AGL measurement in conservation wetlands and over water using a laser altimeter

Drones have become an important tool for conservation work and we have seen them applied in game counting in Kenya, seal monitoring in the Arctic and for anti-poaching work in Southern Africa. One area of particular interest is in getting accurate size measurements of animals without direct human contact. LightWare has been assisting conservation groups with laser altimeters to improve the scaling accuracy of photographs taken from both drones and full size aircraft.

We recently did some work with the Center for Research into Ecological and Environmental Modelling at the University of St Andrews in Scotland. Their requirement was particularly demanding as they needed to fly over sandy beaches, wetlands and the sea in order to capture their images. Using a laser altimeter under these conditions is very challenging as water tends to absorb the laser light and scatter the remaining signals away. Initial test flights over the beach pictured above were done with one of our standard SF02 lasers and gave the following results:

You can see that the laser continuously loses signal when operating at around 30 m above the beach. The lost signal is shown as a reading of 40 m as defined in the recorder. There are periods of several seconds where the reading is lost completely.

The conventional solution to this problem is to use a higher power laser system with larger optics, but in this case we didn't want to add any extra weight. So we decided to go back to the drawing board and develop a new way of handling the intermittent signals, as well as improving the sensitivity of the laser detector. The results are shown below:

In this test the drone was flown over rock-pools and out over the ocean. You can see that the results are dramatically better, with no loss of signal recorded right up to the 40 m recorder limit. Additional tests have shown that the improved capabilities over water have also lead to greater altitude measurements over solid ground with distance readings in excess of 120 m possible.

Tests are continuing with this new product, called the SF11, and a limited number of "beta" units are available to the public at a price of $249.00. Contact Tracy at: info@lightware.co.za for further information.

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Comment by Fnoop on November 14, 2015 at 1:10am

That's great work, I often fly over water.  One question, it looks like the barometer is more reliable, so why not use that instead?

Comment by Laser Developer on November 14, 2015 at 2:05am

@Fnoop, thanks for the kind comment.

So why not use a barometer? Because the barometer doesn't know where the ground is, or where a boat is, or how steep the beach is, or if there is a small island, or a rocky outcrop, or if there is a tree sticking out of a swamp, or if a weather front has just come over changing the air pressure, or even where the water is!

It is a common misconception that a barometer measures the altitude of an aircraft. It doesn't. A barometer measures air pressure and air pressure can be considered a short term proxy for altitude above sea level. For a full scale aircraft traveling at 10,000 feet ASL, this approximation is close enough.

But for a drone taking high resolution images of seals along an uncharted coast line in a remote, hostile environment it is better to know where the "floor" really is :).

Of course, I can't argue with the fact that barometers are cheap!

Comment by Adam Kroll on November 15, 2015 at 6:30pm

Is a firmware update available to handle the intermittent signals for those who have bought existing units?

Comment by Laser Developer on November 15, 2015 at 11:06pm

Absolutely Adam. Please wait until we've completed all beta testing and officially released the SF11 to the public. SF10/B and SF10/C units purchased within the last 12 months should be hardware compatible with the new firmware. As always, LightWare would be delighted to take any opportunity to provide great customer service and if you feel the need to fly over water then contact Tracy: info@lightware.co.za to make arrangements.

Comment by Fnoop on November 25, 2015 at 6:54am

Ah, I see!  Actually one of the things I'm sometimes asked to do is take photos of seals, birds, whales etc, and it's very important not to fly below a certain altitude not to disturb them.  Having a really accurate altitude for this, and for estimating size through photos would indeed be a great feature.

Do you have any plans to try and produce a cheaper more consumer price-friendly version like the lidar lite?

Comment by Laser Developer on November 25, 2015 at 7:16am

@Fnoop - Are you sure you want to use a cheap LiDAR? If you need to justify the budget then consider that independent tests show that the SF11 will give 100m useable range over most terrain for $249.00. That's $2.49 per meter. We don't comment on competitor's products but I'm sure you can do the comparison yourself using real flight data and the retail price of any other LiDAR on the market. You might be surprised :)!

Comment by River Bian on January 21, 2016 at 8:00am

The SFxx datasheet presents that the Lidar operation temperature is 0...40°C, could it works well below 0°C or above 40°C?

Comment by Laser Developer on January 21, 2016 at 12:12pm

@River - Yes the unit has been tested over extended temperature ranges from -20C to + 60C. However, we do not guarantee the accuracy or performance over this greater range.

Comment by Nick Sargeant on January 27, 2016 at 6:06am

Hi @Laser Developer. Just wondering if there has been any progress on this project? How is the testing going? Are there many SF11 units out in the wild?

Comment by Laser Developer on January 27, 2016 at 6:42am

@Nick Sargeant - Hi Nick, it's nice to hear from you. Numerous tests have been conducted on the SF11's with about 25 having done real-world flight service. We are doing an official launch next week so there will be more information published on DIYD and the product is already available from our website. The units are being pre-ordered as fast as we can produce them so you might need to place a backorder to get in the queue (typically 2-4 weeks lead time if we are out of stock).

FYI: Below is flight data taken above solid ground in Australia (thanks to Tridge). This test was to check the performance at the limits of the measuring range to ensure that there were never any "false" readings. The aircraft was making steeply banked turns at 80m and 100m AGL. The barometric altitude (green) is flatter than the laser (purple) because the roll compensation is turned off.

The SF11 does lose some measuring range in the extremely bright background light conditions that we have sometimes seen down-under and it handles this very well without any errors in the data.

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