Lightware SF10/x

How do range finders handle trees? I have processed a lot of data from manned aircraft with laser altimeters and these often record first and last returns (reflections). The first reflection is of course the tree canopy, and the last is the ground.

Has anyone got experience flying the lidar lite and lightware rangefinders over trees? What happens?

I would prefer a laser altimeter to be terrain following and not tree height following.

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Replies

  • @Justin - we've set the SF10 family to capture the first return only. This is the safer option for situations where you don't want to fly into a tree.

    For terrain following where there are gaps in the canopy, you can filter out the short and long results by creating two rolling filters - one that accepts results that are shorter than the mean and one that takes results that are longer than the mean. When there is both canopy and ground present the "short filter" locks onto the canopy whilst the "long filter" locks onto the ground. If there is only ground or only canopy present then the filters drift towards the mean and show the same result.

    There is actually enough processing power and detail in the data for an SF10 to provide multiple return information. This is however well beyond the use case of most of our customers so we haven't provided the functionality. We might introduce these features as the demand for survey applications increases. 

    • Hello,

      Terrain following rather than tree height following would be of much more interest to us. We would set the sonar height in the automission to say 20m above the tallest tree in the area. We would know the tallest tree since we would have previously flown a lidar survey over the area or have discovered it by other means.

      Can you make it an option in the firmware to use the last return rather than the first return?

      When we purchased the Lightware SF10c, we were under the impression that we where getting terrain following, not tree following.

      Cameras and sensors want a near constant distance to the ground surface, not the tree tops.

      • That's certainly possible Justin. I guess the right solution would be to offer multiple return capability so that the user can select options like first, last or all returns. I've added your request to our development schedule and it will go through the formal approval process in the next few weeks.

        • How did the approval process for lightware terrain following instead of tree height following go?

          • We ran a number of tests and got some very good information about what would be involved to add ground tracking. I was hoping to get some new firmware included as part of the latest release of the SF11/C but unfortunately there were too many unproven features for the technical committee to allow them without further trails.

            So here are some of the things that still need to be resolved:

            • Minimum detectable height separation between canopy and ground - at this time the search algorithm that looks for valid return signals doesn't specifically isolate returns that are closely spaced and overlapping. We might have to change this algorithm to improve the signal discrimination.
            • Filter biasing - since there will be a mixture of return types, either single canopy, single ground or two signals, there needs to be a mathematical filter that biases the results towards the ground. At the moment this filter is unbiased and tends towards either the canopy or the ground depending upon which has the higher hit rate.
            • Interpretation of signals with significantly different amplitudes in the presence of noise - in a single return system, the return signal is usually much larger than the noise. Where there is signal attenuation through the canopy, the ground return may be much smaller than the canopy return and at long range this could easily be interpreted as noise rather than a genuine signal. We need to define a clear set of rules for identifying weak ground returns under these conditions.

            These are just of few of the things that we need to consider before offering a terrain following solution that is reliable enough to be of value. We have included all the information as part of the next round of firmware upgrades but there is no fixed implementation date yet.

        • The first or last choice in the SF10/x firmware would be immediately compatible with the pixhawk.

          The multiple returns would not and be harder to deal with by any flight controller.

          Go for the low hanging fruit and first/last option.

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