How automotive radar measures the velocity of objects
viksnewsletter.com
viksnewsletter.com
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.
>> 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.
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?
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.)
To answer these questions, we need to consider several key aspects of radar technology and signal processing. Let's break it down:
Identifying your own pulses: The main ways to distinguish your own reflected pulses from other sources are:
a) Frequency: While there is Doppler shift due to moving targets, the shift is typically small compared to the carrier frequency. Modern radars use sophisticated signal processing to account for and measure this shift.
b) Pulse coding: Radars often use unique pulse patterns or modulation schemes. This allows the receiver to correlate incoming signals with the known transmitted pattern, effectively filtering out other sources.
c) Timing: The time delay between transmission and reception is used to calculate distance. Signals arriving at unexpected times can be filtered out.
d) Direction: Using directional antennas or phased arrays, the radar system knows where it's transmitting and expects returns from that direction.
Dealing with weak reflections and interference: a) High sensitivity receivers: Modern radar systems use very sensitive receivers to detect weak return signals.
b) Signal processing: Advanced digital signal processing techniques can extract weak signals from noise and interfering sources.
c) Frequency diversity: Some systems use multiple frequencies to improve detection and reduce interference.
Discerning from other vehicle transmissions: a) Frequency allocation: Automotive radars typically operate in specific frequency bands. Within these bands, individual systems may use slightly different frequencies.
b) Time diversity: The probability of pulse overlap decreases with shorter pulse durations. Many modern systems use very short pulses or continuous wave techniques.
c) Spatial filtering: Directional antennas help focus on specific areas, reducing interference from other directions.
d) Signal characteristics: Different radar systems may use distinct modulation schemes, pulse repetition frequencies, or other signal characteristics that can be used to differentiate them.
Bad human.
If I wanted a hyper-confident response that is unfettered by such constructs as context and introspection from a bot I already know where to find one.
Sincerely,
Not a fucking bot.
If you had written it, I'd be willing to discuss exactly why I think that about this prose.
But you did not write it, and I have zero interest in conducting a third-party discussion with a bot.
Now consider how many bits of information is collected by these radars per second.
That gives and indication of how much free bandwith there is in the radar bands if the radars are built at the level of sophistication we expect from wifi. (At an OOM level, if not accurately).
Any congestion with current technology would be because the technology is far less optimized and standardized than wifi.
Even with 10 cars trying to scan the same region, that's about 5 OOM of headroom compared to wifi.
Also consider what modern military radiation radarss can do. The F-35 can actively track 50 targets, all in one direction. And it can do that while potentially 100s of aircraft all are sending radar beams into the same space, with some even activelly trying to jam the F-35 radar.
Obviously, really old and cheap radars can have interference issues. But any such limitation is not due to the Physics or even engineering, but rather on the cost of a radar sophisticated enough to handle its environment.
Any idea what the range of influence on the noise floor is/will likely be?
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.
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.
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.
> 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.
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?
https://www.viksnewsletter.com/p/how-i-write-an-engineering-...
The newest weapon in the war against tailgating.
I guess it my be better to stick with old-fashioned caltrops instead of chaff.
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.
If they wouldn’t do that filtering they would trigger a brake for almost all manhole covers and overhead signs and such.
Also a dopler radar can scan the whole sector with every emission, and simply measure the distance to the closest moving objects (and their speed) with each scan.
A radar that creates a 2d image of the sector usually needs to send a targeted beam in one direction at a time. Such a radar is not only much more expensive, it also adds latency.
Visible light is better suited for 2D imaging, anyway.
However, I'm not sure it'd be effective: the whole problem with tailgaters is that they follow too closely, so there might not be enough space for the chaff to decelerate and trigger the tailgater's radar. It would be an interesting test, though. But I think an active radar emitter that detects the tailgater's radar and generates a matching signal that appears like a rapidly-braking car would be better, though obviously more technically difficult.
https://owners.honda.com/utility/download?path=/static/pdfs/...
> The velocity of the target also manifests as a frequency shift in the received chirp due to Doppler effect
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.