7 ms·

How automotive radar measures the velocity of objects

95 points by subbdue 2y ago

Animats 2y ago
"The IF signal has a phase that is the difference between transmit and received signal phases." Yes. That's a neat property of superhetrodyning - phase is preserved. Both the outgoing and incoming signals are down-converted by mixing with the local oscillator. The phase angle difference between out and in is the same at both the transmitted/received frequency and the IF frequency. But down at the IF frequency, you get to work at a lower frequency where it's easier to do A/D conversion and counting. Most software defined radios still have a superhetrodyne front end, so the digital stuff is working at the IF frequency. This is less necessary than it used to be, now that digital circuits can work well into gigahertz ranges.
- kazinator 2y ago
  Same like how you get to work with single-digit-hertz beats when tuning one string to match another.
  - tonyarkles 2y ago
    Heh, when getting a GPS Disciplined Oscillator set up I was feeding the 10 MHz oscillator into an SDR to use as a reference clock and was calibrating against a local cell tower. 7 Hz beat/offset at 1700 MHz!
dvh 2y ago
What will happen if every single car has radar? Wouldn't they interfere? You're stuck in traffic and 300 nearby cars are blasting radars all over the place?
- psunavy03 2y ago
  There are various ways a radar (or any RF signal) can be designed to recognize its own signal from all the background noise. We don't worry about millions of cell phones or WiFi routers sharing bandwidth either.
  - r2_pilot 2y ago
    Actually, in both cases, we do. Cell towers deliberately have different frequencies allocated from their neighboring towers, and Wi-Fi has multiple channels, several of which do not have any overlap.
    - psunavy03 2y ago
      When I said "we don't worry about it," I meant the problem has been acceptably mitigated.
      - mitthrowaway2 2y ago
        What that really means is that several talented people dedicated their entire careers to worrying about it!
  - ssl-3 2y ago
    Neither of those examples answers the question. Co-channel Wifi interference is real. It really puts a damper on range and throughout compared to how it used to be. It is a largely unmitigated clusterfuck, as is the way with CSMA/CA once density increases enough. LTE interference isn't an important thing in practice, in part because because all participating devices have very tightly-controlled timings. It isn't a clusterfuck at all because of the mitigations in place, but it does require centralized coordination to be this way. Radars on cars don't have centralized coordination (do they?). What mechanism prevents their performance from degrading as wifi does?
    - r2_pilot 2y ago
      For your specific question, you can send a pulse train specific to the radar emitter, and you filter out anything that doesn't match your specific pulse train
      - ssl-3 2y ago
        How do I know that each of the pulses I sent out are the same ones I receive in reflection, in terms of frequency (remember, we have moving targets and therefore Doppler shift here), and amplitude? The reflections of my own pulses are pretty quiet compared to the pulses of, say, an oncoming vehicle's own radar transmissions. How do I discern those individually-received pulses from the very similar pulses that are being received from other transmitting vehicles? (Sure, their timings should be be different, and the pulses may be very brief. But they may also overlap rather significantly, and errors can be very bad here. We need some rather good certainty here -- however that certainty is accomplished.)
- kube-system 2y ago
