS/AV GPS/IMU units and the DSS or AIC digital cameras from Applanix & Rollei –also now owned by Trimble!

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iMAR (St.Ingbert, Germany)

(i) iMAR is a well known supplier of inertial navigation units & systems.

(ii) It also offers its complete 3D-ALSsystem for sale to service providers.

(iii) The example shown is operated by Bewag, an Austrian power company.

(iv) It comprises a Riegl laser scanner, an iMAR ring-laser IMU, Javad GPS & a Rollei AIC camera


Tuck Mapping (Virginia, U.S.A.)(i) In North America, quite a number of service providers have built systems that are based on the use of Riegl laser scanner engines – like those of the German system suppliers – but they are not sold.

(ii) This shows the EagleEye system developed by Tuck Mapping, together with the Applanix POS/AV & DSS camera

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Lidar Services International, Calgary, Canada

(i) LSI is also a service provider & has developed its own series of Helix airborne laser scanning systems in-house, based on Riegl’slaser scanning engines.

(ii) The FAA has approved installation pods and kits that the company has developed for its Bell 206B helicopters (for low altitudes) and Cessna 185F fixed-wing aircraft (for higher altitudes).

Cessna 185F

Bell 206B

Airborne Laser Scanners(a) Topographic Scanners; (ii) Custom-Built Systems

TopEye (Sweden)(i) This company (now part of Blom

Group) has for many years acted as a service provider utilizing its own laser scanning systems built in-house for low altitudeoperation (corridor mapping)

(ii) The earlier systems – TopEyeMk I – used a bi-directional oscillating mirror (like OptechALTM and Leica ALS)

(iii) The re-built systems – TopEyeMk II – use a progressive series of overlapping elliptical (Palmer) scans to cover the ground.

(iv) 8 systems built – 6 operated by Blom Sweden; 2 are operated by Aerotec in the U.S.A.

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AHAB (Sweden)Company was set up by 3 ex-SAAB people to re-develop the TopEye & HawkEye laser scanners. It is now an independent supplier.

---------------------------------------------------------------

(i) Its newest product is the very compact and lightweight Dragon Eye system.

(ii) Very high pulse repetition frequency (PRF)of 300 kHz. Palmer elliptical scans.

(iii) Multiple Pulse in the Air (MPiA) capability –optional.

(iv) Full waveform recording – optional.

(v) Operational with TerraTec in Norway.


FugroThis company also operates its FLI-MAP

laser scanners that have been constructed in-house.

The FLI-MAP II system used continuous rotating mirror as its scan mechanism in conjunction with digital and video cameras. Low altitude (H = 150 m!).

The FLI-MAP 400 system has a more powerful laser and has twin digital frame cameras and twin video cameras. Higher altitudes (H = 400 m).

4 GPS receivers (2 on outriggers) are used to measure the attitude (tilt) values.

8 systems are in current use by John Chance (U.S.A.) & Fugro-Inpark (NL) for low altitude operations (corridor mapping).

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Fugro :- Its new FLI-MAP 1000 system entered service in September 2008.

(i) 150 or 250 k pulses per second

(ii) Multiple-Pulse-in-Air (MPiA)technique.

(iii) Much higher max. operational altitude of 950 m. Can be used in fixed-wing aircraft.

(iv) Real-time RGB colour encodingof each measured laser point.

Airborne Laser Scanners(a) Topographic Scanners; (iii) Research Systems

(i) NASA carried out much of the pioneering research work with airborne laser profilers & scanners between 1975 & 1995.(ii) It still carries out work with its LVIS (Laser Vegetation Imaging Scanner). Very high altitude operation (H = 12 km). Very powerful laser rangefinder. Scans in steps – stationary mirror. Forestry & vegetation applications.

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Airborne Laser Scanners(b) Airborne Bathymetric Laser Scanners

General Principles

(i) This involves the use of two laser rangefinders emitting pulses simultaneously at different wavelengths – in the infra-red and green parts of the e-m spectrum.(ii) The infra-red radiation is reflected from the water surface; whereas the pulse of green radiation passes into and through the water and is reflected by the seabed back towards the rangefinder. Depth is derived from the time differences.

Airborne Laser Scanners(b) Bathymetric Scanners; (i) Commercial Suppliers

Optech(i) Various models – SHOALS-200 (1993); -

400 (1998); -1000 (2003); -3000 (2006). The principal users are U.S. government hydrographic agencies.

(ii) Fugro-Pelagos (a commercial company) and the Japanese Coast Guard also operate examples of SHOALS.

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Hawk Eye IIIs being used for coastalSurveys by Blom UK on an international basis.

AHAB (Airborne Hydrography AB)The company has its origins in the Saab group which built the Hawk Eye systems in the mid-1990s. These were used by Swedish hydrographic agencies, then later operated by the Indonesian Navy. The product rights were then sold to AHAB (ex-Saab people).


Tenix(i) The LADS (Laser Airborne Depth

Finder) was developed for the Royal Australian Navy (RAN) in 1993. Mounted on a Fokker F-27. Still operational – has been upgraded recently in 2008.

(ii) LADS-II was operated commercially world-wide by the Tenix LADS Corporation. Mounted on a Dash-8. The company was acquired by Fugroin August 2009.

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Airborne Laser Scanners(b) Bathymetric Scanners; (ii) Research Systems

NASAThe Experimental Advanced Airborne Research Lidar (EAARL) has(i) Only a green laser – no infra-red;(ii) a very short laser pulse (1.3 ns);(iii) a very narrow FOV (1.5 mrad);(iv) full digitized waveform recording.

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