Traditional accelerometer based measurement can only estimate the road geometry and is only really suitable for measurement at high speeds.
The advantage of the RPS PaveScout is that it can accurately calculate the road geometry using the on-board laser profiling system, GPS system and the high precision IMU (inertial measurement unit).
This allows us to measure Roughness in difficult conditions and at much slower traveling speeds producing more detailed and accurate measures.
The standard method of measuring roughness for a network is to utilize a combination of point laser sensors and accelerometer placed vertically above the point laser. Fundamentally this method requires the accelerometer to be doubly integrated to produce a height profile. The predominate force on the accelerometer however is gravity, so when the sensor is tilted (by slowing down, turning a corner or other effects), then the estimation of gravity is no longer correct, resulting in major errors in the estimated height profile, as a component of gravity is measured by the sensor. As a result, when operating at low speeds, turning corners or when the road camber changes, this method has errors. It is for that reason that all calibration for roughness is done at a constant speed, on a straight road, with the road carefully selected for changes in camber. The same cannot be said for a normal road, where a driver is exposed to traffic and all the complexities of a road network.
RPS uses a different approach which allows more accurate roughness measures in difficult conditions and slower speeds. Similar to a reference device, we use the road geometry itself instead of an estimate of the road geometry as used by the accelerometer approach. The geometry in the direction of travel is calculated through the combination of the laser profiling system, the GPS system and the high precision IMU (inertial measurement unit) system. The IMU system corrects for short term GPS error, allowing the roughness calculation to be unaffected by short term GPS dropout or error. The result is an ability to determine roughness both at slower speeds and even around corners. The low speed threshold for accurate roughness can be further reduced through the use of a GPS reference station to correct vehicle GPS readings.
The RPS system also allows us to perform the same measures as a point laser with an accelerometer placed in the same vertical dimension as the point laser. This is because 6 degrees of freedom within the sensor allows us to simulate an accelerometer anywhere within the frame of the vehicle. The result is the best of both worlds - a better form of IRI which is less affected by corners and the same result as historical measures.
Roughness is specifically designed for large high speed roads. For suburban regions, roughness makes less sense due to camber changes in a road surface (due to corners or speed bumps) resulting in a high IRI. For most road authorities these regions are not a concern (or for a speed bump purposely "rough"). This is because vehicles are normally traveling slowly. The system automatically detects these regions, through a combination of turning angle, speed and a geospatial list of features from the road authority. These regions are then smoothed when performing the roughness calculation. The result is a "suburban" IRI, where high IRI corresponds to the degraded areas, instead of being overwhelmed by corners and geometry.