  > What will happen if every single car has radar? If you're on the road in a relatively affluent area where people drive late model cars, this is pretty close to already the case. Automakers have started making these systems standard on many/all of their models in the US for several years now. Toyota, for example, started rolling out these systems a decade ago, and have been standard on all US models since 2018. I'm not sure what these systems use in practice for interference mitigations, but there's a bunch of stuff that could be done, for instance, hopping between different frequencies.
- curiousfab 2y ago
  Typical FMCW radars transmit very short ramps (microseconds) at a very long (relatively) intervals (several ten milliseconds), i.e. a duty cycle of less than 0.1%. In order to create interference between two radars, the ramps have to overlap pretty exactly, within a few nanoseconds of each other. This is very unlikely to happen. Modern radars employ technologies to detect and/or avoid such collisions. Overall it is not really an issue, even with many radars in crowded spaces.
  - abstrakraft 2y ago
    This is true for some earlier lofi radars, but as driver assistance and self-driving have developed, so have the requirements and capabilities of the radar systems. Newer systems generally have shorter PRIs for higher doppler bandwidth, and much higher duty cycles for more energy on target - the FCC limits power, so you've got to get energy from the time axis. Both of these things make the interference problem harder.
  - aidenn0 2y ago
    How is it continuous wave if it has a non-unity duty cycle?
    - abstrakraft 2y ago
      In practice, the difference between pulsed radar and continuous wave radar is a continuum rather than a dichotomy. Historically, FMCW (frequency modulated continuous wave) had a high duty cycle (though not 100%, the ramp generators need finite time to reset (though you can alternate between up and downramps and get closer)). For some applications, though, requirements force you to short ramps and long PRIs, thus low duty cycles, but the name (FMCW) sticks.
- kazinator 2y ago
  I suspect radar like this needs only a tiny time slices to do its work. Say for example that it's only necessary to get updates about the moving object 100 times per second: every 10 ms. The radar pulse durations necessary to do the job can probably be measured in microseconds, though. A 10 ms separation between pulses measured in microseconds is a large amount of empty space.
- deleted 2y ago
  [deleted]
- isopede 2y ago
  Interference is a real problem with FMCW radars, either maliciously in the case of electronic warfare, or accidentally in the case you mentioned, with many radars in the same space using the same frequency band. Wifi and cell phones use time division or frequency division multiplexing techniques, but radars (at least current-gen) generally do not. There are mitigation techniques like randomization of chirp frequencies, choosing different idle times between frames, and signal processing techniques to try to detect interference and filter it out. In the general case, FMCW techniques will always have interference problems. This is one reason amongst many others that military radars do not use FMCW but instead coded pulse compression techniques.
teeray 2y ago
It would be interesting to see how to attenuate or deflect radar emissions from cars to passively disable automatic braking. Won’t brake if the car believes it’s driving on a flat expanse of nothing.
- jvanderbot 2y ago
  The radar cross section of an automobile within 100 meters is probably too high unless it's made of celery or has blackbody paint.
- opwieurposiu 2y ago
  If you had a chaff dispenser, perhaps you could deploy a chaff cloud that would look like a fixed object and trigger the emergency braking of the cars behind you. The newest weapon in the war against tailgating.
  - kayodelycaon 2y ago
