- The Optech engineers report in Professional Surveyor on how they overcame inherent pulse rate limitations.
- At 100 kHz the return pulse is not received before the laser needs to transmit the next one.
- The innovative scanner design solution involves the use of dual lasers.
An article in the current (May 2010) issue of Professional Surveyor magazine provides an interesting discussion of the inherent limitations of airborne laser scanners and how the engineers at Optech came up with an innovative design solution.
Basically the problem is that at as a laser approaches 100 kHz the time between laser shots does not allow for the return pulse to be received before the next serial pulse is transmitted. This can result in the system not knowing how to match return and emitted pulses.
One solution to this is a multi-pulse logic design. In this system you have to anticipate when the return pulse is expected to arrive, thereby allowing you to match return with emitted signals. There can be issues with this approach as it can create blind zones for those regions when a return pulse arrives at the same time as a new pulse is being transmitted.
To overcome this the Optech engineers leveraged previous research that they had done with NOAA on the use of 2 scanners, which led them to the use of 2 lasers in the same scanner. This allows them to effectively double the repetition rate, while eliminating the issues with multipulse logic. One of the lasers can be forward looking which allows better definition of thin vertical objects.
A very interesting article.
