[jiofih]

As far as I’m aware LIDAR cannot see though fog, as the light is dispersed, and even light rain might reduce range significantly.

Tesla cars have radar which can see through any weather condition and detect transparent surfaces, invisible to LIDAR.

[Animats]

There are LIDAR units that can see through fog. If you get data from multiple returns, not just the first, you can tell the difference between fog, rain, and solid surfaces. "First and last" is a big win. A solid obstacle in fog looks like a repeatable "last" (furthest) return, while rain and fog look like random disconnected points.

I think Google's own unit has 8 stored returns.

[Obi_Juan_Kenobi]

> LIDAR cannot see though fog

Simply depends on what wavelength of light you use.

Water-absorbing frequencies are nice because the atmosphere then shields most light, giving you nice SNR from your laser illumination. But better sensors could work around this, using other frequencies that can 'see' through fog.

It's certainly a technological limitation of current systems, but it's not an inherent limitation.

[holoduke]

Resolution of normal radar is much lower unless you have a gigant receiver. Also there is a significant delay of 10 to 100ms That's why a combination of lidar and radio radar is desired.

[usefulcat]

> Also there is a significant delay of 10 to 100ms

I'm curious why there is such an apparently long delay?

[kyzyl]

It really depends on the type of RADAR being used. If it's an FMCW radar, typically you will get a beat signal whose frequency corresponds to the target range. That frequency will vary with range, and in order to be well resolved you have to observe it for something like 1/period. So that puts a fundamental lower bound on how long you have to integrate. There are lots of tricks to improve things, and there are lots of variants of the standard radar hardware/methods, but I suspect that's what OP was referring to.

[6nf]

That doesn't explain 100ms delays though. Speed of light times 100ms round trip is 15,000 km.

[kyzyl]

It depends entirely on what configuration of RADAR you've got, and what you're pointing it at. You can build a system that will result in returns with beat frequencies of just about anything, depending on the Tx modulation and the range. The question is whether it will be useful for your application in terms of range/velocity resolution, latency, integration time etc.

Like I said I was just speculating about why OP specifically mentioned 10-100ms. the light does indeed travel pretty quickly (although, as anybody in the radar/lidar industry will tell you, not nearly quickly enough!), however the round trip time is just the minimum latency you have to eat to get any information about your target. Once you have light coming back, you need to integrate for some about of time to achieve your desired SNR. That time could be very small, or it could be infinite if there are no photons coming back. Let's randomly say that you're using a RADAR with a Tx bandwidth situated such that the round trip time is 1us, and that your target range is s.t. the beat frequency of the return is 1kHz. Your job is to estimate that frequency, so you have to observe the waveform (by integrating samples for an FFT, typically) for at least one cycle of the RF wavelength. That would require that you wait 1us for the light to fly, and then wait another 1ms for the RF to cycle once. So your measurement latency is ~1ms. Now that's not 100ms, but perhaps you need more than one cycle to give a good estimate of the frequency, and then even more because the target is faint and there aren't many photons coming back. You could possibly arrive at some much higher number, like 10-100ms.

I'm not sure if that was OP's point, but that's all I'm saying ;-)

[jiofih]

Also curious, most of what I’ve read about FMCW radar mentions single-digit millisecond latencies.

[kyzyl]

Yeah, it definitely can be shorter than 100ms. See my sibling comment. It just depends on the type of radar being used and the target range/velocity. Certainly for shortish range targets and mmwave radars on reasonably reflective targets you can get a signal with decent SNR in shorter time frames.