5 ms·

> It's widely known that RADAR systems are not able to detect stationary objects, only moving ones. Actually, they are are to detect them but just not where the

> It's widely known that RADAR systems are not able to detect stationary objects, only moving ones. Actually, they are are to detect them but just not where they come from That doesn't make any sense. Radar systems do detect stationary objects just fine. In fact, from the point of view of the moving car, almost nothing is stationary. I wouldn't be surprised if you're correct about the markings on the road, I almost crashed into a temporary barrier driving at night, because there were two sets of lane markings - the old ones, and the ones going around the barrier. Construction workers simply didn't bother to erase the old ones.

sweden 9y ago
It is mentioned in the manual of the car: "Traffic-Aware Cruise Control cannot detect all objects and may not brake/decelerate for stationary vehicles, especially in situations when you are driving over 50 mph (80 km/h) and a vehicle you are following moves out of your driving path and a stationary vehicle or object is in front of you instead." You can also read here that Volvo's system faces the same problem: https://www.wired.com/story/tesla-autopilot-why-crash-radar/ https://www.wired.com/story/tesla-autopilot-why-crash-radar/
- gus_massa 9y ago
  IIRC the radar can detect the object but it has too many false positives, like a small soda can that has a lucky alienation and shape that make it appear as a big object. So the system ignores the static signals of the radar, because randomly stopping each time an small object is confused with a big object is also dangerous. And the system uses other signals to identify and avoid static objects.
- saalweachter 9y ago
  This is not a RADAR problem, it’s a world modeling problem. So you’re sending out pulses and listening for echos, which tells you how far away something is in a particular direction. You correlate subsequent pulses to say whether the object is moving toward you or away. If you have a car 50 m ahead of you every time you ping, everything is good. Now that car suddenly swerves around a car stopped in front of it, and your ping off that object says 60 m. A crash is less likely, your model thinks! The object in front of you is rapidly speeding away! By the time it realizes it isn’t, boom, crash.
  - adamlett 9y ago
    That’s not how it works in my limited understanding. Radars don’t just detect the positions of object, but also their relative speed. For this, they take advantage of the Doppler effect. When a pulse gets reflected, its frequency rises if the object is moving towards the radar, and falls if it’s moving away.
    - saalweachter 9y ago
      You could be right, either in this case or other cases. I'm entirely inferring what autos are doing based on their failure case (not detecting stationary cars when the car in front of you swerves around them); that sounds a helluva lot like mistaking the new car for the old. Again, not saying it's a limitation of RADAR; it sounds like a deficiency in the way they're using it.
    - hexane360 9y ago
      Although there's an inherent trade-off between detecting position and speed. Longer pulses give more accurate frequency/speed detection, while short pulses give more accurate time/position detection. This is the same phenomenon that underlies Heisenberg uncertainty.
random4369 9y ago
> It's widely known that RADAR systems are not able to detect stationary objects What the fuck? That's not at all how radar works. It has no such limitation.
- pja 9y ago
  My understanding is that RADAR systems can detect stationary objects just fine: What's going on is that in-car radar systems can't differentiate between, say, a metal sign on a gantry or at the side of the road and a parked car on the road, because their spatial resolution is poor to non-existent. Hence in-car RADAR systems (often) ignore radar returns that are non-moving relative to the road surface. Otherwise the car would panic brake every time you went past a road sign. The effect of this is that current in car RADAR systems are great at avoiding collisions with vehicles that suddenly brake to a halt in front of you whilst at the same time will happily let you drive a full speed straight into the back of a parked car. This is why I believe current self driving vehicles (apart from Tesla) are all using LIDAR for object detection. (The above is my interpretation of my reading on the current state of the art. If anyone knows more detail, please correct me.)
  - URSpider94 9y ago
    The problem for all self-driving car systems is modeling the road. Let’s say you are climbing a gentle hill, and there’s an overpass ahead. The nose of the car is aiming up, so it will see the overpass dead ahead and not moving. This looks the same to the car as driving down a flat road with a semi perpendicular to the travel lanes and dead stopped. This is true for all imaging systems - RADAR, LIDAR and camera. Same if you are about to enter a right-hand sweeper, pedestrians standing on the sidewalk in front of you would look as if they are directly in your path. Musk mentioned this in one of his blog posts. Tesla is attempting to build a database of obstacles like this and geotag them, so that the car can filter them out.

