A bit about LIDAR

Many moons ago, I was a teenager in New Guinea, working in what we would call "work experience" today. I was in the jungle near Sogeri, and though I didn't realise it at the time, it became good background for the history of the Kokoda campaign that I wrote for younger readers, some years back. It helped me in another way, because we used a "chain", by then a steel tape, to measure distances.

In Port Moresby, I was shown a tellurometer, though I never saw one operating. I was told that in the near future, chaining would be a thing of the past, but the basic methods we used were far more hands-on. They were little changed from the methods used by explorers and surveyors in the 19th century, so that helped me when it came to writing histories of that sort of work.

It's hard to relate to that now, in an era when the truth of plate tectonics can be established by GPS measurements on ground stations. Even recent advances whizz by so fast that we tend to miss them, as a US friend observed the other day:

"[My dad] kept reminding me that he was working on the Atlas missile program and the controls were all in a trailer bigger than his 5th wheel and didn't have as much computing power as Liz's graphing calculator. It's pretty awesome to think that we could have sent men to the moon with less computing capacity than I'm carrying around in my purse."

Well, the tellurometer was good, but grab this, from a release I saw some time back, relating to a story in Nature Photonics*: I tucked it away in a file that was just being searched for something quite different...

"By combining the best of two different distance measurement approaches with a super-accurate technology called an optical frequency comb, researchers at the National Institute of Standards and Technology (NIST) have built a laser ranging system that can pinpoint multiple objects with nanometre precision over distances up to 100 kilometres. The novel LIDAR (for 'light detection and ranging') system could have applications from precision manufacturing lines on Earth to maintaining networks of satellites in perfect formation, creating a giant space-based platform to search for new planets."

LIDAR is a bit like radar: it sends out a flash then detects and analyzes the reflection, updating distances to multiple reference points every 200 microseconds (to get your head around that, it means 5000 times a second!). Now LIDAR has an "ambiguity range", and it has trouble distinguishing between two distances separated by this range. The new system has an ambiguity range of around 1.5 metres, so all the operators have to do is use GPS or some other method which gives you a close enough measure to tell you which of the competing values to accept.

So, we can measure the distances between mountains incredibly accurately but what else can we do? We can launch a bunch of satellites to fly "in formation" at very precise distances, measured by LIDAR. These, says the release, could form images of "black holes with multiple X-ray telescopes on different satellites, and support tests of general relativity through measurements of satellite spacing in a gravitational field".

LIDAR may also be used in automated manufacturing, where many parts need to fit together with tight tolerances, the developers say. This is always the crunch point in press releases: some of them make ambit claims about "shows promise as a cure for cancer" while others miss the boat by not seeing where the real applications may lie.

Recall that Frank Whittle conceived of the jet engine as a power unit for fighters, and not for the jets that criss-cross the world each day, carrying Covid-19, SARS, swine flu and tourists from place to place. Before him, the earliest rocket scientists were simply trying to find a way to get higher in the wispy gas at the edge of our atmosphere than rockets could go.

I would like to be around for another fifty years to see what LIDAR ends up doing, but that won't happen. That's why I spend my days burrowing through 19th century science, where I can look to see what became of this or that greatly praised invention, like putting sulfur in your socks to ward off cholera, but that's another story.

And this is yet another story:
https://www.gizmodo.com.au/2020/05/what-is-lidar-and-why-would-you-want-it-on-your-phone/

* I. Coddington, W. C. Swann, L. Nenadovic and N. R. Newbury. Rapid, precise absolute distance measurements at long range. Nature Photonics. Published online May 24, 2009. I never throw out any notes!