Greetings everyone,

This email is regarding measurement of range resolution in Time Of
Flight (TOF) LADAR systems. I have seen
(http://www.lase.de/produkte/scanner/eld_l_a/en.html) products with
extremely good range resolutions and I have a few related questions:

1. What are the factors that determine and limit the range resolution?
I would think that pulse width is an important factor. Is there any
relation connecting the pulse width to the axial resolution?

2. How do I estimate this value experimentally?

On a related note I'd also like to know what would be the factors that
limit the spatial resolution of such systems.

Any pointers in this regard will be appreciated.

Thanks for your time.
Palani

From a theoretical point of view, it is primarily signal-to-noise ratio
(SNR). Practical limitations must take into account repeatibility of pulse
shape, pulse stretching from extended targets, pulse walk from thresholding
and many other gotchas.

Bill

Thanks for the follow-up.
SNR affects the range accuracy for sure - but does it affect range resolution?

Thanks,
Palani

Most certainly! Of course if the resolution of the counter is lousy,
improved SNR cannot overcome range quantization noise.

If you think of range measurement as an information extraction process, the
maximimum extractible range information is SNR dependent.

Bill

High-end LIDAR systems are military, and finding information on them is
tricky:

www.kamanaero.com/EODC/images/RecentLidarResults.pdf
http://www-rab.larc.nasa.gov/lidar/

There are lots of ways to do time-of-flight measurements besides simply
measuring the transit time: One of the LIDAR systems I have seen
modulated the overlap between the sent pulse and a gated reciever: the
pulse shape is known, so the received power is very sensitive to the
transit time.

I doubt the instrument at the site you show is a "time of flight"
device. Far more likely its a phase sensing instrument, and therefore
uses a variable pulsewidth modulated laser diode. It would use use
thousands of on/off transitions in determining each distance measurement
by comparing the moduation pattern to the returned signal using
cross-correlation techniques. Resolution is a function of measurement
time, speed and size of the registers, and instrument stability. Single
pulse TOF rangefinders on the other hand are generally used for very
long ranges (several km and up) with very high pulse power (kilowatts to
megawatts peak) and range resolution rarely better than a meter. Low
power single pulse rangefinders are rare as the expense of the detection
circuits isn't justified for the low resolution. Almost all of the
inexpensive handheld rangefinders on the market use a simplified form of
phase detection with relatively low modulation rates.

The accuracy of quality surveying distance meters is limited primarily
by the uncertainty of the velocity of propagation of light through the
atmosphere. That varies of with air pressure and humidity which can't
easily be determined over the entire path. Still, they're orders of
magnitude better than a tape or chain.

Do not confuse time of flight meters that are similar to an optical pulse
radar with DME or distance measuring equipment used by surveyors.

Bill