sweden 9y ago

It is mentioned in the manual of the car: "Traffic-Aware Cruise Control cannot detect all objects and may not brake/decelerate for stationary vehicles, especially in situations when you are driving over 50 mph (80 km/h) and a vehicle you are following moves out of your driving path and a stationary vehicle or object is in front of you instead." You can also read here that Volvo's system faces the same problem: https://www.wired.com/story/tesla-autopilot-why-crash-radar/ https://www.wired.com/story/tesla-autopilot-why-crash-radar/

gus_massa 9y ago
IIRC the radar can detect the object but it has too many false positives, like a small soda can that has a lucky alienation and shape that make it appear as a big object. So the system ignores the static signals of the radar, because randomly stopping each time an small object is confused with a big object is also dangerous. And the system uses other signals to identify and avoid static objects.
saalweachter 9y ago
This is not a RADAR problem, it’s a world modeling problem. So you’re sending out pulses and listening for echos, which tells you how far away something is in a particular direction. You correlate subsequent pulses to say whether the object is moving toward you or away. If you have a car 50 m ahead of you every time you ping, everything is good. Now that car suddenly swerves around a car stopped in front of it, and your ping off that object says 60 m. A crash is less likely, your model thinks! The object in front of you is rapidly speeding away! By the time it realizes it isn’t, boom, crash.
- adamlett 9y ago
  That’s not how it works in my limited understanding. Radars don’t just detect the positions of object, but also their relative speed. For this, they take advantage of the Doppler effect. When a pulse gets reflected, its frequency rises if the object is moving towards the radar, and falls if it’s moving away.
  - saalweachter 9y ago
    You could be right, either in this case or other cases. I'm entirely inferring what autos are doing based on their failure case (not detecting stationary cars when the car in front of you swerves around them); that sounds a helluva lot like mistaking the new car for the old. Again, not saying it's a limitation of RADAR; it sounds like a deficiency in the way they're using it.
  - hexane360 9y ago
    Although there's an inherent trade-off between detecting position and speed. Longer pulses give more accurate frequency/speed detection, while short pulses give more accurate time/position detection. This is the same phenomenon that underlies Heisenberg uncertainty.

random4369 9y ago

> It's widely known that RADAR systems are not able to detect stationary objects What the fuck? That's not at all how radar works. It has no such limitation.

pja 9y ago
My understanding is that RADAR systems can detect stationary objects just fine: What's going on is that in-car radar systems can't differentiate between, say, a metal sign on a gantry or at the side of the road and a parked car on the road, because their spatial resolution is poor to non-existent. Hence in-car RADAR systems (often) ignore radar returns that are non-moving relative to the road surface. Otherwise the car would panic brake every time you went past a road sign. The effect of this is that current in car RADAR systems are great at avoiding collisions with vehicles that suddenly brake to a halt in front of you whilst at the same time will happily let you drive a full speed straight into the back of a parked car. This is why I believe current self driving vehicles (apart from Tesla) are all using LIDAR for object detection. (The above is my interpretation of my reading on the current state of the art. If anyone knows more detail, please correct me.)
- URSpider94 9y ago
  The problem for all self-driving car systems is modeling the road. Let’s say you are climbing a gentle hill, and there’s an overpass ahead. The nose of the car is aiming up, so it will see the overpass dead ahead and not moving. This looks the same to the car as driving down a flat road with a semi perpendicular to the travel lanes and dead stopped. This is true for all imaging systems - RADAR, LIDAR and camera. Same if you are about to enter a right-hand sweeper, pedestrians standing on the sidewalk in front of you would look as if they are directly in your path. Musk mentioned this in one of his blog posts. Tesla is attempting to build a database of obstacles like this and geotag them, so that the car can filter them out.