3 ms·
current state of the art LIDAR systems have unbelievably tight windows in terms of space, time, frequency, and pulse coding which collectively allow for excelle
by etrautmann 8y ago
current state of the art LIDAR systems have unbelievably tight windows in terms of space, time, frequency, and pulse coding which collectively allow for excellent background rejection. In practice, this actually isn't that much of an issue. Sunlight and retro reflectors are more difficult challenges to deal with than other LIDARS causing interference.
More specifically, each detector is looking through a soda straw at a tiny spot in space, and rejecting anything that doesn't arrive within a narrow range of time, which also has to have the right light frequency and something like a pseudorandom code sequence which gets match filtered against itself.
In practice, you'd have to send the right wavelength of light to the right 1cm circle in space, send it during just the right few hundred nanoseconds, send it with the right pulse sequence (which one could determine but would be unlikely to happen by chance). Even if it happens once, it would have to happen many times at the imaging rate of the sensor to cause major issues...
- vonmoltke 8y agoIs there an equivalent to the Skolnik books[1] for LIDAR? I keep applying radar principles to LIDAR, but it seems like the function is significantly different if it includes things like a "pseudorandom code sequence". [1] https://books.google.com/books/about/Radar_Handbook.html?id=s35UAAAAMAAJ&source=kp_book_description https://books.google.com/books/about/Radar_Handbook.html?id=...
- al2o3cr 8y agoAFAIK Lidar is using a variant of the well-known radar technique of using pseudorandom noise modulation to avoid some of the gotchas with fixed-frequency pulses: https://apps.dtic.mil/dtic/tr/fulltext/u2/479308.pdf https://apps.dtic.mil/dtic/tr/fulltext/u2/479308.pdf
- Junk_Collector 8y agoSkolnik's books are good but they are pretty old and focus mostly on the hardware/transmission of radar. The code sequencing is on the baseband signal processing side and is a direct result of IQ modulation being a thing in vector signal processing. Lots of modern radar designs use the same style of pseudo-random encoding to prevent interference between units. It's a major push in automotive radar right now (70GHz band) for instance. Think of it as encoding your mac address on each packet you transmit wirelessly so you can tell which signals come from which source.
- vonmoltke 8y agoI don't recall such a scheme being used on either system I worked on. However, skimming the paper in your sibling's post it would be something done in the RF front end, which is probably why I never encountered it. The closest I got to the front end was pseudo-raw IQ from the pulses. It was also probably tied up in the anti-jamming measures, so it wouldn't get wide circulation on the programs.
- Junk_Collector 8y agoYes it's predominately a base band technique, although it has also been written about a fair bit in literature as an efficient power spreading alternative to a classic chirp. In that case, it is predominantly done to help propagation leveraging the extensive work done in communications on "noise-like" signals. In the case of Automotive radar, using it as an identifier to prevent interference is appealing because it allows each radar to use the full 6 GHz of channel bandwidth which greatly improves the performance of the radar despite the fact that there are tons of same channel independent devices operating. Each one would effectively be a jammer otherwise.