    I have a Toyota and the owner's manual says the radar does not detect fixed objects, like parked cars.
    - ssl-3 2y ago
      Fascinating. I guess it my be better to stick with old-fashioned caltrops instead of chaff.
    - kube-system 2y ago
      I've seen other brands say/do the same. I suspect these systems would have tons of false positives otherwise, because cars often drive in close proximity to fixed barriers with high radar reflectivity. I'm sure the best way to not run into other cars on the road is to detect the other objects that are also moving at non-zero velocities. They're likely to be other cars, rather than road signs, poles, etc.
      - kayodelycaon 2y ago
sandworm101 2y ago
Some slight errors in terminology. >> The time between two consecutive chirps is called the pulse repetition rate (PRT), and plays a key role in the accuracy of doppler velocity estimation. This is actually known as the pulse-repetition interval (PRI), or time (PRT). A "rate" is describable by a frequency. An interval is described with a unit of time between repetitions. Radar signal characteristics are a rabbit hole of such definitions. They really do matter once one switches from theoretical discussion to actual math. Confuse a rate with a period and your math for calculating ambiguity zones will fall apart.
- vikramskr 2y ago
  Author here: Thanks for pointing this out. You are right, I will edit the article to fix.
roger_ 2y ago
Only skimmed this but didn’t see any mention of tracking, which is the only way (?) to get the true velocity when there’s non-radial motion.
- enchilada 2y ago
  The article says it’s using doppler effect. > The velocity of the target also manifests as a frequency shift in the received chirp due to Doppler effect
  - KeplerBoy 2y ago
    Sure but there's no doppler effect if the target moves tangentially to your radar.
    - toast0 2y ago
      For automotive radar, I'm not sure the requirement is to determine the true speed of the detected object. The requirement is to determine if the radar vehicle is likely to collide with the detected object. If the detected object is moving tangentially with the radar, relative velocity would be zero, because it's staying a fixed distance from the vehicle while both are in motion, and would seem to not be in the path of a collision.
    - vikramskr 2y ago
      Author here: You can still use angle of arrival estimates from the MIMO antenna system to track the object. https://www.viksnewsletter.com/p/how-radar-measures-angle?r=222kot&utm_campaign=post&utm_medium=web https://www.viksnewsletter.com/p/how-radar-measures-angle?r=...
transpute 2y ago
Wi-Fi 7 doppler radar has been used by AI/NPU laptop to detect nearby humans, https://www.youtube.com/watch?v=3WFx-8agAq4 https://www.youtube.com/watch?v=3WFx-8agAq4 > With a PC with Intel Wi-Fi sensing capabilities in sleep mode, the PC Wake-on-Approach is activated as it detects human presence. Even when a user forgets to lock the PC, a count-down to lock starts with no human present. False detection is prevented even with human presence behind and next to the PC.
deleted 2y ago
[deleted]
DoingIsLearning 2y ago
I really like the figures in this and the previous entry: https://www.viksnewsletter.com/p/how-automotive-radar-uses-chirp-signals https://www.viksnewsletter.com/p/how-automotive-radar-uses-c... If the author is here I would be curious to know your process and tools to generate the graphs and figures?
- vikramskr 2y ago
  Author here: I primarily use Excalidraw. I wrote an article detailing how I go about writing my newsletter here. https://www.viksnewsletter.com/p/how-i-write-an-engineering-newsletter?r=222kot&utm_campaign=post&utm_medium=web https://www.viksnewsletter.com/p/how-i-write-an-engineering-...
  - DoingIsLearning 2y ago
    Great walk through, thanks for sharing!

