An accurate estimate of a vehicle’s own velocity, referred to as ego-velocity, is crucial for truly safe ADAS and, eventually, autonomous driving. Advanced radar imaging deciphers the car’s own speed, leveraging its ability to sense the environment in high resolution specifically in elevation, and its ability to detect stationary objects. While ego-velocity can be estimated using a variety of methods, only Imaging Radar provides the precision and continuity required for true autonomous driving.
To achieve true safety, a vehicle must have a coherent understanding of its environment. One of the greatest challenges to achieving this understanding is being able to map the vehicle’s surroundings while simultaneously localizing it within that map as it is being continuously generated. This challenge is addressed by SLAM, Simultaneous Localization And Mapping.
A key element of SLAM is being able to distinguish between stationary and moving objects in the vehicle’s environment. Since the speed of objects in a moving environment is relative, a vehicle must be able to understand whether an object is moving toward it because the object is moving or because the vehicle is moving, or both. Making this distinction requires the ability to accurately determine the vehicle’s ego-velocity. By accurately estimating and then deducting ego-velocity from the perceived velocity of environmental objects, the vehicle is able to decipher which objects are stationary versus which are moving while the vehicle itself is in motion.
The “simultaneous” part of SLAM refers to the ongoing process of generating a map that is improved by data gathered when the vehicle moves on it, while analysis of how the vehicle moves is enhanced by the continuously improving accuracy of the map itself. Each of these two inseparable actions relies heavily on accurate estimation of ego-velocity. It is impossible to deduce how the vehicle is moving within the map without an appropriately precise estimation of ego-velocity. Likewise, the accuracy of the map itself requires the correct assessment of the speed of all objects on it, which also cannot be deduced without trustworthy ego-velocity.
Environmental awareness and comprehension is fundamental to vehicle navigation, whether performed by man or machine. While human drivers are somewhat aware of their own limitations and usually take large margins to compensate, in the autonomous car it is the role of sensors to provide this added safety. Having a good estimate of ego-velocity, thus, is a crucial task for autonomous driving.
Vehicle sensors currently on the market have each tackled the problem of ego-velocity based on their own strengths, but each also suffers from significant disadvantages.
By contrast, based on its ability to distinguish between radar detections that stem from stationary objects and those that stem from dynamic objects, and by exploiting its high resolution in all dimensions (range, azimuth, elevation, and doppler), an Imaging Radar can make a highly accurate, per-frame, ego-velocity estimation, that is continuous (no thresholding behavior) around 0 km/h, and can overcome problematic scenarios such as bridges, tunnels, traffic jams, and garages. Thus, while ego-velocity can be estimated using a variety of methods, only Imaging Radar provides the precision and continuity required for true autonomous driving.
Ego velocity deducted using Imaging Radar
The Role of Elevation Resolution
To understand the importance of elevation to the process of ego-velocity tracking, let’s assume that the radar-car is driving “forward” at a constant speed of v, and consider a single radar detection at range r, azimuth Φ (bore-sight is Φ=0), and elevation Θ (again Θ=0 is bore-sight) with respect to the radar, whose measured line-of-sight (Doppler) velocity is vr. Up to measurement noise, then, one can extract the desired ego-velocity from the geometrical relation
vr = v cos Θ Cos Φ, as seen in the figure below, where the antenna is shown as the light blue rectangle, Φ is the angle 1, and Θ is the angle 2
Having no access to elevation (as is the case in traditional radars) has two important implications for the ability to calculate ego-velocity. The first implication is that one is forced to discard the portion of elevation in the relation (assume that Θ =0). The other, more subtle, implication stems from the fact that for planar antenna (which is the case with all traditional radars), the azimuth angle Φ is in fact inaccessible to the radar. Instead, one can only measure the angle between the antenna’s “horizontal axis” and the line of sight from the radar to the target, the so called α, designated as angle 3 (in blue), satisfying
While the factor Θ is, in most cases, not too far away from unity, considering the radar field of view in elevation, the distortion induced by discarding elevation data (and, in fact, using a wrong coordinate system) may still accumulate to a significant divergence of the single-frame estimated ego-velocity and the true ego-velocity. Since tracking ego-velocity involves consistency between frames, this divergence may quickly lead to a complete loss of ego-velocity tracking. This typically happens when a large portion of the detected point-cloud originates from elevated objects, for example driving under a bridge.
The ability to track the car’s ego velocity at a basic level is not technically complicated. To do so continuously, independently, and reliably, however, puts high-resolution Imaging Radar miles ahead of the traditional low channel-count radars and alternative sensors on the market to date. The importance of ego-velocity as a feature in the first phase of any advanced driving solution should not be underestimated.Connect to learn more