Long range Laser Altimeter for commercial Drones and Light Aircraft

The SF00 long range laser altimeter

Measurements of the height above ground up to and beyond 250m (820 feet) provide important information to many different types of aircraft. For commercial and military drones operating beyond line-of-sight or light aircraft and helicopters operating in uncontrolled airspace, knowing the height above ground is a critical safety requirement. Accurate AGL measurement is also important for precision photographic work in surveying and wildlife conservation.

Our new SF00, long range laser altimeter includes some interesting features that provide high reliability data even under difficult operating conditions. The long range capability can increase the chance of detecting background noise and the requirement for redundancy in critical systems means that the SF00 has to be able to recognize it’s own laser signature against a background of similar signals. Addressing these challenges has lead to significant improvements in laser altimeter performance for mission critical applications.

Under normal conditions the signals detected by a laser altimeter are clean and have a high SNR so measuring the time-of-flight to centimeter precision is no difficulty. In the image below the blue trace shows the “measurement window” during which a return signal is expected and the yellow trace is a typical return signal. Signals outside the measurement window are ignored.

Clean return signals inside the measurement window

In situations where extreme background light and high temperatures are experienced, noise in the form of stray photons or high energy electrons can create interference on the return signal. Internal control systems continually monitor this noise and make adjustments to the amplifier circuits to minimize its effect but there is always some residual noise in the system. Differentiating between this random noise and the return signals is critical to getting stable results. The SF00 does this by performing a statistical analysis on groups of measurements to establish their standard deviation. Groups that contain noise have a higher standard deviation than groups containing clean signals.

Noise due to background light and temperature effects

In high reliability systems it is not uncommon to use two laser altimeters aimed at the same point on the ground. Under these conditions it is possible for one unit to detect the return signal of the other since the lasers have the same wavelength and firing frequency. The SF00 deals with this in two ways:

Firstly, there is a “channel selection” that changes the firing frequency of the laser. This reduces the probability of the return signal from one unit showing up in the measurement window of the other. However, there is still a finite probability that a subharmonic of the different firing frequencies will continue to permit crosstalk.

The second approach is to randomize the laser firing frequency. This introduces “spread spectrum” like properties into the return signals increasing the probability of “self identification” whilst giving external signals the properties of random noise, and as mentioned above, the SF00 has a method of identifying and removing random noise.

Cross-talk in a high reliability application

At the limit of performance, it is possible to have a weak return signal mixed with random noise and cross talk from other lasers. The SF00 handles this by adding another layer of analysis that examines groups of results and makes a decision on the most probable altitude. If the probability is too low then it finally surrenders and warns of a lost signal condition.

The SF00 includes USB, serial and analog interfaces along with two set point alarms. Power is +5V at 250mA or USB and the maximum measuring range is > 250m at a resolution of 3cm. 

Thanks for reading, L.D.

Views: 3231

Comment by Bart Theys on October 8, 2015 at 6:39am

What is the weight/dimensions and where to buy? :)

Comment by naish on October 8, 2015 at 6:41am

You guys are always head with laser technology. I hope that one day Lightware will be able to compete with Pusleight on cheap short range LRF.  Btw I can't see any info of the SF00 on your website.

Comment by Laser Developer on October 8, 2015 at 6:49am

@Bart - dimensions 120mm x 110mm x 50mm with a weight of 500g. It will be available on the LightWare website in the next week for US$899 but you can pre-order by mailing info@lightware.co.za

@naish88 - thanks for the kind comment. Full information coming next week.


MR60
Comment by Hugues on October 8, 2015 at 7:03am

Nice, this is the missing product for forestry agronomists. They must measure tree heights over large areas, thus flying at altitudes typically 100m or 150m above trees. All of the SF lidars until now had a too short range. 

One remaining minus point is the 500g weight; that is relatively heavy and will undermine autonomy of the drone a lot.

Do you have any pictures or schèmes to show how it can be fixed on a multicopter or fixed wing ? (shape looks fancy but not very practical for frame attachments) ?

Comment by Laser Developer on October 8, 2015 at 7:16am

@Hugues - Thanks for the comments. The long range optics are quite heavy and they require an extremely rigid housing to keep their alignment perfect, especially with vibration and thermal changes. The attached dimension drawing shows the numerous mounting points that are available:

 

Comment by David on October 8, 2015 at 9:08am

This looks like a great product at a pretty reasonable price...what type of performance would you expect if you flew this over a 6ft tall corn crop? 

Comment by frederic reblewski on October 8, 2015 at 9:29am

interesting! what is the power consumption? ( I assume the weight do not include a power source )

Comment by Laser Developer on October 8, 2015 at 9:36am

@David - corn is surprisingly reflective in NIR because of the properties of the chlorophyl. The SF00 should read all the way to 250m.

@frederic - the unit runs from 5V and draws less than 250mA. It is designed to plug straight into a data logger that has a USB port (laptop, Linux box, Pi2 etc) or it can run from a separate 5V supply.


Developer
Comment by Randy on October 8, 2015 at 6:21pm

250m! Really impressive.

Comment by Laser Developer on October 9, 2015 at 6:15am

Thanks @Randy. Most people don't realize that to go from a unit that can measure 50m to a unit that can measure 250m requires a 25 times improvement in performance. You can't do this by increasing the laser power because of the limits imposed by eye safety regulations. Instead, every element of optical, mechanical and electronic design has to be improved. Lots of fun :). I wonder if we can measure to the moon?

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