95 points by subbdue 2y ago

"The IF signal has a phase that is the difference between transmit and received signal phases." Yes. That's a neat property of superhetrodyning - phase is preserved. Both the outgoing and incoming signals are down-converted by mixing with the local oscillator. The phase angle difference between out and in is the same at both the transmitted/received frequency and the IF frequency. But down at the IF frequency, you get to work at a lower frequency where it's easier to do A/D conversion and counting. Most software defined radios still have a superhetrodyne front end, so the digital stuff is working at the IF frequency. This is less necessary than it used to be, now that digital circuits can work well into gigahertz ranges.

kazinator 2y ago
Same like how you get to work with single-digit-hertz beats when tuning one string to match another.
- tonyarkles 2y ago
  Heh, when getting a GPS Disciplined Oscillator set up I was feeding the 10 MHz oscillator into an SDR to use as a reference clock and was calibrating against a local cell tower. 7 Hz beat/offset at 1700 MHz!

dvh 2y ago

What will happen if every single car has radar? Wouldn't they interfere? You're stuck in traffic and 300 nearby cars are blasting radars all over the place?

psunavy03 2y ago
There are various ways a radar (or any RF signal) can be designed to recognize its own signal from all the background noise. We don't worry about millions of cell phones or WiFi routers sharing bandwidth either.
- r2_pilot 2y ago
  Actually, in both cases, we do. Cell towers deliberately have different frequencies allocated from their neighboring towers, and Wi-Fi has multiple channels, several of which do not have any overlap.
  - psunavy03 2y ago
    When I said "we don't worry about it," I meant the problem has been acceptably mitigated.
    - mitthrowaway2 2y ago
      What that really means is that several talented people dedicated their entire careers to worrying about it!
- ssl-3 2y ago
  Neither of those examples answers the question. Co-channel Wifi interference is real. It really puts a damper on range and throughout compared to how it used to be. It is a largely unmitigated clusterfuck, as is the way with CSMA/CA once density increases enough. LTE interference isn't an important thing in practice, in part because because all participating devices have very tightly-controlled timings. It isn't a clusterfuck at all because of the mitigations in place, but it does require centralized coordination to be this way. Radars on cars don't have centralized coordination (do they?). What mechanism prevents their performance from degrading as wifi does?
  - r2_pilot 2y ago
    For your specific question, you can send a pulse train specific to the radar emitter, and you filter out anything that doesn't match your specific pulse train
    - ssl-3 2y ago
      How do I know that each of the pulses I sent out are the same ones I receive in reflection, in terms of frequency (remember, we have moving targets and therefore Doppler shift here), and amplitude? The reflections of my own pulses are pretty quiet compared to the pulses of, say, an oncoming vehicle's own radar transmissions. How do I discern those individually-received pulses from the very similar pulses that are being received from other transmitting vehicles? (Sure, their timings should be be different, and the pulses may be very brief. But they may also overlap rather significantly, and errors can be very bad here. We need some rather good certainty here -- however that certainty is accomplished.)
kube-system 2y ago
> What will happen if every single car has radar? If you're on the road in a relatively affluent area where people drive late model cars, this is pretty close to already the case. Automakers have started making these systems standard on many/all of their models in the US for several years now. Toyota, for example, started rolling out these systems a decade ago, and have been standard on all US models since 2018. I'm not sure what these systems use in practice for interference mitigations, but there's a bunch of stuff that could be done, for instance, hopping between different frequencies.
curiousfab 2y ago
Typical FMCW radars transmit very short ramps (microseconds) at a very long (relatively) intervals (several ten milliseconds), i.e. a duty cycle of less than 0.1%. In order to create interference between two radars, the ramps have to overlap pretty exactly, within a few nanoseconds of each other. This is very unlikely to happen. Modern radars employ technologies to detect and/or avoid such collisions. Overall it is not really an issue, even with many radars in crowded spaces.
- abstrakraft 2y ago
  This is true for some earlier lofi radars, but as driver assistance and self-driving have developed, so have the requirements and capabilities of the radar systems. Newer systems generally have shorter PRIs for higher doppler bandwidth, and much higher duty cycles for more energy on target - the FCC limits power, so you've got to get energy from the time axis. Both of these things make the interference problem harder.
- aidenn0 2y ago
  How is it continuous wave if it has a non-unity duty cycle?
  - abstrakraft 2y ago
    In practice, the difference between pulsed radar and continuous wave radar is a continuum rather than a dichotomy. Historically, FMCW (frequency modulated continuous wave) had a high duty cycle (though not 100%, the ramp generators need finite time to reset (though you can alternate between up and downramps and get closer)). For some applications, though, requirements force you to short ramps and long PRIs, thus low duty cycles, but the name (FMCW) sticks.
kazinator 2y ago
I suspect radar like this needs only a tiny time slices to do its work. Say for example that it's only necessary to get updates about the moving object 100 times per second: every 10 ms. The radar pulse durations necessary to do the job can probably be measured in microseconds, though. A 10 ms separation between pulses measured in microseconds is a large amount of empty space.
deleted 2y ago
[deleted]
isopede 2y ago
Interference is a real problem with FMCW radars, either maliciously in the case of electronic warfare, or accidentally in the case you mentioned, with many radars in the same space using the same frequency band. Wifi and cell phones use time division or frequency division multiplexing techniques, but radars (at least current-gen) generally do not. There are mitigation techniques like randomization of chirp frequencies, choosing different idle times between frames, and signal processing techniques to try to detect interference and filter it out. In the general case, FMCW techniques will always have interference problems. This is one reason amongst many others that military radars do not use FMCW but instead coded pulse compression techniques.

teeray 2y ago

It would be interesting to see how to attenuate or deflect radar emissions from cars to passively disable automatic braking. Won’t brake if the car believes it’s driving on a flat expanse of nothing.

