AN INTRODUCTION TO LASER SCANNING

PART 2 – AIRBORNE LIDAR, THE BASICS OF TRAJECTORIES

TRAJECTORIES• AIRBORNE LIDAR SYSTEMS USE 2D SCANNERS

• THE 3RD DIMENSION IS DERIVED FROM THE MOVEMENT OF THE AIRCRAFT

• THE PRECISE POSITION AND ORIENTATION OF THE LIDAR SYSTEM IN THE AIRCRAFT IS KNOWN AS A TRAJECTORY• TIME

• FROM GNSS• POSITION - LATITUDE, LONGITUDE, ELEVATION

• FROM GNSS• ORIENTATION (ATTITUDE) - HEADING, ROLL, PITCH

• FROM IMU

GLOBAL NAVIGATION SATELLITE SYSTEM (GNSS)• GNSS’ EMPLOY A CONSTELLATION OF ORBITING SATELLITES TO PROVIDE

GEOGRAPHICAL POSITION AND VELOCITY INFORMATION

• MANY SYSTEMS ARE IN USE AT THIS TIME, WITH THE MOST POPULAR INCLUDING:• UNITED STATES - GLOBAL POSITIONING SYSTEM (GPS)• RUSSIA – GLOBAL NAVIGATION SATELLITE SYSTEM (GLONASS)• CHINA – COMPASS / BEIDOU NAVIGATION SYSTEM (BDS) • EUROPEAN UNION – GALILEO

• RECEIVERS THAT CAN TRACK MULTIPLE GNSS CONSTELLATIONS WILL GENERALLY PROVIDE MORE ACCURATE POSITIONAL DATA, ESPECIALLY IN URBAN CANYONS WHERE SKY VISIBILITY IS LIMITED

GLOBAL NAVIGATION SATELLITE SYSTEM (GNSS)• DUE TO ERROR SOURCES SUCH AS ATMOSPHERIC INTERFERENCE AND

SATELLITE CLOCK TIMING, GNSS RECEIVERS ARE OFTEN ONLY ACCURATE TO 1-2M

• ACCURACY CAN BE IMPROVED USING METHODS SUCH AS:• PRECISE POINT POSITIONING (PPP) - <30CM• POST-PROCESSING AGAINST A REFERENCE STATION – DIFFERENTIAL GNSS

(DGNSS) - < 2CM• REAL TIME KINEMATIC (RTK) CORRECTION - < 2CM

• GNSS IS USUALLY LOGGED AT 2-10HZ• ONE POSITION MEASURED EVERY 0.1-0.5 SECONDS

INERTIAL NAVIGATION SYSTEM (INS)

• AN INS IS A COMBINATION OF AN INERTIAL MEASUREMENT UNIT (IMU) AND A CONTROL COMPUTER

• THE INS IS INITIALLY PROVIDED WITH ITS POSITION AND VELOCITY FROM AN EXTERNAL SOURCE, AND FOR AIRBORNE LIDAR THIS IS GNSS

• A KALMAN FILTER COMBINES THE DIFFERENT MEASUREMENTS TAKING INTO ACCOUNT ESTIMATED ERRORS TO PRODUCE A TRAJECTORY

INERTIAL MEASUREMENT UNIT (IMU)

• MEASURES ROTATION AND ACCELERATION• COMBINED WITH POSITION AND VELOCITY FROM GNSS

RECEIVER TO PRODUCE A TRAJECTORY

• TWO MAIN TYPES OF IMU• FIBRE OPTIC GYRO (FOG)• MICRO ELECTRO MECHANICAL SYSTEM (MEMS)

• IMUS USUALLY LOG BETWEEN 100-500HZ

TYPICAL AIRBORNE LIDAR TRAJECTORY

TYPICAL AIRBORNE LIDAR POINTCLOUD

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