Background
In traditional Surveying and Mapping, points are determined by measuring the angle and distance to an object from an instrument positioned over a known location (X, Y, Z or elevation). By performing three-dimensional coordinate geometry (COGO), you are able to calculate the location and elevation of features that you measure either directly with a prism, or indirectly with a reflectorless total station (more remote sensing). Surveyors often use an assumed coordinate system for smaller projects where the absolute position is not as important as the relative accuracy of the features they are measuring. Performing surveys with Mobile LiDAR over the same area takes the practice to the next level by requiring both high relative and absolute accuracies all while operating at highway speeds.
In order to better understand how accurate measurements are collected from a Mobile LiDAR system, it is important to know the instruments onboard and the function of each. As individual sensors, they do not provide an adequate solution. But, when properly integrated as a cohesive system, they provide the foundation for accurate information and project success.
Currently, there are several systems commercially available with varying configurations and measurement systems. A broad overview is provided, with specific references to Optechs Lynx Mobile Mapper and Applanixs Position and Orientation Systems (POS) due to my intimate familiarity with each.
Position and Orientation System (POS)
The word system is defined as a set of connected things or parts forming a complex whole. Those connected things in Mobile LiDAR are three measurement devices: Global Positioning System antenna(s), an Inertial Measurement Unit and a Distance Measurement Instrument. The connection is a little black box with half a dozen cable inputs. What makes it a complex whole is the integration of the measurement devices to establish the vehicles position and orientation with real-time metrics presented to an operator on a heads-up display.
Each component is important in a system. The limitations of one component in a condition or environment will require the other components to compensate to ensure an accurate solution.
Global Positioning System (GPS)
The primary method of location for the Mobile LiDAR vehicle is the Global Positioning System (GPS). Available systems use both single and dual antenna configurations. Although the perceived role of GPS is merely location (easily achieved with a single GPS antenna), the second antenna is an important component and has advantages over single antenna systems. By accurately measuring the baseline between the two antennas difference in X, Y and Z we establish the GPS Azimuth Measurement Subsystem (GAMS). The GAMS solution aids in the determination of heading and further compensates for GPS outages caused by obstructions such as buildings and trees.
The fundamental issue with GPS as a sole source for positioning is the measurement frequency. Typically GPS measurements are 1 Hz one measurement per second (our colleagues in the sky often utilize 2 Hz systems). For those of you breaking out your calculator, that equates to 88 feet per second when traveling at 60 mph. In that single second, the Mobile LiDAR could have measured 100,000 points or more, depending on the system. Therefore, we rely on our second measurement instrument.
Inertial Measurement Unit (IMU)
The Inertial Measurement Unit (IMU) measures the attitude and acceleration of the vehicle, much like an aircraft.By measuring the changes about the X, Y and Z axis, we are able to calculate the vehicles position at increments between GPS positions. The frequency, accuracy and drift of IMUs vary greatly. Unfortunately, the IMU doesnt get a lot of credit and is not often mentioned during Mobile LiDAR discussions (except by those who rely heavily on accuracy or respect its awesomeness); perhaps because it is not a visible component or measuring staggering amounts of points, but nonetheless, it is an extremely crucial component to a successful mission.
My LYNX utilizes a Litton LN-200 IMU, which has solid-state fiber optic gyroscopes and silicon accelerometers. As the IMU-of-choice for various tactical weapons and unmanned aerial vehicles, it is an ideal fit for Mobile LiDAR when measuring minute changes in direction or attitude (roll, pitch, and yaw) is important.
Distance Measurement Instrument (DMI)
The often overlooked member of the measurement family is the Distance Measurement Instrument. Its not a tool our colleagues in the sky use. The DMI has two distinct purposes:1) determine the distance traveled by measuring therevolutions of the wheel (based on calculations of wheel diameter and circumference); and 2) alert the system whenthe vehicle isstopped.Since there is inherent drift in both the GPSand IMU components, the DMI plays a vital role in determining when the vehicle wheel stops revolving. By utilizing the DMI to alert the system that the collection vehicle is in a stationary position, the system can automatically alleviate spatial drift errors that would be induced by the GPS or IMU.