jvanderbot 2y ago
The radar cross section of an automobile within 100 meters is probably too high unless it's made of celery or has blackbody paint.
opwieurposiu 2y ago
If you had a chaff dispenser, perhaps you could deploy a chaff cloud that would look like a fixed object and trigger the emergency braking of the cars behind you. The newest weapon in the war against tailgating.
- kayodelycaon 2y ago
  I have a Toyota and the owner's manual says the radar does not detect fixed objects, like parked cars.
  - ssl-3 2y ago
    Fascinating. I guess it my be better to stick with old-fashioned caltrops instead of chaff.
  - kube-system 2y ago
    I've seen other brands say/do the same. I suspect these systems would have tons of false positives otherwise, because cars often drive in close proximity to fixed barriers with high radar reflectivity. I'm sure the best way to not run into other cars on the road is to detect the other objects that are also moving at non-zero velocities. They're likely to be other cars, rather than road signs, poles, etc.
    - kayodelycaon 2y ago

sandworm101 2y ago

Some slight errors in terminology. >> The time between two consecutive chirps is called the pulse repetition rate (PRT), and plays a key role in the accuracy of doppler velocity estimation. This is actually known as the pulse-repetition interval (PRI), or time (PRT). A "rate" is describable by a frequency. An interval is described with a unit of time between repetitions. Radar signal characteristics are a rabbit hole of such definitions. They really do matter once one switches from theoretical discussion to actual math. Confuse a rate with a period and your math for calculating ambiguity zones will fall apart.

vikramskr 2y ago
Author here: Thanks for pointing this out. You are right, I will edit the article to fix.

roger_ 2y ago

Only skimmed this but didn’t see any mention of tracking, which is the only way (?) to get the true velocity when there’s non-radial motion.

enchilada 2y ago
The article says it’s using doppler effect. > The velocity of the target also manifests as a frequency shift in the received chirp due to Doppler effect
- KeplerBoy 2y ago
  Sure but there's no doppler effect if the target moves tangentially to your radar.
  - toast0 2y ago
    For automotive radar, I'm not sure the requirement is to determine the true speed of the detected object. The requirement is to determine if the radar vehicle is likely to collide with the detected object. If the detected object is moving tangentially with the radar, relative velocity would be zero, because it's staying a fixed distance from the vehicle while both are in motion, and would seem to not be in the path of a collision.
  - vikramskr 2y ago
    Author here: You can still use angle of arrival estimates from the MIMO antenna system to track the object. https://www.viksnewsletter.com/p/how-radar-measures-angle?r=222kot&utm_campaign=post&utm_medium=web https://www.viksnewsletter.com/p/how-radar-measures-angle?r=...

transpute 2y ago

Wi-Fi 7 doppler radar has been used by AI/NPU laptop to detect nearby humans, https://www.youtube.com/watch?v=3WFx-8agAq4 https://www.youtube.com/watch?v=3WFx-8agAq4 > With a PC with Intel Wi-Fi sensing capabilities in sleep mode, the PC Wake-on-Approach is activated as it detects human presence. Even when a user forgets to lock the PC, a count-down to lock starts with no human present. False detection is prevented even with human presence behind and next to the PC.

deleted 2y ago

[deleted]

DoingIsLearning 2y ago

I really like the figures in this and the previous entry: https://www.viksnewsletter.com/p/how-automotive-radar-uses-chirp-signals https://www.viksnewsletter.com/p/how-automotive-radar-uses-c... If the author is here I would be curious to know your process and tools to generate the graphs and figures?

vikramskr 2y ago
Author here: I primarily use Excalidraw. I wrote an article detailing how I go about writing my newsletter here. https://www.viksnewsletter.com/p/how-i-write-an-engineering-newsletter?r=222kot&utm_campaign=post&utm_medium=web https://www.viksnewsletter.com/p/how-i-write-an-engineering-...
- DoingIsLearning 2y ago
  Great walk through, thanks for sharing!