Airborne Laser Scanning Technologies

1. Current Developments in Airborne Laser Scanning Technologies by Prof. Gordon Petrie (Univ. of Glasgow ) IX International Scientific & Technical Conference “ From Imagery to Map: Digital Photogrammetric Technologies” October 5-8, 2009 – Attica, Greece

3. Airborne Laser Scanners Overall 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.

4. Airborne Laser Scanners New 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.

5. Airborne Laser Scanners (i) Scan rates & patterns are also very important. (ii) Bi-directional scanning using oscillating mirrors have 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.

6. Airborne Laser Scanners (i) PRF v. Flying Height (ii) PRF v. Slant Range (iii) Scan Rate v. FOV Leica ALS60 with Control Box The gains in performance arising from these various improvements in the pulse repetition frequencies (PRFs) and scan rates can be seen in the series of graphs that are set out below.

7. Airborne Laser Scanners IGI AEROcontrol Applanix POS/AV Leica IPAS Specification & Accuracy Values for the Leica IPAS GNSS/IMU system using different IMUs (i) Range resolution is 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.

9. 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.

10. 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

11. Airborne Laser Scanners 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

12. Airborne Laser Scanners Summary – 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.

15. Airborne Laser Scanners (a) Topographic Scanners; (i) Commercial Suppliers Optech The 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. ALTM with IMU & AIC Camera

16. Airborne Laser Scanners (a) Topographic Scanners; (i) Commercial Suppliers -------------------------------------------------- (iv) In 2008, the very compact ALTM Orion was released – the M200 model is intended for wide area mapping from 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.

19. 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 Applanix POS/AV GPS/IMU & DSS camera; now it uses its own IPAS10/20 GNSS/IMU (ex-Terramatics) and RCD105 39 MPix camera (from Geospatial Systems). (iv) Circa 140 ALS scanners sold to date.

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

21. Airborne Laser Scanners (a) Topographic Scanners; (i) Commercial Suppliers Reigl Two 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 Aeroscout robotic mini-helicopter LMS-Q680

22. Airborne Laser Scanners (a) Topographic Scanners; (i) Commercial Suppliers Diamond DA42 MPP with scanner pod LMS-S560 System Parallel Pattern Riegl Mainly an OEM supplier of laser scanning engines (rangefinder + scanning mechanism + electronics), but also supplies complete laser scanner systems .

23. Airborne Laser Scanners (a) Topographic Scanners; (i) Commercial Suppliers TopoSys (now Trimble) Harrier 56 IGI LiteMapper System Riegl-based 3D-ALS Scanner System from iMAR Riegl Several 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 .

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

26. Airborne Laser Scanners (a) Topographic Scanners; (i) Commercial Suppliers 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 LiteMapper system in a Russian Mil-8 helicopter!

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

28. Airborne Laser Scanners (a) Topographic Scanners; (i) Commercial Suppliers 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!

29. Airborne Laser Scanners (a) Topographic Scanners; (i) Commercial Suppliers iMAR (St.Ingbert, Germany) (i) iMAR is a well known supplier of inertial navigation units & systems. (ii ) It also offers its complete 3D-ALS system 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

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

31. Airborne Laser Scanners (a) Topographic Scanners; (i) Commercial Suppliers 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’s laser 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

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

36. 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 .

39. Airborne Laser Scanners (b) Bathymetric Scanners; (i) Commercial Suppliers Hawk Eye II Is being used for coastal Surveys 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).

41. Airborne Laser Scanners (b) Bathymetric Scanners; (ii) Research Systems NASA The 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 .