# Hardware store laser rangefinder. How do they do that for \$50?

#### transconductance

Joined Jun 29, 2019
80
I want to ask how a hardware-store laser rangefinder works. I have a Bosch GLM20 and it works very well and definitely has valid sub-inch resolution. I know everyone is going to explain to me that it's time of flight divided by c, etc, etc. The problem is that I find that hard to believe. Can they really build a tens-of-gigahertz timer into something that sells for so little money and runs on so little power?

#### sagor

Joined Mar 10, 2019
898
Not sure about it, but it may not be a "timer". Any wave (radio or light) can be send down a path and reflections received that indicate time/distance. A Fourier Analysis of the wave patterns (peaks and valleys) can reveal the distance. This is called software "TDR" in many similar cases. For example, a RF analyzer testing a length of cable, sends multiple wavelength signals down the cable, and analyzes the return wave pattern to determine the length of the cable (to the end which reflects the radio wave).
Rangefinders may also use time measurements along with other methods, hard to tell without knowing the hardware or how it is designed to measure things....
I'm sure there are simple ways to measure things or calculate distance other than the obvious "measure the time" methods.

#### transconductance

Joined Jun 29, 2019
80
Could be. Radar altimeters work like that. They transmit a frequency-swept signal and compare the transmit frequency to the return frequency. The difference is a function of the round-trip time and thus distance.

My question isn't so much how it can be done. I can think of several ways. My problem is that every search result and reference says they measure the time of flight and I just want to know if that's actually how it works.

#### MisterBill2

Joined Jan 23, 2018
17,835
The swept frequency altimeters were measuring time of flight but in an analog manner. The difference between the sent freq and the returned freq is a direct function of the time of flight, but measuring a difference frequency is much simpler.

#### shortbus

Joined Sep 30, 2009
10,044
This thread would have been the perfect thing/answer to another one. The one where the guy wanted to "see" behind him when riding his bicycle in London. From a couple of weeks ago, didn't bother to search for it to link to though.