In the project PPP-MALS, the Technische Universität Wien and the company Riegl aim to redefine the state of the art in mobile laser scanning. Laser scanning is a method for measuring our environment - whether man-made or natural. From a moving platform, e.g. a drone, vehicle or person-carried device, a laser scanner emits short laser pulses in different directions. For each pulse, the time it takes for the signal to travel through the atmosphere and back after reflecting off an object is measured. Using the speed of light, the time difference ist converted into the distance between platform and object. Global satellite navigation (e.g. Galileo) is used to determine the position of the platform, and intertial navigation can be used to calculate the angular position of the platform in 3D space. By combining all this data, the points of reflection of the laser beams on the Earth's surface is calculated. This creates “point clouds” that cover our environment with 3D points. Typically, the distance between these points is a few centimetres to decimetres, depending on flight altitude, measurement frequency of the laser scanner, etc.
This method is already in use, but problems arise time and again. PPP-MALS aims to ensure that 1) calculations are performed more efficiently, 2) accuracy is increased – specifically so that repeat measurements can be performed with much greater accuracy than before – and 3) a reference is created with which the quality of new methods can be proven beyond doubt. This reference should be sustainable, i.e. usable beyond the duration of the project.
If repeat measurements could be carried out more reliably, even very small signals of environmental change could be detected, e.g. terrain deformations as a result of permafrost thaw cycles, or tree growth within a vegetation period.
The key to the desired improvements is the joint processing of all data in a single optimization step. This has not yet been attempted because 1) it is very complex and 2) it requires expertise from many different areas. Until now, however, the data is processed sequentially: first the satellite navigation measurements, then supplemented by inertial navigation, and finally adding the laser scanner measurements. This means that errors are passed on from one step to the next. If this can be successfully integrated in a single step, the measurements will control and support each other, and the determination of the platform position and the point cloud will become more accurate.
Specifically, the method of precise point positioning (PPP) for satellite navigation is to be combined with inertial navigation and laser scanner measurements in a single, joint optimization step. The trajectory of the platform and the ties between overlapping laser scanning observations on object surfaces are determined simultaneously. This has the additional advantage that no ground stations need to be operated for satellite navigation, which makes the method more universally applicable.
